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Finite Element Analysis for Structural

Performance of Offshore Platforms

Jueren Xie

C-FER Technologies, Canada

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Outline

• Introduction of offshore platforms

• Design considerations

• FEA modeling considerations

• Analysis examples

• Conclusions

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Introduction of Offshore Platforms

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Offshore Platform Facts

• Offshore Platform:A large structure with facilities to drill wells, extract and process oil

and natural gas from under the seabed, and temporarily store product

until it can be brought to shore for refining and marketing

• First Offshore Platform:A drilling/production platform (6 m deep) installed in 1947 off the coast

of Louisiana

• Number of Offshore Platforms Installed:Several thousands around the world (>7,000 as of 2003)

• Depth of Modern Offshore Platform:– Safely producing in depths of water up to 2,100 m

– Safely drilling in depths up to 3,050 mThe historic first offshore discovery out of

sight of land was completed in 1947 by Kerr

McGee Rig 16, a new concept using a

mobile drilling „tender‟. Off southeast

Louisiana, the Ship Shoal 32 well was

drilled to a total depth of 770 m in 4 m of

water 14 km from shore. The field produced

oil until 1984. ©Petrobras - GEO EXPro

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Offshore Platform Types

• Fixed PlatformsBuilt on concrete and/or steel legs anchored directly onto the seabed

(e.g. steel jackets, compliant towers, GBS)

• Tension Leg Platform (TLP)Floating facilities tied down to seabed by tethers

• Semi-submersiblesMarine vessel with good stability and sea-keeping characteristics

• SparsLarge cylinder moored to

seabed

• Floating Production

Systems (FPSO)Large monohull structure

1, 2) conventional fixed platforms; 3) compliant tower; 4, 5) vertically moored tension leg and mini-tension leg

platform; 6) Spar ; 7,8) Semi-submersibles ; 9) Floating production, storage, and offloading facility; 10) sub-

sea completion and tie-back to host facility. - WIKIPEDIA

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Offshore Platform Examples

Troll A GBS platform off the west coast of Norway (Wikipedia 2012)

P-51 semi-submersible platform off the Brazilian coast (Wikipedia 2012)

A gravity-based structure (GBS) is a

support structure held in place by gravity

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Design Considerations

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Design Requirements

• Foundation Design

– In-situ and laboratory tests

• Naval Architecture

– Hydrostatics (to maintain afloat)

– Hydrodynamics (to resist flow motion)

• Structural Design

– Structural Design Validation

– Selection of Construction Materials

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Structural Design Validation

• Scope of Work:– Strength Check (resistance to material yield and/or cracking)

– Stability Check (resistance to buckling)

• Design Loads:– Static Loads

• Dead Weights (weights in air – including structure, equipments and ballast)

• Hydrostatic forces (buoyancy and hydrostatic pressure)

– Dynamic Loads

• Operational (e.g. drilling and crane operations)

• Environmental (e.g. wind, wave, ice, thermal, earthquakes)

• Accidental (e.g. vessel collision, dropping objects, fire/explosion)

• Design Codes: – BS 6235 (1982), API RP 2A-WSD (2000), DNV-OS (2010-2011)

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Simulation-based Design

• Traditional designs were based on experience

• “Simulation-based design” provides more rational

design with increased safety and reduced cost:– FEA in marine and offshore back to late 1950s

– Commercial FEA programs in 1970s and 1980s

– Contact modeling algorithms in 1980s and 1990s

– CAE/CAD in 1990s

• VPD (Virtual Product Development):– FEA for structural mechanics

– CFD for fluid dynamics

– Multiphysics

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FEA Modeling Considerations

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Structural Modeling

• Topsides

– Stick model

• Leg and Base

– Concrete: 3D solid model

– Steel: plate model:

• Beam elements for ring-girders

• Shell elements with rebar for

stiffened plates

• Foundation

– Spring model

Topsides Structure

Base Structure

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Foundation/Support

A Generic GBS Example

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Material Modeling

• Elastic Models(Sufficient for design validation analyses)

– Defined by Young‟s modulus, Poisson‟s ratio, etc.

• Inelastic Models(Often used for failure investigations)

– Concrete Material:

• Smeared cracking model with compressive yielding and tension stiffening

– Steel Material

• Isotropic and/or kinematic hardening

– Soil Material• Non-linear soil spring descriptions

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Mass Modeling

• Self Masses– Refers to dead weight in air of structural members

– Defined as densities of structures and activated during gravity

analysis

• Added Masses– Include entrained masses such as ballasts, water tanks, dumps

and mobilized water volume outside the structure but moving with

the structure

– Defined as directional point masses

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Load Modeling

• Static Analysis– e.g. gravity, push-over, etc.

• Frequency and Modal Analysis– e.g. wind, wave, earthquake, etc.

• Dynamic Analysis– e.g. vessel impact, object dropping, etc.

• CPU– Optimized models can be analyzed using PC

– Targeted CPU time:

• ~ minutes for static analysis

• less than an hour for dynamic analysis

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Analysis Examples

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Topsides Structure

Base Structure

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Foundation/Support

Example Model

Assumed Generic GBS Platform:• Base: 90m × 90m × 25m

• Legs: 18m × 18m

• Stiffn. plate thick.: 30mm

T stiffn. spacing: 1m

• Total weight: ~ 300 mt

• E = 200GPa, Yield = 360MPa

Corbels

Boots

Leg Model

Bulkheads

Base Model

Model Features:• Shell elements with rebar: ~30,000

• Beam elements: ~20,000

• Spring elements: ~10,000

• Mass elements: ~20,000

• Elastic-plastic material model

• Large deformation

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(1) Gravity Loading

• Nearly uniform settlements on foundation

• Strength check – no yielding

• Stability check – no buckling

vertical displacement (m) effective stress (N/m2)

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(2) Natural Vibrations

• Various modes:

– Rotating

– Twisting

– Bending

• Subsequent analyses:

(Modal analyses)

– Wind

– Wave

– Earthquakes

Mode 1 (Frequency = 0.603 s-1

) Mode 2 (Frequency = 0.648 s-1

)

Mode 3 (Frequency = 0.903 s-1

) Mode 4 (Frequency = 1.28 s-1

)

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(3) Vessel Impact

• Impact model:

– 5 kt vessel @ 3m/s

– Contact surfaces

• Impact results:

– Impact period = ~0.3 s

– Max force = 230MN

– Residual plastic strains

– Need check joint forces

(a) prior to impact

(b) at maximum impact

(c) after impact

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Impact force

(MN)

Vesselspeed

(m/s)

Time after initial impact (s)

Speed

Impact force

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Conclusions

• FEA is becoming an important tool for design validation and

failure investigation of offshore platforms

• Feasible modeling approach requires:

– Ability to perform strength and stability checks

– Proper representation of structures and masses

– Capable of capturing primary deformation modes

– Manageable model size

• Load Case - Gravity Loading:

– GBS shows nearly uniform settlement

• Load Case - Natural Vibration:

– Primary modes include rotating, twisting and bending

• Load Case - Vessel Impact:

– Short impact period with significant impact force

– Permanent deformation

– May potentially damage connections to topsides

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Acknowledgement

• C-FER Technologies, Canada(Funding and support)

• Dr. Nader Yoosef-Ghodsi, Senior Research

Engineer, for his technical advice and assistance in

reviewing this paper

• Chris Timms, Manager of Offshore and Structures,

for his assistance in reviewing this paper.

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