inroduction to offshore structures

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Introduction to Analysis of Offshore Structures

OFFSHORE STRUCTURAL ENGINEERINGIntroduction to offshore EngineeringStructural Modeling using SACSLoads on Offshore Structures (Gravity and Environmental)Structural Analyses (in-service & pre-service)y ( p )Design Methods and PrinciplesDesign of Tubular MembersDesign of Tubular JointsDesign of Wide flange Beams / Plate GirdersDesign of Lifting DevicesDesign of buoyancy systemsPile Foundation DesigngPile – Soil InteractionCodes and StandardsCode Provisions (API RP 2A)

5/2/2011 1 Dr. S. NallayarasuDepartment of Ocean Engineering

Indian Institute of Technology Madras-36


Session 2nd May 2011 3rd May 2011 4th May 2011 5th May 2011 6th May 2011

Morning Introduction to Offshore

Lift analysis & Lift Point

Ultimate strength of

Pile design and

Float-over installations

OUTLINE

(9 am to 12 noon)

structures design jacket Driveability

Evening

(2 pm to 5 pm)

Tubular member & Joint design

Non-Tubular member and joint Design

Fatigue analysis and design

Jacket stability and MudmatDesign

Finite Element Analysis

5/2/2011 Dr. S. NallayarasuDepartment of Ocean Engineering


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Hydrocarbon Production and Transportation

Riser

Wel

l Flu

id



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W

Conductors (wells)

Tie-in Spool

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Jackup DrillingMovable Drill Floor



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Introduction to Analysis of Offshore StructuresSEMI-SUBMERSIBLE RIG (COURTESY OF STENA DRILLING)



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Introduction to Analysis of Offshore StructuresDRILL SHIP ‘TRANSOCEAN ENTERPRISE’



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Introduction to Analysis of Offshore StructuresTENDER ASSISTED DRILLING



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Drilling MastJACKEUP PRE-DRILLING

DECK DECK



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PRE-DRILLING OF WELLS JACKEP DRILLI MAST STOWED BACK

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DRILL MASTJACKEUP DRILLING AFTER PLATFORM INSTALLATION

JACKUP LEG

DRILL FLOOR

DECK

Wellhead Platform



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WELL CENTRES

SPUD CAN


Deck Mounted Drilling Rig



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Drill Floor

Drill Mast

Drill

Setback

Hook LoadDRILL LOADS

Living Quarter

Drill Floor

Drill Support Module

Drill Pipes

Moving

Skid Beam



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Wellhead and Process, Living platforms separated by a distance

Wellhead, Process, Living facilities in a single platforms

As we move away from shallow water depth, the safety in design revolves around economy and new technology

platforms separated by a distance a single platforms



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HOSTILE ENVIRONMENT



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Twister



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Fixed PlatformsWellhead Platforms

Generally 4 legged or Tripods (3 legged)Supports minimum topsideSupports Well conductors sizes ranging from 16” to 30”Supports Well conductors sizes ranging from 16 to 30In Persian Gulf, typical water depths around 30-70m, most jacket installed by lifting with weight ranging from 600T to 2000 T

Process PlatformsGenerally 8 legged or 6 leggedLarge topsides either installed by modular lifts or float-overLarge 8 legged Jackets weight vary from 4000T to 6000 T



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Large 8 legged Jackets weight vary from 4000T to 6000 T depending on water depthSouth China Sea water depth ranging from 90m to 120m

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Wellhead platforms



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Introduction to Analysis of Offshore StructuresProcess platforms



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ANODES BOUYANCY TANKSBouyancy Tank Pile Guide

Skirt Sleeve

Anode

Mudmat



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Bouyancy Tanks



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SHEAR PLATE / YOKE PLATE CONNECTION



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INPLACE ANALYSIS - PURPOSEStructural analysis to simulate the behaviour of structure as close as possible and to obtain the response to all loads during its SERVICEp gTo check the global integrity of the structure against premature failureTo check the components (members and joints) against the loads that they are carrying and transmitting to the foundationTo satisfy code requirements against safety



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To satisfy code requirements against safety of structure and supporting foundationCalled Inservice analaysis or Inplace Analysis


Design for In-Service LoadsIn-service analyses

In-place AnalysisDynamic AnalysisDynamic AnalysisFatigue AnalysisSeismic AnalysisShip Impact Analysis

In-service LoadsGravity LoadsE i t l L d



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Environmental LoadsSeismic LoadsAccidental Loads

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Introduction to Analysis of Offshore StructuresINPLACE ANALYSIS

Jacket GeometryMember SizesWave DirectionsHydrodynamic CoefficientsBasic Loads and CombinationsPile-Soil Model (P-Y, T-Z and Q-Z Curves)Analysis MethodsDynamic EffectsPile capacity and Factor Of Safety



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p y yMembers and Joint DesignAllowable Stress Modifiers




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Bottom line : Fabrication and Installation plays a major role !


Design for Pre-Service LoadsPre-service analyses

Load-out AnalysisSea Transportation AnalysisLifting AnalysisLaunch AnalysisUpending AnalysisOn-bottom AnalysisPile Driveability Analysis

Pre-service Appurtenance DesignLif i P d / d b



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Lifting Pad-eye / spreader barLaunch CradleBuoyancy TanksMud-mat

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LOGICAL SEQUENCE

FABRICATIONLOADOUTSEA TRANSPORTATIONLIFTINGLAUNCHINGFLOATATIONUPENDINGON-BOTTOM STABILITYPILE DRIVINGPILE GROUTING



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PILE GROUTINGDECK INSTALLATIONCOMMISSIONINGSTARTUP




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LOADOUT METHODS

LIFTED LOADOUT – For small jacket and decks, where crane either land based or floating is available with adequate capacity, loadout can be done by lifting and placing it on the cargo barge.

SKIDDED LOADOUT – For larger jackets and decks of several SKIDDED LOADOUT – For larger jackets and decks, of several thousand tonnes weight, usually skidded loadout is employed to avoid large capacity cranes. The jacket or deck will be mounted on a temporary skid support either continuous or discrete and will be pulled on to the barge using winches.

TRAILER LOADOUT – Recent times, the trailer loadout is getting more popular due to its robustness and quick preparatory time. Trailers are multi-axle load balancing wheels with appropriate spreader girders on top. The jacket or deck will be moved from quayside on to the barge using these trailers and they will be



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q y g g yremoved.


Barge

Barge Ballasted too much

No SupportA

g

Barge

Barge not enough ballast during rising tide

B



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Barge

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Barge

Ballast Pump Failures during rising tide

No SupportC

D



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Barge


Loadout of Launch Jacket

Mooring LineBollard

WinchPivot andPull Wires

Jacket

FenderPivot and pulley



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Yard

Loadout DirectionSkid Rails

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LOADOUT OF LAUNCH JACKET



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Launch Barge


DECK TRAILER LOADOUTTemporary Brace

COG

TrailerTemporary S



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TrailerSupport

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DECK TRAILER LOADOUTTemporary Brace

COG

TrailerTemporary Support



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Support


DECK SUPPORTED ON LOADOUT BEAM AND TRAILER



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DECK SUPPORTED ON LOADOUT BEAM AND TRAILER



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TRAILER CARRYING A DECK MODULE

Trailer



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TYPICAL TRAILER



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DECK ON LOADOUT BEAM AND TRAILER ON BARGE



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LOADOUT BEAM AND TRAILER IS BEING REMOVED



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JACKET TRAILER LOADOUT

TRAILERS

CARGO BARGEFABRICATION YARD QUAY SIDE



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YARD QUAY SIDE

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TRANSPORTATION METHODS

DRY TOW – The transportation method adopted for most offshore structures are using dump barges and tugs are normally used for towing. Usually tugs will move at a speed of around 6 knots (Approx 3 m/sec).

WET TOW – For faster transportation, self propelled vessels can be used such as conventional ship shape vessels and semi-submersible type carriers. Usually used for trans-continental tow. The speeds can reach as much as 10 to 12 knots. Reduce transportation time by half thus reducing the risk along the route.

SELF TOW – Large jackets, gravity platform structures, spare type structures some times self towed to the site using



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spare type structures some times self towed to the site using tugs. Special care needs to taken for such transportation arrangements.

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TYPICAL TRANSPORTATION ROUTE



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JACKET READY FOR TOW



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DECKS BEING TOWED



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DESIGN ENVIRONMENTAL CONDITIONS

Most Severe environmental conditions during the tow period along the routeDesign environmental conditions include Design environmental conditions include design wave, wind and current10 year return period monthly extremes shall be used as design environmental conditions (Tow period more than 30 days)For smaller tow period, lesser design return



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For smaller tow period, lesser design return period may be accepted

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TYPICAL CARGO BARGE



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Tow Bridle

Tow Arrangement

Barge Tug



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MOTION PARAMETER DEFINITION

Bow

1. Heave

2. Sway

3. Surge

4. Yaw

Stern



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5. Pitch

6. Roll

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TYPICAL BARGE MOTION PARAMETERS



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Source : NDI/30


Jacket on Cargo Barge

Seafastening

Barge

Jacket



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Jacket on Cargo Barge

Jacket

ROLL



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Z

F= ma

MSL X

PITCH

r



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MSL X

Centre of Rotation (COR)

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Barge DraftBarge draft shall be calculated based on the ballast and cargo weight. Usually, it shall be approximately 0.55D where D is the moulded depth of the barge.

In any case following minimum draft shall be used



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Source : NDI/30


Acceleration and Inertial ForceAngular Acceleration

2

2

2

secdeg/in 4T

Qa π=

TWhere

Q = Roll or pitch angle in deg

T = Roll or pitch period in sec

Inertial ForceInertial Force F=m.a.r.

Wh



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Where m = mass of the elementa = angular accelerationr = distance of element from centre of rotation

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Cargo

Roll W Sin(θ)

BARGE



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θ W Cos(θ)




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LIFT METHODS

SINGLE CRANE – Generally single crane lift is adopted for all cases depending on the capacity. Examples of this type of lift is jacket or deck lift in offshore. Most of the vessels in asian region fitted with cranes in the range of 1000 Tonnes to 3500 TonnesTonnes.

DUAL CRANE – For larger jackets, some times dual crane lift is used. Typically Heerama vessel is fitted with dual cranes and can be used for lifts as much as 16,000 Tonnes.

MULTIPLE CRANES – Multiple crane lifts are notmally not used in offshore operation due to problem associated with control and is used in jacket upending in the yard. Some time also used for upending jacket panels and lifting deck panels



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also used for upending jacket panels and lifting deck panels.


DETERMINATE LIFT

A sling arrangement is made such that the system is statically determinate.Sli l d b l l t d i Sling loads can be calculated using equilibrium equations.Sling Mismatch or elasticity of slings does not significantly change the sling load distribution.Most offshore lifts we use this to avoid



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Most offshore lifts, we use this to avoid exact sling length calculations.

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INDETERMINATE LIFT

A sling arrangement is made such that the system is statically indeterminate.Sling loads can not be calculated using equilibrium equations.Typical cases will be lift points at different levels, lifts using matched slings.



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s gs


Auxialiary HookMain Hook

Boom

Jacket

Sling

Crane Vessel

Crane Engine



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Cargo Barge

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JACKET LIFTED OFF THE BARGE



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DECK LIFTED OFF THE BARGE



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DESIGN CONDITIONSThe Crane barge has its own motion characteristics depending on the design.Design environmental conditions to allow the glifting shall be selected to allow the operation of the vessel, and the craneLarge the seastate, the motions will be large leading to additional forces on the barge/crane/structure system.Hence a suitable seastate shall be selected dpending on the specific project and in



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dpending on the specific project and in general, the seastate shall be in the order of 0.5m to 1m wave height with a wind speed not exceeding 20 m/sec.




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MOTION OF CRANE VESSEL

Bow

1. Heave

2. Sway

3. Surge

4. Yaw

Stern



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5. Pitch

6. Roll


MULTI-BODY DYNAMICSCrane Boom StiffnessSling

Stiffness

Cargo

Crane Vessel Movement

Barge Movement

Fender



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Ks Kb - Bending Stiffness

SPRING-MASS SYSTEM

K

s Kb Bending Stiffness

K b



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KwvKwb


DYNAMICS OF CRANE BARGE / CARGO SYSTEM

As the crane vessel is in floating condition, the environmental loads will change the position by roll/pitch and heave motionroll/pitch and heave motion.These motion induces inertia loads on the structure hanging from the crane hook.The effect of Cargo/Crane Boom/Vessel dynamics needs to be studied using suitable model and arrive at the dynamic forces induced

th t



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on the system.

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WHAT IS THE PURPOSE OF LAUNCH ANALYSIS ?

To determine the suitable launch parameters such as barge trim, friction coefficient, required to successfully launch the jacketTo determine the worst case dive depth to make sure the jacket does not touch the seabedTo determine the amount of bouyancy required during launch to avoid undue resultsTo determine the stability of the jacket during the process of diving such, GM, roll and pitch etc.To determine the stresses induced during various phases of motion during launch



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TYPICAL LAUNCH BARGE



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Launch Rail

LAUNCH RAIL AND ROCKER ARMSRocker Arm



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JACKET LAUNCH

Jacket is launched from the launch barge as an alternative to lift. Jacket l h h d f l i h j k i launch method of placing the jacket is suitable for large jacket as the lift capacity of cranes are limited.Jacket will be allowed to slide from the barge into water by means of a push or



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by simply trimming the barge


JACKET LAUNCH ANALYSIS

LAUNCH SIMULATION : Launch analysis is performed to simulate the rigid body motion of the jacket on barge, rocker arm and plunge in to water During various phases of plunge in to water. During various phases of movement, jacket stability, rocker arm reaction and hydrodynamic loads on jacket are simulated.

POST LAUNCH ANALYSIS : During the launch phases, critical phase will be selected



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p , pand a structural analysis will be carried out to simulate the stresses induced during launch.

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LAUNCH PARAMETERS

Barge Trim – The barge trim will depend on the weight of jacket, friction between the barge and jacket. Generally the trim will be around 2 to 4 degrees. The trim will be achieved by suitably ballasting the barge.

Friction – The launch cradle friction with the barge rails will determine whether the jacket will be able to slide on its own weight or require a push. The friction coefficient can be reduced by suitable lubrication of the surfaces.

Winch Speed : Normally a winch speed of 0.6m sec is used.



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Bouyancy – Bouyancy will play a major role in launch of the jacket. A minimum reserve bouyancy of 15% is required


Preparation for LaunchBarge will be arriving at the location of installation and a preset distance of 500m away from the final location of installation, y ,the launch will performed.Barge will be suitably ballasted to achieve a trim angle of 2 to 3 degrees.Seafastening will be cut after obtaining clearance from Marine Warranty Surveyor with regard to weather window and



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gacceptable sea-state for launching.

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BARGE TRIM



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Forward DraftTrim Angle


LAUNCH DURATIONAfter cutting the sea-fastening, jacket will move forward towards the rocker arm. If the jacket does not move artificial push If the jacket does not move, artificial push using strand jack or winch shall be applied to overcome the initial static friction.Once the jacket starts to move, generally the launch duration shall not take more than 2 to 3 minutes



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JACKET LAUNCH TRAJECTORY

Final Floating

Phase 1 to 3Phase 4

Phase 5Push

Barge

Floating Position

Phase 5



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Seabed Clearance


Design for Pre-Service Loads(A jacket After Launch)



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LAUNCH RAIL AND ROCKER ARM

Rocker ArmRocker Arm

Launch Rail

Jacket



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Barge


JACKET ON ROCKER ARM

JACKET COG



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Supports on Rocker Arm

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POST LAUNCH : JACKET TIPPING

Jacket weight

Supports on Rocker arm

weight



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Water Spring Supports


PitchRigging Platform for upending rigging



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COG

Pitch of 1-2 Degrees is acceptable. The rigging platforms shall be above water and allow the rigger to access during operational waves.

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Punch through effect

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JACKET UPENDING IN WATER



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JACKET UPENDING IN AIR



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Introduction to Analysis of Offshore StructuresJACKET UPENDING IN WATER

1 32

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ANALYSIS METHODS

Linear Static AnalysisLinear Static Analysis (Pseudo-Static)Linear Static Analysis (Pseudo-Static)Dynamic Wave Response Analysis (Frequency Domain)Dynamic Wave Response Analysis (Time Domain)Nonlinear Analyses (material or geometric)



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Structural Response – Static AnalysisIf the natural period of the platform is considerably away from fatigue waves, assumption of equivalent static analysis is assumption of equivalent static analysis is acceptableSimple calculations for DAF using SDOF model for each of the wave period can be calculated and applied to the wave loadsSimple Static Analysis either with Pile Soil Interaction or equivalent linearised



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}*{}]{[ DAFFXK =

Interaction or equivalent linearised foundation can be used.

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DYNAMIC AMPLIFICATION FACTOR (SDOF)

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1DAFT T

=⎛ ⎞ ⎛ ⎞2

21 2N n

T TT T

ς⎛ ⎞ ⎛ ⎞− +⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

TN – Natural Period of the structure



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T – Wave Period

ζ – Structural Damping Ratio


8

9

10

Damping = 5%

Damping = 0.1%

DYNAMIC AMPLIFICATION FACTOR (DAF)

2

3

4

5

6

7

Damping = 100%

Damping = 50%

Damping = 15%

DA

F



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0

1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Frequency Ratio β β = TN / T

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Structural Response – Wave Response analysisIf the natural period of the platform is close to the fatigue waves, assumption of equivalent static analysis is not acceptableSimple calculations for DAF using SDOF model for will result in very conservative or non-conservative results depending on the assumptions made on average wave periods for the calculation of DAFHence a Dynamic Wave Response analysis

d t b f d



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needs to be performedDue to iterative calculations in Free Vibration analysis, equivalent linearised Foundation is required


Structural Response – Wave Response analysis

The dynamic wave response analysis requires the dynamic characteristics

0}"]{[}]{[ =+ XMXK

The results of dynamic analysis will be used in Dynamic Wave Response analysis to generate structure response

Free Vibration Analysis



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}{}"]{[}']{[}]{[ FXMXCXK =++

Wave Response Analysis

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Structural Response – Static AnalysisIf the natural period of the platform is considerably away from fatigue waves, assumption of equivalent static analysis is assumption of equivalent static analysis is acceptableSimple calculations for DAF using SDOF model for each of the wave period can be calculated and applied to the wave loadsSimple Static Analysis either with Pile Soil Interaction or equivalent linearised



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}*{}]{[ DAFFXK =

Interaction or equivalent linearised foundation can be used.


DYNAMIC AMPLIFICATION FACTOR (SDOF)

22

1DAFT T

=⎛ ⎞ ⎛ ⎞2

21 2N n

T TT T

ς⎛ ⎞ ⎛ ⎞− +⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

TN – Natural Period of the structure



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T – Wave Period

ζ – Structural Damping Ratio

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8

9

10

Damping = 5%

Damping = 0.1%

DYNAMIC AMPLIFICATION FACTOR (DAF)

2

3

4

5

6

7

Damping = 100%

Damping = 50%

Damping = 15%

DA

F



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0

1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Frequency Ratio β β = TN / T

Introduction to Analysis of Offshore StructuresStructural Response – Wave Response analysis

If the natural period of the platform is close to the fatigue waves, assumption of equivalent static analysis is not acceptableSi l l l ti f DAF i SDOF d l Simple calculations for DAF using SDOF model for will result in very conservative or non-conservative results depending on the assumptions made on average wave periods for the calculation of DAFHence a Dynamic Wave Response analysis needs to be performed



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needs to be performedDue to iterative calculations in Free Vibration analysis, equivalent linearised Foundation is required

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Structural Response – Wave Response analysis

The dynamic wave response analysis requires the dynamic characteristics

0}"]{[}]{[ =+ XMXK

The results of dynamic analysis will be used in Dynamic Wave Response analysis to generate structure response

Free Vibration Analysis



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}{}"]{[}']{[}]{[ FXMXCXK =++

Wave Response Analysis




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DESIGN PHILOSOPHIES

Working Stress Design (WSD or ASD)Limit State Design (LSD)

Load and Resistance Factor Design (LRFD)Plastic Design (PD)



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DESIGN PHILOSOPHY

≤Demand Capacity



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≤Demand Capacity

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Working Stress Design (WSD) or Allowable Stress Design (ASD)

• Service loads are calculated as expected in service• Linear elastic analyses are performed• Allowable stress = Material Strength/Factor of Safety• A design is satisfied if the

Maximum stress < Allowable stress• Limitations

• No 100% sure the load effects will not exceed strengthC ifi t th t th d i ll



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• Case specific, no guarantee that the design covers all cases• Arbitrary choice of Safety Factor?!


STRESS-STRAIN CURVE FOR STEEL



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What is meant by Factor of Safety?

1956-1966: ASCE Task Committee on FOS in their final report indicated that

“The committee has not been successful in its effort to resolve the ‘factor of safety’ question, it is the belief…that the probability approach deserves considerably more study than it has received”



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ALLOWABLE STRESS DESIGN

ASD: Applied stress ≤ Allowable stress

Theoretical

Tested Material

Margin applied to Material stress

≤



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EstimatedLoads

Design Loads

Adjusted Resistance

Material Strength

Design Values

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Design ProcedureDesign Load stresses calculated for each effect and combined appropriately

CompressionTensionBendingShearTorsionHoop

Allowable Stresses taken as as fraction (factor of safety) of yield including the geometric effect such as slenderness, local and global buckling torsional buckling etc



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and global buckling, torsional buckling etc.Design state defined for the worst combined case and a decision made based on design load stresses and allowable stresses


Design LoadsDesign Loads taken as maximum occurring during the life. No variability or probability of accidence includedAt times suitable value is taken from historical data and At times, suitable value is taken from historical data and may reflect true loads during life time

Allowable StressesDesign Yield strength assumed to be a constantFactor of Safety is chosen for each load effect. Allowable stress is taken as a fraction of yield strength with



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y gassumed Factor of Safety

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Plastic Design

• Service loads are factored by a “load factor”• The structure is assumed to fail under these

loads with plastic hinges formedloads with plastic hinges formed• The cross section is designed to resist the

plastic analysis• Members are safe as they will only be subjected to service loads

• Limitations Preclude other stability fatigue etc limit states



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• Preclude other stability, fatigue etc. limit states• Neglecting the uncertainty in material strength• Arbitrary choice of total factor?!


Limit States Design

• Limit states is a condition at which a structureor some part of that structure ceases to perform its intended function

• Functional requirements• Max. deflections, drift, vibration, permanent deformation

• Could be conceptual• Plastic hinge or mechanism formation

F i bili k



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• Fracture or instability, cracks

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Limit States of Strength

• Safety against extreme loads during theintended life of the structure

• Based on safety or load-carrying capacity• Plastic strength, buckling, fracture, fatigueoverturning



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Load and Resistance Factor Design (LRFD)

• LRFD is based on the limit state philosophy• Service loads are multiplied by load factors• Service loads are multiplied by load factors( γ ) and linear elastic analysis is performed

• Materials strength is reduced by multiplyingthe nominal material strength by a resistancefactor (φ)



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Focus on Strength Checking in Design Codes

• Limit states of life, limb and property of humanbeings

• Public Safety of life, limb and property of human beings• Not matters of individual judgments

• Limit states of serviceability

• Usually permit more exercise of judgment on the part of designers



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Basis of LRFD Specification

• Probabilistic models of loads and resistance• Calibration of LRFD criteria to the established ASDCalibration of LRFD criteria to the established ASD

specification for selected members• The evaluation of the resulting criteria by judgment

and past experience aided by comparative design office studies of representative structures



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Safety in LRFD

• Appraise the safety of a structure in terms ofmeasurable probability

• Keep probability of (ultimate strength) failure• Keep probability of (ultimate strength) failuresufficiently and predictably small

• Statistical protection against failure• In statistical terms – probability of failure, orconversely, survival

• Factored (ultimate) loads • Resistance based on characteristic extreme values (e g 5th percentile)



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values (e.g., 5t percentile)


LRFD Checking Equation

RQ φγ ≤∑5/2/2011 Dr. S. Nallayarasu

Department of Ocean EngineeringIndian Institute of Technology Madras-36

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nii RQ φγ ≤∑Partially Safety Factors

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Serviceability in LRFD

• Appraise the structure in terms of itseveryday usefulness

• protect against “non-performance” (buildingfunctionality, occupant comfort, human perception)

• may tolerate higher ‘failure’ probability than for safety

• Real world measurable behavior

• Unfactored (service-level) loads



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( )• Mean value of resistance


Design Rule

nii RQ φγ ≤∑∑• Where Rn is the nominal strength and Qi isthe load effect

• Advantages • Non-case specific, statistical calculations guarantee population behaviorU if f t f f t b th l d d t i l



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• Uniform factor of safety as both load and materialfactors are tied by reliability analysis

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Load and Resistance Factors

• Partial safety factors depend on load andstrength statisticsg

• Load factors to account for• variability (uncertainty) in loads

• Resistance factors to account for• Geometry and material property variability• Imperfections in analysis theory• Consequence of failure



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LRFD Factored Load ≤ Factored Strength

Resistance Factor

LRFD METHOD OF DESIGN

FactoredDesign

Tested Material≤Factored

Load Factor



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EstimatedLoads

Design Loads

DesignResistance

Material Strength

Design Values

≤

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Load Combinations

• When several loads act in combination with the dead load, only one of these takes on its maximumlifetime value while the other is at its “expected”lifetime value, while the other is at its expectedvalues.

• E.g. Load Cases• 1.2D + 1.6L• 1.2D + 0.5L + 1.6W

Max. life timeWind load



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Limit States in OffshoreStructure Design



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Limit States

• ULS – Ultimate Limit States• Ultimate strength behavior

• FLS – Fatigue Limit States• Fatigue and fracture behavior

• SLS – Serviceability Limit States• Displacements and deflections

• ALS – Accidental Limit States• Collision fire blast dropped object etc



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• Collision, fire, blast, dropped object, etc


Partial Safety Factors

• Dead Load: 1.0 – 1.2• Variable Load: 1.3 – 1.4Variable Load: 1.3 1.4• Environmental load: 1.3 – 1.4• The material partial safety factors vary with

material type



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Ultimate Limit States Design

• ULS-A reflecting extreme permanent loads with regular environmental conditions

• ULS-B reflecting large permanent loads with extreme environmental conditions



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API RP2A WSDAPI RP2A –WSD(20th ed. 1993 21st ed. 2000)



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Sections in API RP2A - WSD1. Planning 2. Design Criteria and

procedures

10. Welding11. Fabrication12. Installation

3. Structural Steel Design4. Connections5. Fatigue6. Foundation Design7. Other Structural

Components and Systems8 M t i l

13. Inspection 14. Surveys15. Reuse16. Minimum Structures17. Assessment of Existing

Platforms (21st ed)



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8. Materials9. Drawings and

Specifications

18. Commentary


Design Loading Conditions in API RP2A - WSD

• To produce the most sever effects• Operating environmental conditions combined with deadOperating environmental conditions combined with dead

loads and max/min live loads appropriate to normal operations (typical 1-year to 5-year storm).

• Design environmental conditions combined with dead loads and max/min live loads for combing with extremeconditions (typical 50-year or 100-year storm)



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Allowable Stress in API RP2A - WSD

• As specified in AISC ASD specificationAs specified in AISC ASD specification• Where stresses are due in part to the lateral and

vertical forces imposed by design environmental conditions, the basis AISC allowable stresses may be increased by one -third



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1/3


One-third Increase inAllowable Stresses, why?

• Can sustain higher load for a short durationCan sustain higher load for a short duration• Unlikely to have all the transient load acting

simultaneously • Willing to take high risk• Agree better with LRFD



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Section 2.3.6 Earthquake

In the calculation of member stresses, the stresses due to earthquake induced loading should bedue to earthquake induced loading should be combined with those due to gravity, hydroelastic pressure and buoyancy.For strength requirements, the basic AISC allowable stress and those presented in Section 3.2 may be increased by 70 percent.Permit minor yielding but no significant damage to



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y g g goccur


Lifting

• Dynamic load factorsOpen sea: 2.0 for main members, 1.35 for others

Sheltered: 1.5 for main members, 1.15 for others

• The AISC increase in allowable stresses for short-term loads should NOT be used.

• Safety factor for slings: 4.0



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• Safety factor for shackles: 3.0

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Allowable Stresses for Cylindrical Members

Loading Type Allowable Stress

Axial tension 0.60 Fy

Axial compression <0.60 Fy

Bending <0.75 Fy

Shear 0.4 Fy

Hoop buckling SF = 2.0



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Hoop buckling SF 2.0

Connections SF = 1.7


API RP2A – LRFD (1st ed. 1993)

(Also BS EN ISO 13819)



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Sections in API RP2A - LRFD

A. Planning B. Design RequirementC L d

J. Drawings and Specifications welding

C. LoadsD. Cylindrical Member DesignE. ConnectionsF. FatigueG. Foundation DesignH. Structural Components

and Systems

K. FabricationL. Installation M. InspectionN. Surveys O. Platform ReuseP. Minimum StructuresQ Commentary



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and SystemsI. Materials

Q. Commentary


Resistance Factors inAPI RP 2A – LRFD

Loading Type Resistance FactorAxial Tension 0.95Axial Compression

0.85

Bending 0.95Shear 0.95



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Hoop Buckling 0.80Connections 0.9 - 0.95

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Load Combinations in API RP2A – LRFD

• Factored gravity loads• 1.3D1 + 1.3D2 + 1.5L1 + 1.5L21 2 1 2

• Wind, wave and current loads• 1.1D1 + 1.1D2 + 1.1L1 + 1.35(We + 1.25Dn)• 0.9D1 + 0.9D2 + 0.8L1 + 1.35(We + 1.25Dn)• 1.3D1 + 1.3D2 + 1.5L1 + 1.5L2 + 1.2(Wo + 1.25Dn)

• Earthquake• 1.1D1 + 1.1D2 + 1.1L1 + 0.9E



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• 0.9D1 + 0.9D2 + 0.8L1 + 0.9E


Loading Definitions

• D1 – Dead Load 1, e.g. Self weight• D2 – Dead Load 2, e.g. equipment weight2 , g q p g• L1 – Live Load 1, e.g. weight of fluids• L2 – Live Load 2, e.g. operating forces• We – Extreme wind, wave and current loads• Wo – Operating wind, wave and current loads• Dn – Inertial Load correspond to Wo



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Environmental Considerations

• Normal environmental conditions• Expected to occur frequently during the life of the structurestructure

• Extreme environmental conditions

• Recur with a return period of typically 100 years

• Earthquake environmental conditions

• Ground motion with a reasonable likelihood of notbeing exceeded at the site during the platform’s



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being exceeded at the site during the platform slife (for strength check)

• Ground motion from a rare intense earthquake (fro ductility check)


Design Codes and Standards

Dr. S. NallayarasuDepartment of Ocean Engineering


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Who Produce and Distribute These Documents?

• National governments g• Certificate authorities• Technical standards committees• Companies, universities or individual expertise



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Certification/Classification Authorities

They are independent bodiesThey are independent bodies • American Bureau of Shipping• ClassNK (NK)• Det Norkse Veritas (DNV)• Lloyds Register of Shipping (LR)• Noble Denton



Noble Denton

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Government Requirements

Legalities that need to be met• Norwegian Petroleum Directorate (NPD)• UK Health and Safety Executive (HSE)• US Mineral Management (MMS)



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Codes and Standards

• A set of rules intended to insure safety • Impossible to write rules that fully apply to• Impossible to write rules that fully apply to every situation

• Fundamental premise of the code is to provide general stipulations to leave Sufficient latitude for the exercise ofengineering judgment



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Organizations

• American Petroleum Institute (API)• American Welding Society (AWS)g y ( )• Det Norkse Veritas (DNV)• Norwegian Technology Standards Institution(NTS)

• ISO/TC, Petroleum and Natural GasIndustries

• Subcommittee SC 7, Offshore Structures



• The American Bureau of Shipping (ABS)

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Designer’s Responsibility

• Designer must understand the behavior for which the rule applieswhich the rule applies

• Behavioral understanding comes first, application of rules follows

• The designer has the ultimate responsibility of a safe structure

Engineering Judgment



g g g

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API Recommended Practice

• RP2A Recommended Practice for Planning, Designing and Construction Fixed Offshore g gPlatforms – Working Stress Design, 21st

Edition, 2000• RP2A Recommended Practice for Planning,

Designing and Construction Fixed Offshore Platforms – Load Resistance Factor

t d 993



Design, 1st Edition, 1993

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DNV Offshore Rules & Standards

• OS–C101: Design of Offshore SteelSt t G l (LRFD th d) (2004)Structures, General (LRFD method) (2004)

• OS-C201: Structural Design of Offshore Units(WSD method) (2002)

• Rules for the Design Construction andInspection of Offshore Structures

• Appendix C: Steel Structures (1974) ASD d (1977) LRFD



• (1974) ASD and (1977) LRFD

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American Welding Society

• AWS D1.1 Structural Welding Code W ldi P d t• Welding Procedure etc.

• Strength of tubular joints



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NORSOK Standard

• N-004 Design of steel structures (Rev.1, Dec1998)

• NS 3472 Design Rules for steels structures,(1984)



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ISO Standards

• ISO 13919-2:1995 Petroleum and natural gas industries -- Offshore Structures – Part 2:industries -- Offshore Structures – Part 2: Fixed steel structures

• Contains API RP2A LRFD :1993• Approved as an European Standard (EN ISO)

• ISO 19902, Petroleum and natural gas industries – Fixed steel offshore structures



industries Fixed steel offshore structures

• Issued as Draft International Stanadard

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ISO Standards for Offshore Structures

• ISO 19902, Fixed Steel Structures• ISO 19903, Fixed Concrete Structure• ISO 1990, Fixed Offshore Structures

• Part 1: Monohulls, semi submersibles• Part 2: Tension leg platforms

• ISO 19905, Site-specific assessments of offshore units



• Part 1: Jack-ups • Part 2: Jack-ups Commentary

• ISO 19906, Artic offshore structures5/2/2011 164

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ISO Standards for Offshore Structures

• ISO 19900, Genera requirements for offshoret tstructures

• ISO 19901, Specific requirements for offshorestructures

• Part 1: Metocean design and operating considerations• Part 2: Seismic design procedures and criteria• Part 3: Topsides structure• Part 4: Geotechnical and foundation design considerations

P t 5 W i ht t l d i i i d t ti



• Part 5: Weight control during engineering and construction• Part 6: Marine operations• Part 7: Station keeping systems for floating offshore structures and mobile offshore units

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Health & Safety Executive

• Offshore installations: guidance on design ,construction and certification (fourth edition)19901990.

• Have since been revoked• Some sections of the guidance have beenmade available as individual reports in the Offshore Technology (OTO) series



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inroduction to offshore structures

Documents

morning introduction

offshore structures

joint designfatigue

servicey p design methods

designjacket stability

drillingdeck deck522011

centresspud canintroduction

wconductors wellstie