Experimental and numerical evaluation of a typical dynamic positioning system

A Dynamic Positioning System (DPS) is a complex control system composed of several algorithms, such as controllers, filters and optimal thrust allocation. The design, analysis and performance prediction of a DPS is normally done by a combination of static and dynamic numerical simulations and tank tests with a reduced model. After that, a long commissioning time is required to adjust control and filter parameters. This paper presents a computational dynamic simulator for DP systems, developed in a R&D project carried out at USP, called Numerical Offshore Tank (NOT). Some features of the simulator are the high speed calculation, thanks to a 120 processor cluster and the presentation of the results in a real time 3D-stereo visualization system. Furthermore, important parameters of the operation can be altered during the simulation, emulating a real DP console. It enables the simulation of most of a typical ship manoeuvering with DP especially those related to the offshore oil industry. Regarding offloading, the simulator can deal with multiple body systems taking into account the effects of risers, mooring lines and hawsers, if any. The Newtonian six-degrees-of-freedom model describes the dynamics of each vessel, in which validated models for environmental forces due to current, wind and waves are included. In order to pre-validate the simulator, a simplified experiment was set-up, composed of a reduced model of a DP tanker with 3 thrusters. Several experiments were carried out, and a detailed comparison with simulation results has shown very good agreement. Two loading conditions were considered: under the action of wind and current and the absence of both.