1 Integration Schemes Explicit Integration: Xn+1 – Xn = Dt . F(Xn) Simple, no iteration requiredTime-step limited, i.e. is conditionally stableGood for short-duration phenomena (clashing & snatching)Conditional stability ’guaranteed’ accuracyImplicit Integration: Xn+1 – Xn = Dt . F(Xn+1)Stable, large time-steps, but requires iterationCan lose accuracy,at large time steps - this needs to be assessedCan miss short-duration phenomena (contact, axial waves, etc)An implicit integration scheme for OrcaFlex is under development
2 Time Step Determination Recommended Value:1/20th of the smallest nodal natural period in the modelAlternative Approaches:Increase smallest nodal periodPush our luck & use larger time steps
3 Increasing Nodal Natural Period Which Component Dominates?Axial Stiffness:Reduce stiffness generally, avoiding excessive stretchReduce stiffness locally in areas requiring fine segmentationBending Stiffness:Use longer segments, provided system allows it (check). This has a double effect as decreases the number of nodes as well as increasing time step.Consider modelling termination heads etc. with buoys
4 Increasing Time Step Default Value: Trial and Error Approach: Based on experience rather than theoryErrs on the conservative sideTrial and Error Approach:Try twice recommended valueTry three times recommended valueKeep increasing until code divergesCaveat: Very occasionally produces noisy results - Check
5 General Run Time Considerations Interactions between Objects:Shapes and Seabed: Avoid excessive contact stiffnessLine clashing: Use clash energy as a measure of severity of impactOnly enable line clashing where neededTorsion Modelling:Do you really need it? Do a check run and find outLive Graphs during Simulations:Don’t have more than you need, especially range graphs.Random Sea Cases: Outer Time Step:Recommended value may be unnecessarily small for stiff systems.