Optimisation of the Base Motion of a Vessel to Ensure That a Riser Follows a Planned Trajectory

This paper introduces a spatial model of a riser and describes its application for determining the movement of a base (platform or vessel) in order to implement a planned path for the lower end of a riser. This scheme takes into consideration the fact that the lower end of the riser must bypass obstacles on the seabed despite the action of the marine environment (sea currents, hydrodynamic resistance, buoyancy forces, mass of added water). The task of determining the movement of the base is formulated as a dynamic optimisation process. As a result of this optimisation procedure, parameters are selected to characterise the functions describing the movement of the base at certain time points. These functions are then approximated by third-degree spline functions. A series of computer simulations is carried out for a riser with lower marine riser package and blowout preventer attached to its lower end. The riser is discretised by means of the lumped mass method. The proposed method is verified and validated, and can be used to determine the movement of a vessel when transporting a riser to a new position.