Structural reliability based position mooring

Abstract

This thesis considers control of moored marine structures, referred to as position mooring. Moored marine structures can take on a number of different forms, and two applications are considered in this work, namely aquacultural farms and petroleum producing vessels. It is anticipated that future aquacultural farms will be significantly larger than the existing ones, and placed in much more exposed areas. Hence, there is a significant technology transfer potential between the two seemingly different fields of aquaculture and petroleum exploitation.

Today’s implemented state of the art positioning controllers use predetermined safety regions and gain-scheduling for evaluating the necessary thruster force for the vessel to operate safely. This represents a suboptimal solution; the operator is given a significant number of variables to consider, and the thrusters are run more than necessary. Also, it is likely that a more conservative controller regime does not necessarily increase the overall reliability of the structure as compared with a less conservative but better designed controller.

Motivated by this, a new control methodology and strategy for position mooring is developed. Two controllers using information about the reliability of the mooring system are implemented and tested, both via numerical simulations and model scale experiments. The first controller developed uses a reliability criterion based on the tension in the mooring system as a pretuning device. A nonlinear function based on the energy contained by the system is included in the controller to ensure that the thrusters are run only when needed. The controller is an output-feedback controller, based on measurement of position and estimated values of the velocities and slowly varying environmental loads. The second controller developed contains the reliability criterion intrinsically, thus, less pretuning is needed. The backstepping technique is applied during the design process, and the controller has global asymptotical stability properties.