Adaptive Control of Thruster Modulation for a Dynamically Positioned Drillship

ABSTRACT A primary tradeoff in the design of a vessel dynamic positioning system is tightness of control versus the modulation levels of the automatically controlled propulsion units due to wave motion and noise. An algorithm is described whereby the parameters of the digital controller are adjusted to achieve the desired compromise between control stiffness and thruster modulation, while maintaining the vessel stability margins at their design values. The algorithm is currently implemented in the DP system of a new North Sea drill ship, Nordic Off shore?s Havdrill. As presently mechanized, the algorithm must be adjusted by an operator to respond to changing environmental conditions. However, a scheme is discussed through which the controller would automatically adapt to changing weather and maintain thrust modulation within desired levels. The basic algorithm is applicable to any existing vessel dynamic positioning system which utilizes a digital computer for control. Different degrees of automation are achievable to meet operator requirements for monitor and adjustment of control stiffness or thruster modulation. This permits the operator to quickly change and update the tradeoff to meet changing requirements without altering the basic stability of the positioning system. Analytical results, have been verified through simulation. These study results, including computer data, are presented in the paper. Also included are the results of Sea Trials and their comparison with analytic and simulation results. The inclusion of such an algorithm in the controller of a vessel dynamic-positioning system permits a rapid, reliable adjustment of controller operating characteristics? with the confidence that the basic stability of the system will remain unchanged. INTRODUCTION A primary requirement in the design of a DP control system is that modulation of thrusters and power generators be maintained within tolerable limits. These limits relate to the amount of reserve power which must be generated, fuel consumption, and to thruster and generator equipment wear-out rates. This requirement must be satisfied over a widerange of weather and sea state conditions. If the DP controller design is exclusively oriented toward the satisfaction of these modulation limit's under worst case environmental conditions, over-all control system tightness and holding accuracy may be compromised. It therefore becomes desirable to permit on-board adjustment of DP controller parameters to achieve the maximum control system positioning accuracy consistent with thrust modulation limits. This adjustment can be performed automatically through an adaptive control scheme which responds to changes in varying weather condition, while permitting operator override to update modulation limits as desired CONTROLLER DESIGN APPROACH The synthesis of controller gain and filter parameters in the design of the computer software essentially involves a tradeoff between the required tightness of the dynamic positioning control system and the acceptable thrust modulation levels imposed by both the power generation system anc1by the thrusters themselves. The required control tightness is, of course, dictated by the station keeping position accuracy, or "watch circle," requirement for the given environmental design criteria, i.e., wind, waves, and current.