In the never-ending search for hydrocarbons, many technologies have been developed to find new reservoirs and resources and most areas of the world have been scoured looking for new discoveries. Few expect that any large, undiscovered resources remain to be found near populated areas and in places that would be easily accessed. Instead, new large reserves are being found in more challenging and difficult to reach areas.
One promising area is in the offshore Arctic. However, the Arctic is cold, remote and ice on the water creates considerable challenges for prospecting for hydrocarbons. Over the years, it has generally been regarded that six unprofitable wells must be drilled for every profitable well. If this is actually true, one must hope that the unprofitable wells will not be expensive to drill. However, little in the Arctic is inexpensive.
Currently, in the shallow waters of cold weather places like the Arctic, exploratory drilling from a jack-up or mobile offshore drilling unit (MODU) can be used for about 45-90 days in the short, open-water summer season. Predicting when the drilling season starts and ends is a game of chance and many efforts are undertaken to determine when the jack-up may be safely towed to the drilling location and drilling may be started. Once started, there is considerable urgency to complete the well to avoid having to disconnect and retreat in the event of ice incursion before the well is complete. Even during the few weeks of open water, ice floes present a significant hazard to jack-up drilling rigs on location and legs of the jack-up drilling rig are exposed and quite vulnerable to damage.
Jack-up rigs are mobile, self-elevating, offshore drilling and workover platforms equipped with legs that can be lowered to the ocean floor until a foundation is established to support the hull, which contains the drilling and/or workover equipment, jacking system, crew quarters, loading and unloading facilities, storage areas for bulk and liquid materials, helicopter landing deck and other related equipment.
The jack-up rig is designed to be towed to the drilling site and jacked-up out of the water so that the wave action only impacts the legs, which have a fairly small cross section. However, the legs of a jack-up provide little defense against ice floe collisions and an ice floe of any notable size is capable of causing structural damage to one or more legs and/or pushing the rig off location. If this type of event were to happen before the drilling operations were suspended and suitable secure and abandon had been completed, a hydrocarbon leak would possibly occur. Even a small risk of such a leak is completely unacceptable in the oil and gas industry, to the regulators and to the public.
Thus, once it is determined that a potentially profitable well has been drilled during this short season, a very large, gravity based production system, or similar structure may be brought in and set on the sea floor for the long process of drilling and producing the hydrocarbons. These gravity-based structures are very large and very expensive, but are built to withstand the ice forces year around.
U.S. Pat. No. 4,819,730 discloses a floating work platform that has two derricks provided thereon. However, that patent is directed to a semi-submersible vessel, such as a drill ship. Unlike jack-up rigs, semi-submersible vessels by nature are subject to both the wave and impact of ice floe, and therefore cannot withstand the extreme conditions in the Artic.
EP1094193 discloses a offshore drilling ship that has a dual-activity drilling assembly, so that the time involved in drilling wells in substantially deep water can be reduced. However, this patent is also directed to a drilling ship that cannot maintain high stability required in the Artic under harsh weather conditions. Additionally, the dual-activity drilling assembly disclosed can only drill one well at a time, which is inherently inefficient for exploration or production purposes, especially at a place where only a very narrow window of operation is available.
U.S. Pat. No. 6,491,477 describes a self-elevating drilling unit with dual cantilever assemblies that permit dual drilling on a small platform where each drilling unit can operate on an 8×8 spacing. However, this rig is still subject to the vagaries of weather and does not solve the ice problem that exists in Artic or North sea drilling.
Therefore, there is the need for a drilling system and method that can both withstand the extreme conditions in the Artic and provide highly efficient exploration capacity and mobility.