The Design Ice Island for Impact Against an Offshore Structure

Abstract

ABSTRACT A method is presented by which an ice island may be selected for preliminary design of an offshore structure. The method involves:Estimation of the risk of an ice island impact.Determination of the distribution of kinetic energy amongst impacting ice islands.Selection of a design kinetic energy based on the maximum risk of failure which would be acceptable.Selection of a design ice island. The design ice island is chosen to be the ice island which is most likely to hit the offshore structure with the design kinetic energy. The ice island statistics are reviewed and analysed using the above selection method to define a design ice island. INTRODUCTION Offshore structures have been bull t over the past several years for oil exploration in the Canadian and American Beaufort Seas. It is anticipated that larger, longer-term offshore structures will soon be built for oil production. Amongst the hazards which will have to be considered during the design of an oil production structure which have not been considered for exploration islands are ice islands. Ice islands are the largest individual ice features which could threaten an offshore structure in the Beaufort Sea. Originating from the ice shelves along the northern coast of Ellesmere Island, ice islands occasionally break-off and become incorporated in the permanent polar pack. Their Subsequent motions are controlled by winds, ocean currents, pack ice pressures and coriolis acceleration. The trend in the resulting motion is the familiar Beaufort Gyre. Approximately once every ten years, an ice island under the influence of the Gyre will pass through the Beaufort Sea. Eventually each island entering the Gyre will escape or disintegrate. To date, all ice islands which have been observed to pass through the Canadian Beaufort Sea have passed far north of the exploration acreage. However, an ice island passing through the Beaufort Sea in the future could drift further south through the exploration acreage. If an ice island entered the exploration acreage during the open water season, its great mass coupled with the potentially high drift speeds possible in open water or loose pack would endow the ice island with tremendous kinetic energy. Any offshore structure which was struck would have to dissipate that kinetic energy safely or be destroyed. In anticipation of designing a production structure, two questions must be answered with regard to ice island impacts during the summer. Is the risk of an ice island hitting an offshore structure great enough that the offshore structure should be designed to withstand such a hit? If the answer to the first question is yes, then what are the dimensions and other characteristics of the ice island against which the offshore structure should be designed. The simplest answer to those questions is to design the offshore structure to withstand an impact by the largest ice island imaginable. Although certainly conservative, that answer may be impractical or at least very expensive.