Design Aspects of a Mobile Arctic Caisson

Abstract

Abstract This paper describes the main features of a bottom founded mobile arctic structure which will have the capability of year round operation in a variety of ice conditions in water depths ranging from some 15 to 40 meters. The caisson is essentially a continuous steel annulus supporting a deck carrying drilling and top side facilities. The core of the annulus will be filled with sand to provide most of the resistance to horizontal ice loading. A feature of the design is that means are incorporated for keeping the ballast water, and the sand core, in an unfrozen state throughout the winter. Introduction Gulf Canada Resources have embarked on a program to drill exploratory wells on a number of prospective locations in the Canadian Beaufort Sea (Fig. 1), generally in water depths in the 20 to 40 meter range (70 to 130 feet). To accomplish this, a hybrid drilling system has been selected consisting of one floating platform and one bottom founded unit. The floating platform will be used to "extend" the summer season, while the bottom founded unit will have year round capability. This paper will describe the conceptual and design development of the bottom founded unit (Fig. 2), referred to as the Mobile Arctic Caisson (MAC), and will elaborate on its conformity with the basic design prerequisites:–ability to operate year round given adequate relief well capability;–ability to be transported to the Beaufort Sea via Point Barrow, and be relocated annually with minimum difficulty;–ability to reduce the annual dredge volumes required in deep water sites as a trade off against capital cost of the unit;–ability to drill two wells with limited Re supply. As many of the technical details are proprietary, the paper will address the more general design aspects of the caisson itself. Limited reference will be made to the "top-side" facilities which are relatively standard modular designs. Background Exploratory drilling in. the Beaufort Sea has either been from drill ships operating during the short open water season, or from gravel or sand islands for winter operations. Since the early 1970's over 25 such islands have been built in progressively greater water depths. With the sands available in the Canadian sector, the attainable side slopes of the submerged fill are such that the quantities and costs for an artificial island could become almost prohibitive in the deeper regions of the Gulf acreage. In addition, the western portion of the acreage does not appear to have major deposits of suitable island building materials, which leads to increased haul distances and costs. In the Eastern sector, ample borrow material is available, but at some sites this is overlain by deep silt deposits. This means that on a particular island site much of this unsuitable material could have to be removed, so adding to the overall quantity requirement.