Time Domain Simulation of DP Semi-Submersibles in Numerical Wave-Tank for Large Nonlinear Random Waves

ABSRACT In deepwater, Dynamic Positioning (DP) is proved to be cost-effective for the station keeping and controlling of the floating structures. This paper will show that the DP simulation and control of a floating vessel can be achieved in a numerical wave-tank. The complex interaction of the wind, waves and current forces against the vessel is modeled for the required station positioning with the application of controlled propulsion system. The DP simulation and control in a numerical wave tank will be very useful to the offshore industry in terms of DP system design verification and in terms of the vessel design with the maximum DP forces loading condition. In this paper, we discuss a truly dynamic time-domain simulation method for a fully DP assisted semi-submersible. The computational algorithm for the simulation is based on the concept of a numerical wave-tank. For the basic potential flow solver including diffraction and radiation effects, a boundary integral method in conjunction with time-integration of boundary conditions and equation of body motions is used. Nonlinear effects associated with large amplitude waves and other relevant non-linearities that affect the motions of the body are included in the algorithm. Morison elements with braces are represented without any approximation in the sense that these are determined based on nonlinear wave kinematics and the exact wetted part of the elements. The current and winds are based on empirical relations as is the present standard, but can consider spatial (in particular, depth) variations of velocities. Simulated time histories of motions and required DP thrust are shown for two semi-submersible configurations. The results show that the DP simulation and control in a numerical wave tank is a viable alternate to the real life test. INTRODUCTION As the offshore industry moves into deeper waters, the design of DP systems for controlling the position of floating production platforms has become increasingly important. These platforms often have to work under severe environmental conditions including effects of currents, wind loads and very severe waves. The DP system controls the vessel's motion in three horizontal degrees of freedom – surge, sway and yaw. The vessel is held in relatively fixed position with respect to the ocean floor, without using anchors by the use of propulsive devices controlled by the inputs for the desired location against wind, wave and current environmental forces on the vessel. In general, the wave induced platform motions can be separated into two …