Method and equipment for the flow of offshore oil production wit

Details Method and equipment for the flow of offshore oil production wit Method and equipment for the flow of offshore oil production wit

1998-07-28

2000-07-25

Taylor, Dennis L.

Wells

Submerged well

Wellhead

166338, 166 70, E21B 3305

Patent

active

060926031

DESCRIPTION:

BRIEF SUMMARY
This application is the national phase of international application PCT/GB97/00251, filed Jan. 29, 1997.


FIELD OF THE INVENTION

This invention relates to a method and equipment to assist the flow, to the surface, of hydrocarbon mixtures containing a high concentration of gas. It may be applied to a single offshore oil well or to an offshore manifold which receives the output from various wells for subsequent gathering.


PRIOR ART

The growing exploration for oil in increasingly deeper waters has made it necessary for those skilled in the art to develop new techniques to increase the production of hydrocarbons from offshore wells. It is known that the mixture of hydrocarbons originating from wells can vary substantially in respect of the volumes of its phases, which are normally water, oil and gas.
Once the step of obtaining the greatest possible volume of the mixture of hydrocarbons from a well has been completed, it is then necessary to discharge the mixture to a gathering centre which has primary processing facilities. This may be an offshore platform, a vessel, or even an onshore gathering station. The mixture is discharged to the gathering centre via pipelines which may be rigid or flexible, or even a combination of both.
Very often the reservoir pressure itself is the only energy used to cause the hydrocarbon mixture to flow along these pipes to the gathering centre. However, this has a number of disadvantages because the accumulation of fluids in riser pipes causes an increase in hydrostatic pressure at the wellhead or manifold due to the formation of a column containing a significant volume of fluids. This pressure increase is undesirable because it prevents a large flow of the hydrocarbon mixture from reaching the gathering centre. In the extreme situation the reservoir pressure may be simply incapable of providing a flow to the gathering centre.
When the hydrocarbon mixture contains a large volume of gas there is always the possibility that a number of factors can come together to give rise to the phenomenon of serious intermittency, which causes great oscillations in the pressure levels of the fluid flow. A basic condition for the appearance of serious intermittency is the formation of a liquid seal in the flow lines which encourages gas segregation into the upper part of the pipes. When finally the volume of segregated gas manages in some way to pass along the rising part of the pipe which extends from the sea bed to the gathering centre (known by those skilled in the art as a "riser"), a great increase in pressure is then produced in this rising line. This sudden increase in pressure is undesirable and extremely harmful to installations.
GB-A-2282399 proposes the use of a secondary riser line which is connected to the flow line at a point located at a specific distance from the junction between the lower flow line and the main riser. This secondary riser is connected to the main riser at a point located above the junction between the main riser and the lower flow line.
The function of the auxiliary riser is to relieve the gas pressure in the flow of hydrocarbon mixture which occurs upstream from the point at which the lower flow line joins the main riser, and to inject this gas downstream from that junction point. A control valve may be fitted in the secondary riser, controlled by a sensor installed close to the connection between the flow line and the secondary riser, to control the flow of gas injected into the riser. In this way the effects resulting from the phenomenon of severe intermittency are diminished, or the phenomenon itself may even be prevented, because as the gas is injected into the main riser in a controlled way there is no sudden variation in pressure in the rising flow of fluids to the gathering centre.
This technology was a notable contribution to the control of serious intermittency in multiphase flows. However the formation, in riser pipes, of a column with a significant volume of fluids continues to cause an undesirable increase in pressure at the offshore well-head o

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patent: 5478504 (1995-12-01), de Almeida Barbuto
patent: 5636693 (1997-06-01), Elmer
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