Environment: Current data

Multiple current data sets

Multiple sets of current data can be defined, although only one set of current data is active at any one time. This capability is intended to help when you are analysing a series of load cases with differing current data. You can define the different current data sets in the base data file and, when you generate your load case simulations, you simply set the active current to be one of the pre-defined current data sets.

Multiple sets of current data can be defined

This must be checked if you wish to define multiple sets of current data. If it is not checked then you define just a single current data set.

Active current

Specifies which of the multiple current data sets is active in the model. Only available if you have enabled multiple current data sets.

To define the current data sets click on the edit current data sets button. This opens a separate data form where the current data sets can be defined and named.

Data for a single current data set

Current model

This setting determines how the current field is specified. The two options are variation scheme and tabular. The variation scheme model makes it quick and easy to specify simple current profiles; the tabular model is far more flexible and powerful, and allows current acceleration to be specified as well as current velocity. The current acceleration is combined with the wave acceleration for purposes of computing Froude-Krylov and added mass loads. Current acceleration is always assumed to be zero in the variation scheme model.

Ramped

This allows the static position to be calculated without the effects of current by scaling it by the ramp factor (which defaults to zero in statics and then rises to one during the build-up period of dynamics). This allows the current to be ramped up from zero to its full value. If not selected (the default), then the full current is applied in the statics calculation and throughout the subsequent simulation.

This facility to omit current effects from the statics calculation and introduce them during the build-up is useful where the current may cause lines to come into contact.

Consider, for example, the case of a flexible line to the left of a stiff pipe, with current pushing the flexible up against the pipe. The OrcaFlex static analysis does not include the effects of contact between lines, so if current were included in the static analysis then it would find a static position with the flexible line to the right of the pipe. The simulation would then start with the flexible on the wrong side of the pipe.

By setting the current to ramp during the build-up period, and including clash checking for the two lines, we overcome this difficulty. The static position excludes the effect of current and so leaves the flexible to the left of the pipe. The build-up period will then gradually introduce the current effects but will also account for the contact between the two lines.

Apply vertical stretching

For simplicity, we consider a case where the vertical variation of current is independent of horizontal position (before the option to apply vertical stretching is selected).

When vertical stretching is applied to this case, the vertical profile at the seabed origin is stretched to fit the water depth at points $(X,Y)$ away from the seabed origin. Accordingly, vertical stretching only has any impact when the seabed is not horizontal, because when the seabed is horizontal the water depth is constant everywhere.

The idea behind the stretching is to arrange that the current speed and direction are constant at all points on the seabed, constant at all points at the mean water level, constant at all points half way down the water column, and so on. The current theory topic describes how this is implemented.

Vertical stretching is always applied for the power law vertical variation scheme; it may be deselected in all other cases.