Finite Element Analysis for Structural Performance of Offshore Platforms

Transcription

1 Finite Element Analysis for Structural Performance of Offshore Platforms Jueren Xie C-FER Technologies, Canada

2 Outline Introduction of offshore platforms Design considerations FEA modeling considerations Analysis examples Conclusions

3 Introduction of Offshore Platforms

4 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 m The 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 Petrobras - GEO EXPro

5 Offshore Platform Types Fixed Platforms Built 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-submersibles Marine vessel with good stability and sea-keeping characteristics Spars Large 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) subsea completion and tie-back to host facility. - WIKIPEDIA

6 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

7 Design Considerations

8 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

9 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 ( )

10 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

11 FEA Modeling Considerations

12 Drilling/Utility Leg Drilling/Utility Leg Drilling/Utility Leg Drilling/Utility Leg Structural Modeling Boot Topsides Structure Base Structure Foundation/Support A Generic GBS Example 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

13 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

14 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

15 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

16 Analysis Examples

17 Drilling/Utility Leg Drilling/Utility Leg Drilling/Utility Leg Drilling/Utility Leg Example Model Corbels Topsides Structure Bulkheads Boots Boot Base Structure Foundation/Support Leg Model Base 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 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

18 (1) Gravity Loading vertical displacement (m) effective stress (N/m 2 ) Nearly uniform settlements on foundation Strength check no yielding Stability check no buckling

19 (2) Natural Vibrations Mode 1 (Frequency = s -1 ) Mode 2 (Frequency = s -1 ) Various modes: Rotating Twisting Bending Subsequent analyses: (Modal analyses) Wind Wave Earthquakes Mode 3 (Frequency = s -1 ) Mode 4 (Frequency = 1.28 s -1 )

20 (3) Vessel Impact (a) prior to impact Impact model: 5 kt 3m/s Contact surfaces Impact results: Impact period = ~0.3 s Max force = 230MN Residual plastic strains Need check joint forces (b) at maximum impact Speed Impact force Vessel speed (m/s) Impact force (MN) (c) after impact Time after initial impact (s)

21 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

22 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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