2008 International ANSYS Conference

Transcription

1 2008 International ANSYS Conference FEM AND FSI SIMULATIONS OF IMPACT LOADS ON GRP SUBSEA COMPOSITE COVERS Kjetil Rognlien, MSc Technical Consultant EDR AS, Norway 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

2 Presentation Agenda Background Purpose of study FSI overview Implementation of FSI in ANSYS Specific FSI case Results Conclusion 2008 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary

3 Background Current simulation methods to analyze a products behavior in water is to a great extent based on simplified boundray conditions Dynamic events often modeled as equivalent static Simplifications made by time, not technology, concerns Coupled physics effects are often crucial This raises a need for new simulation methods Fluid Structural Interaction 2008 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary

4 Purpose of Study - Examine different results and effects going from an equivalent static to a dynamic impact simulation model - Study the theory and mechanics of Fluid Structural Interaction (FSI) - Study the implementation of FSI in ANSYS - Create a coupled FSI model to measure the damping effects of the water surrounding a composite plate during impact 2008 ANSYS, Inc. All rights reserved. 4 ANSYS, Inc. Proprietary

5 Fluid Structural Interaction FEM Deflection Convergence? CFD Pressure Main challenges: FEA and CFD different physical problems Solved by different solvers and techniques Must establish communication between solvers Applications: Aerospace industry Medical, blood flow through arteries Subsea pipelines, risers and similar Subsea protection equipment 2008 ANSYS, Inc. All rights reserved. 5 ANSYS, Inc. Proprietary

6 How is this modelled in Ansys? Model the structural and fluid region with correct relative placement Mesh the structure and fluid separately (Ansys and CFX) Specify what surfaces should be FSI interface surfaces on both the structural and fluid geometry Include the simulation setup for the Ansys solver in CFX 2008 ANSYS, Inc. All rights reserved. 6 ANSYS, Inc. Proprietary

7 contd. Specify equal time steps and time domain for Ansys and CFX Solution process: Run first iteration in Ansys Map displacement results from structural interface to CFD interface Calculate fluid flow and resulting pressure on interface in CFX Use pressure from CFX as new boundary condition in Ansys Repeat steps 2-4 until displacements have converged Smaller time steps are needed for low stiffness structures Everything was done within the Workbench environment 2008 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary

8 FSI Model Impact on Fixed Plate Test case: A simple plate structure, partially submerged in water, subjected to an impact from a dropped object Three approaches to finding the maximum allowable impact energy: 2008 ANSYS, Inc. All rights reserved. 8 ANSYS, Inc. Proprietary

9 FSI Model, contd. Static: Apply force to an impact area Use the resulting deflection to calculate stiffness energy Assume all of the impact energy is taken up as stiffness energy Dynamic (Impact): Model plate and impactor as separate parts Give the impactor initial velocity Create a contact pair between the impactor and plate 2008 ANSYS, Inc. All rights reserved. 9 ANSYS, Inc. Proprietary

10 FSI Model, contd. Full Scale FSI: Same as dynamic, but include CFD mesh of water Set up a coupling between the FEA solver and the CFD solver 2008 ANSYS, Inc. All rights reserved. 10 ANSYS, Inc. Proprietary

11 Results Impact VS Static Stiffness energy less than impact energy at any given time Other factors than absorbed stiffness energy contributed to stresses: Varying contact area Local stress concentrations Vibrations in the cover plate during impact Dynamic stress distributions and stress peaks For the element types examined; Solid186, quadratic shape functions Shell181, linear shape functions SolidShell190, linear shape functions the impact method showd an average increase in the strength estimate of about 23%, but with relatively high variations 2008 ANSYS, Inc. All rights reserved. 11 ANSYS, Inc. Proprietary

12 Energies During Impact 2008 ANSYS, Inc. All rights reserved. 12 ANSYS, Inc. Proprietary

13 Pressure Between Impactor and Plate 2008 ANSYS, Inc. All rights reserved. 13 ANSYS, Inc. Proprietary

14 Impact animation ANSYS, Inc. All rights reserved. 14 ANSYS, Inc. Proprietary

15 Impact animation ANSYS, Inc. All rights reserved. 15 ANSYS, Inc. Proprietary

16 Results FSI VS Impact Model FSI simulations showed a considerable damping effect compared to a similar plate in air Much of the vibration and peak stresses in the plate were damped out Compared to the impact in air, the FSI model showed an increased strength estimate of about 20% Strength estimate increased 48% from static model 2008 ANSYS, Inc. All rights reserved. 16 ANSYS, Inc. Proprietary

17 Results FSI VS Impact contd. Parameter Structural FSI % Reduction in FSI Max Displacement in plate [mm] 56,35 54,47 1,84 Max Tensional Stress [MPa] ,5 Max Compressive Stress [MPa] ,8 Max Tsai Wu Index 1,02 0,617 39,5 Max FPFF Index 0,722 0,566 21,6 Displacements were not significantly decreased, but the stress peaks were lowered due to smoother movement Simulation time Steel Composite Structural [s] FSI [s] (Laptop) (Laptop) The FSI Simulations were time-consuming, but able to run on a laptop 2008 ANSYS, Inc. All rights reserved. 17 ANSYS, Inc. Proprietary

18 Stiffness Energies, FSI VS Structural 2008 ANSYS, Inc. All rights reserved. 18 ANSYS, Inc. Proprietary

19 FSI animation ANSYS, Inc. All rights reserved. 19 ANSYS, Inc. Proprietary

20 FSI animation ANSYS, Inc. All rights reserved. 20 ANSYS, Inc. Proprietary

21 Advantages and Disadvantages Impact model VS static: Advantages: Physically closer to real life Includes vibration and stress peaks Shows dynamic response with deformation and stress distribution Lowers maximum stress values, increasing strength estimate Disadvantages: More work required to set up the analysis Run time and result file size greatly increased Introduction of non-linear contacts may make convergence hard to achieve Harder to achieve consistent results with respect to element type 2008 ANSYS, Inc. All rights reserved. 21 ANSYS, Inc. Proprietary

22 Advantages and Disadvantages FSI model VS impact model: Advantages: Includes coupled effects between water and structure Results in a smoother deformation pattern during impact Reduces the local stress concentrations Lowers maximum stress values, hence increasing strength estimate Disadvantages: More work required to set up analysis Run time significantly increased User needs basic knowledge about the physics of fluids More simulation parameters on CFD side introduce potential sources of error, and convergence problems 2008 ANSYS, Inc. All rights reserved. 22 ANSYS, Inc. Proprietary

23 In Conclusion FSI simulation showed great increases in strength estimates Consistent with initial assumption of damping in water Enables more efficient design of protection equipment Money can be saved on materials, production and installation 2008 ANSYS, Inc. All rights reserved. 23 ANSYS, Inc. Proprietary

24 Thank you! Any Questions? 2008 ANSYS, Inc. All rights reserved. 24 ANSYS, Inc. Proprietary