Vessels

Offshore systems of all kinds often have the same two boundaries; the seabed and a vessel. This is also true in OrcaFlex – vessels present a boundary condition to the model.

Vessel motion can be defined by very simple data sources, such as time histories, prescribed or harmonic motion, or may even be externally calculated. Such motions can be used for ROV operations, installation vessel manoeuvres, or to build simple test models. In all these cases, the vessel motion is imposed on the system.

However, OrcaFlex vessels are primarily intended to model rigid bodies that are large enough for wave diffraction to be significant, such as ships, floating platforms, barges, TLPs or semi-subs. In this situation, the vessel motion is based on RAOs, QTFs and other diffraction analysis output, which can be calculated by a separate program and then imported into OrcaFlex. Vessel motion can then be

Many options are available for how the vessel position and motion may be imposed or calculated. For a summary, see vessel modelling overview; for details of the options, see vessel calculation data.

Vessel data and vessel type data

To represent a floating body, the OrcaFlex vessel requires a lot of data to define its properties. The case-specific data, such as its position and how its motion should be calculated, are given on the vessel data form; most of the data, however (e.g. its RAOs, QTFs, added mass etc.), do not depend upon the way in which the vessel is being used, so these data are instead given for a vessel type, and for a given draught of that vessel type. The vessel itself is then set to use that vessel type and draught.

Both the vessel and the vessel type have their lengths specified, and if these differ then all the vessel type data are automatically scaled to the length of the vessel. This system allows a given vessel type to be used for similar, but not identical, vessels, or for more than one vessel. For example a pipe being towed by two similar tugs can be modelled by creating a vessel type called 'Tug' and then creating two vessels, each of type 'Tug' but possibly of different lengths. The RAOs, QTFs, drawing data (defining the tug outline) and many other properties are data of the vessel type, since they apply to both tugs. On the other hand the two tugs differ in their positions and the way they are driven, so these are properties of the individual vessel objects.

Vessel drawing

The vessel is drawn in 3D views of the model, as either a wire frame or using a shaded view drawing file. The drawing allows a simple visual check that amplitudes, phases etc. are consistent with the applied wave, and can also be used to do a visual check for interference between lines and vessel structure.

Drawing data can be defined for both the vessel and for its vessel type, for both wire frame and shaded views. As with all points on the vessel, the drawing coordinates are defined relative to the vessel axes $\Vxyz$.

It is usually convenient to specify drawing data for the vessel type, rather than the vessel, since it can then be shared if that vessel type is used for another vessel, and it will also be automatically scaled if the vessel is a different length. However the vessel can also have its own drawing data, which is drawn in addition to the type data, and this allows you to set up vessel-specific drawing. For example the lead tug in the example discussed above might have a special tow-point fitting that you want to draw. When the vessel is drawn, OrcaFlex first draws the vessel type (scaled to the vessel length) and then draws the vessel. The two parts of the drawing can have different colours, so you can highlight application-specific drawing.