Sammendrag
The positioning capability is a crucial property for safe and reliable operations of a dynamically positioned (DP) offshore vessel. The limiting factor in such is the vessel's thruster configuration and the maximum resultant forces and turning moment this can produce in all directions. Based on the vessel's position and velocity deviations, the DP control law computes a desired resultant load set as correction. This is distributed by the DP thrust allocation algorithm to individual thruster forces based on rated power, thruster type (force direction capability), and hull position. In this paper, a novel and computationally light constrained thrust allocation algorithm which offers eased thruster weighting and prioritization between the degrees of freedom (surge, sway, and yaw), is developed. It is motivated by the importance of maintaining the heading directly against the ice drift, such that the projection of the vessel in the ice is minimized. Thus, first the yaw moment is allocated to the thrusters with a given fraction of the capability at its disposal. Then, the total remainder is distributed to surge and then sway. If the loads on the vessel are beyond the capability of the thrusters, then sway is affected first, then surge, and last yaw. Due to its simplicity, this method provides a convenient tool to evaluate and compare ice loads from model testing with the thrust capability of the DP vessel. In order to enable a DP-ice capability analysis, the quantification of loss of capability is investigated. This introduces three descriptive measures of the ice loads of a given ice condition; peak load, significant load, and mean load. As a case study, a towing tank dataset of an experimental intervention vessel (the CIVArctic vessel) in broken ice is investigated and discussed.