GB2318306A

GB2318306A - Crude oil separator

Info

Publication number: GB2318306A
Application number: GB9621751A
Authority: GB
Inventors: Frank Mohn
Original assignee: Framo Developments UK Ltd
Current assignee: Framo Developments UK Ltd
Priority date: 1996-10-18
Filing date: 1996-10-18
Publication date: 1998-04-22
Also published as: GB9621751D0

Abstract

A separator for separating crude oil from an oil field production mixture comprises a separator tank 2, a degasser 3 and storage tank 20. The separator tank 2 and degasser 3 substantially reduce the pressure of the oil mixture emanating from a production manifold 1 before passing it onto the storage tanks 20. Under reduced pressure the oil passes into the storage tanks 20 where it separates further into its constituent parts under the effects of gravity for one to three days. The crude oil mixture in the separator tank 2 partly separates into its constituent parts comprising of oil, water and gas under the action of gravity. The oil is drawn out and fed into the degassing chamber 3 where the pressure of the oil is reduced to 1.5 bar. The water is drawn out from the bottom of the separator tank 2 and then cleaned by a production water cyclone before being dumped. Gas from the separator tank 2 and also from degasser 3 is pressurised by compressors 8, 9 and then used in the extraction process by being fed back through the production manifold 1. The water is pumped out of tanks 20 via lines 24 and subsequently the separated oil is also pumped out along line 24 oil to export line 17. The separation apparatus is located on a floating oil production and storage vessel.

Description

METHOD OF SEPARATION DESCRIPTION The present invention relates to a method of separation, particularly to a method for separating oil and water which is applicable to a floating production and storage vessel as is currently used in offshore oil fields in the recovery and transport of newly extracted crude oil.

Traditionally crude oil extracted from below the sea bed has a high water content as a result of the recovery process and the environment in which it is extracted.

Production water will be mixed with the oil. The crude oil/water is transferred onto a vessel which is specially adapted to begin purifying the oil by extracting at least some of the extraneous water. Such vessels effect this processing so that they off-load water free refined product at the shore or onto the next transport vessel. This is in order to achieve the correct specification for sales oil.

This system is known as Floating Production and Storage and is often referred to by the acronym FPSO.

The separation of the large proportion of water which is mixed with the extracted oil is usually done by means of a series of separator tanks on the FPSO vessel deck, through which the crude oil is passed one after the other. The mixture is retained in each tank for only a short time (typically between 3 and 15 minutes) and it separates progressively more into its constituent parts, oil, water and gas. Typically the pressure in successive tanks is reduced to facilitate the process. There are normally three such separator stages and it is possible to achieve up to 99.5% water free oil mixture with this arrangement. The separated water is then cleaned and dumped overboard. Other pre-refining steps may also be taken at this stage, for example, excess gases may be removed from the mixture. These gases tend also to separate from the crude oil in the separation tanks and are pumped off through safety valves.

Although it is efficient at oil-water separation, such a series of separator modules is extremely expensive and occupies almost the total deck space on a FPSO vessel.

Maintenance of the separator modules is also expensive.

It is an object of the present invention to provide a simplified system for separation of the oil and water aboard such vessels and to allow for such vessels to be constructed and maintained more cheaply. Apparatus and a method are envisaged.

According to one aspect of the present invention there is provided apparatus for separating crude oil and water comprising a storage tank, means for feeding a crude oil mixture from production in the field into the storage tank, means, at or upstream of the feeding means, for reducing the pressure of the crude oil production mixture, means for pumping separately the separated water and the crude oil mixture out of the storage tank.

Preferably a single pre-separator module is provided upstream of the storage tank. The pre-separator module may comprise a separating tank, means for feeding crude oil from production in the field, into the separating tank means for retaining the crude oil in the separating tank for a predetermined period of time at a predetermined pressure, and means for feeding the crude oil from the separating tank into the vessel storage tank for substantive separation of water from the crude oil mixture.

According to a preferred embodiment the pre-separating module comprises means for reducing the pressure of the crude oil passing through the pre-separator module before the oil reaches the storage tank. The pressure may be reduced by around a factor of ten, as a specific example, the pressure might be reduced from 15 bar to 1.5 bar although it will be understood that many suitable examples of pressure and the required drop may be envisaged and the numerical values for any particular embodiment of the invention will be dependent on many factors.

Additionally means may be provided in the pre-separator module for drawing off separated water from the separating tank and cleaning the drawn off water.

According to a particularly preferred embodiment of the invention, means are provided for re-using the separated gas from the production mixture, for example by re-injecting it through the production manifold. This has the advantage of reducing the volume of new quantities of gas required for production, in fact no new gas is generally necessary and this is environmentally more friendly.

The apparatus of the present invention is particularly advantageously incorporated into a vessel and particularly a Floating Production and Storage Vessel.

According to a second aspect of the present invention there is provided a method of separating crude oil from water in a field production mixture comprising feeding the production mixture into a storage tank, allowing the water and oil to separate by gravity over a long period for example of at least 24 hours and preferably 48 hours or 72 hours and subsequently pumping separated water out of the bottom of the storage tank prior to pumping separated oil out of the tank.

Thus the method according to the second aspect of the present invention comprises allowing separation to occur in a main storage tank of an oil production vessel and pumping separated water out of the storage tank, before off loading the separated oil mixture.

The method for separating crude oil and water may comprise feeding a crude oil mixture from production into a vessel storage tank, and, upstream of the storage tank, reducing the pressure of the production mixture and subsequently' pumping separately the separated water and the crude oil mixture out of the storage tank.

Preferably the production crude oil-water mixture passes through a single pre-separator stage before being fed into the storage tank. In the pre-separator the pressure of the mixture may be reduced for example in a de-gasser which extracts gas from the mixture. The de-gasser may also include means for injecting gas from the general tank ventilation system. Preferably the tank ventilation system is a closed system with no external atmospheric vent and thus no hydrocarbon emission to atmosphere occurs and no atmospheric oxygen is introduced into the tanks. The introduction of oxygen into tanks containing inflammable materials such as oil, is of course extremely dangerous. It was previously avoided using inert gas to mask the mixture in the tanks but this necessitated inert gas generating plants on board vessels and this was expensive and space consuming.

The pre-separator stage retains the oil-water mixture for only a very short time, e.g. between 3 and 13 minutes.

The oil mixture in the storage tank may be drawn off in a continuous operation and the pressure in the storage tank monitored. Valves can be used to switch the flow of oil mixture being exported from the storage tank, into an overflow such as a sloop.

For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made to the accompanying drawing in which the single figure is a schematic representation of the apparatus and of the method of the present invention.

In the figure the production manifold is shown at 1.

A production mixture of oil and gas and water is pumped up from below the sea bed and is fed into a pre-separation module on the vessel deck. The pre-selection module comprises a separator tank 2 and a degasser 3 together with appropriate valves and monitors. The mixture is retained in the separator tank 2 for a short period, which may typically be between 3 and 13 minutes at a pressure of 15 bar. During this time the mixture will separate partly into its constituents under the action of gravity. Thus the contents of the separator tank 2 are shown as layers of water (at the bottom), production oil mixture on top of the water, and gas on top of the oil. After settlement in this separator tank 2 the oil mixture is drawn out and fed into a degassing chamber 3 via a variable pressure reduction valve 4. The pressure of the mixture in the degassing chamber 3 is reduced to 1.5 bar.

Water from the bottom layer of the separator tank 2 is drawn off and cleaned by a production water cyclone 5.

Cleaned water is dumped overboard as indicated by arrow 6 and typically has only about 40 ppm impurities. Some oil from the cleaned water is re-injected into the oil mixture along line 7.

Gas from the top of the separator tank 2 and from the top of the de-gassing chamber 3 is drawn off and is pressurised by compressors 8 and 9 on hydraulic line 10.

This is used again in the extraction process by being fed back through the production manifold 1.

Motors 12 and 13 drive respective compressors 8 and 9.

Oil mixture from the de-gassing chamber 3 is fed via production coolers 14 and via line 16 to the cargo tanks of the vessel incorporating this system.

An injector 19 takes gas from the tank ventilation line 15 and from the hydraulic line 10 and injects it into the degassing chamber 3 to selectively adjust the pressure in the de-gassing chamber 3.

Under reduced pressure the oil mixture passes into the cargo storage tanks 20 where it separates further into its constituents under the effects of gravity. The layers of water, oil mixture and gas are shown in the figure. An outlet is provided from the upper region of the storage tanks 20 and is connected to the tank ventilation line 15 as indicated at 21. The production oil mixture from the preseparation module, is fed via lines 16 and inlet ducts 22 into the cargo tanks 20. There the mixture is allowed to settle for up to 3 days. In due course the outlets 23 are opened and the water which has separated to the bottom of the tanks is pumped out to the sloop tank 25 via lines 24 to sloop line 18 which are connected via valves 26 and ultimately discarded. Subsequently the separated oil, now at least 98% and preferably 99.5% free of water, is pumped out along lines 24 onto the oil export line 17 to be transferred to another vessel or container.

A system such as this can remove up to 99.5% of the water from the production oil mixture.

Claims

1. Apparatus for separating crude oil and water, the apparatus comprising a storage tank, means for feeding a crude oil mixture from production in the field into the storage tank, means, at or upstream of the feeding means, for reducing the pressure of the crude oil production mixture, means for pumping separately the separated water and the crude oil mixture out of the storage tank.

2. Apparatus according to claim 1 further comprising a pre-separator module arranged upstream of the storage tank.

3. Apparatus according to claim 2, wherein the preseparator module comprises a separating tank, means for feeding crude oil, from production in the field, into the separating tank means for retaining the crude oil in the separating tank for a predetermined period of time at a predetermined pressure, and means for feeding the crude oil from the separating tank into the vessel storage tank for substantive separation of water from the crude oil mixture.

4. Apparatus according to claim 2 or claim 3 wherein there is only one pre-separator module.

5. Apparatus according to any one of claims 2 to 4, wherein the pre-separator module comprises means for reducing the pressure of the crude oil passing through the pre-separator module before the oil reaches the storage tank.

6. Apparatus according to claim 5, wherein the pressure reducing means is operable to reduce the pressure by about a factor of ten.

7. Apparatus according to any one of claims 3 to 6 further comprising means provided in the pre-separator module for drawing off separated water from the separating tank and means for cleaning the drawn off water.

8. Apparatus according to any preceding claim wherein means are provided for re-using the separated gas from the production mixture.

9. Apparatus according to claim 8, wherein said reusing means comprise means for injecting the separated gas through the production manifold.

10. A floating oil production and storage vessel comprising apparatus according to any preceding claim.

11. A method of separating crude oil from water in a field production mixture, the method comprising the steps feeding the production mixture into a storage tank, allowing the water and oil to separate by gravity over a long period for example of at least 24 hours and preferably 48 hours or 72 hours and subsequently pumping separated water out of the bottom of the storage tank prior to pumping separated oil out of the tank.

12. A method of separating crude oil from water in a field production mixture comprising allowing separation to occur in a main storage tank of an oil production vessel and pumping separated water out of the storage tank, before offloading the separated oil mixture.

13. A method for separating crude oil and water according to claim 11 or 12 comprising the steps of feeding a crude oil mixture from production into a vessel storage tank, and, upstream of the storage tank, reducing the pressure of the production mixture and subsequently pumping separately the separated water and the crude oil mixture out of the storage tank.

14. A method according to any one of claims 11 to 13, wherein the production crude oil-water mixture passes through a single pre-separator stage before being fed into the storage tank.

15. A method according to claim 14 wherein in the preseparator stage the pressure of the mixture is reduced in a de-gasser which extracts gas from the mixture.

16. A method according to claim 15 wherein the may also includes means for injecting gas from a general tank ventilation system.

17. A method according to claim 16, wherein the tank ventilation system is a closed system with no external atmospheric vent.

18. A method according to claim 14, 15, 16 or 17 wherein the pre-separator stage retains the oil-water mixture for a time period not exceeding 13 minutes.

19. A method according to claim 18 wherein the time period is between 3 and 13 minutes.

20. A method according to any preceding claim wherein the oil mixture in the storage tank is drawn off in a continuous operation and the pressure in the storage tank is monitored.

21. A method according to claim 20 wherein the oil mixture being exported from the storage tank, is switched by one or more valves into an overflow sloop.

22. Apparatus or a method substantially as hereinbefore described with reference to the single Figure of the current application.