Time Stepping |
 
|
Time Stepping |
|
Flexcom has two time stepping algorithms, Fixed Time Stepping and Variable Time Stepping. Generally speaking, a fixed time step is used in analyses where the nature of structural response is broadly similar throughout. For example, in the case of a rigid riser, and/or benign rather than extreme loading conditions. It is well suited to lengthy simulations for a stable model, and is the preferred option provided the required time step is reasonable. A variable step is typically used in analyses where the structural response varies significantly during the course of the simulation. A good example might be that of a steel catenary riser with intermittent seabed contact.
Whether you opt for a fixed or variable time step, the Choice of Time Step is important. It is necessary to choose a time-step which picks up necessary detail in excitation and response. This applies to all time domain dynamic analyses, but it is particularly relevant to those which include significant non-linearities. As a general rule of thumb, it is recommended to use a time step of between 1/12th and 1/20th of the applied wave period in a regular wave analysis.
However, much smaller time steps may be required if the analysis includes any significant non-linearity, such as intermittent contact between a risers and a guide surface for example. Impact situations tend to cause structure response at high frequencies. This problem is exacerbated by the fact that, in general, the element length around the area of contact tends to be small, which can lead to increased structure response at these high frequencies.
Finally, as a general comment, it is important not to ignore any perceived difficulties with your analysis. For example, if you notice that the solution is taking a relatively large number of iterations at each solution step before Solution Convergence is achieved, this may suggest the program is struggling to cope with the time stepping parameters you have chosen. Similarly, if you are running a variable time step analysis, and you notice that the actual time step used never seems to approach the maximum value, then this may also indicate the specified time values are inappropriate for the analysis being performed. Caution is advised against the specification of an excessively large maximum time-step value in an effort to speed up the analysis. The variable time stepping algorithm will try to achieve the maximum value at every possible opportunity, and this can actually make the overall solution less efficient, or even affect solution robustness in certain circumstances.
•*TIME is used to define time parameters for an analysis.