Presentation on theme: "OrcaFlex User Group, 2004 www.orcina.com Slide 1 of 5 OrcaFlex - Speeding Up OrcaFlex Explicit Integration: X n+1 – X n = t. F(X n ) –Simple, no iteration."— Presentation transcript:
OrcaFlex User Group, Slide 1 of 5 OrcaFlex - Speeding Up OrcaFlex Explicit Integration: X n+1 – X n = t. F(X n ) –Simple, no iteration required –Time-step limited, i.e. is conditionally stable –Good for short-duration phenomena (clashing & snatching) –Conditional stability guaranteed accuracy Implicit Integration: X n+1 – X n = t. F(X n+1 ) –Stable, large time-steps, but requires iteration –Can lose accuracy,at large time steps - this needs to be assessed –Can miss short-duration phenomena (contact, axial waves, etc) An implicit integration scheme for OrcaFlex is under development Integration Schemes
OrcaFlex User Group, Slide 2 of 5 OrcaFlex - Speeding Up OrcaFlex Recommended Value: –1/20th of the smallest nodal natural period in the model Alternative Approaches: –Increase smallest nodal period –Push our luck & use larger time steps Time Step Determination
OrcaFlex User Group, Slide 3 of 5 OrcaFlex - Speeding Up OrcaFlex Which Component Dominates? Axial Stiffness: –Reduce stiffness generally, avoiding excessive stretch –Reduce stiffness locally in areas requiring fine segmentation Bending 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 Increasing Nodal Natural Period
OrcaFlex User Group, Slide 4 of 5 OrcaFlex - Speeding Up OrcaFlex Default Value: –Based on experience rather than theory –Errs on the conservative side Trial and Error Approach: –Try twice recommended value –Try three times recommended value –Keep increasing until code diverges Caveat: Very occasionally produces noisy results - Check Increasing Time Step
OrcaFlex User Group, Slide 5 of 5 OrcaFlex - Speeding Up OrcaFlex Interactions between Objects: –Shapes and Seabed: Avoid excessive contact stiffness –Line clashing: Use clash energy as a measure of severity of impact –Only enable line clashing where needed Torsion Modelling: –Do you really need it? Do a check run and find out Random Sea Cases: Outer Time Step: Recommended value may be unnecessarily small for stiff systems. General Run Time Considerations Live Graphs during Simulations: –Dont have more than you need, especially range graphs.