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统计排行 会员列表 社区服务 每日签到 平板模式 手机访问 申请版主 船币充值 船友英雄谱 输入用户名 记住 登录 找回密码 注册 我的快捷通道 大船友首页 行业新闻 招聘求职 技术论坛 圈子 群组 小游戏 船友网校 搜索其实很简单! (^_^) 帖子 搜索 [切换到宽版] 最新帖子 精华区 书签 发帖 回复 « 返回列表 « 1 2 » 2Go 上一主题 下一主题 1435 阅读 17 回复 关注Ta 发消息 只看楼主 更多操作 ESDEP European Steel Design Education Programme 以欧规钢 结构设计为 基础的课程。共分十六卷。 本贴展示第十五卷 ESDEP Course Disclaimer ESDEP ( The European Steel Design Education Programme) was published in 1993 and referred to the pre-Standard version of the Eurocodes (the ENV versi*****). The technical content therefore does not necessarily conform to versi***** of the Eurocodes that are being published (as EN versi*****) from 2002 to 2007. The advice given in ESDEP may be used as general guidance but reference should always be made to the published EN Standards and National Annexes for the actual rules and recommendati*****. Copyright This English language version of ESDEP may be freely used by Universities and Colleges as a source of reference for education and training in steel c*****truction, provided this is not for financial gain. In this context it may be freely copied. Other potential uses of the English version of ESDEP should be referred in writing to the SCI for guidance. Introduction There are links from the 18 Working Groups of the ESDEP course contents to 201 lectures which cover 22 broad subject areas. These are identified by group and lecture number, and each lecture corresponds approximately to a presentation of 50 minutes duration. The lectures include a summary page which lists the objectives and scope. Any pre-requisites are also itemised and a brief summary description of the content is given. References, bibliography and line diagrams are included after the main text. Content The content of the lectures ranges from applied metallurgy to structural systems, and includes mainstream subjects, such as buckling and composite behaviour, as well as specialised secti*****, for instance those dealing with corrosion protection and seismic design. The material covers not only buildings and bridges but also structures such as offshore platforms, tanks, chimneys and masts. The depth of study ranges from basic introduction to very advanced. Material may be useful to both teachers, as a source for lecture presentati*****, and to students, working individually or in groups. WG 1A : STEEL C*****TRUCTION: ECONOMIC & COMMERCIAL FACTORS WG 1B : STEEL C*****TRUCTION: INTRODUCTION TO DESIGN WG 2 : APPLIED METALLURGY WG 3 : FABRICATION AND ERECTION WG 4A : PROTECTION: CORROSION WG 4B : PROTECTION: FIRE WG 5 : COMPUTER AIDED DESIGN AND MANUFACTURE WG 6 : APPLIED STABILITY WG 7 : ELEMENTS WG 8 : PLATES AND SHELLS WG 9 : THIN-WALLED C*****TRUCTION WG 10 : COMPOSITE C*****TRUCTION WG 11 : CONNECTION DESIGN: STATIC LOADING 热搜: 验船师 设计实用手册 密性 IACS 船级社 喷涂 marpol 船友技术论坛 海洋石油支持船 > ESDEP Course-免费资源 ESDEP Course-免费资源 landho 级别: 论坛版主 显示用户信息 鲜花[161] 鸡蛋[2] 楼主 发表于: 2010-11-22 基础理论厅 设计交流厅 船舶修造厅 海洋工程厅 航运海事厅 休闲娱乐厅 管理办公室 新帖速递 (new) 我的主题 我的回复 兔斯基 俏老虎 洋葱头 船友技术论坛_海洋石油支持船 - Powered by phpwind http://bbs.52ship.com/read.php?tid=126910 1 of 54 02-11-2011 PM 11:29

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统计排行 会员列表 社区服务 每日签到平板模式 手机访问 申请版主 船币充值 船友英雄谱

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ESDEP European Steel Design Education Programme以欧规钢结构设计为基础的课程。共分十六卷。

本贴展示第十五卷

ESDEP Course

Disclaimer

ESDEP (The European Steel Design Education Programme) was published in 1993 and referred to the pre-Standardversion of the Eurocodes (the ENV versi*****). The technical content therefore does not necessarily conform to versi*****of the Eurocodes that are being published (as EN versi*****) from 2002 to 2007. The advice given in ESDEP may beused as general guidance but reference should always be made to the published EN Standards and National Annexes forthe actual rules and recommendati*****.

CopyrightThis English language version of ESDEP may be freely used by Universities and Colleges as a source of reference foreducation and training in steel c*****truction, provided this is not for financial gain. In this context it may be freelycopied. Other potential uses of the English version of ESDEP should be referred in writing to the SCI for guidance.

IntroductionThere are links from the 18 Working Groups of the ESDEP course contents to 201 lectures which cover 22 broad subjectareas. These are identified by group and lecture number, and each lecture corresponds approximately to a presentationof 50 minutes duration. The lectures include a summary page which lists the objectives and scope. Any pre-requisitesare also itemised and a brief summary description of the content is given. References, bibliography and line diagramsare included after the main text.

ContentThe content of the lectures ranges from applied metallurgy to structural systems, and includes mainstream subjects, suchas buckling and composite behaviour, as well as specialised secti*****, for instance those dealing with corrosionprotection and seismic design. The material covers not only buildings and bridges but also structures such as offshoreplatforms, tanks, chimneys and masts. The depth of study ranges from basic introduction to very advanced. Materialmay be useful to both teachers, as a source for lecture presentati*****, and to students, working individually or in groups.

WG 1A : STEEL C*****TRUCTION: ECONOMIC & COMMERCIAL FACTORSWG 1B : STEEL C*****TRUCTION: INTRODUCTION TO DESIGNWG 2 : APPLIED METALLURGYWG 3 : FABRICATION AND ERECTIONWG 4A : PROTECTION: CORROSIONWG 4B : PROTECTION: FIREWG 5 : COMPUTER AIDED DESIGN AND MANUFACTUREWG 6 : APPLIED STABILITYWG 7 : ELEMENTSWG 8 : PLATES AND SHELLSWG 9 : THIN-WALLED C*****TRUCTIONWG 10 : COMPOSITE C*****TRUCTIONWG 11 : CONNECTION DESIGN: STATIC LOADING

热搜: 验船师 设计实用手册 密性 IACS 船级社 喷涂 marpol 瓦

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ESDEP Course-免费资源

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鲜花[161] 鸡蛋[2]

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基础理论厅 设计交流厅 船舶修造厅 海洋工程厅 航运海事厅 休闲娱乐厅 管理办公室 新帖速递 (new) 我的主题 我的回复

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WG 12 : FATIGUEWG 13 : TUBULAR STRUCTURESWG 14 : STRUCTURAL SYSTEMS: BUILDINGSWG 15A : STRUCTURAL SYSTEMS: OFFSHOREWG 15B : STRUCTURAL SYSTEMS: BRIDGESWG 15C : STRUCTURAL SYSTEMS: MISCELLANEOUSWG 16 : STRUCTURAL SYSTEMS: REFURBISHMENTWG 17 : SEISMIC DESIGNWG 18 : STAINLESS STEEL

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Course Contents

WG 15A : STRUCTURAL SYSTEMS: OFFSHORELecture 15A.1 : Offshore Structures: General IntroductionLecture 15A.2 : Loads (I) : Introduction and Environmental LoadsLecture 15A.3 : Loads (II) - Other LoadsLecture 15A.4 : - Analysis ILecture 15A.5 : - Analysis IILecture 15A.6 : Foundati*****Lecture 15A.7 : Tubular Joints in Offshore StructuresLecture 15A.8 : FabricationLecture 15A.9 : InstallationLecture 15A.10 : Superstructures ILecture 15A.11 : - Superstructures IILecture 15A.12 : Connecti***** in Offshore Deck Structures

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Previous | Next | ContentsESDEP WG 15ASTRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.1: Offshore Structures:General Introduction

OBJECTIVE/SCOPETo identify the basic vocabulary, to introduce the major concepts for offshore platform structures, and to explain wherethe basic structural requirements for design are generated.

PREREQUISITESNone.SUMMARYThe lecture starts with a presentation of the importance of offshore hydro-carbon exploitation, the basic steps in thedevelopment process (from seismic exploration to platform removal) and the introduction of the major structural concepts(jacket-based, GBS-based, TLP, floating). The major codes are identified.For the fixed platform concepts (jacket and GBS), the different execution phases are briefly explained: design, fabricationand installation. Special attention is given to some principles of topside design.

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A basic introduction to cost aspects is presented.Finally terms are introduced through a glossary.

1. INTRODUCTIONOffshore platforms are c*****tructed to produce the hydrocarb***** oil and gas. The contribution of offshore oil productionin the year 1988 to the world energy c*****umption was 9% and is estimated to be 24% in 2000.The investment (CAPEX) required at present to produce one barrel of oil per day ($/B/D) and the production costs(OPEX) per barrel are depicted in the table below. Condition[/td] [td=1,1,131] CAPEX $/B/D[/td] [td=1,1,76] OPEX $/B[/td] Conventional[/td] Average[/td] [td=1,1,131] 4000 - 8000[/td] [td=1,1,76] 5[/td] Middle East[/td] [td=1,1,131] 500 - 3000[/td] [td=1,1,76] 1[/td] Non-Opec[/td] [td=1,1,131] 3000 - 12000[/td] [td=1,1,76] 8[/td] Offshore[/td] North Sea[/td] [td=1,1,131] 10000 - 25000[/td] [td=1,1,76] 5 - 10[/td] Deepwater[/td] [td=1,1,131] 15000 -35000[/td] [td=1,1,76] 10 - 15[/td]

World oil production in 1988 was 63 million barrel/day. These figures clearly indicate the challenge for the offshoredesigner: a growing contribution is required from offshore exploitation, a very capital intensive activity. Figure 1 shows the distribution of the oil and gas fields in the North Sea, a major contribution tothe world offshore hydrocarb*****. It also indicates the *****hore fields in England, the Netherlands and Germany.

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2. OFFSHORE PLATFORMS 2.1 Introduction of Basic Types The overwhelming majority of platforms are piled-jacket with deck structures, all built in steel(see Slides 1 and 2).

Slide 1 : Jacket based platform - Southern sector North Sea

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Slide 2 : Jacket based platform - Northern sector North Sea

A second major type is the gravity concrete structure (see Figure 2), which is employed in theNorth Sea in the Norwegian and British sectors.

A third type is the floating production unit.

2.2 Environment The offshore environment can be characterized by:

water depth at locationsoil, at seabottom and in-depthwind speed, air temperaturewaves, tide and storm surge, currentice (fixed, floes, icebergs)earthquakes (if necessary)

The topside structure also must be kept clear of the wave crest. The clearance (airgap) usuallyis taken at approximately 1,50 m, but should be increased if reservoir depletion will create significant subsidence.

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2.3 C*****truction The environment as well as financial aspects require that a high degree of prefabrication mustbe performed *****hore. It is necessary to design to limit offshore work to a minimum. The overall cost of a man-houroffshore is approximately five times that of an *****hore man-hour. The cost of c*****truction equipment required to handleloads, and the cost for logistics are also a magnitude higher offshore. These factors combined with the size and weight of the items, require that a designer mustcarefully c*****ider all c*****truction activities between shop fabrication and offshore installation.

2.4 Codes Structural design has to comply with specific offshore structural codes. The worldwide leadingstructural code is the API-RP2A [1]. The recently issued Lloyds rules [2] and the DnV rules [3] are also important. Specific government requirements have to be complied with, e.g. in the rules of Department ofEnergy (DoE), Norwegian Petroleum Direktorate (NPD). For the detail design of the topside structure the AISC-code [4] isfrequently used, and the AWS-code [5] is used for welding. In the UK the Piper alpha diaster has led to a completely new approach to regulation offshore.The resp*****ibility for regulatory control has been moved to the Health and Safety Executive (HSE) and the operator hasto produce a formal safety assessment (TSA) himself instead of complying with detailed regulati*****.

2.5 Certification and Warranty Survey Government authorities require that recognized bodies appraise the aspects of structuralintegrity and issue a certificate to that purpose. The major certification bodies are:

Det norske Veritas (DnV)Lloyds Register of Shipping (LRS)American Bureau of Shipping (ABS)Bureau Veritas (BV)Germanischer Lloyd (GL)

Their requirements are available to the designer [2, 3, 6, 7, 8]. Insurance companies covering transport and installation require the structures to be reviewedby warranty surveyors before acceptance. The warranty surveyors apply standards, if available, on a confidential basis.

3. OFFSHORE DEVELOPMENT OF AN OIL/GASFIELD 3.1 Introduction The different requirements of an offshore platform and the typical phases of an offshoredevelopment are summarized in [9]. After several initial phases which include seismic field surveying, one or moreexploration wells are drilled. Jack-up drilling rigs are used for this purpose for water depths up to 100 - 120 m; for deeperwater floating rigs are used. The results are studied and the economics and risks of different development plans areevaluated. Factors involved in the evaluation may include number of wells required, fixed or floated production facilities,number of such facilities, and pipeline or tanker off-loading. As soon as exploitation is decided and approved, there are four main technical activities, priorto production:

engineering and designfabrication and installation of the production facilitydrilling of production wells, taking 2 - 3 months/wellproviding the off loading system (pipelines, tankers, etc.).

The drilling and c*****truction interaction is described below for two typical fixed platformconcepts.

3.2 Jacket Based Platform for Shallow Water First the jacket is installed. The wells are then drilled by a jack-up drilling unit standing close bywith a cantilever rig extending over the jacket. Slide 3 shows a jack-up drilling unit with a cantilever rig. (In this instance itis engaged in exploratory drilling and is therefore working in isolation.)

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Slide 3 : Cantilevered drilling rig: Self-elevating (jack-up) exploration drillingplatform.

Design and c*****truction of the topside are progressed parallel to the drilling, allowingproduction to start soon after deck installation. For further wells, the jack-up drilling unit will be called once again and willreach over the well area of the production deck. As an alternative to this concept the wells are often accommodated in a separate wellheadplatform, linked by a bridge to the production platform (see Slide 1).

3.3 Jacket and Gravity Based Platform for Deep Water The wells are drilled from a drilling rig on the permanent platform (see Slide 2). Drilling startsafter the platform is built and completely installed. C*****equently production starts between one and two years afterplatform installation. In recent years pre-drilled wells have been used to allow an earlier start of the production. Inthis case the platform has to be installed exactly above the pre-drilled wells.

4. JACKETS AND PILE FOUNDATION 4.1 Introduction Jackets, the tower-like braced tubular structures, generally perform two functi*****:

They provide the substructure for the production facility (topside), keeping it stable above the waves.They support laterally and protect the 26-30 inch well conductors and the pipeline riser.

The installation methods for the jacket and the piles have a profound impact on the design.

4.2 Pile Foundation The jacket foundation is provided by open-ended tubular steel piles, with diameters up to 2m.The piles are driven approximately 40 - 80 m, and in some cases 120 m deep into the seabed. There are basically three types of pile/jacket arrangement (see Figure 3):

Pile-through-leg concept, where the pile is installed in the corner legs of the jacket. Skirt piles through pile sleeves at the jacket-base, where the pile is installed in guides attachedto the jacket leg. Skirt piles can be grouped in clusters around each of the jacket legs. Vertical skirt piles are directly installed in the pile sleeve at the jacket base; all other guidesare deleted. This arrangement results in reduced structural weight and easier pile driving. In contrast inclined piles enlargethe foundation at the bottom, thus providing a stiffer structure.

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4.3 Pile Bearing Resistance Axial load resistance is required for bearing as well as for tension. The pile accumulates bothskin friction as well as end bearing resistance. Lateral load resistance of the pile is required for restraint of the horizontal forces. These forceslead to significant bending of the pile near to the seabed. Number, arrangement, diameter and penetration of the piles depend on the environmental loadsand the soil conditi***** at the location.

4.4 Corrosion Protection The most usual form of corrosion protection of the bare underwater part of the jacket as well asthe upper part of the piles in soil is by cathodic protection using sacrificial anodes. A sacrificial anode (approximate 3 kNeach) c*****ists of a zinc/aluminium bar cast about a steel tube and welded on to the structures. Typically approximately5% of the jacket weight is applied as anodes. The steelwork in the splash zone is usually protected by a sacrificial wall thickness of 12 mm tothe members.

5. TOPSIDES 5.1 Introduction The major functi***** on the deck of an offshore platform are:

well controlsupport for well work-over equipmentseparation of gas, oil and non-transportable components in the raw product, e.g. water, parafines/waxes and sandsupport for pumps/compressors required to transport the product ashorepower generationaccommodation for operating and maintenance staff.

There are basically two structural types of topside, the integrated and modularized topsidewhich are positioned either on a jacket or on a concrete gravity substructure.

5.2 Jacket-based Topsides 5.2.1 Concepts There are four structural concepts in practice. They result from the lifting capacity of cranevessels and the load-out capacity at the yards:

the single integrated deck (up to approx 100 MN)the split deck in two four-leg unitsthe integrated deck with living quarter modulethe modularized topside c*****isting of module support frame (MSF) carrying a series of modules.

Slide 4 shows an integrated deck (though excluding the living quarters and helideck) beingmoved from its assembly building.

Slide 4 : Integrated topside during load out

5.2.2 Structural Design for Integrated Topsides For the smaller decks, up to approximately 100 MN weight, the support structure c*****ists oftrusses or portal frames with deletion of diagonals. The moderate vertical load and shear per column allows the topside to be supported by verticalcolumns (deck legs) only, down to the top of the piles (situated at approximately +4 m to +6 m L.A.T. (Low AstronomicTide).

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5.2.3 Structural Design for Modularized Jacket-based Topsides A major modularized topside weighs 200 to 400 MN. In this case the MSF is a heavy tubular structure (Figure 4), withlateral bracing down to the top of jacket.

5.3 Structural Design for Modularized Gravity-based Topsides The topsides to be supported by a gravity-based substructure (see Figure 2) are in a weightrange of 200 MN up to 500 MN. The backbone of the structure is a system of heavy box-girders with a height of approximately10 m and a width of approximately 12 - 15 m (see Figure 5).

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The substructure of the deck is rigidly connected to the concrete column and acts as a beamsupporting the deck modules. This connection introduces wave-induced fatigue in the deck structure. A recentdevelopment, foreseen for the Norwegian Troll platform, is to provide a flexible connection between the deck and concretecolumn, thus eliminating fatigue in the deck [10].

6. EQUIPMENT AND LIVING QUARTER MODULES Equipment modules (20-75 MN) have the form of rectangular boxes with one or twointermediate floors. The floors are steel plate (6, 8 or 10 mm thick) for roof and lower floor, and grating forintermediate floors. In living quarter modules (5-25 MN) all sleeping rooms require windows and several doors mustbe provided in the outer walls. This requirement can interfere seriously with truss arrangements. Floors are flat orstiffened plate. Three types of structural concepts, all avoiding interior columns, can be distinguished:

conventional trusses in the walls.stiffened plate walls (so called stressed skin or deck house type).heavy base frame (with wind bracings in the walls).

7. C*****TRUCTION 7.1 Introduction The design of offshore structures has to c*****ider various requirements of c*****truction relatingto:

fabrication.weight.load-out.sea transport.offshore installation.module installation.hook-up.commissioning.

A documented c*****truction strategy should be available during all phases of the design and theactual design development should be monitored against the c*****truction strategy. C*****truction is illustrated below by four examples.

7.2 C*****truction of Jackets and Topsides 7.2.1 Lift Installed Jackets The jacket is built in the vertical (smaller jackets) or horizontal position (bigger jackets) on aquay of a fabrication site. The jacket is loaded-out and seafastened aboard a barge. At the offshore location the barge is

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moored alongside an offshore crane vessel. The jacket is lifted off the barge, upended from the horizontal, and carefully set down onto theseabed. After setting down the jacket, the piles are installed into the sleeves and, driven into theseabed. Fixing the piles to the jacket completes the installation.

7.2.2 Launch Installed Jackets The jacket is built in horizontal position. For load-out to the transport barge, the jacket is put on skids sliding on a straight track of steelbeams, and pulled onto the barge (Slide 5).

Slide 5 : Jacket being loaded onto barge by skidding

At the offshore location the jacket is slid off the barge. It immerses deeply into the water andassumes a floating position afterwards (see Figure 6).

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Two parallel heavy vertical trusses in the jacket structure are required, capable of taking thesupport reacti***** during launching. To reduce forces and moments in the jacket, rocker arms are attached to the stern ofthe barge. The next phase is to upright the jacket by means of controlled flooding of the buoyancy tanksand then set down onto the seabed. Self-upending jackets obtain a vertical position after the launch on their own. Pilingand pile/jacket fixing completes the installation. 7.2.3 Topsides for a Gravity-Based Structure (GBS) The topside is assembled above the sea on a temporary support near a yard. It is then taken bya barge of such dimensi***** as to fit between the columns of the temporary support and between the columns of theGBS. The GBS is brought in a deep floating condition in a sheltered site, e.g. a Norwegian fjord. The barge is positionedbetween the columns and the GBS is then deballasted to mate with and to take over the deck from the barge. The floatingGBS with deck is then towed to the offshore site and set down onto the seabed.

7.2.4 Jacket Topsides For topsides up to approximately 120 MN, the topside may be installed in one lift. Slide 6 showsa 60 MN topside being installed by floating cranes.

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Slide 6 : Installation of 60MN K12-BP topside by floating crane

For the modularized topside, first the MSF will be installed, immediately followed by the modules.

7.3 Offshore Lifting Lifting of heavy loads from barges (Slide 6) is one of the very important and spectacularc*****truction activities requiring a focus on the problem when concepts are developed. Weather windows, i.e. periods ofsuitable weather conditi*****, are required for these operati*****.

7.3.1 Crane Vessel Lifting of heavy loads offshore requires use of specialized crane vessels. Figure 7 providesinformation on a typical big, dual crane vessel. Table 1 (page 16) lists some of the major offshore crane vessels.

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7.3.2 Sling-arrangement, Slings and Shackles For lifting, steel wire ropes in a four-sling arrangement are used which directly rest in the four-point hook of the cranevessel, (see Figure 8). The heaviest sling available now has a diameter of approximately 350 mm, a breaking load ofapproximately 48 MN, and a safe working load (SWL) of 16 MN. Shackles are available up to 10 MN SWL to connect thepadeyes installed at the module's columns. Due to the space required, connecting more than one shackle to the samecolumn is not very attractive. So when the sling load exceeds 10 MN, padears become an option.

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Table 1 Major Offshore Crane Vessels Operator [/td] [td]Name [/td] [td]Mode [/td] [td]Type [/td] Heerema[/td] [td=1,2,14%] Thor[/td] [td=1,2,18%] Monohull[/td] [td=1,1,10%] Fix[/td] [td=1,1,42%] 2720[/td]

鲜花[161] 鸡蛋[2]

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McDermott[/td] [td=1,2,14%] [/td] [td=1,2,18%] [/td] [td=1,1,10%] [/td] [td=1,1,42%] [/td]

Micoperi[/td] [td=1,1,14%] [/td] [td=1,1,18%] [/td] [td=1,1,10%] [/td] [td=1,1,42%] 14000[/td] ETPM[/td] [td=1,1,14%] [/td] [td=1,1,18%] [/td] [td=1,1,10%] [/td] [td=1,1,42%] [/td]

Notes:

Rated lifting capacity in metric tonnes.When the crane vessels are provided with two cranes, these cranes are situated at the vessels stern or bow atapproximately 60 m distance c.t.c.

3. Rev = Load capability with fully revolving crane.

Fix = Loadcapability with crane fixed.

7.4 SeaTransport and Sea Fastening Transportation isperformed aboard a flat-top barge or, if possible, on the deck of the crane vessel. The modulerequires fixing to the barge (see Figure 9) to withstand barge moti***** in rough seas. The sea fastening concept isdetermined by the positi***** of the framing in the module as well as of the "hard points" in the barge.

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7.5 Load-out 7.5.1Introduction For load-out threebasic methods are applied:

skiddingplatform trailersshearlegs.

7.5.2 Skidding Skidding is amethod feasible for items of any weight. The system c*****ists of a series of steel beams, acting as track, on which agroup of skids with each approximately 6 MN load capacity is arranged. Each skid is provided with a hydraulic jack tocontrol the reaction. 7.5.3 PlatformTrailers Specialized trailerunits (see Figure 10) can be combined to act as one unit for loads up to 60 - 75 MN. The wheels are individuallysuspended and integrated jacks allow adjustment up to 300 mm.

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The load capacityover the projected ground area varies from approximately 55 to 85 kN/sq.m. The units can drivein all directi***** and negotiate curves.

7.5.4 Shearlegs Load-out byshearlegs is attractive for small jackets built on the quay. Smaller decks (up to 10 - 12 MN) can be loaded out on thedecklegs pre-positioned on the barge, thus allowing deck and deckleg to be installed in one lift offshore.

7.6 Platform Removal In recent yearsplatform removal has become common. The mode of removal depends strongly on the regulati***** of the local authorities.Provision for removal should be c*****idered in the design phase.

8. STRUCTURAL ANALYSIS 8.1 Introduction The majority ofstructural analyses are based on the linear theory of elasticity for total system behaviour. Dynamic analysis is performedfor the system behaviour under wave-attack if the natural period exceeds 3 seconds. Many elements can exhibit localdynamic behaviour, e.g. compressor foundati*****, flare-stacks, crane-pedestals, slender jacket members, conductors.

8.2 In-placePhase Three types of

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analysis are performed:

Survival state, under wave/current/wind attack with 50 or 100 years recurrence period.Operational state, under wave/current/wind attack with 1 or 5 years recurrence period, under full operation.Fatigue assessment.Accidental.

All these analysesare performed on the complete and intact structure. Assessments at damaged structures, e.g. with one member deleted,and assessments of collision situati***** are occasionally performed.

8.3 C*****truction Phase The major phasesof c*****truction when structural integrity may be endangered are:

Load-outSea transportUpending of jacketsLifting.

9. COSTASPECTS 9.1 Introduction The economicfeasibility of an offshore project depends on many aspects: capital expenditure (CAPEX), tax, royalties, operationalexpenditure (OPEX). In a typicaloffshore field development, one third of the CAPEX is spent on the platform, one third on the drilling of wells and one thirdon the pipelines. Cost estimates areusually prepared in a deterministic approach. Recently cost-estimating using a probabilistic approach has been developedand adopted in major offshore projects. The CAPEX of aninstalled offshore platform topside amounts to approximately 20 ECU/kg.

9.2 CapitalExpenditure (CAPEX) The major elementsin the CAPEX for an offshore platform are:

project management and designmaterial and equipment procurementfabricationtransport and installationhook-up and commissioning.

9.3 Operational Expenditure (OPEX) In the North Seaapproximately 20 percent of OPEX are required for offshore inspection, maintenance and repair (IMR). The amount to bespent on IMR over the project life can add up to approximately half the original investment. IMR is the area inwhich the structural engineer makes a contribution by effort in design, selection of material, improved corrosionprotection, accessibility, basic provisi***** for scaffolding, avoiding jacket attachments dangerous to divers, etc.

10. DEEP WATER DEVELOPMENTS Deep waterintroduces a wide range of extra difficulties for the operator, the designer and c*****tructor of offshore platforms. Fixed platformshave recently been installed in water of 410 m. depth, i.e. "Bullwinkle" developed by Shell Oil for a Gulf of Mexico location.The jacket weighed nearly 500 MN. The maximum

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depth of water at platform sites in the North Sea is approximately 220 m at present. The development of the Troll fieldsituated in approximately 305 m deep water is planned for 1993. In the Gulf ofMexico and offshore California several fixed platforms in water depths of 250 - 350 m are in operation (Cerveza,Cognac). Exxon has a guyed tower platform (Lena) in operation in 300 m deep water. An option fordeeper locati***** is to use subsea wells with flowlines to a nearby (approximately maximum 10 km) fixed platform at asmaller water depth. Alternatively subsea wells may be used with flexible risers to a floating production unit. Subsea wellsare now feasible for 300 - 900 m deep water. The deepest wells have been developed off Brasil in moderate weatherconditi*****. The tension legplatform (TLP) seems to be the most promising deepwater production unit (Figure 11). It c*****ists of a semi-submersiblepontoon, tied to the seabed by vertical prestressed tethers. The first TLP was Hutton in the North Sea and recentlyTLP-Jolliet was installed at a 530 m deep location in the Gulf of Mexico. Norwegian Snorre and Heidrun fields have beendeveloped with TLPs as well.

11. CONCLUDING SUMMARY

The lecture starts with the presentation of the importance of offshore hydro-carbon exploitation, the basic steps in thedevelopment process (from seismic exploration to platform removal) and the introduction of the major structuralconcepts (jacket-based, GBS-based, TLP, floating).The major codes are identified.For the fixed platform concepts (jacket and GBS), the different execution phases are briefly explained: design,fabrication and installation. Special attention is given to the principles of topside design.A basic introduction to cost aspects is presented.Finally terms are introduced within a glossary.

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12.GLOSSARY OF TERMS AIRGAP Clearance between the top of maximum wave and underside of the topside. CAISS***** See SUMPS CONDUCTORS The tubular protecting and guiding the drill string from the topside down to 40 to 100m under the sea bottom. After drilling itprotects the well casing. G.B.S. Gravity based structure, sitting flatly on the sea bottom, stable through its weight. HOOK-UP Connecting components or systems, after installation offshore. JACKET Tubular sub-structure under a topside, standing in the water and pile founded. LOAD-OUT The operation of bringing the object (module, jacket, deck) from the quay onto the transportation barge. PADEARS(TRUNNI*****) Thick-walled tubular stubs, directly receiving slings and transversely welded to the main structure. PADEYES Thick-walled plate with hole, receiving the pin of the shackle, welded to the main structure. PIPELINERISER The piping section which rises from the sea bed to topside level. SEA-FASTENING The structure to keep the object rigidly connected to the barge during transport. SHACKLES Connecting element (bow + pin) between slings and padeyes. SLINGS Cables with spliced eyed at both ends, for offshore lifting, the upper end resting in the crane hook. SPREADER Tubular frame, used in lifting operation. SUBSEATEMPLATE Structure at seabottom, to guide conductors prior to jacket installation. SUMPS Vertical pipes from topside down to 5-10 m below water level for intake or discharge. TOPSIDE Topside, the compact offshore process plant, with all auxiliaries, positioned above the waves. UPENDING Bringing the jacket in vertical position, prior to set down on the sea bottom. WEATHERWINDOW A period ofcalm weather, defined on basis of operational limits for the offshore marine operation. WELLHEADAREA Area in topside where the wellheads are positioned including the valves mounted on its top.

13.REFERENCES [1] API-RP2A:Recommended practice for planning, designing and c*****tructing fixed offshore platforms. AmericanPetroleum Institute 18th ed. 1989. The structuraloffshore code, governs the majority of platforms. [2] LRS Code foroffshore platforms. Lloyds Registerof Shipping. London (UK)1988. Regulati***** of amajor certifying authority. [3] DnV: Rules

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for the classification of fixed offshore installati*****. Det NorskeVeritas 1989. Important set ofrules. [4] AISC:Specification for the design, fabrication and erection of structural steel for buildings. AmericanInstitute of Steel C*****truction 1989. Widely usedstructural code for topsides. [5] AWSD1.1-90: Structural Welding Code - Steel. AmericanWelding Society 1990. The structuraloffshore welding code. [6] DnV/MarineOperati*****: Standard for insurance warranty surveys in marine operati*****. Det norskeVeritas June 1985. Regulati***** of amajor certifying authority. [7] ABS: Rulesfor building and classing offshore installati*****, Part 1 Structures. AmericanBureau of Shipping 1983. Regulati***** of amajor certifying authority. [8] BV: Rules andregulati***** for the c*****truction and classification of offshore platforms. Bureau Veritas,Paris 1975. Regulati***** of amajor certifying authority. [9] ANON: Aprimer of offshore operati*****. Petex Publ.Austin U.S.A 2nd ed. 1985. Fundamentalinformation about offshore oil and gas operati*****. [10] AGJBerkelder et al: Flexible deck joints. ASME/OMAE-conference The Hague 1989 Vol.II pp. 753-760. Presentsinteresting new concept in GBS design.

14.ADDITIONAL READING

BS 6235: Code of practice for fixed offshore structures. British Standards Institution 1982. Important code, mainly for the British offshore sector.DoE Offshore installati*****: Guidance on design and c*****truction, U.K. Department of Energy 1990. Governmental regulati***** for British offshore sector only.UEG: Design of tubular joints (3 volumes). UEG Offshore Research Publ. U.R.33 1985. Important theoretical and practical book.J. Wardenier: Hollow section joints. Delft University Press 1981. Theoretical publication on tubular design including practical design formulae.ARSEM: Design guides for offshore structures welded tubular joints. Edition Technip, Paris (France), 1987. Important theoretical and practical book.

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D. Johnston: Field development opti*****. Oil & Gas Journal, May 5 1986, pp 132 - 142. Good presentation on development opti*****.G. I. Claum et al: Offshore Structures: Vol 1: Conceptual Design and Hydri-mechanics; Vol 2 - Strength and Safety forStructural design. Springer Verlag, London 1992. Fundamental publication on structural behaviour.W.J. Graff: Introduction to offshore structures. Gulf Publishing Company, Houston 1981. Good general introduction to offshore structures.B.C. Gerwick: C*****truction of offshore structures. John Wiley & S*****, New York 1986. Up to date presentation of offshore design and c*****truction.T.A. Doody et al: Important c*****iderati***** for successful fabrication of offshore structures. OTC paper 5348, Houston 1986, pp 531-539. Valuable paper on fabrication aspects.D.I. Karsan et al: An economic study on parameters influencing the cost of fixed platforms.

OTC paper5301, Houston 1986, pp 79-93. Goodpresentation on offshore CAPEX assessment.

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Previous | Next | ContentsESDEP WG 15ASTRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.2: Loads (I) : Introductionand Environmental Loads

OBJECTIVE/SCOPETo introduce the types of loads for which a fixed steel offshore structure must be designed. To present briefly the loadsgenerated by environmental factors.

PREREQUISITESA basic knowledge of structural analysis for static and dynamic loadings.SUMMARYThe categories of load for which a pile supported steel offshore platform must be designed are introduced and then thedifferent types of environmental loads are presented. The loads include: wind, wave, current, earthquake, ice and snow,temperature, sea bed movement, marine growth and tide generated loads. Loads due to wind, waves and earthquake arediscussed in more detail together with their idealizati***** for the various types of analyses. Frequent references aremade to the codes of practice recommended by the American Petroleum Institute, Det Norske Veritas, the BritishStandards Institution and the British Department of Energy, as well as to the relevant regulati***** of the NorwegianPetroleum Directorate.

1. INTRODUCTIONThe loads for which an offshore structure must be designed can be classified into the following categories:

Permanent (dead) loads.Operating (live) loads.Environmental loads including earthquakes.C*****truction - installation loads.Accidental loads.

Whilst the design of buildings *****hore is usually influenced mainly by the permanent and operating loads, the design ofoffshore structures is dominated by environmental loads, especially waves, and the loads arising in the various stages ofc*****truction and installation. This lecture deals with environmental loads, whilst the other loadings are treated in Lecture15A.3.In civil engineering, earthquakes are normally regarded as accidental loads (see Eurocode 8 [1]), but in offshore

landho

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engineering they are treated as environmental loads. This practice is followed in the two lectures dealing with loads,Lectures 15A.2 and15A.3.

2. ENVIRONMENTAL LOADSEnvironmental loads are those caused by environmental phenomena such as wind, waves, current, tides, earthquakes,temperature, ice, sea bed movement, and marine growth. Their characteristic parameters, defining design load values,are determined in special studies on the basis of available data. According to US and Norwegian regulati***** (or codes ofpractice), the mean recurrence interval for the corresponding design event must be 100 years, while according to theBritish rules it should be 50 years or greater. Details of design criteria, simplifying assumpti*****, required data, etc., canbe found in the regulati***** and codes of practice listed in [1] - [8].

2.1 Wind LoadsWind loads act on the portion of a platform above the water level, as well as on any equipment, housing, derrick, etc.located on the deck. An important parameter pertaining to wind data is the time interval over which wind speeds areaveraged. For averaging intervals less than one minute, wind speeds are classified as gusts. For averaging intervals ofone minute or longer they are classified as sustained wind speeds.The wind velocity profile may be taken from API-RP2A [2]: Vh/VH = (h/H)1/n (1)where:Vh is the wind velocity at height h,VH is the wind velocity at reference height H, typically 10m above mean water level,1/n is 1/13 to 1/7, depending on the sea state, the distance from land and the averaging time interval. It isapproximately equal to 1/13 for gusts and 1/8 for sustained winds in the open ocean.From the design wind velocity V(m/s), the static wind force Fw(N) acting perpendicular to an exposed area A(m2) can becomputed as follows: Fw = (1/2) r V2 Cs A (2)where:r is the wind density (r » 1.225 Kg/m3)Cs is the shape coefficient (Cs = 1,5 for beams and sides of buildings, Cs = 0,5 for cylindrical secti***** and Cs = 1,0 fortotal projected area of platform).Shielding and solidity effects can be accounted for, in the judgement of the designer, using appropriate coefficients.For combination with wave loads, the DNV [4] and DOE-OG [7] rules recommend the most unfavourable of the followingtwo loadings:a. 1-minute sustained wind speeds combined with extreme waves.b. 3-second gusts.API-RP2A [2] distinguishes between global and local wind load effects. For the first case it gives guideline values of mean1-hour average wind speeds to be combined with extreme waves and current. For the second case it gives values ofextreme wind speeds to be used without regard to waves.Wind loads are generally taken as static. When, however, the ratio of height to the least horizontal dimension of the windexposed object (or structure) is greater than 5, then this object (or structure) could be wind sensitive. API-RP2A requiresthe dynamic effects of the wind to be taken into account in this case and the flow induced cyclic wind loads due to vortexshedding must be investigated.

2.2 Wave LoadsThe wave loading of an offshore structure is usually the most important of all environmental loadings for which thestructure must be designed. The forces on the structure are caused by the motion of the water due to the waves whichare generated by the action of the wind on the surface of the sea. Determination of these forces requires the solution oftwo separate, though interrelated problems. The first is the sea state computed using an idealisation of the wave surfaceprofile and the wave kinematics given by an appropriate wave theory. The second is the computation of the wave forceson individual members and on the total structure, from the fluid motion.Two different analysis concepts are used:

The design wave concept, where a regular wave of given height and period is defined and the forces due to this waveare calculated using a high-order wave theory. Usually the 100-year wave, i.e. the maximum wave with a return periodof 100 years, is chosen. No dynamic behaviour of the structure is c*****idered. This static analysis is appropriate whenthe dominant wave periods are well above the period of the structure. This is the case of extreme storm waves actingon shallow water structures.Statistical analysis on the basis of a wave scatter diagram for the location of the structure. Appropriate wave spectraare defined to perform the analysis in the frequency domain and to generate random waves, if dynamic analyses forextreme wave loadings are required for deepwater structures. With statistical methods, the most probable maximumforce during the lifetime of the structure is calculated using linear wave theory. The statistical approach has to bechosen to analyze the fatigue strength and the dynamic behaviour of the structure.

2.2.1 Wave theoriesWave theories describe the kinematics of waves of water on the basis of potential theory. In particular, they serve tocalculate the particle velocities and accelerati***** and the dynamic pressure as functi***** of the surface elevation of thewaves. The waves are assumed to be long-crested, i.e. they can be described by a two-dimensional flow field, and arecharacterized by the parameters: wave height (H), period (T) and water depth (d) as shown in Figure 1.

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Different wave theories of varying complexity, developed on the basis of simplifying assumpti*****, are appropriate fordifferent ranges of the wave parameters. Among the most common theories are: the linear Airy theory, the Stokesfifth-order theory, the solitary wave theory, the cnoidal theory, Dean's stream function theory and the numerical theory byChappelear. For the selection of the most appropriate theory, the graph shown in Figure 2 may be c*****ulted. As anexample, Table 1 presents results of the linear wave theory for finite depth and deep water conditi*****. Correspondingparticle paths are illustrated in Figures 3 and 4. Note the strong influence of the water depth on the wave kinematics.Results from high-order wave theories can be found in the literature, e.g. see [9].

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2.2.2 Wave StatisticsIn reality waves do not occur as regular waves, but as irregular sea states. The irregular appearance results from thelinear superposition of an infinite number of regular waves with varying frequency (Figure 5). The best means to describea random sea state is using the wave energy density spectrum S(f), usually called the wave spectrum for simplicity. It isformulated as a function of the wave frequency f using the parameters: significant wave height Hs (i.e. the mean of thehighest third of all waves present in a wave train) and mean wave period (zero-upcrossing period) To. As an additionalparameter the spectral width can be taken into account.

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Wave directionality can be introduced by means of a directional spreading functionD(f,s), where s is the angle of thewave approach direction (Figure 6). A directional wave spectrum S(f,s) can then be defined as:S (f,s ) = S(f).D (f,s )(3)

The resp*****e of the structure, i.e. forces, moti*****, is calculated by multiplication of the wave energy spectrum with thesquare of a linear transfer function. From the resulting resp*****e spectrum the significant and the maximum expectedresp*****e in a given time interval can be easily deduced.

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For long-term statistics, a wave scatter diagram for the location of the structure is needed. It can be obtained frommeasurements over a long period or be deduced from weather observati***** in the region (the so-called hindcastmethod). The scatter diagram contains the joint probability of occurrence of pairs of significant wave height and meanwave period. For every pair of parameters the wave spectrum is calculated by a standard formula, e.g. Pierson-Moskowitz (Figure 6), yielding finally the desired resp*****e spectrum. For fatigue analysis the total number and amplitudeof load cycles during the life-time of the structure can be derived in this way. For structures with substantial dynamicresp*****e to the wave excitation, the maximum forces and moti***** have to be calculated by statistical methods or atime-domain analysis.

2.2.3 Wave forces on structural membersStructures exposed to waves experience substantial forces much higher than wind loadings. The forces result from thedynamic pressure and the water particle moti*****. Two different cases can be distinguished:

Large volume bodies, termed hydrodynamic compact structures, influence the wave field by diffraction and reflection.The forces on these bodies have to be determined by costly numerical calculati***** based on diffraction theory.Slender, hydrodynamically transparent structures have no significant influence on the wave field. The forces can becalculated in a straight-forward manner with Morison's equation. As a rule, Morison's equation may be applied when D/L£ 0.2, where D is the member diameter and L is the wave length.

The steel jackets of offshore structures can usually be regarded as hydrodynamically transparent. The wave forces onthe submerged members can therefore be calculated by Morison's equation, which expresses the wave force as the sumof an inertia force proportional to the particle acceleration and a non-linear drag force proportional to the square of the

particle velocity: (4)whereF is the wave force per unit length on a circular cylinder (N)v, |v| are water particle velocity normal to the cylinder, calculated with the selected wave theory at the cylinder axis (m/s) are water particle acceleration normal to the cylinder, calculated with the selected wave theory at the cylinder axis(m/s2)r is the water density (kg/m3)D is the member diameter, including marine growth (m)CD, CM are drag and inertia coefficients, respectively.In this form the equation is valid for fixed tubular cylinders. For the analysis of the motion resp*****e of a structure it hasto be modified to account for the motion of the cylinder [10]. The values of CD and CM depend on the wave theory used,surface roughness and the flow parameters. According to API-RP2A, CD » 0,6 to 1,2 and CM » 1,3 to 2,0. Additionalinformation can be found in the DNV rules [4].The total wave force on each member is obtained by numerical integration over the length of the member. The fluidvelocities and accelerati***** at the integration points are found by direct application of the selected wave theory.According to Morison's equation the drag force is non-linear. This non-linear formulation is used in the design waveconcept. However, for the determination of a transfer function needed for frequency domain calculati*****, the drag forcehas to be linearized in a suitable way [9]. Thus, frequency domain soluti***** are appropriate for fatigue life calculati*****,for which the forces due to the operational level waves are dominated by the linear inertia term. The nonlinear formulationand hence time domain soluti***** are required for dynamic analyses of deepwater structures under extreme, stormwaves, for which the drag portion of the force is the dominant part [10].In addition to the forces given by Morison's equation, the lift forces FD and the slamming forces FS, typically neglected inglobal resp*****e computati*****, can be important for local member design. For a member section of unit length, theseforces can be estimated as follows:FL = (1/2) r CL Dv2 (5)FS = (1/2) r Cs Dv2 (6)where CL, CS are the lift and slamming coefficients respectively, and the rest of the symbols are as defined in Morison'sequation. Lift forces are perpendicular to the member axis and the fluid velocity v and are related to the vortex sheddingfrequency. Slamming forces acting on the underside of horizontal members near the mean water level are impulsive andnearly vertical. Lift forces can be estimated by taking CL » 1,3 CD. For tubular members Cs» p.

2.3 Current LoadsThere are tidal, circulation and storm generated currents. Figure 7 shows a wind and tidal current profile typical of the Gulfof Mexico. When insufficient field measurements are available, current velocities may be obtained from various sources,e.g. Appendix A of DNV [4]. In platform design, the effects of current superimposed on waves are taken into account byadding the corresponding fluid velocities vectorially. Since the drag force varies with the square of the velocity, thisaddition can greatly increase the forces on a platform. For slender members, cyclic loads induced by vortex shedding mayalso be important and should be examined.

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2.4 Earthquake LoadsOffshore structures in seismic regi***** are typically designed for two levels of earthquake intensity: the strength level andthe ductility level earthquake. For the strength level earthquake, defined as having a "reasonable likelihood of not beingexceeded during the platform's life" (mean recurrence interval ~ 200 - 500 years), the structure is designed to respondelastically. For the ductility level earthquake, defined as close to the "maximum credible earthquake" at the site, thestructure is designed for inelastic resp*****e and to have adequate reserve strength to avoid collapse.For strength level design, the seismic loading may be specified either by sets of accelerograms (Figure 8) or by means ofdesign resp*****e spectra (Figure 9). Use of design spectra has a number of advantages over time history soluti*****(base acceleration input). For this reason design resp*****e spectra are the preferable approach for strength leveldesigns. If the design spectral intensity, characteristic of the seismic hazard at the site, is denoted by amax, thenAPI-RP2A recommends using amax for the two principal horizontal directi***** and 0,5amax for the vertical direction. TheDNV rules, on the other hand, recommend amax and 0,7 amax for the two horizontal directi***** (two differentcombinati*****) and 0,5 amax for the vertical. The value of amax and often the spectral shapes are determined by sitespecific seismological studies.

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Designs for ductility level earthquakes will normally require inelastic analyses for which the seismic input must bespecified by sets of 3-component accelerograms, real or artificial, representative of the extreme ground moti***** thatcould shake the platform site. The characteristics of such moti*****, however, may still be prescribed by means of designspectra, which are usually the result of a site specific seismotectonic study. More detail of the analysis of earthquakes isgiven in theLectures 17: Seismic Design.

2.5 Ice and Snow LoadsIce is a primary problem for marine structures in the arctic and sub-arctic zones. Ice formation and expansion cangenerate large pressures that give rise to horizontal as well as vertical forces. In addition, large blocks of ice driven bycurrent, winds and waves with speeds that can approach 0,5 to 1,0 m/s, may hit the structure and produce impact loads.As a first approximation, statically applied, horizontal ice forces may be estimated as follows: Fi = CifcA (7)where:A is the exposed area of structure,fc is the compressive strength of ice,Ci is the coefficient accounting for shape, rate of load application and other factors, with usual values between 0,3 and0,7.Generally, detailed studies based on field measurements, laboratory tests and analytical work are required to developreliable design ice forces for a given geographical location.In addition to these forces, ice formation and snow accumulati***** increase gravity and wind loads, the latter byincreasing areas exposed to the action of wind. More detailed information on snow loads may be found in Eurocode 1 [8].

2.6 Loads due to Temperature Variati*****Offshore structures can be subjected to temperature gradients which produce thermal stresses. To take account of suchstresses, extreme values of sea and air temperatures which are likely to occur during the life of the structure must beestimated. Relevant data for the North Sea are given in BS6235 [6]. In addition to the environmental sources, humanfactors can also generate thermal loads, e.g. through accidental release of cryogenic material, which must be taken intoaccount in design as accidental loads. The temperature of the oil and gas produced must also be c*****idered.

2.7 Marine GrowthMarine growth is accumulated on submerged members. Its main effect is to increase the wave forces on the members byincreasing not only exposed areas and volumes, but also the drag coefficient due to higher surface roughness. Inaddition, it increases the unit mass of the member, resulting in higher gravity loads and in lower member frequencies.Depending upon geographic location, the thickness of marine growth can reach 0,3m or more. It is accounted for indesign through appropriate increases in the diameters and masses of the submerged members.

2.8 TidesTides affect the wave and current loads indirectly, i.e. through the variation of the level of the sea surface. The tides are

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classified as: (a) astronomical tides - caused essentially from the gravitational pull of the moon and the sun and (b) stormsurges - caused by the combined action of wind and barometric pressure differentials during a storm. The combined effectof the two types of tide is called the storm tide. Tide dependent water levels and the associated definiti*****, as used inplatform design, are shown in Figure 10. The astronomical tide range depends on the geographic location and the phaseof the moon. Its maximum, the spring tide, occurs at new moon. The range varies from centimetres to several metres andmay be obtained from special maps. Storm surges depend upon the return period c*****idered and their range is on theorder of 1,0 to 3,0m. When designing a platform, extreme storm waves are superimposed on the still water level (seeFigure 10), while for design c*****iderati***** such as levels for boat landing places, barge fenders, upper limits of marinegrowth, etc., the daily variati***** of the astronomical tide are used.

2.9 Sea Floor MovementsMovement of the sea floor can occur as a result of active geologic processes, storm wave pressures, earthquakes,pressure reduction in the producing reservoir, etc. The loads generated by such movements affect, not only the design ofthe piles, but the jacket as well. Such forces are determined by special geotechnical studies and investigati*****.

3. CONCLUDING SUMMARYEnvironmental loads form a major category of loads which control many aspects of platform design.The main environmental loads are due to wind, waves, current, earthquakes, ice and snow, temperature variati*****,marine growth, tides and seafloor movements.Widely accepted rules of practice, listed as [1] - [13], provide guideline values for most environmental loads.For major structures, specification of environmental design loads requires specific studies.Some environmental loads can be highly uncertain.The definition of certain environmental loads depends upon the type of analysis used in the design.

4. REFERENCES[1] Eurocode 8: "Structures in Seismic Regi***** - Design", CEN (in preparation).[2] API-RP2A, "Recommended Practice for Planning, Designing and C*****tructing Fixed Offshore Platforms", AmericanPetroleum Institute, Washington, D.C., 18th ed., 1989.[3] OCS, "Requirements for Verifying the Structural Integrity of OCS Platforms"., United States Geologic Survey,National Centre, Reston, Virginia, 1980.[4] DNV, "Rules for the Design, C*****truction and Inspection of Offshore Structures", Det Norske Veritas, Oslo, 1977(with correcti***** 1982).[5] NPD, "Regulation for Structural Design of Load-bearing Structures Intended for Exploitation of PetroleumResources", Norwegian Petroleum Directorate, 1985.[6] BS6235, "Code of Practice for Fixed Offshore Structures", British Standards Institution, London, 1982.[7] DOE-OG, "Offshore Installation: Guidance on Design and C*****truction", U.K., Dept. of Energy, London 1985.[8] Eurocode 1: "Basis of Design and Acti***** on Structures", CEN (in preparation).[9] Clauss, G. T. et al: "Offshore Structures, Vol 1 - Conceptual Design and Hydromechanics", Springer, London 1992.[10] Anagnostopoulos, S.A., "Dynamic Resp*****e of Offshore Structures to Extreme Waves including Fluid - StructureInteraction", Engr. Structures, Vol. 4, pp.179-185, 1982.[11] Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing Co., Houston, 1981.[12] Graff, W.J., "Introduction to Offshore Structures", Gulf Publishing Co., Houston, 1981.[13] Gerwick, B.C. Jr., "C*****truction of Offshore Structures", John Wiley, New York, 1986.Table 1 Results of Linear Airy Theory [11]Phase q = kx -wtRelativewater depthd/L

Deep waterd/L ³0,5

Finite water depthd/L < 0,5

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Velocitypotential q

Surfaceelevation zDynamicpressure pdyn

=

za cosqrgza ekz cosq

za cos q

Water particlevelocitieshorizontal u =

vertical w =

za wekz cos qzawekz sin q

Water particleaccelerati*****horizontal u'

= vertical w' =

za w2 ekz sinq-za w2 ekz cosq

Wave celerity

c = Groupvelocity cgr =

Circularfrequency w=

Wave length

L = Wave number

k =

co =

cgr = w=

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Lo =[img]http://www.haiyangshiyou.com

/esdep/media/wg15a/Image182.gif[/img]

ko =[img]http://www.haiyangshiyou.com

/esdep/media/wg15a/Image183.gif[/img]

c =[img]http://www.haiyangshiyou.com

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cgr =[img]http://www.haiyangshiyou.com

/esdep/media/wg15a/Image185.gif[/img]

w=[img]http://www.haiyangshiyou.com

/esdep/media/wg15a/Image186.gif[/img]

L =[img]http://www.haiyangshiyou.com

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kd tanh kd =[img]http://www.haiyangshiyou.com

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Water particledisplacementshorizontal xvertical z

Particletrajectories

-za ekz sinqza ekz cos q

Circular orbits

[img]http://www.haiyangshiyou.com/esdep/media/wg15a/Image189.gif[/img]

[img]http://www.haiyangshiyou.com/esdep/media/wg15a/Image190.gif[/img]

Elliptical orbits

Where z a = [img]http://www.haiyangshiyou.com/esdep/media/wg15a/Image867.gif[/img]

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Previous | Next | ContentsESDEP WG 15ASTRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.3: Loads (II) - Other LoadsOBJECTIVE/SCOPETo present and briefly describe all loads, except environmental loads, and the load combinati***** for which a fixedoffshore structure must be designed.PREREQUISITESA basic knowledge of structural analysis for static and dynamic loadings.

SUMMARYThe various categories of loads, except environmental, for which a pile-supported steel offshore platform must bedesigned are presented. These categories include permanent (dead) loads, operating (live) loads, loads generated duringfabrication and installation (due to lifts, loadout, transportation, launching and upending) and accidental loads. In addition,the different load combinati***** for all types of loads, including environmental, as required (or suggested) by applicableregulati***** (or codes of practice) are given.The categories of loads described herein are the following:

Permanent (dead) loadsOperating (live) loadsFabrication and installation loadsAccidental loads

The major categories of environmental loads are not included. They are dealt with inLecture 15A.2.

1. PERMANENT (DEAD) LOADSPermanent loads include the following:a. Weight of the structure in air, including the weight of grout and ballast, if necessary.b. Weights of equipment, attachments or associated structures which are permanently mounted on the platform.c. Hydrostatic forces on the various members below the waterline. These forces include buoyancy and hydrostaticpressures.Sealed tubular members must be designed for the worst condition when flooded or non-flooded.

2. OPERATING (LIVE) LOADSOperating loads arise from the operati***** on the platform and include the weight of all non-permanent equipment ormaterial, as well as forces generated during operation of equipment. More specifically, operating loads include thefollowing:a. The weight of all non-permanent equipment (e.g. drilling, production), facilities (e.g. living quarters, furniture, lifesupport systems, heliport, etc), c*****umable supplies, liquids, etc.b. Forces generated during operati*****, e.g. drilling, vessel mooring, helicopter landing, crane operati*****, etc.The necessary data for computation of all operating loads are provided by the operator and the equipment manufacturers.The data need to be critically evaluated by the designer. An example of detailed live load specification is given in Table 1where the values in the first and second columns are for design of the porti***** of the structure directly affected by theloads and the reduced values in the last column are for the structure as a whole. In the absence of such data, thefollowing values are recommended in BS6235 [1]:a. crew quarters and passageways: 3,2 KN/m2

b. working areas: 8,5 KN/m2

c. storage areas: gH KN/m2

whereg is the specific weight of stored materials, not to be taken less than 6,87KN/m3,H is the storage height (m).Forces generated during operati***** are often dynamic or impulsive in nature and must be treated as such. For example,according to the BS6235 rules, two types of helicopter landing should be c*****idered, heavy and emergency landing. Theimpact load in the first case is to be taken as 1,5 times the maximum take-off weight, while in the second case this factorbecomes 2,5. In addition, a horizontal load applied at the points of impact and taken equal to half the maximum take-offweight must be c*****idered. Loads from rotating machinery, drilling equipment, etc. may normally be treated as harmonicforces. For vessel mooring, design forces are computed for the largest ship likely to approach at operational speeds.According to BS6235, the minimum impact to be c*****idered is of a vessel of 2500 tonnes at 0,5 m/s.

3. FABRICATION AND INSTALLATION LOADSThese loads are temporary and arise during fabrication and installation of the platform or its components. Duringfabrication, erection lifts of various structural components generate lifting forces, while in the installation phase forces aregenerated during platform loadout, transportation to the site, launching and upending, as well as during lifts related to

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installation.According to the DNV rules [2], the return period for computing design environmental conditi***** for installation as well asfabrication should normally be three times the duration of the corresponding phase. API-RP2A, on the other hand [3],leaves this design return period up to the owner, while the BS6235 rules [1] recommend a minimum recurrence interval of10 years for the design environmental loads associated with transportation of the structure to the offshore site.

3.1 Lifting ForcesLifting forces are functi***** of the weight of the structural component being lifted, the number and location of lifting eyesused for the lift, the angle between each sling and the vertical axis and the conditi***** under which the lift is performed(Figure 1). All members and connecti***** of a lifted component must be designed for the forces resulting from staticequilibrium of the lifted weight and the sling tensi*****. Moreover, API-RP2A recommends that in order to compensate forany side movements, lifting eyes and the connecti***** to the supporting structural members should be designed for thecombined action of the static sling load and a horizontal force equal to 5% this load, applied perpendicular to the padeyeat the centre of the pin hole. All these design forces are applied as static loads if the lifts are performed in the fabricationyard. If, however, the lifting derrick or the structure to be lifted is on a floating vessel, then dynamic load factors should beapplied to the static lifting forces. In particular, for lifts made offshore API-RP2A recommends two minimum values ofdynamic load factors: 2,0 and 1,35. The first is for designing the padeyes as well as all members and their endconnecti***** framing the joint where the padeye is attached, while the second is for all other members transmitting liftingforces. For loadout at sheltered locati*****, the corresponding minimum load factors for the two groups of structuralcomponents become, according to API-RP2A, 1,5 and 1,15, respectively.

3.2 Loadout ForcesThese are forces generated when the jacket is loaded from the fabrication yard onto the barge. If the loadout is carriedout by direct lift, then, unless the lifting arrangement is different from that to be used for installation, lifting forces need notbe computed, because lifting in the open sea creates a more severe loading condition which requires higher dynamic loadfactors. If loadout is done by skidding the structure onto the barge, a number of static loading conditi***** must bec*****idered, with the jacket supported on its side. Such loading conditi***** arise from the different positi***** of the jacketduring the loadout phases, (as shown in Figure 2), from movement of the barge due to tidal fluctuati*****, marine traffic orchange of draft, and from possible support settlements. Since movement of the jacket is slow, all loading conditi***** canbe taken as static. Typical values of friction coefficients for calculation of skidding forces are the following:

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steel on steel without lubrication............................................ 0,25steel on steel with lubrication............................................... 0,15steel on teflon.................................................................. 0,10teflon on teflon................................................................. 0,08

3.3 Transportation ForcesThese forces are generated when platform components (jacket, deck) are transported offshore on barges or self-floating.They depend upon the weight, geometry and support conditi***** of the structure (by barge or by buoyancy) and also onthe environmental conditi***** (waves, winds and currents) that are encountered during transportation. The types ofmotion that a floating structure may experience are shown schematically in Figure 3.

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In order to minimize the associated risks and secure safe transport from the fabrication yard to the platform site, it isimportant to plan the operation carefully by c*****idering, according to API-RP2A [3], the following:

Previous experience along the tow routeExposure time and reliability of predicted "weather windows"Accessibility of safe havensSeasonal weather systemAppropriate return period for determining design wind, wave and current conditi*****, taking into accountcharacteristics of the tow such as size, structure, sensitivity and cost.

Transportation forces are generated by the motion of the tow, i.e. the structure and supporting barge. They aredetermined from the design winds, waves and currents. If the structure is self-floating, the loads can be calculateddirectly. According to API-RP2A [3], towing analyses must be based on the results of model basin tests or appropriateanalytical methods and must c*****ider wind and wave directi***** parallel, perpendicular and at 45° to the tow axis.Inertial loads may be computed from a rigid body analysis of the tow by combining roll and pitch with heave moti*****,when the size of the tow, magnitude of the sea state and experience make such assumpti***** reasonable. For open seaconditi*****, the following may be c*****idered as typical design values: Single - amplitude roll: 20° Single - amplitude pitch: 10° Period of roll or pitch: 10 second Heave acceleration: 0,2 gWhen transporting a large jacket by barge, stability against capsizing is a primary design c*****ideration because of thehigh centre of gravity of the jacket. Moreover, the relative stiffness of jacket and barge may need to be taken into accounttogether with the wave slamming forces that could result during a heavy roll motion of the tow (Figure 4) when structuralanalyses are carried out for designing the tie-down braces and the jacket members affected by the induced loads. Specialcomputer programs are available to compute the transportation loads in the structure-barge system and the resultingstresses for any specified environmental condition.

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3.4 Launching and Upending ForcesThese forces are generated during the launch of a jacket from the barge into the sea and during the subsequent upendinginto its proper vertical position to rest on the seabed. A schematic view of these operati***** can be seen in Figure 5.

There are five stages in a launch-upending operation:a. Jacket slides along the skid beamsb. Jacket rotates on the rocker armsc. Jacket rotates and slides simultaneouslyd. Jacket detaches completely and comes to its floating equilibrium positione. Jacket is upended by a combination of controlled flooding and simultaneous lifting by a derrick barge.The loads, static as well as dynamic, induced during each of these stages and the force required to set the jacket intomotion can be evaluated by appropriate analyses, which also c*****ider the action of wind, waves and currents expectedduring the operation.To start the launch, the barge must be ballasted to an appropriate draft and trim angle and subsequently the jacket mustbe pulled towards the stern by a winch. Sliding of the jacket starts as soon as the downward force (gravity component andwinch pull) exceeds the friction force. As the jacket slides, its weight is supported on the two legs that are part of thelaunch trusses. The support length keeps decreasing and reaches a minimum, equal to the length of the rocker beams,when rotation starts. It is generally at this instant that the most severe launching forces develop as reacti***** to theweight of the jacket. During stages (d) and (e), variable hydrostatic forces arise which have to be c*****idered at allmembers affected. Buoyancy calculati***** are required for every stage of the operation to ensure fully controlled, stablemotion. Computer programs are available to perform the stress analyses required for launching and upending and also toportray the whole operation graphically.

4. ACCIDENTAL LOADSAccording to the DNV rules [2], accidental loads are loads, ill-defined with respect to intensity and frequency, which may

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occur as a result of accident or exceptional circumstances. Accidental loads are also specified as a separate category inthe NPD regulati***** [4], but not in API-RP2A [3], BS6235 [1] or the DOE-OG rules [5]. Examples of accidental loads areloads due to collision with vessels, fire or explosion, dropped objects, and unintended flooding of bouyancy tanks. Specialmeasures are normally taken to reduce the risk from accidental loads. For example, protection of wellheads or othercritical equipment from a dropped object can be provided by specially designed, impact resistant covers. According to theNPD regulati***** [4], an accidental load can be disregarded if its annual probability of occurrence is less than 10-4. Thisnumber is meant as an order of magnitude estimate and is extremely difficult to compute. Earthquakes are treated as anenvironmental load in offshore structure design.

5. LOAD COMBINATI*****The load combinati***** used for designing fixed offshore structures depend upon the design method used, i.e. whetherlimit state or allowable stress design is employed. The load combinati***** recommended for use with allowable stressprocedures are:a. Dead loads plus operating environmental loads plus maximum live loads, appropriate to normal operati***** of theplatform.b. Dead loads plus operating environmental loads plus minimum live loads, appropriate to normal operati***** of theplatform.c. Dead loads plus extreme (design) environmental loads plus maximum live loads, appropriate for combining withextreme conditi*****.d. Dead loads plus extreme (design) environmental loads plus minimum live loads, appropriate for combining withextreme conditi*****.Moreover, environmental loads, with the exception of earthquake loads, should be combined in a manner c*****istent withtheir joint probability of occurrence during the loading condition c*****idered. Earthquake loads, if applicable, are to beimposed as a separate environmental load, i.e., not to be combined with waves, wind, etc. Operating environmentalconditi***** are defined as representative of severe but not necessarily limiting conditi***** that, if exceeded, wouldrequire cessation of platform operati*****.The DNV rules [2] permit allowable stress design but recommend the semi-probabilistic limit state design method, whichthe NPD rules also require [4]. BS6235 permits both methods but the design equati***** it gives are for the allowablestress method [1]. API-RP2A is very specific in recommending not to apply limit state methods. According to the DNV andthe NPD rules for limit state design, four limit states must be checked:

Ultimate limit state For this limit state the following two loading combinati***** must be used: Ordinary: 1,3 P + 1,3 L + 1,0 D + 0,7 E, and Extreme : 1,0 P + 1,0 L + 1,0 D + 1,3 E where P, L, D and E stand for Permanent (dead), Operating (live), Deformation (e.g., temperature, differentialsettlement) and Environmental loads respectively. For well controlled dead and live loads during fabrication andinstallation, the load factor 1,3 may be reduced to 1,2. Furthermore, for structures that are unmanned during stormconditi***** and which are not used for storage of oil and gas, the 1,3 load factor for environmental loads - exceptearthquakes - may be reduced to 1,15.Fatigue limit state All load factors are to be taken as 1,0.Progressive Collapse limit state All load factors are to be taken as 1,0.Serviceability limit state

All load factors are to be taken as 1,0. The so-called characteristic values of the loads used in the above combinati***** and limit states are summarized in Table2, taken from the NPD rules.

6. CONCLUDING SUMMARYIn addition to environmental loads, an offshore structure must be designed for dead and live loads, fabrication andinstallation loads as well as accidental loads.Widely accepted rules of practice, listed in the references, are usually followed for specifying such loads.The type and magnitude of fabrication, transportation and installation loads depend upon the methods and sequencesused for the corresponding phases.Dynamic and impact effects are normally taken into account by means of appropriate dynamic load factors.Accidental loads are not well defined with respect to intensity and probability of occurrence. They will typically requirespecial protective measures.Load combinati***** and load factors depend upon the design method to be used. API-RP2A is based on allowablestress design and recommends against limit state design, BSI favours allowable stress design, while DNV and NPDrecommend limit state design.

7. REFERENCES[1] BS6235, "Code of Practice for Fixed Offshore Structures", British Standards Institution, London, 1982.[2] "Rules for the Design, C*****truction and Inspection of Offshore Structures", Det Norske Veritas (DNV), Oslo, 1977(with correcti***** 1982).[3] API-RP2A, "Recommended Practice for Planning, Designing and C*****tructing Fixed Offshore Platforms", American

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Petroleum Institute, Washington, D.C., 18th ed., 1989.[4] "Regulation for Structural Design of Load-bearing Structures Intended for Exploitation of Petroleum Resources",Norwegian Petroleum Directorate (NPD), 1985.[5] DOE-OG, "Offshore Installation: Guidance on Design and C*****truction", U.K. Department of Energy, London 1985.

8. ADDITIONAL READINGOCS, "Requirements for Verifying the Structural Integrity of OCS Platforms"., United States Geologic Survey, NationalCentre, Reston, Virginia, 1980.Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing Co., Houston, 1981.Graff, W.G., "Introduction to Offshore Structures", Gulf Publishing Co., Houston, 1981.Gerwick, B.C. Jr., "C*****truction of Offshore Structures", John Wiley, New York, 1986.

Table 1 Minimum design live load specificationLoads to be taken into account(kN/m2)

For porti***** of the structure For the structure as awhole

Zone c*****idered Flooring andjoists

Othercomponents

(3)

Process zone (around wells andlarge-scale machines)

5 (1) 5 (1) 2.5

Drilling zone 5 (1) 5 (1) 2.5Catwalks and walking platforms(except emergency exits)

3 2.5 1

Stairways (except emergencyexits)

4 3 0

Module roofing 2 1.5 1Emergency exits 5 5 0STORAGEStorage floors - heavyStorage floors - light

189

126

8 (2)4 (2)

Delivery zone 10 10 5Non-attributed area 6 4 3

(1) Accumulated with a point load equal to the weight of the heaviest part likely to be removed, with a minimum value of5 kN. Point loads are assumed as being applied to a 0,3m ´ 0,3m surface.(2) Applied on the entirety of the flooring surface (including traffic).(3) This column gives the loads to be taken into account for the structure's overall calculation. These values are theinput for the computer runs.Table 2 Characteristic Loads according to NPD [4]

LOAD TYPE LIMIT STATES FOR TEMPORARY PHASES LIMIT STATES FOR NORMAL OServiceability Fatigue Ultimate Progressive Collapse Serviceability Fatigue

Abnormaleffects

Damagecondition

DEAD EXPECTED VALUELIVE SPECIFIED VALUEDEFORMATION EXPECTED EXTREME VALUEENVIRONMENTAL Dependent

onoperationalrequirements

Expectedloadhistory

Value dependent on measurestaken

Dependentonoperationalrequirements

Expectedloadhistory

ACCIDENTAL NOT APPLICABLE Dependentonoperationalrequirements

NOT APPLICABLE

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Previous | Next | ContentsESDEP WG 15ASTRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.4 - Analysis IOBJECTIVE/SCOPETo present the main analysis procedures for offshore structures.

PREREQUISITESLecture 15A.1: Offshore Structures: General IntroductionLecture 15A.2: Loads I: Introduction and Environmental LoadsLecture 15A.3: Loads II: Other Loads

RELATED LECTURESLecture 15A.5: Analysis II

SUMMARYAnalytical models used in offshore engineering are briefly described. Acceptance criteria for the verification of offshorestructures are presented.Simple rules for preliminary member sizing are given and procedures for static in-place and dynamic analysis aredescribed.

1. ANALYTICAL MODELThe analysis of an offshore structure is an extensive task, embracing c*****ideration of the different stages, i.e.execution, installation, and in-service stages, during its life. Many disciplines, e.g. structural, geotechnical, navalarchitecture, metallurgy are involved.This lecture and Lecture 15A.5 are purposely limited to presenting an overview of available analysis procedures andproviding benchmarks for the reader to appreciate the validity of his assumpti***** and results. They primarily addressjackets, which are more unusual structures compared to decks and modules, and which more closely resemble *****horepetro-chemical plants.

2. ANALYTICAL MODELThe analytical models used in offshore engineering are in some respects similar to those adopted for other types of steelstructures. Only the salient features of offshore models are presented here.The same model is used throughout the analysis process with only minor adjustments being made to suit the specificconditi*****, e.g. at supports in particular, relating to each analysis.

2.1 Stick ModelsStick models (beam elements assembled in frames) are used extensively for tubular structures (jackets, bridges, flarebooms) and lattice trusses (modules, decks).2.1.1 JointsEach member is normally rigidly fixed at its ends to other elements in the model.If more accuracy is required, particularly for the assessment of natural vibration modes, local flexibility of the connecti*****may be represented by a joint stiffness matrix.2.1.2 MembersIn addition to its geometrical and material properties, each member is characterised by hydrodynamic coefficients, e.g.relating to drag, inertia, and marine growth, to allow wave forces to be automatically generated.

2.2 Plate ModelsIntegrated decks and hulls of floating platforms involving large bulkheads are described by plate elements. Thecharacteristics assumed for the plate elements depend on the principal state of stress which they are subjected to.Membrane stresses are taken when the element is subjected merely to axial load and shear. Plate stresses are adoptedwhen bending and lateral pressure are to be taken into account.

3. ACCEPTANCE CRITERIA3.1 Code ChecksThe verification of an element c*****ists of comparing its characteristic resistance(s) to a design force or stress. Itincludes:

a strength check, where the characteristic resistance is related to the yield strength of the element,a stability check for elements in compression where the characteristic resistance relates to the buckling limit of theelement.

An element (member or plate) is checked at typical secti***** (at least both ends and midspan) against resistance andbuckling. This verification also includes the effect of water pressure for deepwater structures.Tubular joints are checked against punching under various load patterns. These checks may indicate the need for localreinforcement of the chord using overthickness or internal ring-stiffeners.Elements should also be verified against fatigue, corrosion, temperature or durability wherever relevant.

3.2 Allowable Stress MethodThis method is presently specified by American codes (API, AISC).The loads remain unfactored and a unique coefficient is applied to the characteristic resistance to obtain an allowable

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stress as follows:Condition Axial Strong axis bending Weak axis bendingNormal 0,60 0,66 0,75Extreme 0,80 0,88 1,00

"Normal" and "Extreme" respectively represent the most severe conditi*****:under which the plant is to operate without shut-down.the platform is to endure over its lifetime.

3.3 Limit State MethodThis method is enforced by European and Norwegian Authorities and has now been adopted by API as it offers a moreuniform reliability.Partial factors are applied to the loads and to the characteristic resistance of the element, reflecting the amount ofconfidence placed in the design value of each parameter and the degree of risk accepted under a limit state, i.e:

Ultimate Limit State (ULS): corresponds to an ultimate event c*****idering the structural resistance with appropriate reserve.Fatigue Limit State (FLS): relates to the possibility of failure under cyclic loading.Progressive Collapse Limit State (PLS): reflects the ability of the structure to resist collapse under accidental or abnormal conditi*****.Service Limit State (SLS): corresponds to criteria for normal use or durability (often specified by the plant operator).

3.3.1 Load factorsNorwegian Authorities (2, 4) specify the following sets of load factors:Limit State Load Categories

P L D E AULS (normal) 1,3 1,3 1,0 0,7 0,0ULS (extreme) 1,0 1,0 1,0 1,3 0,0FLS 0,0 0,0 0,0 1,0 0,0PLS (accidental) 1,0 1,0 1,0 1,0 1,0PLS (post-damage) 1,0 1,0 1,0 1,0 0,0SLS 1,0 1,0 1,0 1,0 0,0

where the respective load categories are:P are permanent loads (structural weight, dry equipments, ballast, hydrostatic pressure).L are live loads (storage, personnel, liquids).D are deformati***** (out-of-level supports, subsidence).E are environmental loads (wave, current, wind, earthquake).A are accidental loads (dropped object, ship impact, blast, fire).3.3.2 Material factorsThe material partial factors for steel is normally taken equal to 1,15 for ULS and 1,00 for PLS and SLS design.

3.3.3 Classification of Design Conditi*****Guidance for classifying typical conditi***** into typical limit states is given in the following table:

Condition Loadings Design

Criterion P/L E D A

C*****truction P ULS,SLS

Load-Out P reduced wind support disp

ULS

Transport P transport wind and wave ULS

Tow-out (accidental)

P flooded compart

PLS

Launch P ULS

Lifting P ULS

In-Place (normal)

P + L wind, wave & snow actual ULS,SLS

In-Place (extreme)

P + L wind & 100 year wave actual ULS

SLS

In-Place (exceptional)

P + L wind & 10000 year wave actual PLS

Earthquake P + L 10-2 quake ULS

Rare Earthquake P + L 10-4 quake PLS

Explosion P + L blast PLS

Fire P + L fire PLS

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Dropped Object

P + L drill collar

PLS

Boat Collision

P + L boat impact

PLS

Damaged Structure

P + reduced L

reduced wave & wind PLS

4. PRELIMINARY MEMBER SIZINGThe analysis of a structure is an iterative process which requires progressive adjustment of the member sizes withrespect to the forces they transmit, until a safe and economical design is achieved.It is therefore of the utmost importance to start the main analysis from a model which is close to the final optimized one.The simple rules given below provide an easy way of selecting realistic sizes for the main elements of offshore structuresin moderate water depth (up to 80m) where dynamic effects are negligible.

4.1 Jacket Pile Sizescalculate the vertical resultant (dead weight, live loads, buoyancy), the overall shear and the overturning moment(environmental forces) at the mudline.assuming that the jacket behaves as a rigid body, derive the maximum axial and shear force at the top of the pile.select a pile diameter in accordance with the expected leg diameter and the capacity of pile driving equipment.derive the penetration from the shaft friction and tip bearing diagrams.assuming an equivalent soil subgrade modulus and full fixity at the base of the jacket, calculate the maximum moment inthe pile and derive its wall thickness.

4.2 Deck Leg Sizesadapt the diameter of the leg to that of the pile.determine the effective length from the degree of fixity of the leg into the deck (depending upon the height of the cellardeck).calculate the moment caused by wind loads on topsides and derive the appropriate thickness.

4.3 Jacket Bracingsselect the diameter in order to obtain a span/diameter ratio between 30 and 40.calculate the axial force in the brace from the overall shear and the local bending caused by the wave assuming partialor total end restraint.derive the thickness such that the diameter/thickness ratio lies between 20 and 70 and eliminate any hydrostatic buckletendency by imposing D/t<170/3ÖH (H is the depth of member below the free surface).

4.4 Deck Framingselect a spacing between stiffeners (typically 500 to 800mm).derive the plate thickness from formulae accounting for local plastification under the wheel footprint of the design forklifttruck.determine by straight beam formulae the sizes of the main girders under "blanket" live loads and/or the respectiveweight of the heaviest equipments.

5. STATIC IN-PLACE ANALYSISThe static in-place analysis is the basic and generally the simplest of all analyses. The structure is modelled as it standsduring its operational life, and subjected to pseudo-static loads.This analysis is always carried at the very early stage of the project, often from a simplified model, to size the mainelements of the structure.

5.1 Structural Model5.1.1 Main ModelThe main model should account for eccentricities and local reinforcements at the joints.Typical models for North Sea jackets may feature over 800 nodes and 4000 members.5.1.2 AppurtenancesThe contribution of appurtenances (risers, J-tubes, caiss*****, conductors, boat-fenders, etc.) to the overall stiffness ofthe structure is normally neglected.They are therefore analysed separately and their reacti***** applied as loads at the interfaces with the main structure.

5.1.3 Foundation ModelSince their behaviour is non-linear, foundati***** are often analysed separately from the structural model.They are represented by an equivalent load-dependent secant stiffness matrix; coefficients are determined by an iterativeprocess where the forces and displacements at the common boundaries of structural and foundation models are equated.This matrix may need to be adjusted to the mean reaction corresponding to each loading condition.

5.2 LoadingsThis Section is a reminder of the main types of loads, which are described in more detail in Lectures 15A.2 and15A.3.

5.2.1 Gravity LoadsGravity loads c*****ist of:

dead weight of structure and equipments.

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live loads (equipments, fluids, personnel).Depending on the area of structure under scrutiny, live loads must be positioned to produce the most severe configuration(compression or tension); this may occur for instance when positioning the drilling rig.

5.2.2 Environmental LoadsEnvironmental loads c*****ist of wave, current and wind loads assumed to act simultaneously in the same direction.In general eight wave incidences are selected; for each the position of the crest relative to the platform must beestablished such that the maximum overturning moment and/or shear are produced at the mudline.

5.3 Loading Combinati*****The static in-place analysis is performed under different conditi***** where the loads are approximated by their pseudo-static equivalent.The basic loads relevant to a given condition are multiplied by the appropriate load factors and combined to produce themost severe effect in each individual element of the structure.

6. DYNAMIC ANALYSISA dynamic analysis is normally mandatory for every offshore structure, but can be restricted to the main modes in thecase of stiff structures.

6.1 Dynamic ModelThe dynamic model of the structure is derived from the main static model.Some simplificati***** may however take place:

local joint reinforcements and eccentricities may be disregarded.masses are lumped at the member ends.the foundation model may be derived from cyclic soil behaviour.

6.2 Equati***** of MotionThe governing dynamic equati***** of multi-degrees-of-freedom systems can be expressed in the matrix form:MX'' + CX' + KX = P(t)whereM is the mass matrixC is the damping matrixK is the stiffness matrixX, X', X'' are the displacement, velocity and acceleration vectors (function of time).P(t) is the time dependent force vector; in the most general case it may depend on the displacements of the structurealso (i.e. relative motion of the structure with respect to the wave velocity in Morison equation).

6.2.1 MassThe mass matrix represents the distribution of masses over the structure.Masses include that of the structure itself, the appurtenances, liquids trapped in legs or tanks, the added mass of water(mass of water displaced by the member and determined from potential flow theory) and the mass of marine growth.Masses are generally lumped at discrete points of the model. The mass matrix c*****equently becomes diagonal but localmodes of vibration of single members are ignored (these modes may be important for certain members subjected to anearthquake). The selection of lumping points may significantly affect the ensuing solution.As a further simplification to larger models involving c*****iderable degrees-of-freedom, the system can be condensed to afew freedoms while still retaining its basic energy distribution.

6.2.2 DampingDamping is the most difficult to estimate among all parameters governing the dynamic resp*****e of a structure.It may c*****ist of structural and hydrodynamic damping.Structural DampingStructural damping is associated with the loss of energy by internal friction in the material.It increases with the order of the mode, being roughly proportional to the strain energy involved in each.Hydrodynamic DampingDamping provided by the water surrounding the structure is commonly added to the former, but may alternatively beaccounted as part of the forcing function when vibrati***** are close to resonance (vortex-shedding in particular).Representation of DampingViscous damping represents the most common and simple form of damping. It may have one of the followingrepresentati*****:

modal damping: a specific damping ratio z expressing the percentage to critical associated with each mode (typicallyz = 0,5% structural; z = 1,5% hydrodynamic)proportional damping: defined as a linear combination of stiffness and mass matrices.

All other types of non-viscous damping should preferably be expressed as an equivalent viscous damping matrix.6.2.3 StiffnessThe stiffness matrix is in all aspects similar to the one used in static analyses.

6.3 Free Vibration Mode Shapes and FrequenciesThe first step in a dynamic analysis c*****ists of determining the principal natural vibration mode shapes and frequenciesof the undamped, multi-degree-of-freedom structure up to a given order (30th to 50th).

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This c*****ists in solving the eigenvalue problem: KX = l MXFor rigid structures having a fundamental vibration period well below the range of wave periods (typically less than 3 s),the dynamic behaviour is simply accounted for by multiplying the time-dependent loads by a dynamic amplification factor(DAF):

DAF = where b = TN/T is the ratio of the period of the structure to the wave period.

6.4 Modal Superposition MethodA convenient technique c*****ists of uncoupling the equati***** through the normal modes of the system.This method is only applicable if:

each mass, stiffness and damping matrix is time-independent.non-linear forces are linearized beforehand (drag).

The total resp*****e is obtained by summing the resp*****es of the individual single-degree-of-freedom oscillatorsassociated to each normal mode of the structure.This method offers the advantage that the eigen modes provide substantial insight into the problem, and can be re-usedfor as many subsequent resp*****e calculati***** as needed at later stages.It may however prove time-c*****uming when a large number of modes is required to represent the resp*****e accurately.Therefore:

the simple superposition method (mode-displacement) is applied to a truncated number of lowest modes for predictingearthquake resp*****e.it must be corrected by the static contribution of the higher modes (mode-acceleration method) for wave loadings.

6.4.1 Frequency Domain AnalysisSuch analysis is most appropriate for evaluating the steady-state resp*****e of a system subjected to cyclic loadings, asthe transient part of the resp*****e vanishes rapidly under the effect of damping.The loading function is developed in Fourier series up to an order h:

p(t) = The plot of the amplitudes pj versus the circular frequencies wj is called the amplitude power spectra of the loading.Usually, significant values of pj only occur within a narrow range of frequencies and the analysis can be restricted to it.The relati*****hip between resp*****e and force vectors is expressed by the transfer matrix H, such as:H = [-M w2 + i x C w + K]the elements of which represent:

Hj,k = The spectral density of resp*****e in freedom j versus force is then:

The fast Fourier transform (FFT) is the most efficient algorithm associated with this kind of analysis.

6.4.2 Time Domain AnalysisThe resp*****e of the i-th mode may alternatively be determined by resorting to Duhamel's integral:

Xj(t) =

The overall resp*****e is then obtained by summing at each time step the individual resp*****es over all significant modes.

6.5 Direct Integration MethodsDirect step-by-step integration of the equati***** of motion is the most general method and is applicable to:

non-linear problems involving special forms of damping and resp*****e-dependent loadings.resp*****es involving many vibration modes to be determined over a short time interval.

The dynamic equilibrium at an instant t is governed by the same type of equati*****, where all matrices (mass, damping, stiffness, load) are simultaneously

dependent on the time and structural resp*****e as well.

All available integration techniques are characterised by their stability (i.e. the tendency for uncontrolled divergence of amplitude to occur with increasing

time steps). Unconditionally stable methods are always to be preferred (for instance Newmark-beta with b = 1/4 or Wilson-theta with q = 1,4).

7. CONCLUDING SUMMARYThe analysis of offshore structures is an extensive task.The analytical models used in offshore engineering are in some respects similar to those used for other types of steel structures. The same model isused throughout the analysis process.The verification of an element c*****ists of comparing its characteristic resistance(s) to a design force or stress. Several methods are available.Simple rules are available for preliminary member sizing.Static in-plane analysis is always carried out at the early stage of a project to size the main elements of the structure. A dynamic analysis is normallymandatory for every offshore structure.

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Previous | Next | ContentsESDEP WG 15ASTRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.5 - Analysis IIOBJECTIVE/SCOPETo present the analysis procedures for offshore structures relating to fatigue, abnormal and accident conditi*****, load-outand transportation, installation and local design.PREREQUISITESLecture 15A.1: Offshore Structures: General IntroductionLecture 15A.2: Loads I: Introduction and Environmental LoadsLecture 15A.3: Loads II: Other Loads

RELATED LECTURESLecture 15A.4: Analysis ISUMMARYMethods of fatigue analysis are described including the fatigue model (structural, hydrodynamic loading, and joint stressmodels) and the methods of fatigue damage assessment.Abnormal and accidental conditi***** are c*****idered relating to earthquake, impact and progressive collapse.Analyses required for load-out and transportation and for installation are outlined. Local analysis for specific parts of thestructure which are better treated by dedicated models outside of the global analysis are identified.

1. FATIGUE ANALYSISA fatigue analysis is performed for those structures sensitive to the action of cyclic loadings such as:

wave (jackets, floating structures).wind (flare booms, stair towers).structures under rotating equipments.

1.1 Fatigue Model1.1.1 Structural ModelThe in-place model is used for the fatigue analysis.Quasi-static analysis is often chosen; it permits all local stresses to be comprehensively represented. The dynamiceffects are accounted for by factoring the loads by the relevant DAF.Modal analysis may be used instead; it offers computational efficiency, but may also overlook important local resp*****emodes, particularly near the waterline where direct wave action causes high out-of-plane bending (see Section 5.2). Themode - acceleration method may overcome this problem.

1.1.2 Hydrodynamic Loading ModelA very large number of computer runs may be necessary to evaluate the stress range at the joints. The wave isrepeatedly generated for:

different blocks of wave heights (typically from 2 to 28m in steps of 2m), each associated with a characteristic waveand zero-upcrossing period.different incidences (typically eight).different phases to determine the stress range for a given wave at each joint.

1.1.3 Joint Stress ModelNominal joint stresses are calculated for eight points around the circumference of the brace. The maximum local (hot spot)stress is obtained by multiplying the former by a stress concentration factor (SCF) given by parametric formulae whichare functi***** of the joint geometry and the load pattern (balanced/unbalanced).

1.1.4 Fatigue Damage ModelThe fatigue failure of joints in offshore structures primarily depends on the stress ranges and their number of occurrences,formulated by S-N curves:log Ni = log a + mlog Dsi

The number of cycles to failure Ni corresponds to a stress range. The effect of the c*****tant stresses, mainly weldingresidual stresses, is implicitly accounted for in this formulation.The cumulative damage caused by ni cycles of stress Dsi, over the operational life of the platform (30 to 50 years) isobtained by the Palmgren-Miner rule:

D = The limit of this ratio depends on the position of the joint with respect to the splash zone (typically +/-4m on either side ofthe mean sea level). The ratio should normally not exceed:

1,0 above,0,1 within,

landho

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0,3 below the splash zone.

1.1.5 Closed Form ExpressionThe damage may alternatively be expressed in closed form:

D = wherea, m are coefficients of the selected S-N curve.Ds is the stress range exceeded once in N cycles.k is a long-term distribution parameter, depending on the position of the joint in the structure.N is the total number of cycles.

1.2 Deterministic AnalysisThis analysis c*****ists of time-domain analysis of the structure. The main advantage of this representation is thatnon-linear effects (drag, high order wave theories) are handled explicitly.A minimum of four regular waves described in terms of height and associated period are c*****idered for each headingangle.

1.3 Spectral AnalysisWaves of a given height are not characterised by a unique frequency, but rather by a range of frequencies. If this rangecorresponds to a peak in the structural resp*****e, the fatigue life predicted by the deterministic method can be seriouslydistorted.This problem is overcome by using a scatter diagram, in which the joint occurrence of wave height and period isquantified. Wave directionality may also be accounted for. Eventually the most thorough representation of a sea statec*****ists of:

the frequency spectrum c*****tructed from the significant wave heights and mean zero-crossing periods.the directionality function derived from the mean direction and associated spreading function.

This approach requires that the physical process be approximately linear (or properly linearised) and stationary. Transferfuncti***** TF are determined from time-domain analyses involving various wave heights, each with different period andincidence:

The resp*****e has normally a narrow-banded spectrum and can be described by a Rayleigh distribution.The zero-upcrossing frequency of stress cycles is then approximated by:

Tz = where mn is the nth order moment of the resp*****e.The significant stress range is readily obtained for each sea state as:

ssig = where S(w,q) is the directional wave energy spectrum.

1.4 Wind Fatigue1.4.1 Wind GustsThe fatigue damage caused by the fluctuating part of wind (gusts) on slender structures like flare booms and bridges isusually predicted by spectral methods.The main feature of such analysis is the introduction of coherence functi***** accounting for the spanwise correlation offorces.

1.4.2 Vortex SheddingVortex induced failure occurs for tubes subjected to a uniform or oscillating flow of fluid.Within a specific range of fluid velocities, eddies are shed at a frequency close to the resonant frequency of the member.This phenomenon involves forced displacements, which can be determined by models such as those suggested in [1].

2. ABNORMAL AND ACCIDENTAL CONDITI*****This type of analysis addresses conditi***** which may c*****iderably affect the integrity of the structure, but only have alimited risk of occurrence.Typically all events with a probability level less than the 10-4 threshold are disregarded.

2.1 Earthquake Analysis2.1.1 ModelParticular attention shall be paid to:

foundati*****: the near field (i.e. the soil mass in the direct vicinity of the structure) shall accurately representload-deflection behaviour. As a general rule the lateral foundation behaviour is essentially controlled by horizontalground moti***** of shallow soil layers.modal damping (in general taken as 5% and 7% of critical for ULS and PLS analyses respectively).

2.1.2 Ductility RequirementsThe seismic forces in a structure are highly dependent on its dynamic characteristics. Design recommendati***** are

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given by API to determine an efficient geometry. The recommendati***** call for:providing sufficient redundancy and symmetry in the structure.favouring X-bracings instead of K-bracings.avoiding abrupt changes in stiffness.improving the post-buckling behaviour of bracings.

2.1.3 Analysis MethodEarthquake analyses can be carried out according to the general methods presented in Lecture 15A.4.However their distinctive feature is that they represent essentially a base motion problem and that the seismic loads aretherefore dependent on the dynamic characteristics of the structure.Modal spectral resp*****e analysis is normally used. It c*****ists of a superposition of maximum mode resp*****e andforms a resp*****e spectrum curve characteristic of the input motion. This spectrum is the result of time-histories of aSDOF system for different natural periods of vibration and damping.Direct time integration can be used instead for specific accelerograms adapted to the site.

2.2 ImpactThe analysis of impact loads on structures is carried out locally using simple plastic models [2].Should a more sophisticated analysis be required, it can be accomplished using time-domain techniques presented inSection 6 of Lecture 15A.4.The whole energy must be absorbed within acceptable deformati*****.

2.2.1 Dropped Object/Boat ImpactWhen a wellhead protection cover is hit by a drill collar, or a tube (jacket leg, fender) is crushed by a supply boat, twoload/deformation mechanisms occur simultaneously:

local punch-through (cover) or denting (tube).global deformation along plastic hinges with possible appearance of membrane forces.

2.2.2 Blast and FireOwing to the current lack of definitive guidance regarding explosi***** and fire, the behaviour of structures in such eventshas so far been only predicted by simple models based on:

equivalent static overpressure and plastic deformation of plates for blast analysis.the reduction of material strength and elastic modulus under temperature increase.

In the aftermath of recent mishaps however, more accurate analyses may become mandatory, based on a betterunderstanding of the pressure-time histories and the effective resistance and resp*****e of structures to explosi***** andfire.

2.3 Progressive CollapseSome elements of the structure (legs, bracings, bulkheads) may partially or completely loose their strength as a result ofaccidental damage.The purpose of such analysis is to ensure that the spare resistance of the remaining structure is sufficient to allow theloads to redistribute.Since such a configuration is only temporary (mobilisation period prior to repairs) and that operati***** will also berestricted around the damaged area, reduced live and environmental loads are generally accepted.In this analysis, the damaged elements are removed from the model. Their residual strength may be represented byforces applied at the boundary nodes with the intact structure.

3. LOAD OUT & TRANSPORTATION3.1 Load-OutThe load-out procedure c*****ists in moving the jacket or module from its c*****truction site to the transportation barge byskidding, or by using trailers underneath it.The barge may be floating and is continuously deballasted as the package progresses onto it, or grounded on the bottomof the harbour.3.1.1 SkiddingThe most severe configuration during skidding occurs when the part of the structure is cantilevering out:

from the quayside before it touches the barge.from the barge just after it has left the quay.

The analysis should also investigate the possibility of high local reacti***** being the result of settlement of the skidway orerrors in the ballasting procedure.

3.1.2 Load-Out by TrailersAs the reaction on each trailer can be kept c*****tant, analysis of load-out by trailers only requires a single step todetermine the optimal distribution of trailers.

3.2 Transportation3.2.1 Naval Architectural ModelThe model c*****ists of the rigid-body assembly of the barge and the structure.Barges are in general characterised by a low length/beam ratio and a high beam/draught ratio, as well as sharp cornerswhich introduce heavy viscous damping.For jacket transport, particular care shall be taken in the representation of overhanging parts (legs, buoyancy tanks)which contribute significantly to the righting moment.

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Dry-transported decks and modules may be simply represented by their mass and moments of inertia.This analysis shall provide the linear and angular accelerati***** and displacements of the structure to be entered in thestructural model as inertia forces, and also the partition and intensity of buoyancy and slamming forces.

3.2.2 Structural ModelThe jacket model is a simplified version of the in-place model, from which eccentricities and local reinforcements may beomitted.The barge is modelled as a plane grid, with members having the equivalent properties of the longitudinal and transversalbulkheads.As the barge passes over a wave trough or a crest, a portion only of the barge is supported by buoyancy (long bargesmay be spanning over a whole trough or be half-cantilevered).The model therefore represents the jacket and the barge as two structures coupled together by the seafasteningmembers.

4. INSTALLATION4.1 Launching4.1.1 Naval Architectural ModelA three dimensional analysis is carried out to evaluate the global forces acting on the jacket at various time steps duringthe launch sequence.At each time step, the jacket/barge rigid body system is repositioned to equilibrate the internal and external forcesproduced by:

jacket weight, inertia, buoyancy and drag forces.barge weight, buoyancy and ballast forces.vertical reacti***** and friction forces between jacket and barge.

The maximum reaction on the rocker arm is normally obtained when the jacket just starts rotating about the rocker hinge.

4.1.2 Structural ModelThe structural model is in all aspects identical to the one used for the transportation analysis, with possibly a finerrepresentation of the launch legs.The rocker arm is also represented as a vertical beam hinged approximately at midspan. Interface loads obtained by therigid body analysis are input at boundary conditi***** on the launch legs. All interface members must remain incompression, otherwise they are inactivated and the analysis restarted for that step.Once the tilting phase has begun, the jacket is analysed at least for each main leg node being at the vertical of the rockerarm pivot.

4.2 UpendingNo dedicated structural analysis is required for this phase, which is essentially a naval architecture problem.A local analysis of the lugs is performed for crane-assisted upendings.

4.3 DockingDocking of a jacket onto a pre-installed template requires guides to be analysed for local impact. The same requirementapplied for bumpers to aid the installation of modules.

4.4 Unpiled StabilityThe condition where the jacket may for a while stand unpiled on the seafloor is analysed for the design installation wave.The stability of the jacket as a whole (overturning tendency) is investigated, together with the resistance of the mudmatsagainst soil pressure.

4.5 PilingThe piles are checked during driving for the dynamic stresses caused by the impact wave of the hammer blow. Themaximum cantilevered (stick-up) length of pile must be established for the self-weight of the pile and hammer combined,accounting for first and second order moments arising from the pile batter. Hydrodynamic acti***** are added forunderwater driving.Elements in the vicinity of the piles (guides, sleeves) shall also be checked, see Section 5.1.

4.6 Lifting4.6.1 ModelThe model used for the lift analysis of a structure c*****ists of the in-place model plus the representation of the riggingarrangement (slings, spreader frames).For single lifts the slings converge towards the hook joint, which is the sole vertical support in the model and shall belocated exactly on the vertical through the centre of gracity (CoG) of the model.For heavier dual-crane lifts, the CoG shall be contained in the vertical plane defined by the two hook joints.The mathematical instability of the model with respect to horizontal forces is avoided by using soft horizontal springs atthe padeyes. The force and elongation in these springs should always remain small.

4.6.2 Design FactorsDifferent factors are applied to the basic sling forces to account for specific effects during lifting operati*****.4.6.2.1 Skew Load Factor (SKL)This factor represents the effect of fabrication tolerances and lack-of-fit of the slings on the load repartition in a staticallyundetermined rigging arrangement (4 slings or more). Skew factors may either be directly computed by applying to a pairof opposite slings a temperature difference such that their elongation/shortening corresponds to the mismatch, or

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determined arbitrarily (typically 1/3 - 2/3 repartition).4.6.2.2 Dynamic Amplification Factor (DAF)This factor accounts for global dynamic effects normally experienced during lifting operati*****. DnV [24] recommendsminimum values as follows:Lifted Weight W (tonnes)

up to 100 t

100 t to 1000t

1000 t to 2500t

more than 2500 t

DAF offshore 1,30 1,20 1,15 1,10

DAF inshore 1,15 1,10 1,05 1,05

4.6.2.3 Tilt Effect Factor (TEF)This factor accounts for additional sling loading caused by the rotation of the lifted object about a horizontal axis and bythe longitudinal deviation of the hooks from their theoretical position in the case of a multi-hook lift. It shall normally bebased on 5° and 3° tilt respectively depending on whether cranes are on different vessels or not.4.6.2.4 Yaw Effect Factor (YEF)This factor accounts for the rotation of the lifted object about a vertical axis (equal to 1,05 typically).

4.6.3 C*****equence FactorsForces in elements checked under lift conditi***** are multiplied by a factor reflecting the c*****equence a failure of thatspecific element would have on the integrity of the overall structure:

1,30 for spreader frames, lifting points (padeyes) and their attachment to the structure.1,15 for all members transferring the load to the lifting points.1,00 for other elements.

5. LOCAL ANALYSES AND DESIGNLocal analyses address specific parts of the structure which are better treated by dedicated models outside the globalanalysis.The list of analyses below is not exhaustive and more information can be found in [1-24] which provide a complete designprocedure in each particular case.

5.1 Pile/Sleeve Connecti*****Underwater pile/sleeve connection is usually achieved by grouting the annulus between the outside of the pile and theinner sleeve.The main verificati***** address:

the shear stresses in the concrete.the fatigue damage in the shear plates and the attachment welds to the main jacket accumulated during pile driving andthroughout the life of the platform.

5.2 Members within the Splash ZoneHorizontal members (conductor guide frames in particular) located within the splash zone (+/-5m on either side of themean-sea-level approximately) shall be analysed for fatigue caused by repeated wave slamming.A slamming coefficient Cs=3,5 is often selected.

5.3 Straightened NodesTypical straightened nodes (ring-stiffened nodes, bottle legs nodes with diaphragms) are analysed by finite-elementsmodels, from which parametric envelope formulae are drawn and applied to all nodes representative of the same class.

5.4 Appurtenances5.4.1 Risers, Caiss***** & J-TubesStatic In-Place and FatigueRisers, caiss***** and J-tubes are verified either by structural or piping programs for the action of environmental forces,internal pressure and temperature. Particular attention is paid to the bends not always satisfactorily represented bystructural programs and the location of the touch-down point now known a-priori.A fatigue analysis is also performed to assess the fatigue damage to the clamps and the attachments to the jacket.Pull-InJ-tubes are empty ducts continuously guiding a post-installed riser pulled inside. They are verified by empirical plasticmodels against the forces generated during pull-in by the friction of the cable and the deformation of the pull head, see[22].

5.4.2 ConductorsConductors are analysed in-place as beam columns on discrete simple supports, these being provided by the horizontalframing of the jacket (typically 20 to 25 m span).The installation sequence of the different casings must be c*****idered to assess the distribution of stresses in thedifferent tubes forming the overall composite section.Also the portion of compression force in the conductor caused by the hanging casings is regarded as an internal force(similar to prestressing) which therefore does not induce any buckling tendency, see [23].

5.5 HelidecksThe helideck is normally designed to resist an impact load equal to 2,5 times the take-off weight of the heaviest helicopterfactored by a DAF of 1,30.Plastic theories are applicable for designing the plate and stiffeners, while the main framing is analysed elastically.

5.6 Flare Booms

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Analyses of flare booms particularly c*****ider:variable positi***** during installation (horizontal pick-up from the barge, lift upright).reduced material characteristics due to high temperature in the vicinity of the tip during operation.dynamic resp*****e under gusty winds.local excitation of diagonals by wind vortex-shedding.

6. CONCLUDING SUMMARYWith the trend to ever deeper and more slender offshore structures in yet harsher environments, more elaboratetheories are necessary to analyse complex situati*****. There is a risk for the Engineer having increasingly to rely onthe sole results of computer analyses at the expense of sound design practice.To retain enough control of the process of analysis, the following recommendati***** are given:

× check the interfaces between the different analyses and ensure the c*****istency of the input/output. × verify the validity of the data resulting from a complex analysis against a simplified model, which can also be usedto assess the influence of a particular parameter. × make full use of "good engineering judgement" to criticise the unexpected results of an analysis.

7. REFERENCES[1] Skop R.A. & Griffin O.M., An Heuristic Model for Determining Flow-Induced Vibrati***** of Offshore Structures/OTCpaper 1843, May 1973.[2] De Oliveira J.G., The Behaviour of Steel Offshore Structures under Accidental Collisi*****/OTC paper 4136, May1981.[3] API-RP2A, Recommended Practice for Planning, Designing and C*****tructing Fixed Offshore Platforms/18th edition,September1989.[4] DnV, Rules for the Classification of Fixed Offshore Structures, September 1989.[5] DnV, Standard for Insurance Warranty Surveys in Marine Operati*****, June 1985.[6] NPD, Regulation for Structural Design of Loadbearing Structures Intended for Exploitation of Petroleum Resources,October1984 and Veiledning om Utforming, Beregning og Dimensjonering av Stalk*****truksjoner iPetroleumsvirksomheten, December1989.[7] DoE, Offshore Installati*****: Guidance on Design and C*****truction/London, April 1984.[8] McClelland B. & Reifel M.D., Planning and Design of Fixed Offshore Platforms/Van Nostrand Reinhold, 1986.[9] UEG, Node Flexibility and its Effect on Jacket Structures/CIRIA Report UR22, 1984.[10] Hallam M.G., Heaf N.J. & Wootton L.R., Dynamics of Marine Structures/ CIRIA Report UR8 (2nd edition), October1978.[11] Wilson J.F., Dynamics of Offshore Structures/Wiley Interscience, 1984.[12] Clough R.W. & Penzien J., Dynamics of Structures/McGraw-Hill, New York, 1975.[13] Newland D.E., Random Vibrati***** and Spectral Analysis/Longman Scientific (2nd edition), 1984.[14] Zienkiewicz O.C., Lewis R.W. & Stagg K.G., Numerical Methods in Offshore Engineering/Wiley Interscience, 1978.[15] Davenport A.G., The Resp*****e of Slender Line-Like Structures to a Gusty Wind/ICE Vol.23, 1962.[16] Williams A.K. & Rhinne J.E., Fatigue Analysis of Steel Offshore Structures/ICE Vol.60, November 1976.[17] Anagnostopoulos S.A., Wave and Earthquake Resp*****e of Offshore Structures: Evaluation of ModalSoluti*****/ASCE J. of the Structural Div., vol. 108, No ST10, October 1982.[18] Chianis J.W. & Mangiavacchi A., A Critical Review of Transportation Analysis Procedures/OTC paper 4617,May1983.[19] Kaplan P. Jiang C.W. & Bentson J, Hydrodynamic Analysis of Barge-Platform Systems in Waves/Royal Inst. ofNaval Architects, London, April 1982.[20] Hambro L., Jacket Launching Simulation by Differentiation of C*****traints/ Applied Ocean Research, Vol.4 No.3,1982.[21] Bunce J.W. & Wyatt T.A., Development of Unified Design Criteria for Heavy Lift Operati***** Offshore/OTC paper4192, May 1982.[22] Walker A.C. & Davies P., A Design Basis for the J-Tube Method of Riser Installation/J. of Energy ResourcesTechnology, pp. 263-270, September 1983.[23] Stahl B. & Baur M.P., Design Methodology for Offshore Platform Conductors/J. of Petroleum Technology,November 1983.[24] DnV - Rules for the Classification of Steel Ships, January 1989.

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ESDEP WG 15ASTRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.6: Foundati*****OBJECTIVE\SCOPE

to classify different types of pilesto understand main design methodsto cover various methods of installation

PREREQUISITESLecture 1B.2.2: Limit State Design Philosophy and Partial Safety FactorsLectures 10.6: Shear ConnectionLectures 12.4: Fatigue Behaviour of Hollow Section JointsLecture 15A.12: Connecti***** in Offshore Deck StructuresLecture 17.5: Requirements and Verificati***** of Seismic Resistant StructuresA general knowledge of design in offshore structures and an understanding of offshore installation are also required.SUMMARYIn this lecture piled foundati***** for offshore structures are presented. The lecture starts with the classification of soil.The main steps in the design of piles are then explained. The different kinds of piles and hammers are described. Thethree main execution phases are briefly discussed: fabrication, transport and installation.

1. INTRODUCTION1.1 Classification of SoilsThe stratigraphy of the sea bed results from a complex geological process during which various materials were deposited,remoulded and pressed together.Soil texture c*****ists of small mineral or organic particles basically characterized by their grain size and mutual interaction(friction, cohesion).The properties of a specific soil depend mainly on the following factors:

density.water content.over c*****olidation ratio.

For design purposes the influence of these factors on soil behaviour is expressed in terms of two fundamentalparameters:

friction angle.undrained shear strength Cu.

Since the least significant of either of these parameters is often neglected, soils can be classified within "ideal"categories:

granular soils.cohesive soils.

1.2 Granular SoilsGranular soils are non-plastic soils with negligible cohesion between particles. They include:

sands : characterized by large to medium particle sizes (1mm to 0,05mm) offering a high permeability,silts : characterized by particle sizes between 0,05 and 0,02mm; they are generally over-c*****olidated; they mayexhibit some cohesion.

1.3 Cohesive SoilsClays are plastic soils with particle sizes less than 0,002mm which tend to stick together; their permeability is low.

1.4 Multi-Layered StrataThe nature and characteristics of the soil surrounding a pile generally vary with the depth. For analysis purposes, the soilis divided into several layers, each having c*****tant properties throughout. The number of layers depends on theprecision required of the analysis.

2. DESIGNSteel offshore platforms are usually founded on piles, driven deep into the soil (Figure 1). The piles have to transfer theloads acting on the jacket into the sea bed. In this section theoretical aspects of the design of piles are presented.Checking of the pile itself is described in detail in the Worked Example.

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2.1 Design LoadsThese loads are those transferred from the jacket to the foundation. They are calculated at the mudline.

2.1.1 Gravity loadsGravity loads (platform dead load and live loads) are distributed as axial compression forces on the piles depending upontheir respective eccentricity.

2.1.2 Environmental loadsEnvironmental loads due to waves, current, wind, earthquake, etc. are basically horizontal. Their resultant at mudlinec*****ists of:

shear distributed as horizontal forces on the piles.overturning moment on the jacket, equilibrated by axial tension/ compression in symmetrically disposed piles(upstream/downstream).

2.1.3 Load combinati*****The basic gravity and environmental loads multiplied by relevant load factors are combined in order to produce the mostsevere effect(s) at mudline, resulting in:

vertical compression or pullout force, andlateral shear force plus bending.

2.2 Static Axial Pile ResistanceThe overall resistance of the pile against axial force is the sum of shaft friction and end bearing.2.2.1 Lateral friction along the shaft (shaft friction)Skin friction is mobilized along the shaft of the tubular pile (and possibly also along the inner wall when the soil plug is notremoved).The unit shaft friction:

for sands: is proportional to the overburden pressure,for clays: is calculated by the "alpha" or "lambda" method and is a c*****tant equal to the shear strength Cu at greatdepth.

Lateral friction is integrated along the whole penetration of the pile.

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2.2.2 End bearingEnd bearing is the resultant of bearing pressure over the gross end area of the pile, i.e. with or without the area of plug ifrelevant.The bearing pressure:

for clays: is equal to 9 ´ Cu.

for sands: is proportional to the overburden pressure as explained in Section 6.4.2 of API-RP2A [1].

2.2.3 Pile penetrationThe pile penetration shall be sufficient to generate enough friction and bearing resistance against the maximum designcompression multiplied by the appropriate factor of safety. No bearing resistance can be mobilized against pull-out: thefriction available must be equated to the pull out force multiplied by the appropriate factor of safety.

2.3 Lateral Pile ResistanceThe shear at the mudline caused by environmental loads is resisted by lateral bearing of the pile on the soil. This actionmay generate large deformati***** and high bending moments in the part of the pile directly below the mudline, particularlyin soft soils.2.3.1 P-y curvesP-y curves represent the lateral soil resistance versus deflection. The shape of these curves varies with the depth andthe type of soil at the c*****idered elevation. The general shape of the curves for increasing displacement features:

elastic (linear) behaviour for small deflecti*****,elastic/plastic behaviour for medium deflecti*****,c*****tant resistance for large deflecti***** or loss of resistance when the soil skeleton deteriorates (clay under cyclicload in particular).

2.3.2 Lateral pile analysisFor analysis purposes, the soil is modelled as lumped non-linear springs distributed along the pile. The fourth orderdifferential equation which expresses the pile deformation is integrated by successive iterati*****, the secant stiffness ofthe soil springs being updated at each step.For large deformati*****, the second order contribution of the axial compression to the bending moment (P-Delta effect)shall be taken into account.

2.4 Pile DrivingPiles installed by driving are forced into the soil by a ram hitting the top. The impact is transmitted along the pile in theform of a wave, which reflects on the pile tip. The energy is progressively lost by plastic friction on the sides and bearingat the tip of the pile.

2.4.1 Empirical formulaeA c*****iderable number of empirical formulae exist to predict pile driveability. Each formula is generally limited to aparticular type of soil and hammer.

2.4.2 Wave equationThis method of analysing the driving process c*****ists of representing the ensemble of pile/soil/hammer as aone-dimensional assembly of masses, springs and dashpots:

the pile is modelled as a discrete assembly of masses and elastic springs.the soil is idealized as a massless medium characterized by elastic-perfectly-plastic springs and linear dashpots.the hammer is modelled as a mass falling with an initial velocity.the cushion is represented by a weightless spring (see Figure 3).the pile cap is represented by a mass of infinite rigidity.

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