Dynamic Calculation Procedure

This section contains the parameters related to time domain integration, damping, dynamic forces, iterations and convergence.

1. Time domain procedure

  • Method

    • Nonlinear: System stiffness, damping and mass matrices are updated during the simulation. Should be chosen for analyses where contact is important.

    • Linear: System stiffness, damping and mass matrices are kept constant during the simulation. Loads and response will in general be nonlinear. Significantly faster than nonlinear analysis. Not recommended for analyses where contact is important, e.g. seafloor or pipe-in-pipe contact

2. Random number generator

  • Mersenne twister: Recommended

  • Legacy: Available for comparison with older SIMA versions. May result in wave realizations with inaccurate statistics if a large number of wave components are used.

3. Integration and damping parameters

The time integration procedure. The simulation default procedure is recommended for most cases.

Newmark procedure. The options are:

  • Set to simulator default: recommended, except for decay analyses. Adds a small amount of numerical damping.

  • Use average contact acceleration: recommended for decay analyses

  • Specify parameters for time integration:

    • Nonlinear analysis: Inverse Beta and Gamma

    • Linear analysis: Inverse Beta, Gamma and Theta

Mass proportional damping factors: These will primarily give damping at long response periods. It will also give damping for rigid body motions. Normally not recommended!

  • Global: The same damping factor is used for all local degrees-of-freedom

  • Local: Different damping factors are applied to the tension, torsion and bending degrees-of-freedom.

Stiffness proportional damping factors: These will primarily give damping at short periods.

  • Global: The same damping factor is used for all local degrees-of-freedom

  • Local: Different damping factors are applied to the tension, torsion and bending degrees-of-freedom.

  • Damping Option

    • Total stiffness damping: Use the total element stiffness matrices; i.e. material plus geometric contributions. The damping may be negative if elements are in compression.

    • Material stiffness damping: Use the material element stiffness. Damping will not vary with tension.

An additional damping option is available in nonlinear analysis to either update the damping during the analysis or to keep it constant. See Damping Matrix Calculation under Nonlinear force model.

4. Common force model

  • Start Up Duration: Loads will be ramped up to fill value during the start up period following a S-shaped curve.

5. Nonlinear force model

Nonlinear analysis only

  • Include Slug Flow: Either a simple period slug may be specified or the riser contents may be read from a specified file.

  • Riser / rupture release: Release all or a selected ball joint connector at a specified time in the simulation. Boundary change may alternatively be be used .

  • Damping Matrix Calculation

    • Constant proportional: The damping is kept constant during the analysis.

    • Updated proportional: The damping matrix is updated during the analysis.

6. Nonlinear integration procedure

Nonlinear analysis only

  • Equilibrium frequency: frequency pf equilibrium iterations.

  • Continuation: Option to continue simulation even if the equilibrium criterium is not achieved.

  • Mac iterations: Maximum number of equilibrium iterations at a simulation time step.

  • Convergence norm:

    • Displacement: Convergence measured by the displacement norm.

    • Both: Convergence measured by both the displacement and the energy norms.

  • Displacement accuracy: Convergence criterium for the displacement norm

  • Energy accuracy: Convergence criterium for the energy norm

  • Time step subdivision: Option to try sub-stepping if the time step does not converge.

  • Integration info: Option to print information about converge. Currently found on the _dynmod.res file.

Sub-stepping may be useful for getting the simulation through brief intervals with e.g. contact problems or extreme response. It should however be used with caution as it introduces transients when the time step is changed. The simulation will also be quite inefficient if a significant portion of the time steps are subdivided.

7. Linear force model

Linear analysis only

  • Hydrodynamic Force: Option for hydrodynamic load iterations

  • Force Iteration Convergence: Convergence criterium for hydrodynamic load convergence

  • Max number of load iterations: Maximum number of hydrodynamic load iterations at a time step