Vessel data: Inertia compensation

OrcaFlex models may contain floating bodies that carry a relatively large superstructure. By this general description we mean that the superstructure represents a significant proportion of the total mass and inertia of the floater. Detailed modelling of the superstructure may be a further requirement of the analysis, such as when analysing transport or installation of heavy topsides, or floating wind turbine systems.

To represent the floating body, a diffraction analysis is carried out to provide input for definition of an OrcaFlex vessel type. A typical diffraction analysis would require the total mass and inertia of both the hull and the superstructure as input.

When the diffraction analysis output is used in the OrcaFlex analysis, we see the possibility for double-counting of the superstructure inertia. The inertia is counted as part of the floater in the OrcaFlex vessel type, and that same inertia can also be explicitly represented by the OrcaFlex objects used for the detailed superstructure model.

Explicit modelling of the superstructure in OrcaFlex would then require that the mass and inertia of the superstructure are subtracted from the vessel type that represents the floating body. This should result in the vessel type representing only the hull of the floater. The data on this page are provided for input of the superstructure mass and inertia, in order to have OrcaFlex make the necessary subtractions.

A convenient method for obtaining superstructure mass and inertia values might be to make use of the OrcaFlex compound properties report. You cannot include vessels in the collection of objects that report their overall properties. However, the vessel object will still be available for use as the reporting reference origin. This arrangement is precisely what we believe is required - reporting the overall inertia of the explicit superstructure modelling objects relative to the vessel axes.

Compensating mass and moment of inertia tensor

The compensating mass and moment of inertia tensor with respect to axes through the centre of compensating mass in the vessel x, y and z directions.

Centre of compensating mass

The coordinates of the centre of compensating mass, relative to vessel axes.