Pendulation control for dynamical positioning capable ship; considerations on actuator usage

Ship-mounted cranes are used in the offshore industry for, among others, placement of wind turbine components. The pendulation of the load can be decreased with (active) tugger lines. This allows for larger operational weather conditions. In this work we investigate the effect on the actuator usage when the pendulation controller and the ship’s dynamic positioning (dp) system are combined. This is done by comparing three situations: i) tightly tuned dp system with independent pendulation control, ii) dp system tuned based on the pendulation controller, and iii) integration of both controllers.
The controllers are all designed with the H2 control methodology. Which disturbances should be countered, and which are ignored is explicitly specified, and allows us to indicate that the dp system should not act on the high frequency wave forces, while the pendulation controller should. For a fair comparison, the design penalties are modified such that the load variance is equal in all cases.
A frequency domain comparison and a set of detailed time domain simulation shows that extra system information can be used to minimise the actuator usage. The majority of the savings is attained by tuning the pendulation controller and the dp system such that the dp system has as low as possible bandwidth, while still countering low-frequency second order wave effects and saturation of the swing controller. Furthermore, for the pendulation controller’s actuator usage, the integration of the controllers helps to minimise its high frequency contents.