Offshore Drilling From Ice Platforms

Abstract

The successful development of a method for drilling offshore from a floating ice platform has enabled exploration wells to be drilled economically in the Canadian Arctic Islands. This method avoids the long wait for sophisticated offshore drilling vessels to be developed, financed, and built to operate in the severe ice conditions prevalent in the area. Introduction The six gas fields discovered to date in the Sverdrup basin contain an estimated 13 Tcf of reserves (Fig. 1). For years geologists have recognized that much of the remaining undiscovered 90 to 260 Tcf of gas reserves in the Canadian Arctic Islands lie offshore. The Arctic Ocean in this area is covered with ice for 11 to 12 months of the year. Presently available drillships and semisubmersible drilling vessels are unable to reach these ice-infested waters. A drilling system mounted on an air-cushion vehicle that can move on top of the ice has been proposed, but is not yet under construction. Jack-up drilling platforms are not applicable because of excessive water depths. In any case, it is doubtful that a bottom-supported structure could withstand the powerful forces exerted by the thick ice sheet. After several years of study it was determined that the ice sheet remains relatively stationary between the Arctic Islands from January to June each year. Further studies revealed that by progressively flooding and freezing in thin layers over a period of 1 to 2 months, an artificial ice platform could be built that was thick enough and strong enough to safely carry the weight of a small conventional land rig. In March 1974, the first Arctic offshore gas well was drilled in 421 ft of water at W. Hecla N-62, some 7 miles west of Sabine Peninsula, Melville Island. In 1975 the technique was successfully used again off the east coast of Sabine Peninsula to drill East Drake I-55 in 467 ft of water and to extend the proven limits of the Drake Point gas field 7 miles offshore (Fig. 2). Plans are presently being made to drill three more offshore wells from ice platforms in 1976. One of these will be a 6,000-ft wildcat test. Measurement of Ice Movement The single most critical factor before ice-platform drilling can be undertaken is to know that the rig will not move significantly in relation to the ocean floor during the drilling period. In March 1971, a study of horizontal ice movement at several prospective locations within 5 miles from shore was undertaken by triangulation from two onshore sites. A precision theodolite was used to measure angles to targets on the ice (Fig. 3). In later years, tellurometers were used to measure distance to the on-ice targets. This method supplemented theodolite measurements and allowed ice movement to be measured in obscure weather and up to 15 miles offshore. The same offshore locations were checked in subsequent years to ensure that there was no excessive fluctuation from one year to the next. In 1974 a geodometer was used in an attempt to measure ice movement at points up to 40 miles offshore. This instrument proved unsatisfactory because of unsuitable conditions in the Arctic. The data gathered from these surveying methods generally indicated that horizontal ice movement was less than 20 ft over the period January to June and usually was less than 10 ft during the February-March-April drilling period. During 1974 and 1975 an acoustic method of measuring horizontal ice movement at points 40 to 50 miles offshore was developed (Fig. 3). JPT P. 379