When drag anchors are no longer suitable for anchoring floating wind turbines to the seabed because of unfavorable soil conditions, it is necessary to use drilled and grouted anchor piles instead. For the installation of such anchor piles it is necessary to drill large diameter holes, with diameters ranging between 1 and 5 meters. The drilling method used to construct such boreholes is the reversed circulation drilling (RCD) method. Floating application of this method has only been done up until now in mild wave conditions. To make this operation more economically feasible, it is necessary to increase the workability of this operation in more challenging environmental conditions. A motion compensation device is a vital aspect in this.
A systematic approach was used to determine which type of motion compensation device is best suited for this operation. Numerical models were set up in the commercial software OrcaFlex, where a wide range of different motion compensation types were tested. These motion compensation systems vary in the amount of degrees of freedom compensated and the actuation method. The actuation methods are passively actuated gas springs and position-based motion control using hydraulic actuators.
First, it was identified what combination of degrees of freedom and actuation method is best suitable. This was determined by performing a regular wave analysis and comparing the performances of the proposed motion compensation systems. The performance was indicated by the ability of the motion compensation system to control the weight on bit, reduce the maximum stress as well as the fatigue damage accumulation in the drill string. From this analysis it was concluded that a passive actuated concept that compensates for the heave, roll, and pitch of the vessel is the most effective scheme.
The optimal type of motion compensation system was then pursued by designing two different systems capable of compensating for the heave, roll, and pitch motion of the vessel. The first, relatively simple, type consists of a vertical gas spring and a universal joint where the drill rig can rotate freely. The other type is a passive variant of the Barge Master platform motion compensation system. This second concept was designed using a multi-objective optimization algorithm, the NSGA-II algorithm. The difference between these two systems is that the platform concept has rotational stiffness, while the simple type is free in this degree of freedom.
It was finally concluded that both concepts are viable solutions, and that both systems represent a motion compensation system that compensates for the heave, roll, and pitch motion of the vessel. The performance regarding the indicators is very similar, therefore choosing one of the concepts would come down to the other advantages and disadvantages of the concepts.