FIELD

This invention relates to hydraulic circuit and a method of supplying hydraulic fluid. In one embodiment, it relates to a method of supplying hydraulic fluid to an underwater hydrocarbon extraction facility.

BACKGROUND

It is desirable to have the highest possible safety standards in underwater hydrocarbon extraction facilities. To achieve this, all components should have, wherever possible, a fail-safe operation wherein a loss of electrical or hydraulic power to the facility will result in the shut down of the flow of production fluid from the facility or any hydrocarbon wells within the facility.

Prior art hydraulic circuits make use of only a single directional control valve (DCV) between the input and output. This arrangement means that prior art subsea control modules (SCMs) using such circuits have a single point of failure, and consequently prior art SCMs cannot be given a safety integrity level (SIL) of greater than SIL2.

Additionally, in prior art SEMs if it is desired to vent the hydraulic fluid from the hydraulic circuit, the use of a single DCV and a single vent line could cause the venting operation to take quite a long time.

To overcome some of the problems associated with prior art hydraulic circuits the present invention provides a hydraulic circuit wherein the energisation of two directional control valves (DCV) is required in order for a hydraulic output to be provided with hydraulic fluid. De-energisation of either of the two DCVs results in the venting of hydraulic fluid from the circuit, and so any tree valves supplied by the circuit will revert to their fail-safe positions, stopping the flow of production fluid in the tree and safely isolating the tree. Each DCV is SIL2 capable, and so the added level of redundancy increases the hardware fault tolerance such that each SCM incorporating the circuit of the present invention is SIL3 capable on its own. The present invention also makes use of two separate vent lines, and so the venting operation when both DCVs are de-energised is quicker than systems which use a single vent line.

While the hydraulic circuit of the present invention is particularly suitable for a subsea control module, it could be used anywhere in a subsea control system where isolation valves are important for safety and / protection of the environment.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention there is provided a hydraulic circuit comprising a hydraulic input, a hydraulic output, a first directional control valve arranged between the hydraulic input and hydraulic output, and a second directional control valve arranged between the hydraulic input and hydraulic output in series with the first directional control valve, wherein the first directional control valve has a first position in which it inhibits the passage of hydraulic fluid from the hydraulic input to the hydraulic output and a second position in which it permits the passage of hydraulic fluid, the second directional control valve has a first position in which it inhibits the passage of hydraulic fluid from the hydraulic input to the hydraulic output and a second position in which it permits the passage of hydraulic fluid, and wherein the first and second directional control valves each require an energy input to be maintained in their respective second positions.

In accordance with a second aspect of the present invention there is provided a method of supplying hydraulic fluid from a hydraulic input of a hydraulic circuit to a hydraulic output of the hydraulic circuit, the method comprising the steps of: providing a first directional control valve arranged between the hydraulic input and hydraulic output; providing a second directional control valve arranged between the hydraulic input and hydraulic output in series with the first directional control valve, wherein the first directional control valve has a first position in which it inhibits the passage of hydraulic fluid from the hydraulic input to the hydraulic output and a second position in which it permits the passage of hydraulic fluid, the second directional control valve has a first position in which it inhibits the passage of hydraulic fluid from the hydraulic input to the hydraulic output and a second position in which it permits the passage of hydraulic fluid, and wherein passage of hydraulic fluid from the hydraulic input to the hydraulic output and a second position in which it permits the passage of hydraulic fluid, and wherein the first and second directional control valves each require an energy input to be maintained in their respective second positions; the first and second directional control valves each require an energy input to be maintained in their respective second positions; energising the first and second directional control valves; and supplying hydraulic fluid from the hydraulic input to the hydraulic output.

The hydraulic circuit could further comprise a first vent line, wherein the first directional control valve causes hydraulic fluid from the hydraulic circuit to vent via the first vent line in its first position. The hydraulic circuit could further comprise a second vent line, wherein the second directional control valve causes hydraulic fluid from the hydraulic circuit to vent via the second vent line in its first position. The hydraulic circuit could comprise a return loop that runs from the hydraulic output to a point intermediate the first and second directional control valves. At least one of the first and second vent lines could include a flow meter.

The hydraulic circuit could form part of a control arrangement for an underwater hydrocarbon extraction facility. In this case, the hydraulic circuit could be received within a subsea control module. The hydraulic input could be a hydraulic line in an umbilical. The hydraulic output could operate at least one Christmas tree valve. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 schematically shows a hydraulic circuit in accordance with a first embodiment of the present invention in a first configuration; Fig. 2 schematically shows the hydraulic circuit of Fig. 1 in a second configuration;

Fig. 3 schematically shows the hydraulic circuit of Fig. 1 in a third configuration; and

Fig. 4 schematically the hydraulic circuit of Fig. 1 in a fourth configuration. DETAILED DESCRIPTION

A hydraulic circuit 1 according to an embodiment of the invention is schematically shown in Fig. 1. The hydraulic circuit 1 runs from a hydraulic input A to a hydraulic output B. The embodiment shown in Figs. 1-4 is for use in a subsea control module (not shown) of an underwater hydrocarbon extraction facility, and so the hydraulic input A is received from a hydraulic supply line in an umbilical cable which runs from a surface location to the sea bed. The hydraulic output B leads to a plurality of valves in a Christmas tree at the wellhead of an underwater hydrocarbon well. The hydraulic circuit 1 includes a first directional control valve (DCV) 2 and a second DCV 3 in series with the first DCV 2. Both DCVs 2, 3 are shown in their open positions in Fig. 1. As can be seen, in this position both DCVs 2, 3 permit the passage of hydraulic fluid from the hydraulic input A to the hydraulic output B.

The DCVs 2, 3 are biased by a spring into their closed positions. This means that in order for the DCVs to be maintained in their respective open positions energy must be input. Hereafter, the open position will be referred to as the energised position and the closed position as the de-energised position. However, it should be noted that a DCV can become stuck in the open position without being energised due to a fault, e.g. jamming. The hydraulic circuit 1 also includes a return loop 4. After the final branch off for tree valves at the hydraulic output B the hydraulic circuit loops back upon itself to a point in between the first DCV 2 and the second DCV 3. The return loop 4 includes a check valve 5, which ensures that hydraulic fluid does not flow the wrong way around the return loop 4 (i.e. clockwise as pictured in Fig. 1) when hydraulic fluid is being provided from the hydraulic input A to the hydraulic output B.

Fig. 2 schematically shows the hydraulic circuit of Fig. 1 in a second configuration. Like reference numerals have been retained where appropriate.

In Fig. 2, the first DCV 2 has become de-energised (for example, due to an interruption in electrical power to the subsea control module (not shown) containing the hydraulic circuit 1). It has therefore moved from a second, energised position, shown in Fig. 1, to a first, de-energised position, shown in Fig. 2.

In the second position, the first DCV 2 blocks the passage of hydraulic fluid from the hydraulic input A to the hydraulic output B. Additionally, it connects the return loop 4 to a vent line 6. The vent line 6 runs from the hydraulic circuit 1 to the sea D.

As the check valve 5 prevents hydraulic fluid from passing the wrong way, i.e. clockwise, around the return loop 4, hydraulic fluid can only flow anti-clockwise around the return loop 4 and out to sea through the vent line 6. As this occurs, hydraulic power to the hydraulic output B is lost, and so any tree valves being supplied by the hydraulic output B return to their fail-safe positions, stopping the flow of production fluid in the tree (not shown).

The vent line 6 includes a flow meter 7. By monitoring the flow of fluid in the vent line it can be determined whether or not the first DCV 2 is performing as intended. If fluid is detected flowing in the vent line 6, the flow meter 7 may trigger an alarm for a topside well operator who can perform further investigation on the hydraulic circuit 1 and the DCVs 2, 3 to see if any repairs are required.

The vent line 6 also includes a pair of check valves 8, 9 which ensure that hydraulic fluid may only pass from the hydraulic circuit 1 to the sea D, and prevent the ingress of sea water to the hydraulic circuit 1.

Fig. 3 schematically shows the hydraulic circuit of Fig. 1 in a third configuration. Like reference numerals have been retained where appropriate.

In Fig. 3, the second DCV 3 has become de-energised (for example, due to an interruption in electrical power to the subsea control module (not shown) containing the hydraulic circuit 1). It has therefore moved from a second, energised position, shown in Fig. 1, to a first, de-energised position, shown in Fig. 3.

In the second position, the second DCV 3 blocks the passage of hydraulic fluid from the hydraulic input A to the hydraulic output B. Additionally, it connects the hydraulic output B to a vent line 10. The vent line 10 runs from the hydraulic circuit 1 to the sea E. As hydraulic fluid is being provided, via the first DCV 2 to the check valve 5, hydraulic fluid is prevented from passing anti-clockwise around the return loop 4. As a result, hydraulic fluid is vented from the hydraulic output B to the sea E via the vent line 10.

Similarly to the vent line 6, the vent line 10 includes a flow meter 11. By monitoring the flow of fluid in the vent line it can be determined whether or not the second DCV 3 is energised. If fluid is detected flowing in the vent line 10, the flow meter 1 1 may trigger an alarm for a topside well operator who can perform further investigation on the hydraulic circuit 1 and the DCVs 2, 3 to see if any repairs are required.

The vent line 10 also includes a pair of check valves 12, 13 which ensure that hydraulic fluid may only pass from the hydraulic circuit 1 to the sea E, and prevent the ingress of sea water to the hydraulic circuit 1.

Fig. 4 schematically shows the hydraulic circuit of Fig. 1 in a fourth configuration. Like reference numerals have been retained where appropriate.

In Fig. 4, the first and second DCVs 2, 3 have become de-energised (for example, due to an interruption in electrical power to the subsea control module (not shown) containing the hydraulic circuit 1). Therefore, they have both moved from their respective second positions, shown in Fig. 1, to their respective first positions, shown in Fig. 4.

With both DCVs 2, 3 in their respective first positions, the first DCV 2 blocks the passage of hydraulic fluid from the hydraulic input A. Additionally, the first DCV 2 connects the return loop 4 to the vent line 6. The second DCV 3 connects the hydraulic output B to the vent line 10. Hydraulic fluid in the return loop 4 may pass, via the check valve 5, to the vent line 6. Hydraulic fluid from the hydraulic output B may pass to the vent line 10. In both cases, hydraulic fluid from the hydraulic circuit 1 is vented to the sea D, E via the vent lines 6, 10. As can be seen from each of Figs. 2, 3 and 4, the de-energisation of either the first or second DCV 2, 3, or the de-energisation of both DCVs 2, 3, will result in the venting of fluid from the hydraulic circuit 1, and the prevention of the supply of hydraulic circuit from the hydraulic input A to the hydraulic output B. In other words, in order for hydraulic fluid to be supplied from the hydraulic input A to the hydraulic output B, the energisation of both DCVs 2, 3 is required.

While the above embodiment is shown with a flow meter in each vent line 6, 10, in practice only one of the vent lines could include a flow meter. Additionally, while it is typical to include a pair of check valves in each vent line, this could be reduced to a single check valve, or increased to more than two.

The invention is suitable for use with a hydraulic input received from an umbilical, as described above, and also suitable for use where the hydraulic input is received from a supply consolidated from multiple supply lines.

While the above embodiment shows the loop 4 returning from after the final branch off for tree valves, the loop 4 could return from any point after the first branch off for tree valves.

ADVANTAGES OF THE INVENTION

As discussed in the background section above, the invention provides an improvement in safety when compared to prior art hydraulic circuits. Circuits using only a single DCV have a single point of failure, whereas the present invention includes a redundant DCV, which increases the safety integrity level rating of the whole circuit, and subsea assets including such a circuit.

Additionally, prior art circuits include only a single vent line. The provision of a second vent line decreases venting time, and can allow a tree at a subsea well to be shut down more quickly than those including only a singly vent line.

The invention is not limited to the specific embodiments disclosed above, and other possibilities will be apparent to those skilled in the art.