Shutting down an underwater fluid production well

Field of the Invention

This invention relates to shutting down an underwater fluid production well.

Background of the Invention

When electric power is lost to an underwater fluid production well (for example an underwater hydrocarbon production well), the well shuts down. Currently, on subsea control modules with electrically operated hydraulic dump valves, all valves close instantly, when electrical power is lost, which can result in damage to the surface controlled sub-surface safely valve, because fluid was flowing at the time that this valve was closed. A solution to this problem is to close the low pressure valves first, thus shutting off the production fluid flow, before closing the high pressure valves, but such a sequence cannot be controlled without complex hydraulic sequencing. Currently, for subsea oil wells located at short distances from the topside system, the problem can be solved by venting the umbilical cable of low pressure hydraulic supply, followed by the high pressure supply, and for longer offset solutions, complex hydraulic sequencing has been employed using flow restrictors to attempt to hold the high pressure system pressure up for longer than the low pressure system.

US 2011/0270431 discloses a system for controlling production shut down of an underwater fluid production well, the well having a sensor for producing an output signal indicative of the state of the well and a valve which is actuable to shut down production activity of the well, the system comprising means for receiving the output signal; and a processor for processing the received signal to determine if a shut down is required and outputting a shut down signal to the valve; wherein the receiving means and processor are located at the well.

As further prior art there may be mentioned US5335730, which discloses the precharacterising features of claim 1.

Summary of the Invention

According to this invention from one aspect, there is provided a production control system for an underwater well, comprising:

• first electrically operated means for supplying first hydraulic fluid, for opening a first control valve of the well;
• second electrically operated means, for supplying second hydraulic fluid at a higher pressure than said first fluid, for opening a further control valve of the well;
• electronic circuitry for providing electrical power for operating said first and second means; and
• means for controlling the sequence of closing said control valves as a result of a loss of electrical power from said electronic circuitry, said controlling means comprising:

  • electrical power storage means;
  • detection means responsive to said loss of power from said electronic circuitry; and
  • means coupled with said detection means for using electrical power from said storage means to keep said further control valve open for a period after closure of said first control valve and close it after said period,

characterised in that

said electronic circuitry comprises at least one subsea electronics module (SEM) in a subsea control module at a tree of the well, and

the electrical power storage means and said means coupled with said detection means are in said subsea control module.

Typically, said electrical power storage means is charged by electrical power from said electronic circuitry.

Said first electrically operated means could comprise a first directional control valve, said second electrically operated means comprising a second directional control valve.

Typically, said detection means comprises means responsive to the pressure of hydraulic fluid supplied from said first electrically operated means.

Said first control valve typically comprises a production fluid control valve.

Said further control valve typically comprises a surface controlled sub-surface safety valve.

The system could be such that, in response to closure of said first control valve, first hydraulic fluid is vented therefrom and, in response to closure of said further control valve, said second hydraulic fluid is vented therefrom. In this case, said first hydraulic fluid could be supplied to said first electrically operated means from a directional control valve, via which venting of that fluid from said first control valve occurs, said second hydraulic fluid being supplied to said second electrically operated means from another directional control valve, via which venting of that fluid from said further control valve occurs.

According to this invention from another aspect, there is provided a method of shutting down a production control system for an underwater well, the system comprising:

• first electrically operated means for supplying first hydraulic fluid, for opening a first control valve of the well;
• second electrically operated means, for supplying second hydraulic fluid at a higher pressure than said first fluid, for opening a further control valve of the well;
• electronic circuitry for providing electrical power for operating said first and second means;
• electrical power storage means;
• detection means responsive to said loss of power from said electronic circuitry; and
• means coupled with said detection means for using electrical power from said storage means to keep said further control valve open for a period after closure of said first control valve and close it after said period, the method comprising:

  • controlling the sequence of closing said control valves as a result of a loss of electrical power from said electronic circuitry by, in response to said loss of power from said electronic circuitry, using electrical power from said storage means to keep said further control valve open for a period after closure of said first control valve and close it after said period,

characterised in that

said electronic circuitry comprises at least one subsea electronics module (SEM) in a subsea control module at a tree of the well, and

the electrical power storage means and said means coupled with said detection means are in said subsea control module.

Brief Description of the Drawing

Fig. 1 illustrates diagramatically the relevant parts of a well control system according to an embodiment of this invention.

Detailed description of the Invention

Referring to Fig. 1, two subsea electronics modules (SEMs) 1 and 2 are housed within a subsea control module mounted on a typical well tree. An additional sequenced shutdown module 3 external to the modules 1 and 2 (but internal to the subsea control module in a pressure isolated vessel), contains a rechargeable battery 4 and an electronic printed circuit card 5. The latter carries charging circuitry 6 to charge a battery 4, which is connected to a power supply unit (PSU) 7, which powers a central processor unit (CPU) 8 which includes a flash memory, interfaces suitable for accepting inputs from pressure switches 9, 10 and 11 and an interface to a line 12 for providing an electrical signal to open a directional control valve (DCV) 13. The battery 4 is charged by charging circuitry 6 from either subsea electronics module 1 or subsea electronics module 2 via a supply line (typically 24V) 14 or 15, the battery 4 also supplying power to the power supply unit 7 under normal operating conditions.

Reference numerals 16 and 17 designate hydraulically latched directional control valves for supplying low pressure hydraulic power from a low pressure consolidated (LPC) source to a production master valve (PMV) 18 and a production wing valve (PWV) 19 respectively, the hydraulic pressures at the outputs of valves 16 and 17 being detected by pressure switches 9 and 10 respectively. Directional control valves 16 and 17 are opened by respective electrical enabling pulses on lines 20A and 21A from subsea electronics module 1 or lines 20B and 21B from subsea electronics module 2 and in normal operation are thereafter hydraulically latched.

Reference numeral 22 designates a hydraulically latched directional control valve for supplying high pressure hydraulic power from a high pressure consolidated (HPC) source to a surface controlled sub-surface safety valve (SCSSV) 23, hydraulic pressure at the output of the valve 22 being detected by pressure switch 11. In normal operation, directional control valve 22 is opened by an electrical enabling pulse on a line 24A from subsea electronics module 1 or a line 24B from subsea electronics module 2, thereafter remaining hydraulically latched.

Reference numeral 25 designates a directional control valve for supplying low pressure hydraulic fluid from the low pressure consolidated source to valves 16 and 17, in normal operation it being kept open by an electrical signal on a line 26A from subsea electronics module 1 or a line 26B from subsea electronics module 2 and in its closed position venting fluid from the source to a low pressure (LP) return. Directional control valve 13, when open, supplies hydraulic power from the high pressure consolidated source to valve 22, in normal operation it being kept open by an electrical signal from either a line 27 from subsea electronics module 1 or a line 28 from subsea electronics module 2 via the central processor unit 8 and line 12 from the latter. In its closed position, valve 13 vents hydraulic fluid from the high pressure consolidated source to a high pressure (HP) return.

During normal operation, control of closing the low pressure operated production fluid flow valves (i.e. the production master valve 18 and the production wing valve 19) is effected from either subsea electronics module 1 or subsea electronics module 2 by control of the hydraulically latched valves 16 and 17 and the valve 25. The latter switches the hydraulic power supply for the valves 18 and 19 from the low pressure consolidated hydraulic power source to the low pressure return. A transition from low to high of the electrical signal on line 27 from subsea electronics module 1 or line 28 from subsea electronics module 2 causes valve 13 to be opened, enabling high pressure consolidated hydraulic power to the hydraulically latched valve 22 which can then be controlled by either the subsea electronics module 1 or subsea electronics module 2 in the normal manner, i.e. via line 24A or 24B. Transition from high to low of the electrical signal from either subsea electronics module 1 or subsea electronics module 2, whilst their electric power is still available, will result in the valve 13 being driven to the closed or vent position, i.e. allowing venting of the hydraulic actuator of the valve 23. Note that the consolidated low pressure and high pressure hydraulic sources result from separate twin sources which are consolidated within the subsea control module.

In the event of electric power failure to both subsea electronic modules 1 and 2, the timed sequence of the shutdown module 3 comes into operation, powered by the battery 4. At power loss to subsea electronic modules 1 and 2, the valves 16, 17 and 25 will close to their venting positions allowing the production fluid valves 18 and 19 to close and vent to the low pressure return. The surface controlled sub-surface safety valve 23 will remain open, since the directional control valve 22, being hydraulically latched since the venting directional control valve 13 remains powered (from the central processor unit 8 under power from battery 4), prevents hydraulic fluid venting from the valve 23. After an initial period, if the pressures to which the pressure switches 9 and 10 respond have fallen below a threshold set in the logic in the central processor unit 8, thus indicating that the production fluid flow valves 18 and 19 are closed, the directional control valve 13 is closed by the central processor unit 8, allowing the valve 23 to vent to the high pressure return and thus close without damage, as the production flow has been previously stopped. The pressure switch 11 provides confirmation to the central processor unit 8 of the status of the valve 23. After a second time period, the directional control valve 13 is closed irrespective of the responses from the pressure switches 9, 10 and 11, as a safety precaution. Typically the sequence is generated by the central processor unit 8 from software stored in its flash memory, which could also measure and report the charge state of the battery 4. In order to provide a secure implementation of the control loop, the electronics and software within the shutdown module 3 are designed with the target of achieving SIL1 rating.

The embodiment of this invention could be varied using valve position detectors or pressure transducers rather than pressure switches. Dual batteries could be used, one being charged while the other is ready to use. Depending on system requirements, valve 13 could be a hydraulically latched directional control valve to reduce power consumption.

Advantages of using the Invention

An advantage is that potential damage to a surface controlled sub-surface valve can be prevented by the controlled shut down on electric power failure to the well, this being particularly applicable with oil field developments which are located at a long offset from the topside control system.