Modelling and Control of Riser Angles and Stresses in Dynamic Positioning

Abstract Conventional controller designs for dynamic positioning of ships and floating marine structures have so far been based on the principle on automatic positioning in the horizontal-plane about some desired position and heading co-ordinates. A three degrees of freedom multivariable controller either of linear or nonlinear type, normally with feedback signals from surge, sway and yaw position and velocities, has been regarded as adequate for the control objective. For semi-submersibles feedback from roll and pitch angular rotation velocity may also be included. However, for certain marine operations this control philosophy may not be the most appropriate approach ensuring a safe and cost effective operation. For drilling and work over operations the main positioning objective is to minimise the bending stresses along the riser and the riser angle magnitudes at the well head on the subsea structure and at the top joint as well. A positioning strategy solely based on position and velocity feedback may not be the most optimal solution for these applications. In this paper a new hybrid dynamic positioning controller, that also accounts for riser angle offsets and bending stresses is proposed. It is shown that a significant reduction in riser angle magnitude can be achieved. Simulations with a drilling semi-submersible demonstrate the effect of the proposed control strategy.