Rozprawy doktorskie na temat „Ice”

Safe winter navigation is a hot topic. Not only is the traffic density increasing but theenvironmental considerations are also getting bigger. An oil leakage from a big oiltanker can be of catastrophic proportions in the wrong area and more trafficincreases the risk of an accident. A project that aims for safer winter navigation isSafeWIN. The aim of this project is to develop a forecasting system for compressiveice and thus make winter navigation safer.This thesis is part of above mentioned project and aims to investigate what influenceice compression and ice class has on winter navigation. Vessels are exclusivelyAFRAMAX size tankers sailing on Primorsk in the Gulf of Finland during 2006. Transitdata comes from AIS tracks recorded by the Swedish Maritime Administration. Adatabase with tanker transits has been created and this information is the source forthe studies in this thesis. Included in the database are wind data, ice particulars andtransit information such as speed, and time at different activities during the transit.Average values for a transit has been investigated for comparison and to get a pictureof an average transit.Velocity, waiting time and time with assisting icebreaker are parameters that arebelieved to show how a tanker performs in winter navigation. These parameters arecompared with ice compression and ice class separately to see if there is acorrelation. Ice compression has also been investigated for correlation towards windforce to see if stronger wind generates stronger compression.Using the velocity in different ice compressions an estimate of ice resistance that stemfrom ice compression has been extracted by means of Lindqvist’s formula.

For the nearest years the design load level for offshore structures in Arctic regions is likely to be controlled by first-year sea ice ridges and rubble fields if the icebergs are not present in the area. Drifting ridges may hit fixed or moored surface structure such as platforms or ships, or they may gouge the seabed endangering pipelines and wellheads. Both the temporal and spatial properties of the consolidated layer and the unconsolidated part (the ice rubble) of the ice ridge are important input into ridgeload models. A better understanding of the ice rubble behaviour will enable us to determine the ice-ridge load more accurately. This thesis deals with two separate but connected subjects, namely: the ice strength field heterogeneity of both first-year sea ice ridge and level ice and the mechanical behaviour of the ice rubble.

Field mechanical testing of first-year sea ice by uniaxial compression has been done in order to improve the knowledge of the ice fields strength heterogeneity. The inplane ice strength non-homogeneity of different ice fields on the landfast level ice in the Spitsbergen fjords were investigated. A typical ice strength variability was found to be about 20-40% for the vertical ice samples and 10-20% for the horizontal ones. The weak zones were found within the entire ice fields. It was established that the strength of the ice samples taken from a certain depth from the ice cover surface at the different locations of the same ice field varied by a factor of 3 to 4. Furthermore, two first-year sea ice ridges have been examined with respect to the 2D spatial strength distribution: one in the North-western Barents Sea and one in the Arctic Ocean nearby Spitsbergen. The ice ridges are characterized by high strength heterogeneity of 40-55%, where the ice strength varied more than 3 times along both vertical and horizontal directions.

Special Finite Difference program “Inhomogeneity” was used to study the influence of the ice strength heterogeneity on the ice loads. It was shown that the ice heterogeneity might be one of the reasons for the scale effect.

In order to investigate the nature of freeze bonds between the ice blocks, series of field and laboratory small scale tests were conducted with submerged ice blocks. The small scale field tests were carried out in Adventfjorden on Spitsbergen. An opening was made in the landfast level ice. The level ice was sawed into cubic blocks which then were submerged down into the water. Some of the ice blocks were cut in two parts and then frozen together in order to simulate the freeze bonds between the ice blocks. The other blocks were submerged without forming the adfreeze bond. In addition to that, laboratory tests with both artificial ice (fresh and sea water) and natural sea ice were conducted in the cold laboratories at the University Centre in Svalbard (UNIS). The temporal development of the freeze bonding strength and the local strength of the ice blocks in the ice rubble, their changes with block size, confinement and ice properties were studied. The average freeze bonding strength from the field tests was found as 32 ± 18 kPa after 48 hours of submerging. The corresponding freeze bonding strength from the laboratory tests was 67 ± 52 kPa for the natural sea ice and 274 ± 142 kPa for the artificial freshwater ice within 60 hours of testing. Moreover, the ratio of freeze bonding strength to the strength of the submerged ice was found in the range of 0.008 to 0.082 with an average around 0.03 after 48 hours of submerging in the field. The corresponding strength ratio varied from 0.14 to 0.38 for the artificial fresh water ice and from 0.015 to 0.40 for the sea ice within 60 hours of testing in the laboratory.

A pseudo-discrete continuum model has been developed to study the behaviour of the ice rubble and in particularly its initial failure mechanism that is associated with the breakage of the freeze bonding contacts (rubble skeleton). The model is a combination of discrete particle assembly generation and Finite Element analysis of this assembly. The model provides a possibility to simulate the contacts between the ice blocks. A parametric analysis simulating 2D direct shear tests shows that the pseudo-discrete continuum model is very sensitive to both strength and morphology of the freeze bonds between the ice blocks. An attempt to extend the model to study large deformations within the ice rubble (that is associated with rotation, rearrangement of the ice blocks and their breakage) was considered.

The lateral "corners" where Kamb and Whillans Ice Streams (KIS and WIS) discharge into the Ross Ice Shelf share common geometries and ice mechanical settings. At both corners of the now-stagnant KIS outlet, shear margins of apparently different ages confine regions with a relatively flat, smooth surface expression. These features are called the "Duckfoot" on the northern, right-lateral side and the "Goosefoot" on the other. It has been suggested, on evidence found in ice internal layers, that the flat ice terrains on KIS were afloat in the recent past, at a time when the ice stream grounding line was upstream of its present location. The overdeepening in the bed just upstream of the KIS grounding line supports this view of the past geometry.

The right-lateral margin at the outlet of the currently active WIS, the location of Subglacial Lake Englehardt (SLE), appears to have many similarities with the right lateral margin of KIS, though with a less developed looking inboard margin. This paper presents a mechanical analysis using surface and bed topography and velocity datasets comparing the Duckfoot flat ice terrain with the terrain around Subglacial Lake Englehardt. At both locations mechanical thinning along shear margins and lows in the bed topography redirects basal water routing towards the features. Here, I consider the history of these features and their role in ice stream variability by comparison of the relict and modern features and via numerical modeling of ice shelf grounding and ungrounding in response to variations in ice flow.

We propose two scenarios for the development of flat ice terrains/subglacial lakes at the outlets of ice streams. In the first, development of a lake in the hydraulic potential low along a shear margin forces a margin jump as shearing develops along the inboard shore of the margin lake. This thesis presents evidence for an inboard (relative to the main outboard shear margin) zone of shear along the inboard shoreline of SLE, suggesting that subglacial lakes along shear margins are capable of facilitating shear margin jumps. In the second, grounding line advance around a relative low in the bed, creating adjacent margins along the lakeshores, forms a remnant lake. Discerning which of these scenarios is appropriate at the KIS outlet has implications for understanding the history of the ice stream grounding line.

An ice flow model is used to place these local conditions in a regional context by studying the effect of internal perturbations, such as ice rise stagnation or inward margin jumps, on grounding line position. Bathymetry is important in determining ice stream flow in the ways that might not be otherwise realized in 1-D flow model studies. In the numerical modeling experiments, grounding line advance across the KIS outlet is mediated by the overdeepening in the bed and proceeds not in the direction of ice flow but transverse to flow. This finding adds complexity to both a flowline view of grounding line migration and the theory that grounding lines are unstable in the presence of inward sloping bed topography.

This thesis presents a palaeoglaciological study aimed to determine the location, geometry and temporal evolution of palaeo-ice streams of the north-easternmost Laurentide Ice Sheet. The work was accomplished through the geomorphological interpretation of satellite imagery over 3.19 x 10⁶ km² of the Canadian Arctic, using a glaciological inversion scheme. Ice streams were active in this region during most of the time between the Last Glacial Maximum and the last deglaciation. A web of ice streams and inter-ice stream areas existed. Three major ice stream networks are identified: the M'Clintock Channel, Gulf of Boothia – Lancaster Sound and Hudson Strait. The M'Clintock Channel bears the most complex landform record, comprising three generations of palaeo-ice streams. Their location was weakly controlled by the subglacial topography and their geometry was determined by frozen-bed portions of the ice sheet, thus providing evidence for pure ice streams in the Laurentide Ice Sheet. In contrast, the more pronounced relief of the Gulf of Boothia – Lancaster Sound corridor supported topographically controlled ice streams. The landform record on emerged land along Hudson Strait is insufficient to support the existence of ice streams. It is therefore proposed that ice streams were constrained within the deep parts of the strait while flanked by cold-based zones on the margins. Small transient ice streams on Baffin and Prince of Wales islands drained local remnant ice caps during the collapse of the ice sheet. Analysis of the controls on the location and flow of palaeo-ice streams suggests that the interaction between the subglacial topography and thermal state of the substrate plays a more fundamental role than the geology. It is concluded that the behaviour of ice streams cannot be explained in terms of environmental controls alone, but the complex dynamics of ice stream shear margins and onset zones must be considered.

The principal objective being to include realistic mechanical parameters for marine ice in ice shelf flow models, uniaxial compression experiments have been performed on various types of marine ice samples. Technical developments are an important component of this thesis has they were necessary to equip the laboratory with the appropriate tools (pneumatic rig, automatic ice fabric data handling).

Results from experimental compression on isotropic marine ice show that it represents the higher boundary for meteoric ice viscosity throughout the whole temperature range, thereby validating Cuffey and Paterson's relationship with an enhancement factor equals to 1.

Marine ice is however often quite anisotropic, showing elongated crystals and wide single maximum fabric, that should impact its mechanical properties. Experiments on pre-oriented marine ice samples have therefore been carried out combining the study of epsilon_{oct} vs. tau_{oct} with a thorough analysis of microstructural data 'before' and 'after' the experiment.

Depending on the orientation of the sample in the applied stress field and on the intensity of the latter, anisotropic marine ice can be harder or softer than its isotropic counterpart, with n=4 often observed in Glen's flow law. Associating the experimental geometrical settings to potential natural equivalent, results suggest that anisotropic marine ice would strengthen ice shelf flow in most areas (for a same given temperature), apart from suturing areas between individual ice streams as they merge to form the ice shelf, where it could become weaker than meteoric ice in certain circumstances.

Finally, preliminary sensitivity studies, using a simple ice shelf model with our experimental parameters of Glen's flow law have allowed us to discuss the potential impact of rift location, rift size and thermal regime in the ice shelf behavior.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished