OrcaFlex Manual - Orcina

Theory, Dynamic Analysis
5.6 DYNAMIC ANALYSIS
132
w
The dynamic analysis is a time simulation of the motions of the model over a specified period of time, starting from
the position derived by the static analysis.
The period of simulation is defined as a number of consecutive stages, whose durations are specified in the data.
Various controlling aspects of the model can be set on a stage by stage basis, for example the way winches are
controlled, the velocities and rates of turn of vessels and the releasing of lines, links and winches. This allows quite
complex operational sequences to be modelled.
Before the main simulation stage(s) there is a build-up stage, during which the wave and vessel motions are
smoothly ramped up from zero to their full size. Ramping of current is optional (see Current Data). This gives a
gentle start to the simulation and helps reduce the transients that are generated by the change from the static
position to full dynamic motion. This build-up stage is numbered 0 and its length should normally be set to at least
one wave period. The remaining stages, simply numbered 1, 2, 3, … are intended as the main stages of analysis.
Time is measured in OrcaFlex in seconds. To allow you to time-shift one aspect of the model relative to the others,
different parts of the OrcaFlex model have their own user-specified time origins. See the diagram below.
For example, simulation time is measured relative to the simulation time origin, which is specified on the Wave page
on the environment data form. The simulation time origin is at the end of the build-up stage, so negative simulation
time is the build-up stage and the remaining stages are in positive simulation time. The figure below shows a
simulation using a build-up of 10 seconds, followed by two stages of 15 seconds each.
Each wave train also has its own time origin, and similarly for time-varying wind and any time history files that you
use. All of these time origins are defined relative to the global time origin (which is not user-specified), so if
necessary you can use the time origins to time-shift one aspect of the model relative to the others.
By default all of the time origins are zero, so all of the time frames coincide with global time. For most cases this
simple situation is all you need, but here is an example where you might want to adjust a time origin.
� You might want to arrange that a wave crest, or a particularly large wave in a random sea, arrives at your vessel
at a particular point in the simulation. If you use the View Profile facility and find that the wave arrives at the
vessel at global time 2590s, then you can arrange that this occurs at simulation time 10s (i.e. 10 seconds into
stage 1) by either setting the simulation time origin to 2580 or else setting the wave train time origin to -2580.
The former shifts the simulation forwards to when the wave occurs, whereas the latter shifts the wave back to
the period the simulation covers.