Hydrodynamic Research Topics for DP Semi Submersibles

Abstract

The study of the behavior of dynamically positioned semi submersibles in the time domain can deliver important information on the positioning accuracy and thruster loading of the structure. It is not sufficient to only consider a static equilibrium of environmental and thruster forces. Dynamic effects in the stationkeeping performance of the semi submersible may play an important role.The environmental loads contain both mean and low frequency varying components. Results from calculations and regular wave tests show that the use of diffraction analysis to calculate the wave drift forces on a semi submersible will lead to an underestimation of the loads.The total thrust force delivered by the azimuthing thrusters on a semi submersible is influenced by a large number of variables. Interaction effects, such as thruster-hull, thrustercurrent and thruster-thruster interactions, are the result of complex physical phenomena. Results of model tests show, that the thrust loss due to thruster-hull interaction effects can be up to 40% of the open water thrust.Other examples of dynamic effects in the stationkeeping behavior of a DP semi submersible include filtering of measured position signals, the application of wind feed forward, the thruster response times and the use of forbidden sectors for the azimuthing thrusters. It is possible to include these effects in DP model tests.