Numerical Modelling of the Hydrodynamic Effects of Marine Operations in Broken Ice

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.