Vessel types: Other damping

There are various sources of drag or damping (the terms are often used interchangeably) on vessel motion. OrcaFlex models most of these explicitly, using data on the vessel type data form for each form of damping: current and wind loads, wave radiation damping, wave drift damping. In addition to these, OrcaFlex also offers other damping, as a way to incorporate other sources of damping which do not fall into any of these categories. Viscous roll damping, for example, is a wave frequency effect which is not covered by any of these specific damping forms and so could be modelled using other damping.

Other damping is specified using the following data on the vessel type data form, all of which are automatically Froude scaled to the vessel length if it differs from that of the vessel type. The other damping load will only be applied if other damping is included.

Reference origin

The point on the vessel to which the damping coefficients refer. The relative velocity used to calculate the other damping load is the value at this reference origin, and the other damping load is applied at this point. It is given relative to vessel axes.

Note: The other damping coefficients are the diagonal terms of the damping matrix, so no coupling effects (surge-pitch, sway-roll, etc.) are applied at the reference origin, and coupling effects will only be included due to the offset of the reference origin from the vessel origin. The reference origin should therefore be close to the overall centre of damping load, where such coupling effects are small.

Calculated from primary

Specifies the part of primary motion velocity used to calculate the other damping load, which in turn determines which part of the response is to be damped. This can be wave frequency motion, low frequency motion or total motion. How this data is interpreted is described in full detail in the other damping theory.

Frame of reference

The axis directions used when calculating the damping load. For an illustration, consider the $x$-component of damping force. This direction receives a damping load calculated using the linear and quadratic damping coefficients labelled 'surge'.

If primary low frequency heading is chosen, the surge damping coefficient is applied to the component of input velocity along a vessel $x$-axis which only swings in the horizontal plane following vessel low frequency yaw. The resulting force then acts along that slowly-yawing direction.

By contrast, if diffraction is chosen as the damping frame of reference, then the $x$ input velocity component is taken along a vessel $x$-axis that rotates with the whole of the vessel angular motion: roll, pitch and yaw. This component is scaled by the surge coefficient, and the resulting force applied using the same, unfiltered $x$ direction.

The remaining option, matching calculation mode, will base the other damping frame of reference on the calculation mode chosen for each vessel. In filtering mode, the primary low frequency heading frame will be used, and in QTF modification mode the diffraction frame will be used.

Other damping is unusual, because there is no specific physical model represented by these data. Once you have a model that will provide damping coefficients, a frame of reference must also be selected.

The effect of changing frame of reference is discussed on the vessel frames of reference theory page. You might find it useful to consider the data source from which you obtained the other damping coefficients, as sometimes the physics represented by your damping model will suggest an appropriate frame of reference for the OrcaFlex calculation.

In the absence of any other guidance, it is simplest to try to use a single consistent set of axes for your vessel effects. However, the frames of reference for vessel included effects derived from diffraction analysis all depend on vessel calculation mode, so we have provided a choice for other damping frame consistent with this.

Damping coefficients

Damping coefficients may be given for all six vessel degrees of freedom, and are given relative to the frame of reference described above. Two sets of coefficients are available, one for linear damping and one for quadratic.

Note: The vessel type symmetry convention affects the way in which the quadratic coefficients are used. If the symmetry is circular, then a cross-flow drag model, with a vertical axis, is applied for the quadratic term. For other symmetries, the quadratic coefficients are used with a drag model that treats each direction of motion independently.