dc.description.abstract | For sustainable offshore field development and safe navigation in the Arctic seas,
reliable numerical models that are capable of simulating the interactions between
structures, water and sea ice are needed. The Discrete Element Method (DEM) has been
widely used by many researchers worldwide to model the dynamics of broken-ice fields
and the dynamics of structures surrounded by ice. However, despite the large number of
ice-related applications of DEM described in the scientific literature, prior development
of the method has focused mainly on improving the modelling of contact interactions
between ice floes and structures, whereas the effects associated with fluid dynamics
have been largely neglected. This thesis introduces several hydrodynamic models that
can be incorporated into DEM to improve the simulation of marine operations in broken
ice and to enable new applications of the method in ice-related problems.
It was necessary to introduce several hydrodynamic models because the flow regimes
around a structure differ significantly, i.e., the flow regime upstream of the structure is
different from that downstream and from that in the wake of a propeller if the marine
structure is equipped with propellers. Thus, three approaches were considered in this
thesis:
Potential theory was adopted to model the hydrodynamic effect on ice floes
upstream of a structure.
The Vortex Element Method (VEM) was employed to simulate the
hydrodynamics in the downstream wake.
A special technique based on empirical formulas was developed to predict the
dynamics of ice in the propeller wash of a ship.
The novel synthesis of DEM and a potential-flow model presented in this thesis enabled
simulations of the hydrodynamic interactions in multi-body systems, e.g., structures in
broken-ice fields. Unlike standard potential-flow codes, this method can handle the
actual motions of bodies as they arbitrarily move and rearrange themselves in the
system. Specially designed laboratory experiments proved the applicability of this
combined model in predicting the hydrodynamic interaction forces between a structure
and ice floes upstream.
For the first time, the formation of vortices in the flow downstream of an offshore
structure was shown to have an effect on the spreading of broken ice in the wake of the
structure. This effect was efficiently simulated by employing VEM, demonstrating a
new application of the method in ice-related problems.
The propeller-wash effect has been used for decades in Arctic marine operations to
remove ice locally. However, a comprehensive numerical model that can accurately
simulate such operations is presented in this thesis for the first time. A specially
designed full-scale experiment was conducted to calibrate the model, and a set of
independently collected full-scale data was used for a validation study in which the
experimental results were compared with numerical predictions. This study proved the
high accuracy of the model in simulations of an offshore operation in which the
propeller flow of a vessel was employed to clear channels in multi-layered ice rubble.
A number of findings were discovered while validating the developed models and when
performing case studies to demonstrate the capabilities of the models. These findings
are mainly associated with the hydrodynamic effects studied in relation to typical Arctic
offshore operations such as station-keeping in broken ice and ice management and may
therefore be useful to better understand the hydrodynamic processes involved in such
operations. Selected findings that show the importance of hydrodynamics in the
considered marine operations in ice are summarised as follows:
The presence of a bluff structure in a drifting broken-ice field may impose
repulsive hydrodynamic forces on the ice upstream, which may change the freedrift
velocities of the floes by more than 20% in a typical station-keeping
scenario.
The alternating flow due to vortex shedding in the wake of a structure may
contribute to the spreading of broken ice downstream of the structure, which
may eventually lead to full clogging of the wake.
The average hydrodynamic force on an ice piece in a propeller jet was found to
be nearly twice as high as the drag force on an equivalent body in a uniform
flow at the same Reynolds number. It was also found that the jet-induced force
on the ice was proportional to the square of the axial velocity of the propeller jet. | nb_NO |