| dc.description.abstract | 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. | nb_NO |
