flexible riser installation challenges

8/11/2019 Flexible Riser Installation Challenges

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The 15th

Marine Industries Conference (MIC2013)

29-31 October 2013 Kish Island

Flexible Riser Installation Challenges

Ramin Valizadeh1,

Morteza Hoseinpour2, Alireza Ghaemi3, Farshid Ebrahimi4

1Offshore structural engineer, Petro Barasun International Co.;[email protected] structural engineer, Petro Barasun International Co.;[email protected]

Hydraulics structural engineer, Petro Barasun International Co.;[email protected] structural engineer, Petro Barasun International Co.;[email protected]

Abstract

The Soroosh / Nowrooz integrated field development is located in the Persian Gulf with the Nowrooz satellite field

located 50km to the north of Soroosh. The processed fluids are exported to an FSU in 45m of water (LAT) for storageand off-take by shuttle tankers. The present development phase comprises the export of crude from each of the fields to

a single FSU (KHALIJ-e-FARS).

Petro Barasun International Company (PBI) was selected to execute the installation of flexible riser between subsea

pipeline PLEM and FSU TURRET. The client in this project is Iranian Offshore Oil Company and the project consist of

installation of 49" oil FFRP export Pipeline to new FSU with about 200 meters length with a related PLEM (Pipeline

End Manifold.

The purpose of the paper is to define the primary considerations affecting flexible riser installation and to determinefurther and highlight which areas of product specification may be addressed with a view to improving the economics of

installation operations. The paper first defines flexible riser characteristics and identifies the necessary installation

hardware prior to providing commentary on the methods of installation. A basic cost model is introduced to illustrate

the major cost elements of the installation process. Areas of potential hazard and risk are briefly highlighted and asummary of conclusions is provided.

Keywords:flexible riser installation, installation methods, bending stiffness, installation economics.

Introduction

The installation of flexible risers presents a series of practical problems, some of which are of sufficient note to be ofimportance to the designer and to the end user. Their flexibility may easily be brought into question as soon as the

installation contractor begins to handle the end product. Without the benefits of the manufacturing plant, machinery and

conditions it soon becomes obvious that the material is often far from flexible and that what flexibility it possesses onlycreates problems for the contractor.

Whilst the contractor is in the business of overcoming these problems, it is of obvious benefit to all parties that the

relevant direct experience be fed back into the design cycle. This then allows the designer, manufacturer and end user toassess and interpret any benefits that may be achievable by alteration in the specification or design parameters. To date,

the manufacturers have often been left largely to their own devices and conscience with regard to specification, design

and construction. They have expended finance, resources and time in large quantities on the research and development

of their products.

The end products they have created are considered fit for their purpose by the relevant authorities and end users. The

products fill a market requirement and have assisted in pushing forward the technical innovations of subsea

developments. The majority of their R&D work is of a confidential and commercially sensitive nature.

The installation of the end product is an area in which independent installation contractors have been, and will continue

to be, involved, often in association with the manufacturer. It is, however, not immediately apparent that full use is

made of the accumulated experiences gained during installation operations. This is not to say that the current endproduct is in any way deficient or that large scale improvements are possible, only that as part of the expansion of the

subject knowledge it is important to consider the products inherent installation problems.

The authors are invited to use the following guidelines in preparing papers, exactly. We are sure that the authors will

respect the following guidelines in order to raise the conference quality.

The length of the full paper must be between 6 to 10 pages. Please use the following guidelines in preparing your full

papers.

CHARACTERISTICS

The detailed specifics of materials and the methods of construction of a flexible riser are not in themselves relevant to

the installation operation. They do, however, determine the riser characteristics which govern the methods of

installation. Some of these are so basic that the main reason to mention them here is to ensure they are not ignored.

LengthThe continuous length of the riser product is a characteristic of obvious importance. The length required is an outcome

of the riser system design for its specific application and will depend intrinsically on the water depth, the motion
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characteristics of the permanent topside vessel and the designs weather criteria for the site. The length required istherefore normally a physical necessity which is independent of any installation considerations.

The riser length affects the overall riser weight and the dimensions of the reels and winches necessary. There is nodefinable maximum limit on the length of riser that can be installed. However, it is self-evident that the shorter the

length then the more easily it may be installed. Generally, there would be no anticipated difficulties with a riser length

up to 1000 m. Over this length the weight may become a limiting factor, particularly if a riser system such as a Lazy- orSteep-S is being installed as these will require ancillary mid-water and gravity bases to be carried. The installation time

does not increase in direct proportion to the riser length, but it does increase. If multiple riser systems are being installed

then the overall installation time will increase dramatically if additional vessel trips are required because of deck

loading constraints.

Diameter

The internal diameter of a riser is effectively a feature of the flow requirements. The external diameter is a design and

manufacturing feature necessary for the specified internal diameter and flow requirement.

The external diameter is a characteristic which impacts installation in several ways. The riser diameter affects the

overall riser weight and the dimensions of reels and winches: this has a bearing on vessel selection. The diameter also

affects the weight/unit length, the minimum bending radius and the bending stiffness, all of which are of primary

concern to the installation contractor as they dictate the handling characteristics of the riser. Generally, experience

indicates that whilst a 6 in (1 in = 25.4 mm) diameter riser is relatively easy to handle, an 8 in may be difficult.

Weight

The weight/unit length of a flexible riser is determined by the design, materials and construction and is dependent on

the diameter required and the service application. The greater the weight/unit length, the more difficult the handlingproblems associated with over boarding, laydown and pull-in.

The overall weight of the complete riser affects deck loading, sea fastening and craneage requirements. Generally, any

weight-saving possible, either overall or per unit length, will have a consequent benefit towards ease of handling and

vessel requirements during installation.

Table 1 presents typical weight/unit length values for various diameter risers. The values are dependent on design

factors such as service conditions and operating pressures.

Table 1: Typical Values for Flexible Risers Used in Production Systems

External Diameter 4 6 8 10

Weight/Unit length (kg/m) 15 22 75 150

Weight/Unit MBR (m) 0.7 1.0 1.75 2.10

Minimum bending radius

The minimum bending radius (MBR) is a primary characteristic of a flexible riser and it is governed by the design,materials and construction of the product. The MBR is a definitive radius to which the riser may be bent without

damage. It has been suggested that safety factors should be incorporated and further that a factor of 1 -5 should be used

for installation purposes except when the riser is to be bent and supported permanently. Table 1 presents typical MBR

values for various diameter risers.

The MBR of a riser defines the storage drum and winch dimensions necessary. It also controls the dimensions of any

gutter used to feed the riser over the deck edge. It is used as a governing criterion for analysis of the catenary and itsseabed touchdown point. Effectively, it is the main characteristic that must be monitored during installation operations.

A large MBR in excess of 2 m must be accommodated by design, analysis and careful preparation of the installation

procedures. The handling and monitoring requirements increase and the installation time increases. Note that using the

suggested safety factor of 1-5 entails the installation contractor attempting to maintain the actual bend radius in excessof 3 m. During laying operations, this is simply a matter of adjusting the parameters of length paid out and/or vessel

position. However, during seabed handling operations for pull-in and tie-in, it is difficult if not impossible to ensure thatthis is maintained.

Maximum allowable tension

The maximum allowable tension that can be applied to a flexible riser itself is normally so high as to be irrelevant to the

installation. However, there may be a lower maximum allowable tension that should not be exceeded for the riser to end

fitting connection. Effectively, if the catenary self-weight for the water depth can be applied on the end fitting then this

is most unlikely to be exceeded by pull-in loads. Typical values for the actual tensile strength of various riser diameters

are given in Table 2 together with their maximum allowable tensions using a safety factor of 2.


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Table 2: Typical Values for Riser Tensile Strength

Riser diameter Tensile strength Max. allowable

(in) (ton) (ton)

4 55 27-5

6 79 39-58 90 450

Tensions during installation operations need to be fully analyzed; however, typically for a 6 in riser, being installed in

120 m water depth, these would need to be maintained below 15 tones.

Bending stiffnessThe bending stiffness of a flexible riser is a characteristic, which has a major impact during installation, particularly onhandling operations subsea. It is often not included as standard datum and the information is not always made available

to the installation contractor. The bending stiffness affects the lifting and manoeuvring of the riser on the seabed during

laydown and during pull-in operations. Typically bending stiffnesses in excess of 90 kN m2 will result in handling

difficulties during laydown, pull-in and tie-in operations.

INSTALLATION EQUIPMENT

The equipment requirements for the installation of a flexible riser are dictated by the riser characteristics.

Winches

A suitable main winch is required to safely deploy a flexible riser. This may take the form of a base and motor, which

engages into a separate and removable drum, thus allowing the change out of a whole drum complete with the

individual riser stored on that drum. If this is not possible then it is necessary to transfer the riser from a delivery orstorage drum on to the main installation winch.

A complete riser length will be a considerable weight that will be dependent on its characteristics and the weight of its

ancillary end fittings. For a Lazy-S system in a 120 m water depth, this will be in the region of 40 tones for a 4 in riser

and up to 80 tones for an 8 in riser.

With the MBR of a riser being of the order of 1-75 m, and this governing the drum diameter, it can be appreciated that

this necessitates a dimensionally large and powerful winch.

Generally, hydraulic winches are utilized offshore and they will be capable of providing upwards of 15 tones line pull at

variable speeds up to 25 m/min. Actual-laying speeds will be in the region 10-15 m/min.

As the flexible riser and pipe market is relatively small, there is a very limited element of choice in the selection ofwinches. Whilst there may be a requirement to optimize winch design, it is not viewed as economically justified.

For safe deployment, it is necessary that the winch be provided with a positive braking mechanism.

Gutters

In order to deploy safely a flexible riser over the side of a vessel deck it is necessary to utilize a gutter or chute

arrangement to lead the riser safely and ensure that a safe bending radius is maintained both over the deck edge and at

either side on entry and exit. Depending on the vessel shape, an angle in excess of 90 may be required to be radiused to

ensure that the riser does not contact any sharp or sudden edge in the event of the vessel backing up on the catenary.

INSTALLATION VESSELS

The minimum vessel requirements are dictated by the necessary installation equipment and the riser characteristics.

Deck space

Flexible riser installation requires a large deck space particularly as it will usually be necessary to carry the variousancillary items that make up the complete riser system, such as a mid-water arch and gravity base structures. The

installation winch itself will take up a large area of deck and there must be sufficient area of deck between the winch

and the gutter to allow handling and rigging of end fittings and positioning of the riser system ancillary items.

Riser systems may easily consist of three or more risers in a single group and these may be required to be installed

simultaneously. The deck space requirements can easily stretch the limits of even large construction vessels.

Effectively, the minimum deck space, which would be considered feasible for the majority of operations, would be

1000 m2.

With deck space and loading as primary requirements, the semi- submersible construction vessels offer the most viable

solution.

Deck loading

As noted above, the installation vessel may be required to accommodate several complete riser systems on deck and all

up deck loadings may be of the order of 700 tones. This may also be concentrated over relatively small areas


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necessitating a deck loading capacity in the region of 5 tones/m2. Attention must be paid to the positioning of the

equipment and the sea fastening requirements.

Craneage

An anticipated rating would be 100 tones or more, capable of a single fall operation to the seabed depth and maintaining

in excess of 50 tones at a radius of 10-15 m from the vessels side. An adequate hoisting speed would be in the order of10 m/min. Slewing and topping speeds are not critical. Vessel cranes will be used for handling and over boarding

operations. It may be necessary to use the crane for lifting and handling drums complete with their spooled riser and this

could easily require larger capacity craneage, dependent on available deck space and crane positions.

Diving capability

The diving capability required for a typical North Sea operation as outlined in Section 3.1 above would be anticipated

as a 300 m rated diving system, capable of supporting 16 divers or more and with twin three-man bells. An air diving

station would be advantageous to allow for potential shallow water intervention during over boarding operations.

Additionally, a remotely operated vehicle (ROV) capability is necessary. An eyeball ROV is adequate but because deck

space is at a premium it would be advantageous for the vessel to have a built-in system

Vessel motions

Flexible riser deployment operations will only be carried out in favorable weather conditions with a similar forecast.

However, good vessel motion characteristics can increase the weather operating window which can have obvious

advantages in the case of rapid and unexpected changes in weather and sea-state.

Additionally, seabed pull-in and tie-in operations may be carried out in relatively adverse conditions and the motion

characteristics can determine whether the vessel will be operating or on weather downtime. The advantages of a semi-submersible vessel are obvious.

Positioning

As the installation vessel should be capable of diving operations, this should in itself ensure that an adequate dynamic

positioning (DP) system is available. It is extremely advantageous to have a long baseline acoustic array with real time

video output in both the vessel DP control room and the dive control station. The output should have all major featureson screen including the position of seabed structures.

INSTALLATION ANALYSIS

A sequential analysis of the riser during critical periods of the installation is necessary to confirm that basic criteria

determined from the riser characteristics are being met.

Criteria

The primary criteria to be monitored during riser installation are:

location of the touchdown point;

location of the vessel; and

length of catenary.

These points, together with the riser basic data, will allow full analysis of the riser using standard catenary theory and

hence ensure that adequate safety margins can be maintained for both bending radii and tensions.

Statics or dynamics

The use of standard centenary static analysis is the current practice for determination of geometry and forces during

installation.

Riser installation operations by their nature are limited to extremely good weather conditions in the region of sea-states1 to 2. Unless a static analysis suggests that the load limits are being approached then the introduction of dynamic

analysis does not seem to provide any specific benefits to the operational aspects. However, the time scheduled for the

installation should also be taken into account and it may be considered beneficial to incorporate dynamics, particularly

when multiple riser systems are being installed.

OPERATIONAL METHODS AND CONDITIONS

Operational methods are initially defined by experience and analysis against the previously determined background of

the riser characteristics and selected installation equipment and vessels. Only a general commentary on operational

methods can be presented here and it is to be remembered that operational conditions may necessitate deviation from

proposed methods.

Overboarding

Overboarding of end terminations and ancillary systems such as mid-water arches and gravity bases can presentrelatively complex rigging operations. Most overboarding operations utilize the vessel craneage but ancillary winches

and stopper wires are often required. Chinese fingers are used when it is necessary to stop the riser or to attach holdback


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winches. It is to be noted that Lazy- and Steep-Wave configurations minimize overboarding and rigging requirements

and significantly reduce both deployment times and risks associated with overboarding.

Deployment

Deployment of the riser itself is relatively simple, requiring gradual payout of the installation winch in conjunction with

vessel positioning.Deployment of the final end termination or other items to the seabed may require diver intervention to monitor final

positioning and to de-rig lifting slings and attachments.

Pull-ins

Subsea pull-in operations may necessitate divers working in adverse conditions of poor visibility and high tidal currents.

They are in effect rigging problems which are aggravated by the fact that the lifting and moving of the end termination

and riser lifted length cause mud movement which may reduce visibility to nil. For Lazy-Wave and Lazy-S riser

systems, use of the vessel crane attached to the end fitting is the most commonly used method. Further pull-in wires will

be attached to the seabed structure close to the fixed tie-in location, incorporating tirfors, or if necessary run to a surface

constant tension winch. Lifting bags may be attached at intervals along the riser length to assist in reducing pull-in

tensions and frictional resistance.

For the Steep-Wave and Steep-S riser configurations, the riser end connection is required to be pulled down into its

seabed riser base connector (RBC). The methods for carrying out this operation have yet to be optimised and have a

tendency to exceed the pull-in times for Lazy riser configurations.

Alignments and tie-in

Final alignment and tie-in of a seabed flange can present one of the biggest and potentially most time-consumingaspects of installation of the Lazy riser configurations.

A flexible riser must be laid correctly to the seabed with sufficient but not excess overlength to facilitate pull-in. If this

is not the case then the result after pull-in may be significant misalignment of the flanges. It can then require a certain

amount of persuasion to enable the flanges to mate successfully, particularly with large diameter flexibles that in

practice demonstrate considerable rigidity and resistance to bending. Judicial use of dead man anchors, tirfor lines and

lifting bags can be used, but pulling the riser length transversely across the seabed is far from easy as mud build-up willalways occur in the area of the touchdown point. In the resulting poor visibility, the divers spatial reference pointsdisappear and it is impossible to achieve a clear and definite understanding of the riser position.

Once alignment has been achieved, the tie-in becomes similar to that of a rigid pipe flange and provided that the access

around the flange is easy then the operation can be achieved in a timely manner.

For the Steep riser configurations the controlled pull-down to the RBC will positively guide and locate the riser end

connection into place eliminating alignment times.

INSTALLATION ECONOMICS

This section does not aim to provide an indication of the economic merits of subsea production nor to address the

relative proportion ofoverall development costs which are attributed to installation. It is rather the intention to definethe major cost elements inherent in the installation operation.

Operational costs

Operational costs constitute the major proportion of the total installation cost. Operational costs may be split into

various degrees of refinement but essentially consist of the following:

a) vessel costsinclusive of vessel day rate, fuel and lube and marine crew;

b) personnel costsinclusive of project personnel day rates, individual equipment and victualling; and

c)

equipment costsinclusive of project specific equipment day rates.Inclusive costs for North Sea operations may be running in excess of 50 000 per day. With operational costs forming

75% of the overall budget there is a substantial incentive to reduce operational times to a minimum.

Engineering and preparation costs

Because of the high costs associated with the operational aspects, it becomes the aim that all engineering andpreparation works be targeted at reducing the operational time. Well considered, prepared and presented procedures will

do much to reduce overall costs. Preparing the groundwork for logistics, and ensuring that all needs are foreseen prior to

the operational requirement, are primary concerns. An increase in engineering resources is easily justifiable even if it

results in minor savings in operational time. A smooth operation is a cost-effective operation.

Comparative costs for riser systems installation

It is not possible to provide definitive costs for riser installation operations unless a specific project is fully assessed and

this would not greatly assist during the initial design considerations of a project with different design criteria. However,a comparison of the relative costs is given in Table 3 and it is believed that this may prove useful as a guideline for

initial cost assessment for the differing types of riser systems.


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The model is simplistic in approach but could be further refined to incorporate weighting for relative risk and factors to

uplift for multiple or group installation.

Table 3:Relative Costs for Riser System InstallationFree hanging Lazy-S Lazy-Wave Steep-S Steep-Wave

Relative vessel days 0-40 0-80 0-60 100 0-80Relative vessel cost/day 0-75 100 1 00 100 100

Relative cost of installation a 30 0-80 0-60 1 00 0-80

To allow the designer and operator to put this into perspective against design and manufacturing costs:

1)

Assume overall engineering costs as equivalent to 50% of the vessel cost per installation day.

2) Assume additional equipment costs as equivalent to one-third of the vessel cost per installation day.3)

Assume that installation of a free hanging catenary riser takes two days which is a realistic starting base.

4)

Assume that an installation vessel capable of installing a free hanging riser system costs 35 000 per day

(approximately 1990 cost).

Relative installation costs would then be as detailed in Table 4. It is important to note that these are orders of magnitude

only.

Table 4:Relative Installation Costs for Riser SystemsRiser system Installation cost (order of magnitude) ()

Free Hanging 127 500Lazy-S 340 000

Lazy-Wave 255 000

Steep-S 425 000

Steep-Wave 340 000

The illustrated costs are for a single riser but costs would not be pro rata for multiple or group installation.Certain costs are not included, notably mob and demob costs. It is obviously of importance to review the uplifts for

engineering and additional equipment as these are subject to prevailing market conditions. The model should only be

used for indicative purposes. The model should be reviewed, adjusted and revised as necessary for any specificapplication. The use of any cost model is not recommended as a replacement for cost analysis based on detailed

schedules and defined resources. The potential for time and hence cost savings that have been previously discussed

cannot easily be illustrated without a detailed cost analysis.

HAZARDS AND RISKS

Heavy rigging operations in a marine environment hold inherent hazards for both personnel and equipment.

Methods and procedures for the installation work should be specifically reviewed with regard to safety aspects and

operational personnel must be aware of any potential risks.With regard to the flexible riser itself, there are two major mechanisms of potential damage:

1)

Mechanical damage by over-tension or by bending. Correct implementation of procedures and monitoring during

installation effectively negates the possibility of damage occurring, unless caused by vessel DP drive-off or during

local rigging operations. It is impossible to guarantee safeguards against DP drive or other hazards throughout riser

installation operations. The risks should, however, be assessed and contingency procedures prepared accordingly.

Local rigging operations on the vessel deck or on the seabed must be carefully planned and controlled.2)

Coating damage by abrasion or impact. Whilst every effort will be made to reduce the possibility of coating

damage it must be recognized that this is a problem inherent with the handling of a riser. There is a relatively highrisk of minor coating damage occurring. Materials and repair procedures should therefore be available.

Methods and materials for permanent subsea repairs are understood not to be available and this aspect could be

actively progressed. Meanwhile, it is essential that the criteria for acceptable/ unacceptable damage be predefined.

CONCLUDING REMARKS

The basic parameters which determine the handling characteristics of a flexible riser are determined prior to the

consideration of the installation aspects. This may be detrimental to the overall economics of the riser system.

Specific areas which will result in the reduction of installation costs are shorter overall length, smaller overall diameter,

lower MBR and lower weight/unit length. The areas which potentially require to be addressed further are the MBR andbending stiffness. Any reduction in the bending stiffness of the riser will have positive benefits throughout the

installation and later maintenance operations.

References[1] American Petroleum Institute (API), Recommended Practice for Flexible Pipes API-RP 17B, API, Washington DC,

USA, 1988.


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[2] Cocks, P. J., Testing and Structural Integrity of Flexible Pipes. Engineering Structures, Oct 1989, Butterworth and

Co.

[3] Veritec, Guidelines for Flexible Pipes, Rev. 2, Veritec A/S, Oslo, Norway, 1987.

[4] Johnston, D. R. A., Installation aspects of flexible riser systems, Eng. Struct., Vol. 11, 1989.

[5] Frank Lim, INSTALLATION OF RISERS IN DEEP WATERS, 4th PetroMin Deepwater & Subsea Technol ogy

Conference & Exhibition, Hotel Istana, Kuala Lumpur, Malaysia, 20th - 21st June 2006.[6] Patel, M. H., Seyed, F. B., Review of flexible riser modeling and analysis techniques, Engineering Structures,Vol. 17, No. 4, pp. 293-304, 1995.

[7] Headworth, C., Aylwin, E., Smith, M., Enhanced Flexible Riser Installation Extending Towed Production System

Technologies, Marine Structures, Vol. 5, pp 455-464, 1992.

[8]http://www.globalccsinstitute.com/

[9]http://www.onepetro.org
http://www.globalccsinstitute.com/http://www.onepetro.org/http://www.onepetro.org/http://www.globalccsinstitute.com/

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