Environment: Wave calculation

Wave calculation method

OrcaFlex offers you a choice of methods for calculating wave kinematics,

By default, the choice you make here for the wave calculation method applies to all 3D buoys, 6D buoys and lines in the model; it is possible, however, to override this choice and specify that an individual 3D buoy, 6D buoy or line should use a different wave calculation method. Further details can be found in the help for the relevant model objects.

The instantaneous position (exact) method calculates the fluid kinematics at the instantaneous position of each relevant model object, at every time-step of the simulation. This method has always been used historically by OrcaFlex, and can be regarded as the 'exact', most accurate method. The other two methods apply differing optimisations, and may lead to a considerable performance gain when the number of wave components is large.

The likelihood is that the static position (interpolated) method will yield the fastest runtime of the three methods, but this performance gain may come at the cost of reduced accuracy, especially if the static position assumption is not a good one. The instantaneous position (interpolated) option has more general applicability, yet can still result in significant performance improvements. When using either optimisation, care must be taken to ensure that the predicted wave kinematics are an acceptable approximation to those predicted by the exact method.

The relative performance of the instantaneous position (interpolated) and static position (interpolated) methods in comparison with the exact method depends heavily upon the number of wave components in the model. With just a few wave components present, the expectation is that the bulk of the computational work will be unrelated to wave kinematics. In this scenario we expect the performance benefit to be minor and it will be better to use the more accurate instantaneous position (exact) method. However, in a model containing a large number of wave components (several hundred or more), the performance gain of the instantaneous position (interpolated) and static position (interpolated) methods may be considerable.

Instantaneous position (exact)

Wave kinematics (sea surface elevation, fluid velocity and fluid acceleration) are calculated fully for each relevant model object, at every time-step of the simulation and at the object's instantaneous position.

Instantaneous position (interpolated)

The wave kinematics are computed only at the vertices of a regular grid, which extends in both time and space. The kinematics at all other times and positions are then determined approximately by linear interpolation, based on the values at nearby grid vertices.

Note: The instantaneous position (interpolated) optimisation cannot be used in conjunction with extrapolation stretching. It is also not compatible with sea state disturbance RAOs; any objects that experience sea state disturbance will use the instantaneous position (exact) method to compute their wave kinematics instead.

Static position (interpolated)

The standard wave kinematics calculation will be performed once at the start of the simulation for each relevant model object (i.e. when each object is in its static position). The kinematics at future times will then be computed for the same positions, i.e. assuming that each object remains stationary throughout the simulation, which can be done without repeating the full calculation. This can be a useful approximation for models containing objects that remain relatively still during the course of a simulation.

Note: The static position (interpolated) optimisation cannot be used in conjunction with either Wheeler stretching or extrapolation stretching.

Both of the interpolated options have a user-defined time interval and spatial interval associated with them. The interpretation of these parameters varies slightly, depending upon the optimisation being used, as described below.

Wave calculation time interval

The default value of this parameter is zero, which means that the wave kinematics will be recalculated at every time-step and no interpolation between times will be employed. Care should be taken when choosing the time interval to ensure that it is sufficiently small, such that the interpolation between times gives a good approximation to the true kinematics. This choice could possibly be based on the specific wave periods present in the sea state. The time interval can be ignored when all objects use the instantaneous position (exact) calculation method: it has no meaning in this context.

Wave calculation spatial interval

Care should be taken when choosing this parameter to ensure that it is sufficiently small, such that the interpolation between nodes gives a good approximation to the true kinematics. This choice could possibly be based on the specific wavelengths present in the sea state. It may also be related to the geometric properties of the lines in the model (e.g. curvature) if the static position (interpolated) calculation method is being used. The spatial interval can be ignored when all objects use the instantaneous position (exact) calculation method: it has no meaning in this context.

Note: The instantaneous position (interpolated) and static position (interpolated) optimisations only apply to wave trains comprising linear wave components; the fluid kinematics of nonlinear wave trains are always computed using the instantaneous position (exact) method.

Wave kinematics cutoff depth

In addition to the choice of wave calculation, OrcaFlex provides the option of specifying a cutoff depth below which the wave kinematic fluid velocity and acceleration are taken to be zero. This may reduce the computational effort associated with model objects in regions where the wave-induced fluid motion is negligible. This saving may be especially noticeable for models containing a large number of wave components.

The cutoff depth is measured downwards from the mean sea surface. The default value is infinity, which is equivalent to not having a cutoff depth at all.

Note: The wave kinematics cutoff depth has no effect on fluid velocity due to current. A vertical current profile may also be defined independently of this cutoff depth if required, but any efficiency gains from applying the cutoff depth to wave kinematics will be made regardless of whether or not there is any current below that depth.