Design Of Hydrostatically Supported Sand Islands For Arctic Drilling

ABSTRACT The search for oil and gas has already extended to the Arctic areas of the world. To date conventional sand islands have been used for exploration drilling purposes in water depths of up to 43 feet. In deeper water exploration has only been possible using floating drilling equipment which can only operate during the short summer season of open water. This paper briefly outlines the geotechnical principles and development to date of hydrostatically supported sand islands. This construction technique, which utilizes hydrostatic water pressure to stabilize dredged sand at near vertical underwater slopes, would allow sand islands to be quickly and econimically built in water depths of up to 200 feet. The hydrostatically supported sand island is a gravity structure and, therefore, is only suitable for use on competent seabed soils. This paper presents two different designs which are currently proposed for use as drilling structures in such areas as the Beaufort Sea. One design is intended for use as a movable exploration structure and the second for a permanent production island that would remain on location for 30 to 50 years. The near vertical side slopes of the hydrostatically supported sand islands reduce the amount of sand required to manageable amounts, and allow the construction on location to be safely completed during the short Arctic summers. The sand provides sufficient mass to resist ice pressures. INTRODUCTION In the Beaufort Sea conventional islands have been built for use as exploration platforms. In these inhospitable regions offshore structures are exposed to continuously moving ice sheets during the winter and intermittent ice flows during the short summer season of open water. (References 7 and 10). These islands with their enormous mass appear to be the most desirable method of resisting the ice pressures and thus providing a stable platform for drilling in both winter and summer. To date the largest island completed in the Canadian Arctic is ISSERK built by Imperial oil Ltd. in 1977 in a water depth of 43 feet with 15 feet of freeboard. It has been reported that this island required a full summer construction season using 2,500,000 cubic yards of fill and was of the sacrificial beach type with side slopes of approximately 20 horizontal to 1 vertical. A suction dredge capable of producing 50,000 cubic yards of sand per day was used. In deeper water the volume of sand increases dramatically. For example, a similar island in 100 feet of water would require at least 25,000,000 cubic yards of sand and 150 .feet at least 75,000,000 cubic yards of sand. These quantities of sand are prohibitive in relation to both the cost and the time required for construction particularly when the construction weather window cannot he relied upon beyond 70 days. In addition the resources of suitable island building sand are finite in any area of the world. Floating drilling platforms have also been used in the Beaufort Sea area of Canada but can only operate during the brief open water summer season.