Fluid handling – Processes – Affecting flow by the addition of material or energy
Patent
1989-12-07
1991-02-12
Cohan, Alan
Fluid handling
Processes
Affecting flow by the addition of material or energy
137571, F17D 117
Patent
active
049916149
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for transport of hydrocarbons from an offshore source of hydrocarbons over long distances, as stated in the preamble of the independent method claim.
The invention also relates to a plant for such transport of hydrocarbons, as stated in the preamble of the independent device claim.
The invention, in fact, relates to a method with the aim of rendering possible transport of hydrocarbon liquid (oil) and hydrocarbon gas (gas) through one and the same pipeline over long distances in connection with offshore oil and gas production.
Offshore oil and gas production today is commonly carried out as follows:
Production wells are drilled from a platform into the reservoir. The platform is placed above wave tops on a support standing on the sea floor or floating on the surface of the sea The wellhead valves closing the reservoir pressure are provided on the platform, commonly straight above production wells.
The oil being highly pressurized in the hydrocarbon reservoir contains large volumes of dissolved gas. The capability of the oil to retain dissolved gas decreases with dropping pressure and rising temperature. When oil flows up from a reservoir through the production well and the well head valve on the platform causing a pressure drop gas is, thus, released from oil. What appears after the well head valve is, thus, a mixture of oil and gas.
This mixture of oil and gas is supplied to a processing plant which is generally located on the platform. The functions of such a processing plant essentially are separation of oil and gas and rendering oil suitable for transport and gas suitable for transport or return to the reservoir.
Since such processing requires power and hydrocarbons are flammable a series of auxiliary functions and emergency systems must be provided around the processing plant. Operation of processing, auxiliary, and emergency systems, furthermore, requires operators who, in turn, require quartering and a series of other functions. Plants,.thus, tend to be large and expensive both as regards investments and operation. The expense problem is enhanced at greater depth of the sea when the platform with plant has to be supported by an expensive stationary or floating basis.
Great development projects are running at present with the object of cost reduction. Among others, technology was developed which permits well head valves to be located on the sea floor--so called subsea production plants. This is of considerable economic importance because the number of rigs necessary for draining a hydrocarbon reservoir may be reduced. A subsea production plant is located above an area of the hydrocarbon reservoir that cannot be reached by the aid of production wells from a platform.
Production wells of a subsea production plant are drilled from floating or jackup drilling vessels. Oil and gas from the hydrocarbon reservoir flows up and past well head valves on the sea floor, and then passes as a two-phase flow (oil and gas in a mixture) in a pipeline connecting the subsea production plant with the platform. Such two-phase flows cause formation of slugs of liquid involving heavy liquid knocking, uncontrolled flowing conditions, and considerable pressure drop in the pipeline. The distance between the subsea production plant and the platform, thus, must not be large. At present, a practical limit is assumed to be approximately 15 kilometers.
Technical concepts to increase said distance will have a great economical potential. In its utmost consequence the platform may then become redundant, since well head valves may be placed on the sea floor close to the hydrocarbon reservoir, and processing, auxiliary, and emergency systems may be provided on the shore.
Large development projects are in progress these days in order to solve the problem of transporting oil/gas mixtures over large distances. Some of these projects aim at supplying pressure to the oil/gas mixture by placing two-phase pumps on the sea floor to compensate for the great pressure drop. Other projects aim at separating
REFERENCES:
patent: 4725287 (1988-02-01), Gregoli
Cohan Alan
Kvaerner Engineering A/S
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