Modeling Ice Rubble Fields Around Arctic Offshore Structures

ABSTRACT Ice rubble fields, extensive accumulations of fractured ice broken out of a moving ice sheet, form around most Arctic offshore structures located in a dynamic ice environment. The extent and influence of such a rubble field are principally governed by the water depth, ice movements, properties of the ice and geometry of the structure. The authors have developed a computer simulation program which calculates the rubble field extent and properties that will accumulate around various offshore structures located in the path of moving ice. Of particular value are statistics on rubble extent, sliding resistance, pressure attenuation and reduction of the force transmitted to the structure. This tool provides the basis for a more rational calculation of force and ice scour reductions, compared to traditional calculations, as well as the basis for an assessment of the operational problems of product export and re-supply over and through free floating or grounded rubble. Pre- and post-processors are used to develop statistics on the ice movement data, and to plot the results of rubble extent, rubble cross-sections, and pressure transmission calculations. The model has been applied to numerous artificial offshore structures, and for verification purposes, to several naturally occurring features. INTRODUCTION The southern Canadian Beaufort Sea has seen active offshore exploration for oil and gas since the early 1970's, and the results have encouraged increasing activity in that area. Similarly, the discovery wells are located in the offshore Alaskan Beaufort Sea, but activity there is principally on, or protected by, the barrier islands and land fast ice. The new lease sales in Alaska may however open areas further offshore to development and will therefore require consideration of the more dynamic Arctic ice environment. The first Canadian Arctic offshore hydrocarbon exploration structure was the Imperial Oil Ltd. artificial island Immerk B-48, constructed in 3 m of water in the near-shore zone and partially protected by Pelly and Hopper Islands. Subsequent structures include the Esso Resources Ltd. (a wholly owned subsidiary of Imperial Oil) Issungnak in 19 m of water and the Dome Petroleum Ltd., Gulf Canada Resources Inc. et al Tarsiut in 23 m of water, both fully exposed to wind, waves and ice. In all, about 20 such artificial islands have been built in the Canadian Beaufort Sea, each providing a winter platform for drilling while at the same time acting as a test structure for refining the designs of those to follow. In a dynamic ice environment rubble fields will form around these exploration structures, as well as around many of the production structures currently proposed. The effect of the rubble on the ice/structure interaction has been discussed in previous papers1,3,12 including an analysis of the rubble fields around a wide structure in deep water by one of the authors2, however until now the modeling of rubble accumulation around a complex structure in a dynamic ice environment has not been presented.