Experimental and Numerical Investigations of Dynamic Positioning in Discontinuous Ice
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This thesis studies dynamic positioning (DP) operations of a conceptual Arctic drillship in discontinuous sea ice conditions. The stationkeeping behaviour of the vessel under the influence of dynamic ice actions is investigated in both intact and managed ice environments, with an emphasis on DP operations in broken ice. The problem is approached by a combination of experimental and numerical methods. The experimental work was performed in the large ice tank of the Hamburg Ship Model Basin in 2011 and 2012, where almost 250 different scenarios were tested in various ice conditions using a scale model of the conceptual Arctic drillship. The governing characteristics of the global ice load signals were identified from the model testing data, and a connection was established between the major physical processes occurring in the ice cover and the ice loads acting on the vessel. Then, these findings were used to analyse the limitations of conventional open-water DP control systems in ice. It was found that conventional systems require improvements for successful stationkeeping in tight ice conditions. Finally, it was concluded that model testing is a promising method for studying and analysing DP operations in both intact and managed sea ice conditions. The numerical portion of the thesis presents a novel approach to high-fidelity simulations of the vessel-ice interaction process. This approach is based on a 3D formulation of the nonsmooth discrete element method. A physics engine middleware is used for collision detection, and an iterative multibody solver is employed to calculate the contact forces among the simulated objects. The numerical model enables the simulation of progressive failure and fragmentation of the ice floes, together with the submergence and sliding of the broken ice pieces around the vessel, which makes it possible to simulate both intact and managed ice conditions within a single software framework. The outcomes of the numerical simulations were compared with experimental data, and the results confirmed that the model is able to capture the major physical processes identified in both full- and model-scale experiments with reasonable fidelity and computational performance. Furthermore, the model was successfully applied to a wide range of engineering challenges and novel DP solutions, including DP in managed ice, DP in level ice, physical ice management, automatic heading control of a vessel in managed ice, DP-ice capability plot derivation and DP in curvilinearly drifting managed ice. Although DP is clearly a promising stationkeeping technology for Arctic offshore operations, more full-scale data are needed to qualify the experimental and numerical techniques for predicting the global sea ice loads on DP vessels.