Sea Bottom Scouring in the Canadian Beaufort Sea

Abstract

ABSTRACT Echo sounding, side scan sonar and seismic profi1ing records have shown that the continental shelf of the Canadian Beaufort Sea has been subjected to extensive scouring by ice features. The scouring phenomena is extremely important in the design and protection of offshore well s and future pipelines. Analyses of records collected by industry and government in the early 1970's and reported by Hnatiuk and Brown in 1977 have been refined with the inclusion of additional data collected by industry and government in 1975 and 1976. Here, the results of the information synthesis are presented in terms of regional maps showing relevant scouring parameters and their variation with location and water depth. A quantitative evaluation of scour return period is al so presented on the basis of sedimentation assumptions. This information is compared with a recent analysis of sidescan scour mosaics collected repetitively over four areas ranging in water depths from 45 to 150 feet and with time interval s ranging from 2 to 7 years between the repetitive seafloor maps. The rate of addition of new scours determined from the repetitive mosaic approach supports the regional assessment of Beaufort Sea scour but suggests more episodic and a really frequent scour events along with more active scouring in water depths approaching 150 feet. This information is discussed in terms of the Beaufort Sea ice regime. INTRODUCTION The continental shelf of the Beaufort Sea which extends some 75 to 100 miles from shore to a water depth of 600 feet, has long been considered to have the potential for major accumulations of hydrocarbons. During the period 1963 to 1969, permits to explore were issued for the majority of the Canadian mainland shelf. Offshore drilling commenced from artificial islands in late 1973 and oil was discovered in 1975. Subsequent drilling from artificial islands, caisson islands and drillships has confirmed the presence of major reservoirs of oil and gas, many of which will likely be developed for production. Gouging or "scouring" of the seabed by sea ice pressure ridges or glacial ice features has been a concern in exploratory drilling operations and will be of even greater concern when gathering lines, pipelines and subsea development well s are being designed. Two critical aspects of the concern are depth of scour and return period. Currently when drillships are operating in areas of active scouring, "glory holes" or excavations are prepared in which the blowout preventors are placed for protection from the ice. The first good records of sea bottom scouring in the Canadian Beaufort Sea were obtained during the summer of 1970 (1). These side scan sonar and echo sounder records plus those obtained during 1971 and 1972 were analyzed through Projects funded by members of the Arctic Petroleum Operators' Association (2,3) and reported at the 1976 OTC (4).