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

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.

Horizontal current variation

Current speed is allowed to vary with horizontal position. The variation is given as a dimensionless multiplicative factor. If it is used in conjunction with variation of current speed with depth, the factor will be applied at all depths.

A value of '~' means that there is no horizontal current variation. A numeric value (e.g. 0.5) allows you to apply a scaling factor to the vertical current speed profile.

To model current speed variation with horizontal position you must define a horizontal variation factor variable data source. The horizontal variation factor is assumed to be constant in the direction normal to the given axis.

Negative factors can be used which allow you to model reversing currents.

Vertical current variation

Apply vertical stretching

If the option to apply vertical stretching is not selected, the vertical variation of current is independent of horizontal position.

If vertical stretching is applied, the vertical profile, as defined 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 current method; it may be deselected for interpolated current.

Current method

Can be interpolated or power law. The interpolated method uses a full 3D profile with variable speed and direction. The power law method uses an exponential decay formula.

Data for interpolated method

Speed and direction

The magnitude and direction of a reference current (generally taken as the surface current). The actual current at a given Z level is then defined relative to this reference current by a current profile.

The direction is the direction in which the current is progressing – for example, 0° and 90° mean currents flowing in the $X$ and $Y$ directions, respectively.

The speed and direction can be fixed, vary with simulation time or be given by an external function.

Profile

A current profile is defined by specifying multiplicative factors and rotations at various depths, relative to the reference current. At each depth in the table the current speed is the reference current speed multiplied by the factor for that depth; the direction is the reference direction plus the rotation specified. Current speed and direction are interpolated linearly between the given depths. The current at the greatest depth given is applied to any depth below this. Similarly, the current at the least depth given is applied to any depth above this.

Negative factors may be used to model reversing currents.

Note: OrcaFlex interpolates rotations over the shortest route. For example if consecutive rows in the table specify rotations of 350° and 10° then OrcaFlex interpolates passing through 355°, 0° and 5°.

If you prefer to enter current speeds and directions directly, rather than using a reference current and reference-relative profile, simply set the reference current speed to 1 and the reference direction to 0.

Data for power law method

Speed at surface and at seabed

The current speed at the still water level and at the seabed. These limiting values are applied at all horizontal positions, with exponential decay between these limits as described by the theory. As a result, vertical stretching is always applied for power law currents.

Note: The current speed at the seabed cannot be greater than that at the surface.

Direction

The current direction is the same at all levels. The direction specified is that in which the current is progressing – for example, 0° and 90° mean currents flowing in the $X$ and $Y$ directions, respectively.

Exponent

This determines how the current decays. With a smaller value, the decay is spread more evenly across the water depth. With a higher value, the decay mostly occurs close to the seabed.

View vertical profile graph, view vertical profile 3D view

The vertical profile graph plots Z against current speed, which can be useful to help visualise and check your vertical current speed variation.

The vertical profile 3D view displays a 3D view with a number of arrows showing the current velocity vectors at a range of depths. These vectors are non-dimensionalised: they do not show absolute current speeds, but they enable comparison of current speeds at varying depths. This view is particularly useful for visualising and checking current profile rotation data.