Vessel theory: Inertia compensation

The inertia of an OrcaFlex vessel is initially defined by the vessel type it uses. The vessel inertia used in simulation can be reduced from this initial value if inertia compensation data are specified. Here we provide details of how the compensating inertia affects the vessel.

As would be expected, the mass considered for the vessel is found by subtracting the compensating mass from the mass provided by the vessel type. A new centre of mass is considered for the vessel reduced mass.

The subtraction of compensating moments of inertia away from the values provided by the vessel type requires that the two inertia tensors are both defined using a common origin. OrcaFlex will shift the compensating inertia tensor as necessary to ensure a correct subtraction.

The compensating inertia will also affect the hydrostatic stiffness applied to this vessel. Moment arm contributions corresponding to the calculations described for a vessel type with centre of buoyancy specified will always be subtracted from the original hydrostatic stiffness. This calculation will use the compensating mass at the centre of compensating mass.

After this, if the vessel type does have an explicit centre of buoyancy, then the usual moment arm contributions that OrcaFlex removes from the hydrostatic stiffness will also be calculated. This calculation will use the reduced vessel mass at the new centre of mass.