Dynamic Positioning of Marine Vessels in Ice
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- Institutt for marin teknikk 
This thesis is a collection of papers focusing on various aspects of dynamic positioning of marine vessels in ice. Most emphasis is put on the dynamic broken, or managed, sea-ice environment where pioneering operations have shown that conventional dynamic positioning systems are capable given light ice conditions. When the conditions toughen, or the ice drift direction changes quickly, these systems struggle and may fail. Yet, it is reported that manual control renders su cient stationkeeping possible. To understand the operational environment the vessel-ice interactions are studied using model scale experiments and numerical simulations. It is found that the multi-body interactions of the vessel-ice interaction contain complex processes that may introduce a signi cant and highly varying disturbance. To handle this it is concluded that the core control system must be reviewed with focus on increasing reactiveness to external perturbations together with an operation strategy complying with the ice dynamics. Increasing reactiveness is approached in three ways; by extending conventional model based design methods to capture the ice dynamics, by introducing hybrid control theory to allow for instantaneous change of estimated variables, and nally, by incorporation of inertial measurements to form an acceleration feedforward in the control system. All are investigated theoretically and experimentally and show varying feasibility. For closed-loop experiments in managed ice, a framework using a state-of-the-art high - delity numerical model is developed and applied. Weather vaning coupled with the reactive algorithms is investigated for operating compliantly with the ice dynamics. It is advantageous as the optimal vessel heading is found through the vessel motion response, and not an explicit ice drift measurement. Finally, motivated by the oblique heading and ice load coupling a novel recursive thrust allocation algorithm for prioritization of selected degrees of freedom is proposed and investigated.