Nonlinear 3D FE-Stability analysis of suction pile Ralf Lampert 1 , Stefan Eckardt 2 , Roger Schlegel 2 , Kjetil Rognlien 3 , Frode Halvorsen 3 1) Dynardo US, Inc. San Francisco, USA; 2) Dynardo GmbH Weimar, Germany; 3) EDRMedeso AS, Norway * Acknowledgments to Project Partner: TechnipFMC plc , DNV GL
Dynardo – dynamic software & engineering GmbH • Founded: 2001 • More than 60 employees, offices at Weimar, Vienna and San Francisco • Leading technology companies Daimler, Bosch, E.ON, Nokia, Siemens, EADS are supported Software Development CAE-Consulting • project work • support • know how transfer / seminar Dynardo is engineering specialist for CAE-based • embedded engineer sensitivity analysis, optimization, robustness • workflow and process automation evaluation and robust design optimization Nonlinear 3D FE-Stability analysis of suction pile 3
Task - Stability analysis of suction pile construction Suction piles are anchoring structures in the seabed. They are used e.g. for foundation construction for oil & gas applications or wind turbines. For installation, an internal underpressure is created inside the pile, so that the pipe is pulled into the seabed. For proof of stability of the suction pile the soil-structure-interaction is important. The finite element method has especially proven to be suitable for the simulation and verification of suction pile constructions. Nonlinear 3D FE-Stability analysis of suction pile 4
Workflow - 3D FE-Stability analysis of suction piles Steps for Solution with ANSYS optiSLang : 1. 3D ANSYS model build up for the suction pile and soil 2. Definition of the model & soil parameter, Definition of the nonlinear load history (primary stress state, pore pressure state, suction procedure) 3. Introduction of imperfection (from Eigenvalue Buckling Analysis) 4. Non-linear stability analysis considering structural imperfections and soil-structure interaction with soil (Mohr-Coulomb) material models for different load cases 5. Model validation & Sensitivity analysis for checking model quality and uncertainty of boundary conditions 6. Proof of serviceability and stability Nonlinear 3D FE-Stability analysis of suction pile 5
Calculation Flow in ANSYS optiSLang Nonlinear 3D FE-Stability analysis of suction pile 6
Finite Element Model Properties boundary conditions Geometry, Mesh prefered brick mesh, 0.5 – 1 Mio DOF u vertical = 0 suction pile & soil n Soil Layers: - deformation boundary conditions perpendicular to the surface on all based on sides geotechnical report - on the bottom side soil layer with u v = 0 on bottom surface - on the top side without coupling between top plate and inner soil Nonlinear 3D FE-Stability analysis of suction pile 7
Material modeling ANSYS Geomechanical Toolbox is a library of elasto-plastic material models, which describe the behavior of natural (rock, soil, wood) and of artificial (steel, concrete, masonry) materials for civil engineering and geotechnical applications Unique features: • consistent elasto-plastic algorithms allowing the efficient numerical handling of multiple failure surfaces within the framework of multi-surface plasticity • powerful combination of different yield conditions/failure surfaces • very realistic nonlinear material models which are based on engineering material parameters with a clear physical meaning: e.g. friction angle, dilatancy angle, cohesion, tensile strength, compressive strength • identification and visualization of failure mechanisms based on plastic activities RES shear shear Φ C tension tension σ N f t material law for joints material law for rock/concrete nonlinear hardening and shear and tensile failure shear and tensile failure softening behavior Nonlinear 3D FE-Stability analysis of suction pile 8
Soil structure interaction Modeling nonlinear soil behaviour with multisurface Mohr Coulomb / Rankine material models Modeling contact joint between soil and suction pile with nonlinear friction contact elements or anisotropic Mohr Coulomb friction model with tension cut off aproach t RES F tan c 0 Re s N F f 0 1 N t -s N Nonlinear 3D FE-Stability analysis of suction pile 9
Model Check: Comparison Earthpressure SX horizontal stresses compared with analytical calculation (TIME=1) Layer 1: f 1 , r 1 pathplot Layer 2: f 2 , r 2 Layer 3: f 3 , r 3 Left: SX (global coordinates) Right: Pathplot (SX) Nonlinear 3D FE-Stability analysis of suction pile 10
Model Check: physical plausibility for nonlinear load history Loadstep1: Gravity (t=1) Loadstep2: (suction pressure) (t=2) ux horizontal displacement of complete soil (cylindrical coordinate system) Nonlinear 3D FE-Stability analysis of suction pile 11
Model Check: physical plausibility for nonlinear load history x(analytical) = ~3800Pa In (core) G I K x(analytical) = k 0 · r ·g·z =~8000Pa Out (soil) A C E Three positions with two elements each – one from the inside and one form the outside Left: radial stress SX right: radial displacement UX Nonlinear 3D FE-Stability analysis of suction pile 12
Model Check: parameter studies for inner soil For conservative calculation we reduced the strength (inner friction angle) and stiffness (G, E) of the inner soil (to avoid unrealistic „container effect“ for inner soil). Although a range of 30-35° for friction angle F in geotechnical report is given, we investigated a reduction up to a F of 0°. This decrease is a result of the inner suction pressure, which will generate a perfusion of the soil and therefore loosen the soil . This leads to following parameters and their ranges for a given cohesion: friction angle F core (inner soil): 0-30° • density r core (inner soil): 0-50% reduction • young’s modulus E core (inner soil): values corresponding to F • poisson ration n core (inner soil): values corresponding to F • Nonlinear 3D FE-Stability analysis of suction pile 13
Model Check: parameter studies for inner soil Load vs. ux displacement (cylindrical coordinate system): 1) considering a chosen imperfection This figure shows the ux radial displacements of the suction pile start design without reduction of inner soil at the point of its largest deflection. reducing f, n (additionally) reducing r Blue line depicts the reference Point of Failure calculation (nonlinear spring model) Overview of some Calculations: Load-Displacement (ux – radial displacement) Nonlinear 3D FE-Stability analysis of suction pile 14
Mesh sensitivity: on the DNV GL simplified plane strain model test 𝜏 𝑜𝑝𝑠𝑛𝑏𝑚 = 5.14 𝑡 𝑣 Nonlinear 3D FE-Stability analysis of suction pile 15
Mesh sensitivity: on the DNVGL simplified plane strain model test skirt Nonlinear 3D FE-Stability analysis of suction pile 16
Mesh sensitivity: on the DNVGL simplified plane strain model test 𝜏 𝑜𝑝𝑠𝑛𝑏𝑚 = 5.14 𝑡 𝑣 total displacements plastic strains volumetric elastic strains USUM (m) EPPLEQV EPEL VOL Nonlinear 3D FE-Stability analysis of suction pile 17
Practical project application – Johan Sverdrup oil field A quarter-model of the a full ITS (Integrated Template Structure) assembly used in the analyses. The surrounding soil is modelled with a width of 14x14m and a depth of 2m below the suction pile’s tip. Nonlinear 3D FE-Stability analysis of suction pile 18
Practical project application – Johan Sverdrup oil field Results at structural collapse Buckling mode equivalent plastic strain radial displacement von Mises Stresses Nonlinear 3D FE-Stability analysis of suction pile 19
Practical project application – Johan Sverdrup oil field Soil Spring Stifness – Layer 1 162% pressure 100% Old Spring – Oval NewSpring – 6 Waves New – Springs for 6 waves – Mod 1.5 New – Springs for 6 waves – Mod 2 Continuum Model 0,0 0,5 1,0 1,5 2,0 2,5 3,0 displacement (m) Nonlinear 3D FE-Stability analysis of suction pile 20
ANSYS optiSLang – variation analysis Nonlinear 3D FE-Stability analysis of suction pile 21
Excellence of • is a general purpose tool for variation analysis using CAE-based design sets and/or data sets for the purpose of • sensitivity analysis • design/data exploration • calibration of virtual models to tests • optimization of product performance • quantification of product robustness and reliability • Robust Design Optimization and Design for Six Sigma support of process integration serves arbitrary CAX tools with workflow generation process automation Nonlinear 3D FE-Stability analysis of suction pile 22
RDO – Robust Design Optimization Design Understanding Design Understanding Design Improvement Design Improvement Investigate parameter sensitivities, Investigate parameter sensitivities, reduce complexity and reduce complexity and Optimize design performance Optimize design performance generate best possible meta models generate best possible meta models CAE-Data Robust Design Measurement Data Model Calibration Model Calibration Design Quality Design Quality Identify important model parameter Identify important model parameter Ensure design robustness Ensure design robustness for the best fit between simulation for the best fit between simulation and reliability and reliability and measurement and measurement Nonlinear 3D FE-Stability analysis of suction pile 23
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