Sea Ice Forces and the State of Technology of Offshore Arctic Platforms

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering. The oil industry is in the infancy of offshore arctic operations. The capability to conduct exploration during part of the year has been demonstrated, and in the next few years production will begin in shallow waternear Prudhoe Bay. Year-round operations in deep water are at least a few yearsin the future. This paper is a summary evaluation of the industry's currentcapability, and of the ice forces that will control the design of offshorestructures in depths of 100 to 200 ft [30 to 60 m] in the arctic. The technology of arctic offshore operations still is being developed, and we willlearn a lot before production begins in deepwater arctic areas. It will be surprising if the opinions expressed in this paper do not change in the next 5or 10 years. Much of the industry's arctic experience has been gained in the CanadianBeaufort Sea. The. Alaskan Beaufort presents greater challenges. In the wordsof Weeks and Weller: . . . ice conditions are generally more severe north of Alaska with more multiyear ice, more ridging, and large ice motions occurring." The oil industry's technical capability to explore and produce in the Alaskan Beaufort Sea varies with a number of factors but mainly with water depth. Fig. 1 shows the variation of ice conditions on a cross section of thecontinental shelf of the Beaufort Sea. There is a dramatic change in iceconditions at about 50 to 70 ft [15 to 21 m] water depth, where the groundedridge zone is found. The grounded ridge zone marks the change from the relatively stationarylandfast ice to much more mobile ice farther offshore. The amount of motion andthe speed of motion increase by about a factor of 10 at this boundary. Indeeper water, the faster sea-ice motion will result in larger ice forces actingon fixed structures. Fig. 2, a map of the Diapir field (OCS Sale 87) area, gives the water depth ranges of the Beaufort Sea over which various exploration and production technologies will be applicable. The general categories given on the map, however, deserve further explanation. Depth Range 0 to 100 ft [0 to 30 m] Current Technology is Adequate Exploration. Three basic systems available for drilling in less than 100 ft[30 m] of water in the Alaskan Beaufort Sea include earthfill islands, caisson-retained islands, and submersible drilling vessels. In less than about 60 ft [18 m] of water, earthfill islands can be built. Beyond the edge of the landfast ice, dredged or barge-hauled gravel may berequired, rather than gravel trucked over ice roads, because the sea icefarther offshore moves too much to maintain an ice road over a period of weeks. From 60 to 100 ft [18 to 30 m] of water, caisson-retained systems (Fig. 3)may be used. In these systems, large box structures, called caissons, are usedas retaining walls at the surface of the island. This reduces the volume offill required. Caisson-retained islands may have a cost advantage over 100%earthfill islands, depending on the relative costs of the caissons and of dredged fill. The first two caisson-retained islands were built in the CanadianBeaufort Sea at Tarsuit and Kadluk. Submersible drilling vessels, such as the Global Marine concrete is land drilling system (CIDS) especially built for arctic ice loading, also may beused in this range of water depth. The first CIDS, designed for a maximum 60-ft[18-m] water depth, was deployed in the Alaskan Beaufort Sea in Summer1984. P. 21^