GB2523079A

GB2523079A - Hydraulic accumulator

Info

Publication number: GB2523079A
Application number: GB1400443.6A
Authority: GB
Inventors: James George Oag; Rae Andrew Younger
Original assignee: SPEX SERVICES Ltd
Current assignee: SPEX SERVICES Ltd
Priority date: 2014-01-10
Filing date: 2014-01-10
Publication date: 2015-08-19
Anticipated expiration: 2034-01-10
Also published as: GB2523079B; GB201400443D0

Abstract

A hydraulic accumulator, i.e. an apparatus that stores energy and supplies it in the form of hydraulic power, is provided. The hydraulic accumulator comprises: a reservoir 34 for containing hydraulic fluid which has an outlet 40 for connection to a hydraulic system conduit, the reservoir being at least partially defined by a movable wall or piston 32 separating the reservoir 34 from a pressure chamber 36; and a deflagrant propellant source 44, such as potassium or sodium perchlorate, which when ignited supplies pressurised gas to the pressure chamber 36 to apply pressure to the reservoir 34 via the movable wall or piston 32. The hydraulic accumulator of the present invention is of smaller size than other hydraulic accumulators known in the art and this is especially suitable when it is used to supply hydraulic power to blowout preventers in subsea oil wells.

Description

HYDRAULIC ACCUMULATOR

Field of the invention

This invention relates to hydraulic accumulators and is particularly, but not exclusively, applicable to hydraulic accumulators for use as a power source in relation to oil and gas wells.

Background of the invention

In oil and gas wells, it is conventional to provide a blow-out preventer (BOP) which is intended to act to close the well bore in the event of over pressure downhole. The BOP is generally based on one or more hydraulic rams which are triggered in the event of overpressure to close across the well bore. The rams are normally supplied with hydraulic pressure from a remote source, but to ensure rapid operation in an emergency, a hydraulic accumulator is included in the hydraulic circuit in the vicinity of the rams.

There are a number of problems with conventional arrangements. It is difficult to provide sufficient energy storage in a conventional hydraulic accumulator to ensure reliable closure, especially if the BOP is required to shear thick walled tubulars. Simply increasing the size of the accumulator, or providing multiple accumulators, raises problems of available space; the BOP is typically part of a crowded wellhead installation. There are also difficulties in supplying hydraulic pressure from a remote source. If the wellhead is, for example, in deep water it is necessary to have either a long hydraulic circuit from the surface or to provide a subsea hydraulic pump; it would be desirable to enable a BOP which can operate entirely independently of external power sources.

Summary of the Invention

Accordingly, the present invention provides a hydraulic accumulator comprising a reservoir for containing hydraulic fluid, the reservoir having an outlet for connection to a hydraulic system conduit, the reservoir being at least partially defined by a movable wall separating the reservoir from a pressure chamber, and at least one deflagrant propellant source which when ignited supplies pressurised gas to the pressure chamber to apply pressure to the reservoir via the movable wall.

Prior to ignition, the hydraulic accumulator may be substantially unpressurised.

Preferably, there are a plurality of propellant sources.

The propellant sources may be ignited sequentially.

The movable wall may be a flexible diaphragm.

In preferred embodiments, however, the movable wall is a first piston, and the reservoir and pressure chamber are defined by a cylinder in which the first piston is slidable.

In some embodiments, a number of propellant sources are mounted on a second piston, the first piston and the second piston being slidable within the cylinder in one direction only. The pistons may engage with an axial rod or with the wall of the cylinder via one-way clutches.

In other embodiments the hydraulic fluid reservoir comprises a first hydraulic fluid reservoir having passive pressurisation means and a secondary cylinder, and the propellant gases act on a piston slidable within the secondary cylinder to supply hydraulic fluid from the secondary cylinder to the first hydraulic fluid reservoir.

Preferably, the secondary cylinder communicates with the first hydraulic fluid reservoir via a one-way valve. A further hydraulic fluid reservoir may communicate with the secondary cylinder.

Preferably, the secondary cylinder, the propellant source(s) and, if present, the further hydraulic fluid reservoir are removable from the first hydraulic fluid reservoir as a unit for replacement by another unit.

From another aspect, the present invention provides a method of providing pressurised hydraulic fluid, comprising igniting a propellant charge to apply pressure to a volume of hydraulic fluid via a movable wall of a hydraulic fluid reservoir.

In certain embodiments the pressure of the hydraulic fluid is maintained as the propellant gases cool by restraining return movement of the movable wall.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example, with reference to the drawings, in which: Figs. 1 a and lb are schematic cross-sections of one form of hydraulic accumulator embodying the invention; Figs. 2a to 2e are perspective views illustrating the structure and operation of a second embodiment; Figs. 3a to 3g are similar views of a further embodiment; Figs. 4a to 4e are similar views of another embodiment.

Detailed Description of the Drawings

Referring to Fig. 1, a hydraulic accumulator comprises a pressure vessel 10 which is divided by a flexible diaphragm 12 into a first volume containing a hydraulic fluid 14 and a second volume 16 for receiving a gas. The first volume communicates via a conduit 18 with any desired hydraulically-driven apparatus (not shown), for

example a BOP.

A propellant charge 20 is located within the pressure vessel 10 and can be ignited via a cable 22 by any suitable control system. Ignition of the propellant charge 20 produces hot gas at a high pressure and therefore pressurises the hydraulic fluid 14 as indicated in Fig. lb to supply power to the connected apparatus.

It is important to note that the propellant is not explosive in operation, rather it combusts at a subsonic rate (deflagrates) to produce large volumes of hot gas.

The system of Fig. 1 is included to illustrate the general principle of the present invention. Although it could be used to good effect in some circumstances, it does have some deficiencies. The pressure generated during deflagration of the propellant rises over a few seconds, allowing a moderately rapid recharge of the accumulator while being sufficiently slow not to introduce sharp pressure spikes or any damaging water hammer type effects in the hydraulic system. However, a limitation is that the majority of the energy released from the propellant is released as heat, which is quickly lost to the environment. Initially, the produced gas will be at about 2000°K (3140°F), but the heat is quickly lost, and the cooling gas volume and the pressure will reduce. If the pressure is directly applied to the hydraulic system, this will cause a peak pressure to the system, followed by a drop in pressure as the gas cools.

The produced hot gas has a volume may times that of the gas after it has cooled. Depending on the quantity and pressure of gas required, a cold gas system will require a significant quantity of propellant. Ideally the desired accumulator pressure would be sustained without a subsequent drop in pressure from gas cooling. The following embodiments are directed towards this.

Referring to Fig. 2, Fig. 2a shows the initial state and Figs. 2b -2e show sequential operation. The accumulator comprises a cylinder 30 divided by a free piston 32 into a first volume containing hydraulic fluid 34, and a second volume 36.

The cylinder 30 is closed at one end by a cap 38 having an outlet connector 40 for connection to a hydraulic circuit (not shown). The cylinder 30 is closed at the other end by a cap 42 which mounts a propellant assembly 44 within the second volume 36.

The propellant assembly 44 comprises a number of small charges which can be ignited individually. When hydraulic pressure is required, a first propellant charge fires to apply pressure to the piston 32 and charge the hydraulics (Fig. 2b). If more pressurisation is required, further charges are ignited as seen in Figs. 2c and 2d.

Fig. 2e shows the hydraulics fully recharged.

Suitable propellants include Potassium Perchlorate and Sodium Perchlorate or single, double or treble based propellant materials (which may be solid or liquid).

Suitable detonators include any RF immune detonator initiation systems (which might include EBW or EFI systems or similar), or any suitable ignition source to ignite propellant.

The arrangement of Fig. 2 will, of course, lose pressure as the temperature drops but the pressure/time characteristic is improved over that of Fig. I and will be suitable for some applications.

Fig. 3 shows an embodiment in which pressure is retained when the gas cools. Those parts in Fig. 3 which are similar to those of Fig. 2 are denoted by like references. The propellant assembly 44 is mounted on a piston 46, and the pistons 32 and 46 are slidably movable on an axial rod 48 via one-way clutches.

Alternatively, one-way clutches or equivalent devices could operate between the pistons and the cylinder wall.

Fig. 3a shows the initial propellant charge firing. The resulting propellant gases force the lower piston down, compressing the hydraulics, as seen in Fig. 3b.

As the gases cool, the lower piston 32 remains in position and the upper piston 46 is drawn down as seen in Fig. 3c. If further pressurisation is required, the next propellant charge fires, as seen in Fig. 3d, and the process is repeated as seen in Figs. 3e -3g.

This embodiment captures nearly all of the energy released by the propellant for each recharge. This is achieved by maintaining the combustion chamber at a near-constant volume after each propellant burn. There may be some performance benefit for subsequent recharges after the initial operation, because the spent products of combustion will be reheated by the subsequent combustion cycles.

This embodiment has a number of advantages such as it is very efficient and uses minimal propellant. Additionally, the embodiment has high fidelity and control over hydraulic pressure. It is compact in operation, lends itself to subsea operation and as it only has two moving components it is highly reliable.

Fig. 4 shows another embodiment including a cylinder 30 and a free piston 32.

The lower end of the cylinder 30 is closed as before by a cap 38 having an outlet connector 40. The upper end of the cylinder 30 is closed and the space 36 above the piston 32 contains a pressurised gas.

A removable assembly 50 is connected to the cylinder 30, and comprises a secondary cylinder 52 in which a free piston 54 is slidable. An end cap 56 carries a propellant assembly 44 which mounts a number of small propellant charges. A hydraulic fluid reservoir 58 communicates with the lower end of the secondary cylinder 50. The secondary cylinder 50 communicates with the cylinder 30 via a one-way check valve.

The system is deployed fully charged (Fig. 4a). When hydraulic pressure is required, for example when a BOP is operated and pressure falls, a single propellant charge is fired and generates an increase in pressure and volume which acts on the piston 54 (Fig. 4b). The piston 54 drives fluid through the check valve to charge the main cylinder 30 (Fig. 4c). As the gas cools, the piston 54 retracts, drawing fresh hydraulic fluid from the reservoir 58 into the lower part of the cylinder 50 (Fig. 4d).

If required, another propellant charge is fired and the cycle is repeated to further charge the hydraulics (Fig. 4e).

After use, the assembly 50 can be removed and replaced with a fully charged assembly, without needing to replace the entire hydraulic accumulator system.

The embodiment of Fig. 4 has a number of advantages. For example the system gets close to capturing all of the energy released by each propellant. It is very efficient, and uses minimal propellant. The embodiment is compact and as such lends itself to subsea applications. Additionally, propellant can be replaced without needing to replace whole system and minimal moving components, gives high reliability.

It should be noted that the term "hydraulic accumulator" is normally used to denote a device which is used in conjunction with another source of hydraulic pressure such as a pump to provide a reservoir of hydraulic energy. However, the present invention could be used as a primary source of hydraulic energy without a pump being present, and the term "hydraulic accumulator" as used herein should be interpreted to cover both of these situations. a

Claims

CLAIMS1. A hydraulic accumulator comprising a reservoir for containing hydraulic fluid, the reservoir having an outlet for connection to a hydraulic system conduit, the reservoir being at least partially defined by a movable wall separating the reservoir from a pressure chamber, and at least one deflagrant propellant source which when ignited supplies pressurised gas to the pressure chamber to apply pressure to the reservoir via the movable wall.
2. A hydraulic accumulator according to claim 1, in which there are a plurality of propellant sources which can be ignited sequentially.
3. A hydraulic accumulator according to claim 1 or claim 2, in which the movable wall is a flexible diaphragm.
4. A hydraulic accumulator according to claim 1 or claim 2, in which the movable wall is a piston, and the reservoir and pressure chamber are defined by a cylinder in which the piston is slidable.
5. A hydraulic accumulator according to claim 4, in which a number of propellant sources are mounted on a second piston, the first-mentioned piston and the second piston being slidable within the cylinder in one direction only.
6. A hydraulic accumulator according to claim 5, in which the pistons engage with an axial rod or with the wall of the cylinder via one-way clutches.
7. A hydraulic accumulator according to claim 1 or claim 2, in which the hydraulic fluid reservoir comprises a first hydraulic fluid reservoir having passive pressurisation means and a secondary cylinder, and in which the propellant gases act on a piston slidable within the secondary cylinder to supply hydraulic fluid from the secondary cylinder to the first hydraulic fluid reservoir.
8. A hydraulic accumulator according to claim 7, in which the secondary cylinder communicates with the first hydraulic fluid reservoir via a one-way valve.
9. A hydraulic accumulator according to claim 7 or claim 8, including a further hydraulic fluid reservoir communicating with the secondary cylinder.
10. A hydraulic accumulator according to any of claims 7 to 9, in which the secondary cylinder, the propellant source(s) and, if present, the further hydraulic fluid reservoir are removable from the first hydraulic fluid reservoir as a unit for replacement by another unit.
11. A method of providing pressurised hydraulic fluid, comprising igniting a propellant charge to apply pressure to a volume of hydraulic fluid via a movable wall of a hydraulic fluid reservoir.
12. A method according to claim 11, in which the pressure of the hydraulic fluid is maintained as the propellant gases cool by restraining return movement of the movable wall.