Heave, pitch, and roll damping of platforms and ships due to positioning thrusters

Semisubmersible platforms and large volume floating structures are usually designed in such a way that their heave, pitch and roll motions due to first order wave forces shall be minimum. The most important component in the motions of such platforms is in many cases the resonant motions induced by second order wave forces. Since the motions occur at resonance, and since the relative damping is often very small, the resulting motion amplitude is very sensitive to the hydrodynamic damping of the structure. This paper demonstrates that the application of positioning thrusters may introduce damping of the same order of magnitude as the viscous damping of such structures. This means that it may be necessary to include the effect of positioning thrusters when predicting or evaluating the motion characteristics of semisubmersibles and other dynamically positioned structures. The effect can also be used to reduce the motions of such structures when particularly critical operations are to be carried out, e.g. crane operations, transfer of personnel, etc.