GB2400912A

GB2400912A - Indirect fluid pressure determination in extraction wells

Info

Publication number: GB2400912A
Application number: GB0309132A
Authority: GB
Inventors: David Webster
Original assignee: ABB Offshore Systems Ltd; Vetco Gray Controls Ltd
Current assignee: Baker Hughes International Treasury Services Ltd
Priority date: 2003-04-23
Filing date: 2003-04-23
Publication date: 2004-10-27
Also published as: GB0309132D0

Abstract

The pressure at a location in a fluid system is determined by supplying fluid to the system to increase system pressure and measuring system pressure when fluid flow between the fluid supply and the location is detected. The fluid system may be a sub-sea hydrocarbon extraction well where the pressure at a location on the seabed is measured remotely/indirectly from above sea level. Flow detectors 5 provided on the seabed are filled with hydraulic fluid and the pressure of this fluid is measured at the surface using a pressure gauge 10. The pressure at a node of the well is determined by increasing the fluid pressure until it exceeds that at the node, when fluid will then flow through the flow detector. Upon detection of flow the real pressure at the node is calculated by summing the hydraulic fluid pressure and the static fluid pressure (calculated from depth of well).

Description

24009 1 2 Fluid Pressure Determination This invention relates to fluid

pressure determination, for example to provide a back-up pressure measuring system for sub-sea fluid extraction wells such as hydrocarbon exkaction wells.

Fluid extraction wells, for example sub-sea hydrocarbon extraction wells, rely on pressure sensors to provide data to enable the operator to safely control the well. Typically these sensors will be of the quartz crystal or strain gauge type. They do occasionally fail, and when they do so the safety of the well and its operators is put at risk. In some cases the well has to be shut down, which not only loses revenue but also, particularly in the case of sub-sea wells, results in the need for expensive repair action. Such repair action is expensive because generally the pressure sensors are not retrievable from the well "tree" in which they reside. Some recorded failures of pressure sensors have been as a result of processing errors during the manufacture of the sensor, which means that the simple provision of a back-up pressure sensor does not necessarily solve the problem of losing pressure data.

It is an object of the present invention to provide a pressure measurement system which does not employ pressure sensors at the well.

According to a first aspect of the invention, there is provided a method for determining the pressure at a location in a fluid system, the method comprising the steps of providing a fluid supply means to supply fluid to the system to increase the pressure in the system, using flow detection means to detect when fluid is caused to flow at a point between the fluid supply means and the location, and using pressure measurement means to measure the pressure in the system when fluid flow is detected.

In a particular embodiment, the location is at a seabed installation and the pressure is measured above sea-level.

Preferably, a pressure transfer barrier is arranged between the location and the flow detection means.

The pressure at the location may be determined by taking the measured pressure in the system and adding to it the static pressure of the fluid between the pressure measurement means and the location.

Preferably, the static pressure is calculated using the height difference between the pressure measurement means and the location.

The pressure at a plurality of locations may be determined by measuring the pressure in the system in response to detection of flow at a respective plurality of points.

Preferably, a fluid pressure sensor is used also to directly measure the pressure at the location.

According to a second aspect of the present invention, there is provided apparatus for determining fluid pressure at a location in a fluid system, comprising a fluid supply for supplying fluid to the system, a flow sensor arranged to detect fluid flow between the fluid supply and the location and a pressure sensor arranged between the fluid supply and the flow sensor, such that as fluid is supplied to the system by the fluid supply, the fluid pressure at the location may be determined from the fluid pressure measured by the pressure sensor when the flow sensor detects fluid flow.

In a particular embodiment the location is at a seabed installation and the pressure sensor is above sea-level.

Preferably, a pressure transfer barrier is arranged between the flow sensor and the location.

The flow sensor may comprise a flap that responds to fluid flow to operate a switch.

Alternatively, the flow sensor may comprise a turbine which rotates in fluid flow.

Preferably, a plurality of flow sensors are employed such that the pressure at a plurality of respective locations may be determined.

Preferably, a fluid pressure sensor is also provided in the vicinity of the or each location to measure fluid pressure at the location.

In a further aspect of the invention there is provided a fluid control system comprising the aforementioned apparatus.

In a yet further aspect of the invention there is provided a hydraulic control system for a sub-sea well comprising the aforementioned apparatus.

The invention will now be described by way of example with reference to Figure 1, which shows a schematic diagram of an embodiment of the invention.

The figure shows a fluid extraction well, in particular a hydrocarbon extraction well, comprising a well tree 1 located in an installation on the seabed. A well hydraulic control system 2, located within the tree 1, has a multiplicity of pressure sensors fitted at nodes in the system where it is desirable to monitor pressures, either of the extracted fluid or of the control system hydraulics. A selection of these, generally those vital to safe control of the well, are connected to one side of pressure transfer barriers 3. Only one pressure transfer barrier 3 is shown in the figure for clarity. Each pressure sensor may be connected to a dual port with a single-in, dual- out adapter, which provides a port for the pressure sensor and an outlet node to connect to the pressure transfer barrier 3. The barrier 3 shown in sectioned view is a sealed cylinder with a sliding piston 4 inside, which is sealed to the wall of the cylinder.

The other sides of the pressure transfer barriers 3 are connected to respective flow detectors 5, which are mounted on a manifold 6 within a sub-sea control module (SCM) 7. The flow detectors 5 are filled with hydraulic fluid, typically water glycol, which is fed to them from hydraulic power unit (HPV) 8 via a pressure control valve 9, both of which are located at a surface facility. The pressure of the hydraulic fluid is measured at the surface facility by a pressure gauge 10. The hydraulic fluid is fed to the well tree, and hence the flow detectors 5, by an umbilical 11. The flow detectors 5 are simple switches, and are generally much more reliable than pressure switches. Each may comprise for example a flap that moves with a flow and operates a microswitch, or alternatively a turbine coupled to a rotation detector. The electrical output of each flow detector is connected to a sub-sea electronics module (SEM) 12 housed within the SCM 7. The electrical signal resulting from the detection of a flow can be added to the conventional telemetry data typically sent to the surface via umbilical 11. The detection of a flow is indicated at the surface on a display system 13.

To determine the pressure at a node in the well hydraulic control system 2, an operator increases the pressure of hydraulic fluid from the HPU 8 by operating pressure control valve 9.

When this pressure exceeds that at the node, fluid will flow from the HPU 8, through the respective flow detector 5, as the piston 4 moves in the pressure barrier 3 (to the left as shown in Figure 1). When the display system 13 indicates fluid flow through detector 5, the operator notes the pressure as shown on pressure gauge 10. In order to deduce the real pressure at the monitored node it is necessary to add the static pressure of the fluid in umbilical 11 to the pressure shown on pressure gauge 10. The static pressure can be easily calculated since the depth of the well from the sea surface and the specific gravity of the hydraulic fluid are both known. Alternatively, the pressure gauge 10 can be calibrated to take into account the static pressure of the hydraulic fluid in the umbilical, thus giving a direct measurement of the pressure at the monitored node. By controlling the valve 9, the hydraulic pressure in the system may be increased until there is flow detected at each detector 5, so that the pressure at each node can be determined.

Although this method of determining the pressure is typically less accurate than using fitted pressure sensors at the well tree, it provides a low-cost, high reliability back-up pressure measurement system which gives the well operator confidence that the well can continue to operate safely in the event of pressure sensor failures. Since the SCM 7 is replaceable using a remotely operated vehicle (ROV), further back-up is provided in the unlikely failure of a flow detector.

The above described embodiment is exemplary only, and other ways of carrying out the invention within the scope of the claims may be readily envisaged. For example, pressure gauge may be connected to the electronic control system, so that when flow is detected, the pressure shown by the gauge is automatically recorded. Any suitable flow detector or pressure transfer barrier may be used. Although the invention is shown with reference to a fluid extraction well, in particular a hydrocarbon extraction well, it may be used in any fluid system to determine pressure at a location therein.

Claims

Claims 1. A method for determining the pressure at a location in a fluid

system, the method composing the steps of providing a fluid supply means to supply fluid to the system to increase the pressure in the system, using flow detection means to detect when fluid is caused to flow at a point between the fluid supply means and the location, and using pressure measurement means to measure the pressure in the system when fluid flow is detected.
2. A method according to claim 1, wherein the location is at a seabed installation and the pressure is measured above sea-level.
3. A method according to claim 1 or claim 2, wherein a pressure transfer barrier is arranged between the location and the flow detection means.
4. A method according to any preceding claim, wherein the pressure at the location is determined by taking the measured pressure in the system and adding to it the static pressure of the fluid between the pressure measurement means and the location.
5. A method according to claim 4, wherein the static pressure is calculated using the height difference between the pressure measurement means and the location.
6. A method according to any preceding claim, wherein the pressure at a plurality of locations is determined by measuring the pressure in the system in response to detection of flow at a respective plurality of points.
7. A method according to any preceding claim, wherein a fluid pressure sensor is used to directly measure the pressure at the location.
8. Apparatus for determining fluid pressure at a location in a fluid system, comprising a fluid supply for supplying fluid to the system, a flow sensor arranged to detect fluid flow between the fluid supply and the location and a pressure sensor arranged between the fluid supply and the flow sensor, such that as fluid is supplied to the system by the fluid supply, the fluid pressure at the location may be determined from the fluid pressure measured by the pressure sensor when the flow sensor detects fluid flow.
9. Apparatus according to claim 8, wherein the location is at a seabed installation and the pressure sensor is above sea-level.
10. Apparatus according to claim 8 or claim 9, comprising a pressure transfer barrier arranged between the flow sensor and the location.
1 1. Apparatus according to any of claims 8 to 10, wherein the flow sensor comprises a flap that responds to fluid flow to operate a switch.
12. Apparatus according any of claims 8 to 10, wherein the flow sensor comprises a turbine which rotates in fluid flow.
13. Apparatus according to any of claims 8 to 12, comprising a plurality of flow sensors such that the pressure at a plurality of respective locations may be determined.
14. Apparatus according to any of claims 8 to 13, wherein a fluid pressure sensor is provided in the vicinity of the or each location to measure fluid pressure at the location.
15. A fluid control system comprising the apparatus of any of claims 8 to 14.
16. A hydraulic control system for a subarea well comprising the apparatus of any of claims 8 to 14.
17. A method substantially as hereinbefore described with reference to Figure 1.
18. Apparatus substantially as hereinbefore described with reference to Figure 1.