Dynamic Positioning of Marine Vessels in Ice
Doctoral thesis
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Date
2016Metadata
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- Institutt for marin teknikk [3472]
Abstract
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.