2008 International ANSYS Conference · 2008 International ANSYS Conference ... LOADS ON GRP SUBSEA...

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2008 International ANSYS Conference

FEM AND FSI SIMULATIONS OF IMPACT LOADS ON GRP SUBSEA COMPOSITE COVERS

Kjetil Rognlien, MScTechnical ConsultantEDR AS, Norway


Presentation Agenda

Background Purpose of study FSI overview Implementation of FSI in ANSYS Specific FSI case Results Conclusion


Background

Current simulation methods to analyze a products behavior in water is to a great extent based onsimplified 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


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


Fluid Structural Interaction

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

FEM Deflection

CFD Pressure

Convergence?


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


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


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:


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


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


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


Energies During Impact


Pressure Between Impactor and Plate


Impact animation 1


Impact animation 2


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


Results FSI VS Impact contd.

Displacements were not significantly decreased, but the stress peaks were lowered due to smoother movement

Parameter Structural FSI %ReductioninFSI

MaxDisplacementinplate[mm] 56,35 54,47 1,84

MaxTensionalStress[MPa] 675 530 21,5

MaxCompressiveStress[MPa] 400 325 18,8

MaxTsaiWuIndex 1,02 0,617 39,5

MaxFPFFIndex 0,722 0,566 21,6

Simulationtime Steel Composite

Structural[s] 324 2606

FSI[s] 11957(Laptop) 75095(Laptop)

The FSI Simulations were time-consuming, but able to run on a laptop


Stiffness Energies, FSI VS Structural


FSI animation 1


FSI animation 2


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


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


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


Thank you!

Any Questions?

FEM AND FSI SIMULATIONS OF IMPACT LOADS ON GRP SUBSEA COMPOSITE COVERSPresentation AgendaBackgroundPurpose of StudyFluid Structural InteractionHow is this modelled in Ansys?contd.FSI Model Impact on Fixed PlateFSI Model, contd.FSI Model, contd.Results Impact VS StaticEnergies During ImpactPressure Between Impactor and PlateImpact animation 1Impact animation 2Results FSI VS Impact ModelResults FSI VS Impact contd.Stiffness Energies, FSI VS StructuralFSI animation 1FSI animation 2Advantages and DisadvantagesAdvantages and DisadvantagesIn ConclusionSlide Number 24

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