Stresses were measured in multi-year sea ice near a grounded floe-island. Compressive stresses as high as 250 p.s.i. were observed, in the form of impulses. A second installation in annual sea ice near a grounded pressure ridge showed tensile stress of 100 p.s.i., followed by crack formation. These stresses were localized to a range of the order of 100 meters. Localized compressive stresses in multi-year ice near grounded offshore structures may exceed 250 p.s.i., and impulsive loading should be expected.
The techniques which have been used for offshore exploratory drilling in ice-infested waters fall into three categories:
drillships or semisubmersibles during the ice-free summer season;
drilling from thickened, floating ice sheets
construction of artificial gravel or ice islands as drilling sites.
The first two techniques have been used in deeper water, whereas the third approach has been used in shallow water. Novel offshore structures and drilling systems have been proposed for ice-covered waters, but only model studies of these possibilities have been made. For production sites offshore, the long-term stability and accessibility of a fixed offshore structure or island is an attractive advantage.
Offshore structures affixed to the sea floor in ice-covered waters are subjected to large forces caused by the movement of sea ice against their boundaries. The magnitude of these forces depends upon several factors:
the nature of the forces driving the ice sheet;
the strain rates during the ice-structure interaction;
the mechanics of elastic and plastic deformation of the ice sheet;
the failure strength of the ice as found in nature;
the details of stress concentration in the ice sheet near the fixed structure;
the modes of failure and sequences of failure that occur near the structure;
the dynamic response of the structure.
Two different regimes of ice cover should be distinguished: The first is a quasi-continuum of ice floes composing a rather solid ice sheet, such as is found in winter. The second is a broken sheet with ice floes separated from each other, free to move and impact against fixed structures, such as is encountered during breakup, or during the summer intrusion of the pack ice.
The research reported here was oriented towards the measurement of the ice stress produced by nature in the vicinity of a grounded obstacle. We were also interested in the causes of ice stress, and the spatial and temporal patterns of ice stress and ice fracture. Clearly, a great deal of instrumentation is required to fully answer such questions, and furthermore, since events of high stress are infrequent, an extended period of data collection is needed to form a data base of statistical significance, from which extreme cases can be confidently predicted. The initial results reported here should therefore be taken as examples of stress behavior in natural sea ice, which may suggest further models and measurements.
