Position-moored drilling vessel in level ice by control of riser end angles

This paper focuses on the control of marine riser of a position-moored drilling vessel operating under level ice regime with different ice thicknesses. The strategy is to ensure that the riser end angles (REAs) are within the safe limits for drilling and work-over operations by keeping the vessel at an appropriate position. Using a control criterion based on REAs, the desired vessel position is computed to counteract the environmental disturbances. An optimal set-point chasing algorithm is used to control the mooring lines by adjusting their lengths to bring the vessel to the desired position. The ice load model of Nguyen et al. (2009) was used to simulate the vessel–ice interaction. Numerical simulation results of a moored drilling vessel first exposed to open water and then to level ice of different thicknesses indicated that the proposed control strategy performs satisfactorily in level ice regime. The riser end angles and bending stresses along the riser are reduced considerably when in automatic control mode.

Research Highlights
► We propose a method for controlling PM vessels to achieve small riser end angles in ice-covered sea.
► Vessel offsets are eliminated by changing mooring lines’ lengths based on an optimal algorithm.
► Proposed method obtains better performances than conventional DP controls designed for open water.