Sveinung Løset

Thermal energy conservation in icebergs and tracking by temperature

Calculations using a two-dimensional numerical model which simulates the heat balance and temperature distribution of icebergs show that the temperature in the central region of an iceberg is almost unaffected by the thermal conditions imposed on its boundary. Hence the original temperature of the iceberg at the time of calving is retained in its core owing to the insulating quality of the ice. When correlated with glacioclimatic conditions, this temperature conservation may be used to backtrack an iceberg to its parent ice cliff. Nine different icebergs were manually temperature-profiled in the Barents Sea during late winter 1988–1990. A scenario is considered wherein one of these icebergs under the forces of winds and currents is assumed to set off from Franz Josef Land in late autumn 1987. The numerical heat balance model is used to compute the temperature distribution, heat balance, and ablation of this iceberg as it drifts through the Barents Sea. The calculations show that the heat balance of the part of the iceberg exposed to air has marginal influence on the ablation and thus on the expected lifetime of the iceberg. Ablation of the submerged body also occurs during winter, when the water temperature is well below the melting point of glacial ice. This is caused by freezing-point depression leading to an ablation of about 0.5 m per month of the underside and sidewalls of the submerged body. For water temperatures above 0.6°C the ablation results in a steep temperature gradient reaching the core temperature 3 m into the ice. With a tolerance of 0.4°C the temperature is virtually free of any thermal boundary influence 12 m into the ice. The simulated and measured temperature profiles of the iceberg are compared with measurements of the thermal regime of Austfonna, Nordaustlandet, and the Churlenis Ice Dome on Franz Josef Land. The model calculations indicate that the ice cliffs on Svalbard can be excluded as parent ice cliffs for these icebergs and that the core temperature of the bergs appears to be a conservative tracer.

Modelling floating offshore units moored in broken ice: comparing simulations with ice tank tests

A numerical simulation model that simulates the behaviour of a vessel moored in broken ice has been used to compare results from numerical simulations with ice tank tests. In the simulations a tanker moored on an eight legged catenary mooring of the Submerged Turret Loading (STL) concept was towed through a broken ice field. The setup of this situation was as similar to the ice tank setup as possible. The results predicted by the model are in qualitative agreement with the experimental results, but the amplitudes tend to be overpredicted. The predicted frequency of oscillation in surge is slightly lower than what was measured. The deviations between predicted and measured results may in part be explained by the differences in model and ice tank setup, and in part by poor representation of various load limiting mechanisms. However, with the qualitative agreement achieved, and the differences between model and experimental setup in mind, it is reasonable to believe that the numerical model may yield realistic results in further full-scale simulations.

Discrete element modelling of a broken ice field — Part I: model development

Abstract This paper describes a two-dimensional discrete element model which simulates the dynamics and interaction forces between distinct ice floes in a broken ice field. The ensemble of ice floes, is modelled as a granular material consisting of circular discs of different diameters, constrained in a domain with displacement controlled or periodic boundaries. The dynamic behaviour of the ensemble is determined by calculating the motion of the individual discs caused by interaction forces and body forcing (shear forces from air and water drag, surface tilt and Coriolis forcing). The interaction forces arise either directly from contacts between adjacent discs or by squeezing of brash ice which may be present between the distinct floes. Multiple contacts between discs are allowed and a viscoelastic-plastic rheology is applied at contacts. A computer algorithm is developed by which a cell structure is used to quickly identify neighbouring discs in the domain.

Modelling floating offshore units moored in broken ice: model description

Abstract A simulation model that predicts the behaviour of a vessel moored in broken ice has been developed. The kinematics and dynamics of a broken ice field subject to external forcing are simulated using discrete element modelling. In the model each icefloe is represented by a circular disk while boundaries, land formations and structures may have any piecewise linear shape. Disk–disk or disk–structure collisions are modelled as linear viscous-elastic, or as a Coulomb friction contact in case of relative tangential sliding. The ice model was coupled to a model of a vessel moored on a catenary mooring, i.e., a model accounting for the wind and hydrodynamic forcing on a slender vessel, as well as restoring forces from mooring lines. The coupled numerical model is described in this paper.

Sea ice and iceberg observations in the western Barents sea in 1987

Abstract A multisensor ice data acquisition programme for the western Barents Sea was carried out during three field campaigns in the mid winter and fall of 1987. The main purpose of the programme was to obtain comprehensive information about the ice in the area at that time. The reasoning was that prior to any oil/gas exploration and production in the Barents Sea, the physical environment has to be quantitatively surveyed in order to ensure safe operations related to human safety, the regular operability and safety of the structure and protection of the environment. Prior to this field investigation programme in 1987 data on sea ice and icebergs for engineering purposes for the western Barents Sea were meagre. The present paper highlights some of the findings with emphasis on ice edge speeds, ice edge displacement and ice drift. For icebergs, the paper focuses on population, size distributions and geometric parameters. Further, the paper discusses the production of icebergs in the Barents Sea and highlights drift and observed extreme drift speeds of two icebergs which were tagged with satellite positioning buoys in 1987. Drift speeds exceeding 1 m s−1 are often reported for short periods, but this study logged a maximum speed of 1.38 ms−1 and an unprecedented mean speed of 1.13 ms−1 during a 31 hour period with strong tailwind.

Discrete element modelling of a broken ice field — Part II: simulation of ice loads on a boom

Abstract A two-dimensional discrete element model which simulates the dynamics and interaction forces between individual ice floes in a broken ice field has been described in the paper “Discrete Element Modelling of a Broken Ice Field — Part I: Model Development”. The present paper elaborates on the application of this model using three examples. The first example presents a simulation of the forces exerted on a boom when it is pulled through a broken ice sheet. This example refers to small-scale tests, in which the kinematic behaviour and calculated forces are compared with, and calibrated against, tests with such a boom in the ice tank at the National Research Council Canada (NRC, Ottawa). The two other examples, utilizing the calibration from the small-scale testing, study the full-scale interaction between a broken ice field and the boom. Example 2 shows that for an ice thickness of 1.0 m and a boom speed of 0.2 m/s, the average forces on a 250 m wide boom were 50 kN and 119 kN for ice concentrations of 0.4 and 0.6, respectively. In Example 2 the floe diameters ranged from 20 m to 36 m. The values of the parameters in the visco-elastic-plastic rheology (linear spring constant, viscous damping coefficient and friction coefficient) appeared to have marginal influence on the force exerted on the boom. The average force increases with increasing ice concentration, ice thickness, boom speed and boom width. Keeping the ice concentration, ice thickness and boom speed fixed, the average force on the boom is proportional to the boom width.

Numerical modelling of the temperature distribution in tabular icebergs

Abstract The temperature distribution in icebergs affects the mechanical strength properties of the ice and is therefore crucial in engineering applications when estimating loads on offshore structures from impinging icebergs. The temperature distribution also affects the rate of deterioration and thereby the expected life time of an iceberg. Hence a numerical model which simulates the temperature distribution and ablation of icebergs is developed. A full description of a two dimensional version of this model is given. The various terms which affect the thermal energy balance and ablation of icebergs are discussed. Melting, refreezing and run-off are considered. The temperature distributions of the model are compared to manual and unmanned temperature measurements on icebergs in the Barents Sea. The results show steep temperature gradients and extremely stable core temperatures. In the present simulation only the outer 3–4 metres of the decaying surface layer is significantly affected by the transient boundary conditions. The corresponding affected surface layer below the waterline is 2 metres.