A theoretical and field study of load transmission through grounded ice rubble

Marshall, Alfred R. (1990) A theoretical and field study of load transmission through grounded ice rubble. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (20MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Oil exploration from artificial Islands in the Beaufort Sea has shown that grounded ice rubble fields often accumulate around such structures, preventing moving ice from acting directly against the structure. A simple, rigid body model, has commonly been used to estimate the load "seen" by the structure, with the result that no loading is expected until global rubble sliding occurs. It was realized that such an approach is not entirely realistic and so a collaborative field program was carried out, followed by theoretical modelling. The field work was done at a drilling location in the southern Beaufort Sea on ESSO's Caisson Retained Island (CRI). The three groups involved were ESSO Resources Canada Ltd., National Research Council of Canada (NRC) and Memorial University of Newfoundland (M.U.N.). The M.U.N. data and subsequent theoretical work is presented here, with the ESSO and NRC data summarized in the site description. -- Memorial University collected data from pressure sensor rosettes, a strain array, thermocouple arrays, rubble profiling, and ice property measurements. The most important data came from the pressure sensor rosettes and the thermocouple arrays. These indicated that average sea ice pressures against the rubble reached 350 kPa, and that a substantial and rapidly formed refrozen layer existed within the rubble, with a thickness exceeding 3 m near the rubble field periphery. -- Based on the field measurements, a theoretical model was developed to investigate the role of the rubble refrozen layer in load transmission through rubble. The rubble field was modelled as a system of springs and dampers to represent the elastic, frictional, and viscous stiffness of the various load paths. The deformations and load distributions were calculated using a commercial finite element package called ABAQUS. Each material property was assumed to have a range of values, the limits of which were determined from the literature, theoretical considerations and field measurements. -- The sensitivity of the output to each of the inputs was examined and the following was concluded; -- 1. From the analysis it appears that the elastic response of the rubble leads to the highest loads on the embedded structure because the transmitted load tends to decrease with time (creep). -- 2. The ice rubble properties that are the most important for further study are the delayed elastic and shear properties of un-refrozen rubble, as well as their variability within natural rubble fields. -- 3. Significant loads may be transmitted through ice rubble to a structure before global sliding of the rubble field occurs. -- 4. The presence of the refrozen layer gives the potential for substantial loads to be imposed on the structure during rubble field movement. -- 5. Possible future improvements to the ABAQUS computer model include the incorporation of; refrozen layer discontinuities, berm slope, non-rigid structure behaviour, kinetic and static friction values (rather than an average), and water and ice mass. Furthur research is also required on ice/rubble failure modes to esablish design pressures at the rubble field boundaries. -- 6. The sample 3D calculations show that viscous deformation may also be important, although it is also clear that the 3D method presented needs improvement. -- KEY WORDS: Sea ice; sea-ice; ice rubble; ice loads; landfast ice; rubble fields; Kaubvik; artificial islands; Beaufort Sea; pressure sensors; strain array; thermocouple array; refrozen layer; consolidated layer; ABAQUS; computer model.

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