A1260498607OCMol

1
Integration Schemes

• Explicit Integration Xn1 Xn Dt . F(Xn)
• 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 Xn1 Xn Dt . F(Xn1)
• 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
2
Time Step Determination

• 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

3
Increasing Nodal Natural Period

• 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

4
Increasing Time Step

• 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
5
General Run Time Considerations

• 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

• Live Graphs during Simulations
• Dont have more than you need, especially range
  graphs.

• Random Sea Cases Outer Time Step
• Recommended value may be unnecessarily small for
  stiff systems.