GB2369787A

GB2369787A - Subsea Separator

Info

Publication number: GB2369787A
Application number: GB0123414A
Authority: GB
Inventors: Donald Ferns
Original assignee: Kvaerner Oil and Gas Ltd
Current assignee: Kvaerner Oil and Gas Ltd
Priority date: 2000-09-29
Filing date: 2001-09-28
Publication date: 2002-06-12
Anticipated expiration: 2021-09-28
Also published as: GB2369787B; GB2369787A9; GB0023967D0; GB0123414D0

Abstract

A subsea separator, particularly for separating crude oil, comprises three compartments, the first compartment (12) having an upwardly leading pipe (15) to allow gas to be drawn off to the surface, a downwardly leading pipe to conduct aqueous liquids to the second compartment (17), and a weir (14) over which hydrocarbon liquids can flow into the third compartment (19); the second and third compartments both having pipes (18, 22) leading from a low point of the compartments to draw off aqueous liquids and hydrocarbon liquids respectively to the surface. There is provided a means to detect the fluid interface levels within the compartments remotely by an operator on the surface; these means may comprise vertically spaced bleed holes in the compartments, each bleed hole being connected to an umbilical leading to indicators on the surface. The compartments may be arranged in separate vessels or as discrete parts of a single vessel. There may also be provided a means to control the rate at which aqueous liquids are led to the surface.

Description

SUBSEA SEPARATOR

The invention relates to a subsea separator for regulating and separating different phases of a stream of mixed hydrocarbon fluids.

In particular the invention relates to a surge/slug catcher and separator for use on the seabed. The separator is intended to regulate flow along a pipeline; and to separate gas and liquid hydrocarbons, and water, from a stream of mixed hydrocarbon fluids (e. g. arising directly from a subsea well).

Isolated subsea wells produce crude oil directly from beneath the seabed. This crude oil may contain grains of sand, water, and a mixture of hydrocarbon fluids. As the crude oil is generally produced from regions of high pressure, significant amounts of gas may be dissolved in the oil. The crude oil is transported from the well or wells to processing facilities by pipeline.

Gaseous hydrocarbons may occupy some of the space in the pipeline. This space will depend upon the pressure of the gas and its flow rate. Liquids will occupy the remaining space. Problems may arise if there is a significant build-up of liquids in the pipeline (resulting from flowrate changes).

In order to regulate flow along the pipeline, it is desirable to have buffer storage zones in which surges of liquids can be allowed to build up at times of high flow rates. The build up of liquids can be drawn off from the buffer zones subsequently, when flowrates decrease.

In order to process the hydrocarbon fluids effectively, it is desirable to remove the sand and water, and to split the gaseous phase from the liquid phase near to the subsea well.

Subsea separators have been proposed to effect this removal and splitting.

One such proposal is described and illustrated in UK patent specification 2,242, 373. In this case a single separation vessel has weirs to trap sand and water in discrete compartments at the base of the vessel. By constant monitoring of a pressure indicator and a level indicator, it is possible to control flow valves so that the separation process is maintained in a state of equilibrium. In this case a microprocessor is arranged to control most routine and Emergency Shut Down (ESD) functions. Human intervention is required only to initiate and curtail production.

However, successful operation of this proposal would depend to some extent on the ability to detect and to control accurately the levels of fluid interfaces within the discrete compartments.

This could result in the effective regulation of liquid surges. However, the use of subsea instrumentation to detect pressure and level changes introduces complexity, maintenance requirements, and a risk of breakdowns.

The present invention is concerned with stabilising and reducing hold-ups to the flow of hydrocarbon fluids for processing in topsides facilities above the sea surface. It is also concerned with a subsea separator having no moving parts (and specifically requiring no subsea pumps), and no subsea instrumentation.

The invention provides a subsea separator comprising three compartments, the first compartment having an upwardly leading pipe to allow gas to be drawn off to the surface, a downwardly leading pipe to conduct aqueous liquids to the second compartment, and a weir over which hydrocarbon liquids can flow into the third compartment; the second compartment having a pipe leading from a low point in that compartment to allow aqueous liquid to be drawn off to the surface; and the third compartment having a pipe leading from a low point in that compartment through which hydrocarbon liquids can be led to the surface, in which there is means to detect fluid interface levels within the compartments and to display indications of those levels so that an operator can appropriately control the rates at which the gas and hydrocarbon liquids are led to the surface.

It is preferred that the means to detect the interface levels comprise vertically spaced bleed holes in the compartments, each bleed hole being connected to an umbilical leading to the surface, and the umbilical leading to indicators on the surface.

The compartments may be arranged as discrete vessels, or to be contained in two discrete vessels, or as discrete parts of a single vessel.

Preferably there is additional means to control the rate at which aqueous liquids are led to the surface.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross section of a subsea separator and storage unit; Figures 2a and 2b are diagrams of surface draw off arrangements; and Figure 3 is a diagrammatic view of a multi core umbilical leading to the surface.

A subsea separator 10 is arranged to accept fluids from an incoming pipeline 11. The pipeline leads up to a vapour/liquid gravity separator 12. The separator 12 has a weir 14 to trap liquids. Gas occupies the space above the liquids and beyond the weir. Gaseous hydrocarbons leave the separator 12 up a pipe 15 at the top of the separator. Liquid hydrocarbons pass over the weir 14 and flow out of the separator 12 through a pipe 16. Aqueous phase liquids (water and possibly glycol) pass down into a surge vessel 17 generally beneath the separator 12.

The aqueous phase surge vessel 17 is located below the vapour/liquid gravity separator 12. The vessel 17 is open to the separator 12 such that anyvapour or hydrocarbon liquid drawn down into the vessel 17 will float back up to the separator due to its lower density.

The vessel 17 is sized to contain the largest calculated aqueous phase liquid surge/slug expected from the incoming pipeline 11. If this maximum calculated volume is sufficiently small, and if space allows, the vessel 17 may be incorporated into the base of thevapour/liquid gravity separator 12. The aqueous liquids phase (e. g. water and any added glycol) settles to the bottom of the vessel 17, and is recovered from near the bottom of this vessel up a pipe 18 for subsequent treatment.

Any grains of sand in the crude oil are trapped behind the weir 14, and sink to the bottom of the separator 12 or the vessel 17.

Liquid hydrocarbons flowing out of the separator 12 pass through pipe 16 to a surge vessel 19. The hydrocarbon liquid surge vessel 19 is located below the vapourlliquid gravity separator 12. Hydrocarbon liquid from the separator 12 flows over the weir 14 or via a liquid overflow pipe arrangement (not shown) into the vessel 19. Thevapour space of this vessel 19 is in communication with the vapour space of the separator 12 via a vapour balance line 21.

The vessel 19 is sized for the maximum calculated liquid surge/slug (both aqueous phase and hydrocarbon liquids phase combined) which is expected from the pipeline 11. If this maximum calculated volume is sufficiently small, and if space allows, the vessel 19 may be incorporated into the vapour/liquid gravity separator 12. The hydrocarbon liquids phase collected in the vessel 19 is recovered from near the bottom of this vessel up pipe 22 for subsequent treatment.

If the vessel 17 is full of liquid hydrocarbons but vessel 19 is empty, there could be an aqueous phase surge which would flood a volume of aqueous phase liquid into vessel 19, followed by a hydrocarbon liquid surge which would add still more liquid to the vessel 19 The subsea separator 10 is applicable for regulating the flow and controlling the recovery of liquids from a pipeline 11 via a collection vessel 19 to surface facilities (25). To this end the subsea separator incorporates collection vessel 19 below the point of use of the liquids (e. g. the collection vessel may be located in subsea slug catchers or underground storage vessels).

Pressure from the crude oil source drives the liquids to the surface without the use of pumps.

In particular, this separator may be applicable to situations where both a vapour phase and a liquid phase are to be recovered to the same pressure destination from the collection vessel ; and where the pressure of the vapour phase is greater than the hydrostatic head of the liquid phase from the collection vessel to the point of use.

Figure 2a shows how the liquid and vapour streams emerging from the pipes 15 and 22 (leading up from the respective collection vessels) are dealt with at the sea surface. Pipe 22a transfers liquid, and pipe 15a transfers gas. The liquid flow controller 23 regulates the flow of liquid, and a valve 24 controls the gas pressure, so that the streams of liquid and gas can be recombined at 25.

Figure 2b shows a modification for a situation in which the liquid flow rate is to be set as a specific proportion of the vapour flow rate. In this case a sensor 26 in the vapour pipe 15b regulates a ratio controller 27 to achieve the required proportional flow rate.

Levels of the various fluids in the different vessels (or in different parts of a single vessel) are detected through bleed holes in the walls of the vessel (s). An arrangement for detecting levels is shown in Figure 3. A multi core umbilical 30 has multiple micro-pipelines 31 leading from bleed holes 32 in a seabed vessel (e. g. vessels 12,17 or 19) to valves 33 above the sea surface. In this particular example there are six micro-pipelines. The level of liquid/vapour interface within the vessel can be determined as between the lowest bleed of gas and the highest bleed of liquid.

Claims

Claims

1/A subsea separator comprising three compartments, the first compartment having an upwardly leading pipe to allow gas to be drawn off to the surface, a downwardly leading pipe to conduct aqueous liquids to the second compartment, and a weir over which hydrocarbon liquids can flow into the third compartment; the second compartment having a pipe leading from a low point in that compartment to allow aqueous liquid to be drawn off to the surface; and the third compartment having a pipe leading from a low point in that compartment through which hydrocarbon liquids can be led to the surface, in which there is means to detect fluid interface levels within the compartments and to display indications of those levels so that an operator can appropriately control the rates at which the gas and hydrocarbon liquids are led to the surface.

2/A separator as claimed in claim 1, in which the means to detect the interface levels comprise vertically spaced bleed holes in the compartments, each bleed hole being connected to an umbilical leading to the surface, and the umbilicals leading to indicators on the surface.

3/A separator as claimed in claim 1 or claim 2 in which the compartments are arranged as discrete vessels.

4/A separator as claimed in claim 1 or claim 2 in which the compartments are arranged to be contained in two discrete vessels.

5/A separator as claimed in claim 1 or claim 2 in which the compartments are arranged as discrete parts of a single vessel.

6/A separator as claimed in any one of the preceding claims in which there is additional means to control the rate at which aqueous liquids are led to the surface.

7/A separator substantially as hereinbefore described by way of example with reference to and as shown in Figures 1 and 3, and Figure 2a or Figure 2b of the accompanying drawings.

Amendments to the claims have been filed as follows

1/A subsea separator for use on a seabed and comprising three compartments, the first compartment having an upwardly leading pipe to allow gas to be drawn off to the surface of the sea, a downwardly leading pipe to conduct aqueous liquids to the second compartment, and a weir over which hydrocarbon liquids can flow into the third compartment; the second compartment having a pipe leading from a low point in that compartment to allow aqueous liquid to be drawn off to the surface; and the third compartment having a pipe leading from a low point in that compartment through which hydrocarbon liquids can be led to the surface, in which there is means to detect fluid interface levels within the compartments without the need for instrumentation and control systems, and to display indications of those levels above the surface so that an operator can appropriately control the rates at which the gas and hydrocarbon liquids are led to the surface.

2/A separator as claimed in claim 1, in which the means to detect the interface levels comprise a-series of bleed holes at different heights in each of the individual compartments, each bleed hole being connected to an umbilical leading to the surface, and the umbilical leading to indicators on the surface.

1 3/A separator as claimed in claim 1 or claim 2 in which the compartments are arranged as discrete vessels.

4/A separator as claimed in claim 1 or claim 2 in which the compartments are arranged to be contained in two discrete vessels.

5/A separator as claimed in claim 1 or claim 2 in which the compartments are arranged as discrete parts of a single vessel.

6/A separator as claimed in any one of the preceding claims in which there is additional means to control the rate at which aqueous liquids are led to the surface.

7/A separator substantially as hereinbefore described by way of example with reference to and as shown in Figures 1 and 3, and Figure 2a or Figure 2b of the accompanying drawings.