Hydrodynamic Analysis of Dynamically Positioned Vessels

Present-day offshore operations involve many types of floating structures such as drilling units and support vessels, which have to be kept on station by means of thruster systems. Important aspects in the design and operation of these structures are the motions of the vessel, power requirements and performance of the thrusters and the control characteristics of the DP system. Computational analysis of such systems is hampered by non-linearities such as the second order slowly varying wave forces and the net thrust characteristics.
A mathematical simulation model has been developed on basis of a separate treatment of environmental excitation, fluid reaction forces and positioning forces. By describing the fluid reactive forces by means of retardation functions and inertia coefficients, non-linear and arbitrarily in-time-varying excitations of the vessel are allowed. Both first and second order wave excited forces are computed for arbitrary wave trains by use of the results of 3-D diffraction theory and impulse response techniques. The net thrust of the propeller arrangement is calculated on basis of the open water characteristics taking thruster interactions into account.
Results are shown for a large tanker exposed to wind, waves and current. The simulation results demonstrate the importance of the slowly varying wave forces and of the thruster interactions.