Static Calculation Load Sequence

The static analysis starts with the stress-free confutation and proceeds through the static steps of each static load group in turn. One or more static load types may be applied in each load group. Use the checkbox Run With Previous to apply it together with the load type specified above.

Iterations are carried out at each load step to find a static equilibrium before continuing. The static loading sequence is therefore the key factor for a successful static analysis. The following loading sequence will often be a good starting point:

  • Load group 1: Volume loads. Weight and buoyancy

  • Load group 2: Specified displacements. Displacements of supernodes with Fixed or Prescribed constraint rom stress-free to final static position

  • Load group 3: Specified forces. Static load components. Unless these are used to hold the structure in equilibrium, e.g. top tension of a vertical riser, in which case they should be applied together with the Volume Loads.

  • Load group 4: Current loads and in-plane seafloor contact

Depending on the model, other loads may need to be applied. Available static load types are:

  • Volume forces

  • Specified displacements

  • Specified forces

  • Current forces

  • Tensioner contact forces activation

  • Roller and tubular contact forces

  • Activate pipe-in-pipe contact forces activation

  • Initially pre-stressed segments

  • Body forces from SIMO bodies

  • Activate bottom friction forces. In-plane seafloor spring and friction forces

  • Activate global and geotechnical springs

  • Bending stiffness

  • Temperature variation

  • Pressure variation

  • Activate material memory formulation. Isotropic/kinematic hardening

  • Boundary change

  • Run winch(es)

  • Marine growth. Changes to cross-sectional properties due to marine growth profile.

  • Wind forces

Volume forces will be applied in dynamic analysis if not specified in the static loading sequence.
If current, wind, floater forces and specified force are specified but not activated, the current load, wind load, floater forces and specified force will be activated in dynamic analysis.
If not activated in static analysis, in-plane seafloor spring and friction contact will be activated in dynamic analysis.
Some static loads will be incremented over the specified number of load steps for the load group while others will be activated at the beginning of the load group. For example volume forces and current forces are incremented over the specified load steps while element memory and contact forces are activated at the beginning of the load group in which they are specified.

Under compression or loads that are large compared to the system’s stiffness may lead to convergence problems and possibly to the analysis failing. Both the cross-sectional stiffness and the geometric stiffness from tensions elements will contribute to the system’s stiffness and the stability of static analysis. Boundary changes may be used to hold parts of the system until the system is more stable.

Dynamic time domain analysis is in general more stable as both mass and damping contribute to effective dynamic stiffness and thus improving convergence.

The initial elongation of fibre rope segments is applied in the same way as user-defined stress-free segment lengths, i.e. gradually during a static load group. Initially pre-stressed segments is specified, the fibre rope elongation will be applied in this load group together with any stress-free segment lengths specified by the user. Otherwise, the elongation will be automatically applied in the first load group.