The offshore areas of the United States are estimated to contain significant quantities of resources in yet-to-be-discovered fields. US Government estimates of oil and gas resources in undiscovered fields on the OCS (2006, mean estimates) total 86 billion barrels of oil and 420 trillion cubic feet of gas. These volumes represent about 60 percent of the oil and 40 percent of the natural gas resources estimated to be contained in remaining undiscovered fields in the United States. The 1.76 billion acres of the Outer Continental Shelf (OCS) is a significant source of oil and gas for the Nation's energy supply. The approximately 43 million leased OCS acres generally accounts for about 20 percent of America's domestic natural gas production and about 30 percent of America's domestic oil production.
With the discovery of fields producing hydrocarbons located in the sea at ever-greater depths the utilization of rigid structures fixed to the seabed to support production installations has become more and more costly. Consequently, the use of floating structures (better known among specialists as Stationary Production Units -- SPUs to receive production installations has been emerging as an ever more-frequent alternative that normally presents a lower cost than fixed structures.
These Stationary Production Units (SPUs) are connected to ascending fluid production and export tubes also known among specialists by their English language designation as "risers." Risers conduct the production of producing wells to the SPU, or link the SPU to other installations that receive the production of the fluids produced.
Stationary Production Units must have such characteristics as to allow the utilization of ascending fluid production and export tubes ("risers") in catenary curves made of steel, also known among specialists by the English language abbreviation "SCR," derived from "steel catenary risers." These steel catenary riser tubes present a lower manufacturing and installation cost in comparison to the flexible riser tubes used to that point. Furthermore, in the case of production at greater depths, they lighter, which reduces the load to be supported by the floating structure.
Frequently the economic feasibility of an enterprise for the exploitation of hydrocarbon-producing maritime field depends upon the reduction of total installation and operation costs. Specialists have proposed new Stationary Production Unit (SPU) shells to meet this requirement.
Significant technological innovations have emerged in recent years in anchoring systems and in ascending fluid production and export tube ("riser") systems. Such innovations, however, have been applied to conventional structures, such as semi-submersible platforms and tanker ships in the majority of cases, or to a few structures of a mono-columnar type of enormous draft, known to the specialists as a SPAR type platform, or else to platforms with far-reaching legs, known as TLP type platforms.
Semi-submersible platforms have the principal characteristic, in addition to being anchored, of remaining in a substantially stable position, presenting small movements when they suffer the action environmental forces such as the wind, waves and currents.
This type of floating structure has some disadvantages to being utilized as an SPU. An outstanding disadvantage is its limited load-bearing capacity, which often limits the use of equipment in processing plants installed on the platform deck. Another is a low storage capacity for storing the oil that comes on board, also arising from the limited load-bearing capacity.
Another problem to be noted is the fact that the hull of semi-submersible platforms are structures that are difficult to manufacture, and require techniques different from those employed in ship construction. This type of floating structure also has difficulty in absorbing variations in the load on deck that arise from project changes, which results in a great disadvantage in its use.
Project alterations arise from necessities that emerge during the development of a project, and from employing more equipment in the processing plant as the result of re-evaluation of the production capacity of the field producing the petroleum where the floating structure will be employed.
The use of tanker ships adapted to receive a production plant on their deck has become quite common in recent years, specifically due to the great availability of this type of floating structure on the market. The great advantages presented by such ships are their great capacity to receive loads and their great capacity for storage.
Tanker ships, however, have the disadvantage of not being axi-symmetrical structures and the load on the ship's structure is going to vary in function of the locations where the various items of production equipment are mounted. The great disadvantage in using a ship to house a production plant on its deck is related to the difficulties that this type of floating structure presents to operating in a stabilized manner in relation to environmental conditions, i.e., winds, waves and marine currents.
For ships to be able to receive ascending fluid production and export tubes in a stable manner, i.e., without significant movements in such tubes, it is very common to employ a rotational structure in the ship that is provided with a body anchored to the seabed. That body remains in a substantially fixed position in relation to the seabed, presenting quite limited movements.
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