Broken Ice Field Research Articles

A generalized probabilistic model of ice load peaks on ship hulls in broken-ice fields

On some occasions, especially when traveling in a broken ice field, the process of ice induced loads on ship hulls cannot be well modeled by traditional statistical models which are usually applied for this purpose. Examples of such are the one-parameter exponential and the Weibull's models. The fitting achieved by application of these models could generally result in underestimation of the predicted extreme values. Therefore a so-called generalized model, i.e. the three-parameter exponential model, is proposed in order to improve the process of fitting. The proposed model is actually a proportional combination of two one-parameter exponential models. It tends to give more conservative predicted extreme values as compared to the one-parameter exponential and the Weibull's models. Various approaches for estimation of the parameters are treated, i.e. the method of moments, the non-linear least square method, the non-linear least square method based on application of Kernel density estimation, and the maximum likelihood estimators. The fitting by means of the non-linear least square method was observed to give the best results. However, more stable predictions are provided by the maximum likelihood estimators.

A numerical model for real-time simulation of ship–ice interaction

A ship advancing in level ice will introduce several failure processes to the ice sheet, such as localised crushing and breaking due to bending stresses. The resulting ice fragments will interact with each other, with water and with the hull of the ship. They may rotate, collide, or slide along ship's hull, and eventually they will be cleared away. The situation is different in a broken ice field, i.e., large ice floes may behave similar to level ice while smaller floes will mostly be pushed aside, rotated or submerged. Modelling of such a complex system is very demanding and often computationally expensive which would typically hinder the chances for real-time simulations. This kind of simulations can be very useful for training personnel for Arctic offshore operations and procedures, for analysing the efficiency of various ice management concepts and as a part of the onboard support systems for station keeping. The challenge of meeting the real-time criterion is overcome in the present paper. The paper describes a numerical model to simulate the process of ship–ice interaction in real-time . New analytical closed form solutions are established and used to represent the ice breaking process. PhysX is used for the first time to solve the equations of rigid body motions in 6 degrees of freedom for all ice floes in the calculation domain. The results of the simulator are validated against experimental data from model-scale and full-scale tests. The validation tests exhibited a satisfactory agreement between the model calculations and experimental measurements.

Variation in the wave resistance of an amphibian air-cushion vehicle moving over a broken-ice field

The nonstationary rectilinear motion of an amphibian air-cushion vehicle (AACV) on a water surface covered with finely broken ice is considered for various modes of velocity variation. The influence of the water depth, flotation parameters, and mode of motion on the wave resistance of the vehicle is analyzed. Maneuvering methods for increasing or decreasing the wave resistance of AACVs are proposed.

Smoothed particle hydrodynamics for cohesive grains

A rheology based on the Mohr–Coulomb yield criterion for cohesive materials is implemented in the framework of Smoothed Particle Hydrodynamics (SPH). We enhance the classical SPH approaches by using some conservative renormalization techniques and a new smoothing kernel which improves the stability of the code near discontinuities. We apply this approach to the simulation of broken-ice fields floating on the water surface and moving under the effect of wind forces. When broken-ice fields are regarded as a continuum, their rheological behavior can be described by a model known as viscous-plastic. The ice field is modeled as a viscous fluid for very small strain rates and is assumed to flow plastically otherwise. In the plastic regime the stress states are described in terms of the Mohr–Coulomb yield criterion.

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.

Smoothed Particle Hydrodynamics for the Simulation of Broken-Ice Fields: Mohr–Coulomb-Type Rheology and Frictional Boundary Conditions

A rheology based on the Mohr–Coulomb yield criterion is implemented in the framework of smoothed particle hydrodynamics (SPH). We apply this approach to the simulation of broken-ice fields floating on the water surface and moving under the effect of wind forces. When broken-ice fields are regarded as a continuum, their rheological behavior can be described by a model known as viscous-plastic. In this approach the ice field is modeled as a viscous fluid for very small strain rates and is assumed to flow plastically otherwise. It is in the plastic regime that the stress states are described in terms of the Mohr–Coulomb yield criterion. Besides broken-ice fields, numerous other problems in the field of quasi-static granular flows can be characterized by this type of rheological behavior. We first show how the momentum equations for a Mohr–Coulomb-type rheology are implemented in the framework of SPH. For most granular flow systems, the moving interior particles are bounded by fixed frictional walls and one must also model these boundary conditions. A Coulomb-friction condition is applied within the framework of SPH. This type of boundary is implemented by means of a wall that exerts a normal potential force of repulsion on the SPH particles, combined with a tangential force proportional to the normal force. The approach can be applied to model flows adjacent to straight walls as well to more complicated boundaries.

Laboratory Testing of a Flexible Boom for Ice Management

Combatting oil spills in the Arctic is a major challenge. Drilling or producing oil or gas in the marginal ice zone (MIZ) may allow booms to be deployed upstream of an offshore structure to clear the water of ice, thereby enabling conventional oil spill countermeasures to be used. Such a boom would be kept in place by two ice-going service vessels or by moored buoys. SINTEF NHL and NRC have performed a number of small-scale tests with a flexible boom in the NRC ice basin in Ottawa. The purpose of the tests was to measure the effectiveness of using a flexible boom for collecting ice, and to determine the loads associated with collecting the ice. In the tests, various boom configurations were towed against a broken ice field consisting of ice pieces typically 50–100 mm across and 30 mm thick. The ice concentration was usually 10/10, but it was reduced to 8/10 and 5/10 for two tests. The boom was towed at speeds of 20 and 50 mm-s−1. Both the width of the boom and the slackness of the boom were varied over reasonable ranges. Two six-component dynamometers were used to support the boom. Thus, the force components on each end of the boom were measured. Further, two video cameras were used to record the effectiveness of each boom configuration. In this paper, the full results of this test program are presented and the application of the test results to the full-scale situation are discussed. The tests show that, under certain conditions, the use of boom is feasible for ice management in oil-contaminated water.

OHMSETT TESTS OF A ROPE-MOP SKIMMER IN ICE-INFESTED WATERS1

ABSTRACT The increase in petroleum development activities in the arctic region has raised concerns over potential oil spills during the broken ice season. Currently, exploratory drilling for oil and gas is restricted during this season due to the lack of proven oil spill cleanup methods for broken ice fields. Test programs have been conducted at the U.S. Environmental Protection Agency's Oil and Hazardous Materials Simulated Environmental Test Tank (OHMSETT) to determine the feasibility of cold weather testing and to evaluate various oil spill cleanup methods considered for use in the arctic. This paper describes a test program to determine the practicality of using a catamaran-mounted rope-mop skimmer for spill cleanup in broken ice fields. An Oil Map Pollution Control, Ltd., prototype arctic skimmer was tested in the test tank under controlled conditions during January 30 to February 7, 1984. Freshwater ice cubes of 250 to 280 millimeters (mm) were used in the tests to approximate a broken ice field. During tests, a predetermined ice condition was established across the encounter width of the rope mops and oil was distributed over the ice. The oil and ice were channeled into the skimmer by two booms, which were joined to the skimmer at the bow. Nine tests were conducted at a tow speed of 1 knot using Circo 4X light oil. During the tests, ice concentrations were varied from 0 to 75 percent of the surface area, and oil slick thickness varied from 3 to 8 mm. The test results demonstrated the spill cleanup capability of the skimmer in ice-infested waters having up to 50 percent ice coverage. At higher ice concentrations, the skimmer was ineffective due to ice jamming at the skimmer inlet.

The Bouchard No. 65 Oil Spill in the Ice-Covered Water of Buzzards Bay

This paper describes the Buzzards Bay spill off the Massachusetts coast. The spill occurred in broken ice conditions. Tidal currents transported most of the oil into the broken ice field and beneath large ice floes. Cleanup was made more difficult by a heavy snowfall 7 days after the spill. Introduction On Jan. 28, 1977, the barge Bouchard No. 65 grounded, releasing about 81,150 gal (307 m) No. 2 home heating oil into Buzzards Bay, MA, which was 90% ice covered. Field measurements and observations began at the spill site on Jan. 29 and continued until Feb. 25, when only negligible amounts of oil remained in the bay. This paper presents these data and puts the results in perspective presents these data and puts the results in perspective with the present state of the art.Most current data on oil pollution in ice-covered waters have been obtained from spills under uniform ice conditions. The Buzzards Bay spill, which occurred in broken ice, merited special attention. Of particular interest was the initial transporting of the particular interest was the initial transporting of the oil under the ice, its concentration in the rafted ice, the interaction of the oil with the hummocks and pressure ridges, its spreading from concentrated pressure ridges, its spreading from concentrated pools on the surface of ice floes, its penetration of pools on the surface of ice floes, its penetration of the snow and ice, the weathering of the oil, its final transporting by ice floes during breakup, and the cleanup techniques used. With growing interest in petroleum reserves located in cold regions, the application of the Buzzards Bay experience to other geographical areas could be useful. However, some limitations must be considered when applying this experience to other cold regions, such as coastal Alaskan waters. By continuing to respond with study teams to "spills of opportunity" in cold regions, knowledge of the behavior of oil spilled in these regions can be improved substantially, and our ability to prepare effective countermeasures properly will be enhanced greatly. Environmental Setting Buzzards Bay, located on the Massachusetts coast (Fig. 1) is a shallow, nonestuarine bay, about 46 km long and 19 km wide, with an average depth of 11 m. Connecting Buzzards Bay with Cape Cod Bay to the north is the man-made Cape Cod Canal (Fig. 2). Winter 1977 was unusually severe for most of the northeastern U.S., with unusual icing conditions reported for many parts of the New England coast from December through February. Buzzards Bay, which normally does not ice over, was 90% ice covered by Jan. 28.The ice that formed was less saline than normal sea ice, primarily because the ice had gone through four periods of above-freezing temperatures and rain, periods of above-freezing temperatures and rain, which lowered its salt content. This low-saline ice was not as porous nor as elastic as normal first-year sea ice.Several types of sea-ice formations were found in Buzzards Bay. Shorefast ice, with a characteristic thickness of 0.3 m, covered the waters of the protected coves. protected coves. JPT P. 1092

Visualization Of Marginal Ice Zone Dynamics

Ice floe dynamics in marginal ice zones are simulated using a discrete element model. The ice floes are modelled as random sized, inelastic circular discs floating on the water surface. Contact conditions are represented by a constant restitution and friction coefficients. The ice floes are driven by a uniform wind field. The simulations predict ice floe displacements, rotations, compaction and pressures. This paper focuses on animation and visualization of the simulation output. INTRODUCTION Marginal Ice Zones (MIZ) are broken ice fields which often form at the edge of the polar ice pack, next to open ocean. They form as a result of several processes that take place at the pack ice edge including the incidence of waves and increase of temperatures. A MIZ typically extends over a width of the order of 100 km, and consists of distinct floes with diameters varying from a few meters to several kilometres. Discussions of the morphology and behaviour of MIZ were given by Wadhams [1] and [2], and Lepparanta and Hibler [3]. Models of MIZ dynamics have been developed using a viscous-plastic rheology by Lepparanta and Hibler [4], and a cavitating fluid analogy by Flato and Hibler [5]. The discrete nature of the ice floes has also prompted the use of continuum models of flowing granular materials to treat MIZ (e.g. Shen et al. [6] and Lu et al. [7]). The discrete element approach is used here to simulate MIZ dynamics. This approach has been used to model a variety of problems in Transactions on Information and Communications Technologies vol 5, © 1993 WIT Press, www.witpress.com, ISSN 1743-3517 364 Visualization and Intelligent Design soil and rock mechanics, flow of granular materials, and ice-structure interaction. Some of recent papers on this subject were compiled by Mustoe [8], The present study examines the response of an assembly of ice floes to wind drag, water drag and Coriolis force. Interaction forces between the floes are calculated from an inelastic contact model. The simulation predicts the spatial and temporal distributions of ice velocities, concentrations, trajectories and pressures. Animation of the numerical simulations played an essential role throughout this study. It provided an effective means for debugging and verification of the model. Also with the very large amount of information generated by the simulations, animation was necessary for illustrating the salient trends of MIZ behaviour. The present simulations are based on a model developed by Savage [9]. The following sections briefly discuss the discrete element model of the floes, the equations of motion of individual floes, and the boundary conditions. Presentation of a simulation run follows with emphasis on the animation approach and visualization of the results. DISCRETE MODEL OF ICE FLOES Contact model The ice cover is modelled as an assembly of circular, random sized, inelastic discs. At the contact between two discs, linear loading and unloading springs are used to calculate the normal force as illustrated in Figure 1. The normal force per unit length is given by