Wells – Submerged well
Reexamination Certificate
1999-05-20
2001-07-31
Bagnell, David (Department: 3673)
Wells
Submerged well
C166S357000, C166S267000, C166S372000, C166S369000
Reexamination Certificate
active
06267182
ABSTRACT:
This application is the national phase of international application PCT/GB97/01200 filed May 1, 1997 which designated the U.S.
Field of the Invention
This invention relates to a method and equipment to assist the flow, up to the surface, of hydrocarbon mixtures containing a high gas concentration. It may be applied to a single offshore oil well or to an undersea gathering line (manifold) which receives production from various wells for subsequent delivery.
In the flow of oil, as occurs in undersea production lines, large pressure head losses occur due mainly to the large simultaneous flows of gas and oil, and which give rise to great shear stresses in the flow.
In the technique of deep offshore production another factor which gives rise to high pressure gradients is the great difference in level between the wellhead and the platform, which very frequently makes it necessary to use extensive vertical pipes to deliver the products to the surface; such pipes are known to those skilled in the art as “risers”.
These factors result in high pressures at the wellhead or in the undersea gathering line (manifold), significantly reducing production.
U.S. Pat. No. 5,460,227 discloses a subsea primary gas separation of the production of an offshore oil well, the use of a separated gas flow line to a gathering center, and a pressurization of the separated liquid to transport it to the gathering center.
U.S. Pat. No. 5,199,496 discloses the use of Bernoulli effect for transporting the fluid mixture of crude oil, water and gas to a gathering center.
EP-A-0579497 discloses pressurizing the gas in a separator to enhance both separation and transport of the separated liquid.
U.S. Pat. No. 5,044,440 discloses flowing production fluid form a well head to a subsea primary separator and separating out the liquid phase from that separator and compressing the separated gas phase to facilitate its transport.
Summary of the Invention
The object of this invention is to provide equipment and a method to assist flow of the multiphase production from an oil producing well to any gathering station.
It is a further object of this invention to promote the primary separation of the liquid and gas phases of fluids produced by an oil producing well and to encourage flow of these two separated phases along separate flow lines to a gathering station.
A subsea primary separating means is used to perform a primary separation between the liquid and gas phases right on the ocean bed. Where the separating means is a separating vessel the gas phase is carried away to the gathering station by a separated gas flow line connected to the top of the primary separating vessel and the liquid phase is carried by at least one flow line connected to the lower part of the primary separating vessel.
Pressurized gas is injected into the at least one flow line at intervals to encourage flow of the liquid phase settling out in this line to the gathering station.
The liquid phase is caused to flow to a gathering station through at least one flow line. At least one U-shaped pipe length connects the pressurized gas line to the at least one flow line so that pressurized gas can be injected into the at least one flow line for a required period of time.
This volume of pressurized gas makes it possible for the liquid phase which has collected in the at least one flow line to flow to the gathering center. A check valve installed in each of the at least one flow line prevents the pressurized gas from entering the primary separating means.
If it is desired to increase the flow efficiency, a mechanical interface may be used to push the liquid phase separating out in the at least one flow line to the gathering center. In this case the pressurized gas line must be provided with means to allow the mechanical interface to be inserted at the launching point and with means to allow the mechanical interface to travel along within the pressurized gas line and into the flow line. The flow line must in turn have means to receive the mechanical interface without interrupting production.
Thus one aspect of the present invention provides equipment for offshore oil production with primary gas separation and flow, by means of pressurized gas injection comprising: a subsea primary separating means which receives the production from an offshore oil well delivered from a wellhead through a wellhead flow line; a separated gas flow line which connects the upper part of the primary separating means to a collecting vessel located at a gathering center; at least one flow line which connects the lower part of the subsea primary separating means to a gathering center; wherein at least one U-shaped pipe length which is fitted with a shut-off valve and connects said at least one flow line to a pressurized gas line which is fitted with a pressurized gas check valve near to the point where the pressurized gas line connects to the wellhead to avoid back flow of gas originating from the annulus of the well; wherein said at least one flow line has a check valve located between the subsea primary separating means and the point of connection to said at least one U-shaped pipe length in order to prevent the injected pressurized gas from exerting a back pressure which would adversely affect production from the oil producing well.
The separated gas flow line may include a pressure control valve, which makes it possible to control the level of the liquid phase of fluid produced which collects in the subsea primary separating means.
If it is necessary to remove any accumulations of condensate within the separated gas flow line for the separated gas, the pressurized gas line can be connected to it, this making it possible for a traveling mechanical interface device, which causes the condensed fluids to flow to the gathering station, to pass through the separated gas flow line. In this case the shut-off valve in the at least one U-shaped pipe length must be a clear-flow shut-off valve, and a check valve must be fitted to prevent the pressurized gas from passing into the primary separating means.
Another aspect of the invention provides a method for offshore oil production with primary gas separation and flow by the injection of pressurized gas comprising the following steps:
allowing fluids produced by a well to flow through a wellhead flow line from a wellhead to a subsea primary separating means where primary separation between liquid and gas phases takes place;
the liquid phase separates out in the bottom part of the subsea primary separating vessel and also accumulates in an outlet flow line and in first and second flow lines;
as the liquid phase accumulates in the lengths of the flow line which are horizontal or have only a small gradient, the back pressure exerted by that volume of fluids on the production from the well will be small;
injecting pressurized gas along a gas injection line to cause flow, to a gathering center, of the liquid phase which has accumulated in outlet, first and second flow lines;
wherein it further comprises the following steps:
when the back pressure exerted by the volume of fluids accumulated in the outlet flow line and in the first and second flow lines begins to increase and to adversely affect production from the well, then a shut-off valve among first and second shut-off valve respectively located in second and third U-shaped pipe length which connects the pressurized gas line to first and second flow lines opens for a predetermined period so as to allow a volume of pressurized gas to pass into that flow line among first and second flow line connected to the U-shaped pipe length among second and third U-shaped pipe length where a shut-off valve among first and second shut-off valve has been opened;
a check valve among first and second valves fitted in respective first and second flow line close to the point of connection between each of said first and second flow lines and respective second and third U-shaped pipe length prevents the injected pressurized gas from exerting any back pressure which affects the production of the well;
the injected press
Bagnell David
Kreck John
Nixon & Vanderhye PC
Petroleo Brasileiro S. A. - Petrobras
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