Substantial known reserves of oil and gas exist in Arctic regions, and many of these reserves lie below the surface of the Arctic oceans or other bodies of water in the Arctic regions. In the past, techniques have been successfully used for tapping offshore oil and gas reserves in sub-Arctic regions by using either offshore platforms erected on the ocean floor, semi-submersible moored drilling platforms, or moored drilling vessels. However, year-round use of these techniques in Arctic regions can be hazardous because of the ice problem which exists there during much of the year. For example, a permanent polar ice pack exists over much of the Arctic Ocean and varies in extent depending upon the time of year. During the Arctic winter, the size of the ice pack expands to positions very close to, and in some instances in direct contact with the shoreline. In those areas where the permanent ice pack does not come into direct contact with the shoreline, ice sheets which are fast to or fixed to the shoreline (known as land-fast ice) cover these areas and may extend 25 miles or more offshore.
The permanent ice pack slowly rotates and circulates in the Arctic Ocean, and land-fast ice also moves during the period in which it exists in the Arctic. The motion of a land-fast ice sheet is, overall, in random directions and in random amounts in response to tides, currents, winds and temperature changes. Ice sheets in the Arctic area may move as much as 60 feet per day and can exert substantial forces on a structure, such as a drilling or production platform, extending through the ice sheet from a supporting connection to the ocean floor. Thus, year-round use of conventional offshore drilling equipment in Arctic regions generally has been prevented. Moreover, it is uneconomical to use conventional offshore drilling equipment in Arctic locations only during the short ice-free season because of the time and cost involved in moving the equipment into and out of the Arctic location.
U.S. Pat. No. 3,749,162 discloses ice melting techniques intended to make it possible to explore, drill and produce oil and gas from submerged areas in Arctic regions. Generally, this patent discloses a buoyant vessel of conventional flat hull form maintained in a pool of water in a moving ice sheet over a submerged well site. Heat is applied to the ice from the flat hull for the purpose of melting the portion of the ice sheet which moves toward the hull. Well drilling operations are performed from the vessel while the vessel remains buoyant in the pool over the well site. The pool and the vessel remain fixed above the submerged well site. My laboratory tests disclosed below have shown that an ice melting system using such a flat hull design can produce reliable ice melting for only very low rates of ice movement, say about four feet per day or less.
U.S. Pat. No. 3,831,385 discloses an offshore structure having a heated conical base. The coned shape forces advancing ice sheets to lift and break in a flexural mode rather than crushing. The heated conical surface produces a thin film of melted ice water between blocks of ice and the metal surface, thereby reducing the frictional coefficient between sliding blocks and the cone. The combined net result is to reduce ice impingement forces on the structure when compared with an unheated conical surface or a vertical columnar surface. This patent does not contemplate complete melting of advancing ice to reduce impingement forces essentially to zero.
It is difficult to initiate and maintain high melting rates of moving ice simply by applying heat at relatively low temperature levels to the ice from a flat surface, as in U.S. Pat. No. 3,749,162, including the type of essentially smooth conical surface disclosed in U.S. Pat. No. 3,831,385.