GB2373546A - Apparatus for pressurising a hydraulic accumulator - Google Patents

Abstract

Apparatus for pressurising a hydraulic accumulator <B>A</B>, comprising a vessel <B>1</B> for receiving hydraulic fluid and an expandable compartment <B>2</B> inside the vessel, for receiving pressurised gas. The apparatus comprising pressurising means <B>5</B> which is made up of a large cylinder <B>6</B> connected to a small cylinder <B>7</B>. The large cylinder <B>6</B> is fitted with a large piston <B>8</B> which has a seal <B>9</B>. The small cylinder is fitted with a small piston <B>10</B> which has a seal <B>11</B>. The two pistons are connected by a rod <B>13</B>. The pressurising means is coupled to the expandable compartment of the hydraulic accumulator by coupling means <B>4</B>. The pressurising means is responsive to ambient pressure and adapted to pressurise the interior of the compartment to a level equal to ambient pressure plus an increased amount.

Description

HYDRAULIC ACCUMULATORS The present invention relates to hydraulic accumulators. Hydraulic accumulators are used in hydraulic systems primarily to provide a source of reserve hydraulic power or, since hydraulic fluid is essentially incompressible, as a reservoir to reduce pressure fluctuations in a hydraulic system. Referring to Fig. 1, an accumulator A normally comprises a rugged vessel 1, capable of withstanding substantial pressure, which incorporates an expandable compartment 2, typically a rubber bag. This compartment 2 provides isolation from the hydraulic fluid within the vessel 1 of a compressible medium, typically nitrogen gas, contained within it. The compartment 2 is generally pre-charged via a valve 3 at a pressure to suit the hydraulic system operating pressure requirements. When hydraulic fluid is pumped into the accumulator A, the gas is compressed, allowing a flow of hydraulic fluid into the accumulator, such that the pressure of the hydraulic fluid equals the pressure of the gas in the compartment 2. This reservoir of hydraulic fluid can therefore receive fluid from or provide fluid to the hydraulic system at the system pressure.

Clearly, the vessel 1 has to be strong enough to withstand the pressure difference between the hydraulic system pressure and that of the environment surrounding the vessel. Normally, the pressure surrounding the vessel is atmospheric, i. e. 105 Pa (1 bar). However, when the accumulator is situated in an environment which is at a higher ambient pressure than normal atmospheric pressure, then further demands are made on the strength requirements of the vessel. A typical situation where high environmental pressures are encountered are those of the sub-sea fluid extraction industry, where hydraulic accumulators are installed at sea depths of typically 3000 metres with a consequential environment pressure of some 3 x 107 Pa (300 bar).

Under these conditions, the system pressures are at least as high as the environment pressure. Before installation and submersion to the operating depth, the hydraulic accumulator has to be charged to the sub-sea operating pressure plus the pre-charge, i. e. typically 4.25 x 107 Pa (425 bar). Thus, the required increase in wall thickness of the hydraulic vessel to achieve, typically, a safety factor of 3.25 : 1 creates serious problems, in that the cost of the vessel rises substantially and the weight increases, which reflects into the costs of supporting it and making it difficult to handle.

According to the present invention, there is provided apparatus for pressurising a hydraulic accumulator comprising a vessel for receiving hydraulic fluid and an expandable compartment in the vessel for receiving a pressurising gas, the apparatus comprising pressurising means and coupling means for coupling said pressurising means to such an expandable compartment, said pressurising means being responsive to ambient pressure and adapted to pressurize the interior of said compartment in use of the apparatus to a level equal to ambient pressure plus an increased amount.

Said pressurising means may comprise: a first cylinder with a first piston in it, the first piston being exposed on one side to ambient pressure; and a second cylinder, of smaller diameter than the first cylinder, with a second piston in it, the second piston, on one side, being connected with the other side of the first piston and the second cylinder, on the other side of the second piston, being in fluid communication with said coupling means.

In this case, said pressurising means may comprise a further cylinder with a further piston in it, the further piston being exposed, on one side, to ambient pressure and the further cylinder, on the other side of the further piston, being coupled with the second cylinder on said other side of the second piston by a fluid path which includes a valve which opens above a predetermined pressure to allow flow in a direction from said other side of the second piston to said other side of the further piston. There may be a further valve, connected in parallel with the first valve, which opens above a predetermined pressure to allow flow in a direction opposite to the first-mentioned direction.

The present invention also comprises a combination of apparatus according to the invention and such a hydraulic accumulator, said coupling means coupling the pressurising means with said expandable compartment. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which : Fig. 1 is a sectional view of a typical conventional hydraulic accumulator ; Fig. 2 is a sectional view of a hydraulic accumulator with an example of pressurecharging apparatus according to the present invention, at the sea surface ; Fig. 3 is a sectional view of the hydraulic accumulator with the pressure-charging apparatus, on the sea bed; Fig. 4 is a sectional view of a hydraulic accumulator with pressure charging apparatus according to another example of the present invention, which retains a pre-charge pressure, on the sea bed; and Fig. 5 is a sectional view of such apparatus feeding several accumulators, on the sea bed.

Referring to Fig. 2, compartment 2 of accumulator A, connected to a hydraulic system H, is connected via a self-sealing coupler 4 to another vessel 5 comprising two cylinders 6 and 7. The larger cylinder, 6, is fitted with a piston 8, sealed with a seal 9 and exposed on one side to ambient pressure, and the smaller cylinder, 7, is fitted with a piston 10, sealed with a seal 11, both pistons being able to slide up and down the inside of their respective cylinders. A connecting rod 13 is attached to the piston 8, such that when piston 8 is depressed, piston 10 is also depressed via the connecting rod 13. The self-sealing coupler 4 is arranged such that the compartment 12 formed by the cylinder 7 and the piston 10, typically pre-filled with nitrogen gas, is coupled to the compartment 2 of the accumulator A. The vessel 5 is detachable from the accumulator by parting the self-sealing coupler 4, which then seals both the compartments 2 and 12. Details of a suitable self-sealing coupler are not show as the same is available as a standard device. Conventionally, the static pressure of hydraulic system H approximately matches that of the surrounding pressure by virtue of the static head of hydraulic oil fed via an umbilical 14 from an oil reservoir 15 at the surface.

Fig 3. shows the vessel 5 and accumulator A, attached to hydraulic system H, lowered to the sea bed, the system H typically being attached to a Christmas tree T at the well head. During lowering, the pressure of the environment (seawater in the example) depresses the piston 8, and thus also the piston 10 via the connecting rod 13. The pressure in the compartment 12 will be greater than the surrounding pressure acting on the top of the piston 8 due to the pressure gain that results from the difference in diameter of the two pistons 8 and 10. Thus, with the appropriate ratio of diameters of the two pistons, for the required depth of operation, the apparatus can pressurise the compartment 12 and thus the compartment 2 in the hydraulic accumulator, via the coupler 4, to the surrounding pressure, plus the required pre-charge.

Ideally, the apparatus should be capable of providing the required pressure charge equal to the surrounding pressure plus the pre-charge over a wide range of depths, so that a single design of the apparatus can be employed to install a multiplicity of systems at a variety of depths. Thus, for example, the pressure charging apparatus could have a ratio of piston diameters chosen such that at a depth of 1500 metres of sea water at a pressure of 1.5 x 107 Pa (150 bar) the hydraulic accumulator A is charged to 1.5 x 107 Pa (150 bar) plus a pre-charge of 1.25 x 107 Pa (125 bar), i. e. a total of 2. 75 x 107 Pa (275 bar). However, if this same apparatus was used a depth of, say, 3000 metres at 3 x 107 Pa (300 bar), then the resultant pressure in the hydraulic accumulator will be considerably greater than the required 3 x 107 Pa (300 bar) plus the required 1.25 x 107 Pa (125 bar) pre-charge. This is due to the pressure gain, resulting from the piston diameter ratio, being too great.

Fig. 4 shows the vessel 5 of the pressure charging apparatus connected to a second cylindrical vessel 16 fitted with a piston 17 sealed by a seal 18 and exposed on one side to ambient pressure. Connection of the compartment 12 of the vessel 5 to a compartment 19 of the second vessel 16 on the other side of piston 17 is effected via two valves 20 and 21 connected in parallel. One of the valves, 20, is a pressure release valve which is set to open at the required pre-charge pressure, typically 1.25 x 107 Pal (125 bar), allowing flow in the direction of the arrow X. The other valve 21 is a simple release valve allowing flow in the direction of the arrow Y. When the assembly, comprising the vessels 5 and 16 and accumulator A attached to the hydraulic system H, is lowered to the sea bed, the pressure in the compartment 12 and thus the hydraulic accumulator A rises until it equals the hydrostatic pressure of the environment plus the pressure setting of the valve 20 (e. g. the hydrostatic pressure plus 1.25 x 107 Pa (125 bar) ). At this depth, the valve 20 opens thus allowing any excess pressure above the required pre-charge level to be released by the ejection of 'excess'gas into the vessel 16, as the assembly is further lowered to a greater depth.

The pressure of the ejected gas lifts the piston 17 as necessary against the static pressure head of the environment above this piston.

At any water depth, the pressure in the accumulator A will be equal to that of the environment plus the pre-charge pressure. Thus, the pressure that the vessel 1 of accumulator A has to withstand remains that of the pre-charge pressure only under static conditions, or the pressure of the hydraulic system when operational. Since the vessel 1 of accumulator A does not now have to withstand the full pressure of the environment, typically an additional 3 x 107 Pa (300 bar) at 3000 metres water depth, plus the pre-charge pressure, typically 1.25 x 107 Pa (125 bar), i. e. a total of 4.25 x 107 Pa (425 bar), it can be manufactured with much thinner vessel walls.

Thus, a consequence of the above examples is that the accumulator A can be a standard off the shelf item. This is much lighter and smaller than a special thick walled device and is thus has advantages where space is a limitation and is easier to handle, particularly by a remotely operated vehicle in a sub-sea environment.

Once the system has been installed in the high-pressure environment, the apparatus comprising vessels 5 and 16 is detached from the accumulator A at the self-sealing coupler 4 and returned to the ambient environment. In this example, the apparatus comprising vessels 5 and 16 is simply raised to the sea surface. As the assembly is raised to the surface and the environmental pressure reduces, the retained high pressure in the compartment 12 of vessel 5 returns the pistons 8 and 10 to their original positions as shown on Fig. 2. Likewise, the excess gas in the vessel 16 is returned to the vessel 5 via the release valve 21, allowing the piston 17 to return to its original position at the base of the vessel 16. Thus, the apparatus comprising vessels 5 and 16 is an installation tool only, commonly known as a running tool in the fluid extraction business. Thus, a single running tool can be used to install a multiplicity of systems, over a range of depths, and its cost then becomes negligible compared to the costs of fitting conventional thick walled and heavy accumulators in high pressure environments. A further advantage is that the reduced pressures that an accumulator has to withstand results in lower pressures being required to test it. Consequently, the risk of an accumulator wall failure is much reduced and the

potential danger, from bursting very high-pressure accumulators, is removed. in The present invention can be applied to a plurality of accumulators where more than one accumulator is fitted to a system, by simply coupling the compartments 2 of the accumulators together by appropriate pipe work connected to the running tool at a single self sealing coupler 4. This is illustrated in Fig. 5 in which are shown by way of example four accumulators Al, A2, A3 and A4..

It should be noted that the scale of the running tool in the figures is not intended to be accurate. The size of the tool will depend on the volume of the compartment 2 or the sum of the volumes of compartment 2 in the system and the pressure of the environment that the system will operate in, i. e. the depth of water in the above examples. It should also be noted that the required ratio of the pistons/cylinders will be based on adiabatic conditions of the gas as the rate of lowering of the tool to the sea bed should be slow enough to allow loss of compression heat or gain of expansion heat.

Should it be necessary to raise an accumulator or assembly containing one or more accumulators to the surface for repair or replacement purposes, use of the running tool would be necessary to release the pressure in the accumulator/s in a controlled manner as the accumulator or assembly is raised to the surface, i. e. the pressure is reduced in phase with the change of depth. Failure to release the pressure in such a controlled

manner may result in rupture of an accumulator, since apparatus according to the ZD invention can allow the use of devices not designed to withstand the full environmental pressures involved.

Claims (7)

1. CLAIMS 1. Apparatus for pressurising a hydraulic accumulator comprising a vessel for receiving a hydraulic fluid and an expandable compartment in the vessel for receiving a pressurising gas, the apparatus comprising pressurising means and coupling means for coupling said pressurising means to such an expandable compartment, said pressurising means being responsive to ambient pressure and adapted to pressurise the interior of said compartment in use of the apparatus to a level equal to ambient pressure plus an increased amount.
2. 2. Apparatus according to claim 1, wherein said pressurising means comprises: a first cylinder with a first piston in it, the first piston being exposed on one side to ambient pressure; and a second cylinder, of smaller diameter than the first cylinder, with a second piston in it, the second piston, on one side, being connected with the other side of the first piston and the second cylinder, on the other side of the second piston, being in fluid communication with said coupling means.
3. 3. Apparatus according to claim 2, wherein said pressurising means comprises a further cylinder with a further piston in it, the further piston being exposed, on one side, to ambient pressure, and the further cylinder, on the other side of the further piston, being coupled with the second cylinder on said other side of the second piston by a fluid path which includes a valve which opens above a predetermined pressure to allow flow in a direction from said other side of the second piston to said other side of the further piston.
4. 4. Apparatus according to claim 3, including a further valve, connected in parallel with the first valve, which opens above a predetermined pressure to allow flow in a direction opposite to the first-mentioned direction.
5. 5. A combination of apparatus according to any preceding claim and such a hydraulic accumulator, said coupling means coupling the pressurising means with said expandable compartment.
6. 6. Apparatus for pressurising a hydraulic accumulator, substantially as herein described with reference to Figs. 2 to 5 of the accompanying drawings.
7. 7. A combination of a hydraulic accumulator and apparatus for pressurising the hydraulic accumulator substantially as herein described with

   reference to Figs. 2 to 5 of the accompanying drawings. z