NO20190053A1 - an underwater accumulator system designed to generate hydraulic power - Google Patents

Description

The invention relates to an underwater-based accumulator system which is designed to generate hydraulic power by absorbing and transferring energy from a driving medium in the form of a liquid which is supplied with a pressure which is approximately equal to or lower than the surrounding water pressure.

The accumulator system consists of one or more accumulator units which are fitted with a valve device which means that the accumulator units can maintain a set excess pressure in relation to the ambient pressure even if the ambient pressure changes significantly. Compared to traditional, gas-based accumulator systems, an accumulator system according to the invention will have significant advantages in that it is independent of liquid supply via umbilical, has high performance in relation to weight and size, and is very user-friendly.

Background for the invention

There is a growing need to develop better technology for generating hydraulic energy at great ocean depths. Future oil extraction will take place from installations at ever greater sea depths, and a prerequisite for the safe operation of such installations is that you have almost immediate access to a significant amount of hydraulic power at all times. In the current situation, there is considered to be a great need for better and more compact solutions, especially at depths greater than 1500 msw. Gasless accumulators designed to exploit the pressure difference between an ambient water pressure and an almost pressure-free void in a pressure-resistant tank have great potential, especially as this technology becomes more efficient with increasing depth. The purpose of the invention is thus to produce an accumulator solution which utilizes this potential as best as possible in the form of good user-friendliness and optimal hydraulic capacity in relation to weight and size.

Known technique

Hydraulic equipment used in underwater oil operations is mainly based on being operated by hydraulic fluid, as the equipment is designed to utilize the excess pressure of the hydraulic fluid relative to ambient pressure. Traditional hydraulic actuators are based on the pressure in a compressed gas being transferred via a displaceable piston that acts as a barrier between the compressed gas and the hydraulic fluid. Gas compressed to very high pressures loses pressure quickly during expansion, and consequently has a limited ability to transfer energy. This ability is further degraded by thermal effects which create a temperature drop which results in a reduction in the transfer pressure.

In order to get more energy out of gas accumulators, depth-compensated accumulators have been developed where care has been taken to establish forces that cancel out the effect of a change in ambient pressure. This is achieved by means of a piston arrangement where two oppositely directed surfaces sense respectively ambient pressure or an approximately zero pressure in a gas cylinder. This solution requires expensive machining, and will not prevent the gas pressure from reaching a pressure level where its compressibility is significantly reduced compared to an ideal gas. This is because some tools require a hydraulic pressure of 345 bar and higher.

In recent years, gas-free concepts for generating hydraulic energy have been developed. NO 333477 B1 and NO 343020 B1 describe two different solutions where the pressure difference between surrounding water and an almost pressureless void is utilized. The solutions are based on this void being established in a pressure-resistant tank using specially designed displacement pumps.

NO 333477 B1 describes a solution in which it is proposed that said pressure difference be used for direct operation of equipment that uses hydraulic actuators. As this pressure difference depends on the sea depth, this solution will require specially adapted hydraulic equipment, and such a design is essentially unsuitable for the operation of traditional hydraulic equipment.

NO 343020 B1 is currently believed to represent the most applicable solution when it comes to gas-free concepts. It relies on a pressure booster pump to release and transform the stored energy into hydraulic power that can operate traditional hydraulic tools. For both of these solutions, the release of hydraulic energy is conditional on liquid with approximately ambient pressure being returned to the aforementioned pressure-resistant tanks.

Description of the invention

The operation of the accumulator units and an accumulator system according to the invention is described in the following with reference to figures 1 - 8, where

- Fig.1 shows a principle price for a first embodiment of an accumulator unit according to the invention.

- Fig.2 shows a principle price for a self-regulating valve device for control of

the hydraulic pressure

- Fig. 3 shows how the hydraulic pressure changes with depth for a given configuration of an accumulator unit

- Fig.4 shows an embodiment of a valve device for manual adjustment of the hydraulic pressure

- Fig. 5 shows a simplified preferred embodiment of an accumulator system according to the invention

- Fig.6 shows a pressure converter which is included in a preferred embodiment of the accumulator system

- Fig. 7 illustrates how to recover the hydraulic capacity of an accumulator system in operational use

- Fig. 8 shows an alternative embodiment of an accumulator unit according to the invention

Figure 1) shows a diagram of a first embodiment of an accumulator unit according to the invention. It comprises a housing 4) with an inlet 13) for receiving the drive medium, and an outlet 8) for releasing pressurized hydraulic fluid. The propellant's supply pressure towards the inlet 13) is assumed to be approximately equal to the surrounding water pressure PAMB. The housing is composed of cylindrical guides for a piston device 3). This divides the housing 4) into three separate chambers I-III, and is composed of a piston rod with cross-section A2 and a piston with pressure surface A1. The piston device cooperates with two sliding seals 2.5) which prevent leakage between the chambers.

The accumulator unit comprises a valve device (12) which in this description will be referred to as a depth compensation valve. This is arranged in a housing 11) which forms an end cap in the accumulator unit. Its operation will be explained later with reference to fig.2. The dashed line between a port 10) in the depth compensation valve and a port 9) in the housing 4) indicates a pressure connection that must be established for the preferred embodiment of the depth compensation valve to function.

The propellant is supplied to chamber III with a pressure PS, and will consequently create an upwardly directed force FS = PS*A1 against the piston device via the pressure surface A1. This force pushes the piston arrangement upwards, and generates counter forces in chambers I and II which are equal in sum. However, the counterforce from chamber II is negligible because this chamber appears as an almost pressureless void. This is achieved when the port 9) is blinded off after the piston device has been pushed up towards the upper end position. The friction in the sliding seals 2,5) is considered insignificant in this context, so that the upwardly directed force FS is almost fully used to pressurize the hydraulic fluid in chamber I. The force balance between upwardly and downwardly directed forces against the piston arrangement is thus given by; 1. FS = PS*A1 = (PH + PAMB)*A2,

where PH is the excess pressure of the hydraulic fluid in relation to the ambient pressure PAMB.

Of these there are;

2. PH = PS*A1/A2 - PAMB

In the preceding description, we have chosen to have the hydraulic fluid stored in chamber I, leaving chamber II depressurised. In principle, you can easily change the use of these chambers.

Formula 2 shows that PH can be kept at a defined, fixed level if PS is increased or decreased linearly with the ambient pressure. This is achieved according to the invention by means of said depth compensation valve, which is designed to control the relationship between PS and PAMB so that PH maintains a constant level. This means that the depth compensation valve controls the pressure drop on the supplied driving medium in relation to the ambient pressure. By inserting DP= (PAMB - PS ) into formula 2 we find;

3. DP = PAMB (A1-A2)/A1 – PH*A2/A1

An accumulator unit will only be able to generate a desired hydraulic pressure when it is arranged at a certain minimum depth which is determined by the relation A1/A2. The minimum depth is defined by the desired hydraulic pressure being achieved with PS = PAMB, i.e. at DP=0. It is neither desirable nor necessary for the depth compensation valve to operate before this depth is exceeded. Using formula 3, we find that an accumulator unit according to the invention will be able to deliver a constant hydraulic pressure after the minimum depth has been exceeded if DP is regulated as follows;

4. DP = PAMB (A1-A2)/A1 – PH*A2/A1

= (PAMB – Pamb) * (A1-A2)/A1

where Pamb is ambient pressure at the minimum depth

The depth compensation valve 12) is thus designed to control DP in accordance with formula 4.

The operation of a preferred embodiment of this valve is explained with reference to fig.2.

The valve is built into the housing 11) and comprises an axially displaceable valve body 14). This has a cylindrical protrusion 16) with a sliding seal 15) which forms a leak-free barrier between a first chamber IV which is open to the supply line 13) and a second chamber V which is connected to the depressurized chamber II of the accumulator unit via port 10). The valve body 14) controls, in cooperation with a fixed seat 18), the opening between the first chamber IV and chamber III in the accumulator unit.

Fig.2 illustrates a situation where the seat 18) is pressed against a soft sealing disc 19) on the valve body 14). The cross-section of the seat 18) is defined by an area A3, and the cylindrical outgrowth 16) of the valve body is defined by a cross-section A4. This means that the liquid in contact with chamber IV has a pressure equal to PAMB, and will affect the valve body 14) with a downwardly directed force of size FIV= PAMB*(A3-A4). As chamber V is connected to chamber II in the accumulator unit, chamber V is virtually depressurized, and the force exerted on the valve body 14) from chamber V is thereby limited to the downwardly directed force FV from an installed spring 17). The pressure in chamber III is PS = (PAMB–DP), and this will exert an upwardly directed force of magnitude FIII = (PAMB–DP)*A3 against the valve body 14).

Upward and downward forces against the valve body will be in equilibrium.

That is (PAMB-DP)*A3 = PAMB*(A3-A4) FV

Of this; 5. DP =PAMB*A4/A3 FV/A3

We require formula 4 and formula 5 to be identical.

It conditions; DP = PAMB*A4/A3 FV/A3 = PAMB (A1-A2)/A1 – PH*A2/A1

We see that this will be achieved if the geometry of the accumulator unit and the depth compensation valve are adapted to each other so that

A4/A3 = (A1-A2)/A1 and FV/A3= PH*A2/A1

We see that FV = 0 will result in PH = 0, which means that the hydraulic pressure is set to be in equilibrium with the surrounding water pressure. Consequently, a positive force effect from the spring 17) is required for a positive hydraulic pressure to be generated, i.e. that the hydraulic fluid has an overpressure in relation to the ambient pressure. We see that the hydraulic pressure will increase proportionally with the spring tension FV.

Example 1;

We want an accumulator unit to be able to generate 345 bar at sea depths ≥ 1000

msw – i.e. 1000 msw is set as the minimum depth. Subsequent calculations are simplified by setting the atmospheric pressure equal to 1 bar, and by setting the water pressure increase per 10 meters

depth increase is set to 1 bar. By putting PS = 101 bar into formula 2, there is;

345 bar = 101 bar *A1/A2 – 101 bar - of which A1/A2 = 4.42

By inserting this value into formula 4 there is further;

DP = PAMB*(1-A2/A1)- Pamb*(1-A2/A1) = 0.774*PAMB – 78.2 bar

As mentioned above, compliance between the accumulator unit and the depth compensation valve requires that ; FV/A3= PH*A2/A1= 78.2 bar. If we choose A3 = 12 cm<2>, the accumulator unit will generate a hydraulic pressure of 345 bar if the set spring tension is of size FV= 78.2 bar*12cm<2> – corresponding to approx. 930 kp. If the spring tension is set lower than this, the hydraulic pressure is reduced accordingly.

When the pressure in chamber IV is lower than Pamb, the valve body 14) is pushed to its lowest position by the spring tension, and ensures a maximum opening between chamber IV and chamber III. When the pressure exceeds Pamb, the valve body will seek the position where the supply of drive medium to chamber III is just large enough to maintain the desired power balance. In a normal operating situation, the pressure control is based on an equilibrium between axially acting forces on the valve body 14). Deviations in this equilibrium will produce large corrective forces which produce momentary, but still soft corrections of the valve body's position. There may be a need for oscillation damping measures if the accumulator unit has to be adjusted to be able to cope with large and rapid variations in the consumption pattern. Such measures can be carried out using known techniques, and will not be touched upon in this description.

The depth compensation valve 12) functions as a one-way valve in the sense that it does not allow liquid to be extracted from chamber III via the outlet 13). A one-way valve 20) is therefore arranged in the housing 11) so that this is possible.

It should be mentioned that the force FV against the valve body 14) does not necessarily have to be established by means of a spring. The essential thing is that a downward directed force must be established which is adapted so that PH is maintained at the desired level.

Fig. 3A and 3B show the relationship between depth and hydraulic pressure for an accumulator unit according to the specifications in example 1. The dashed line in fig.3A shows how PH increases as a function of depth before the depth compensation valve comes into operation. The example is based on the accumulator unit being able to produce a hydraulic pressure PH= 345 bar at a minimum depth of 1000 msw. If the adjustment of the spring span is in accordance with this, the depth compensation valve will come into operation at 1000 msw, preventing a further increase in PH. If the spring tension is doubled, the hydraulic pressure will rise to PH=690 bar before it flattens out. This will then only happen at a depth of 2,000 meters - which, under these conditions, will then represent the minimum depth for an accumulator unit which, with A1/A2 = 4.42, will deliver PH= 690 bar.

Fig. 3 B shows correspondingly how the absolute pressure of the pressure downstream of the depth compensation valve, i.e. PS, changes with depth for the three settings. If no depth compensation valve was fitted to the accumulator unit, PS and ambient pressure would be identical - given by the dashed line.

Fig. 4 shows an alternative embodiment where the depth compensation valve acts as a traditional spring-loaded one-way valve. In this embodiment, the depth compensation valve has no self-regulating function, as a set spring tension defines a fixed pressure drop that the depth compensation valve seeks to maintain. This means that the depth compensation valve makes it possible to achieve the same hydraulic pressure at a different depth by adjusting the spring force - something that would be possible at sea when using an ROV. Components which, in principle, have the same function as in a self-regulating design are given the same position number.

A significant disadvantage of using a depth compensation valve that is not self-regulating is that a relatively modest missetting of pressure drop versus depth can result in large deviations of PH in relation to the desired value. The deviation from a desired PH value factor A1/A2 times as large as the incorrect setting of the pressure drop (cf. formula 2 on page 3). A self-regulating depth compensation valve acc. fig.2 is based on the geometry of the valve to match the geometry of the accumulator unit, and any deviation from the desired PH value remains the same even if the depth changes.

A priority area of use for an accumulator system according to the invention will be to generate hydraulic power for operations of limited duration such as for example maintenance operations (work-over) of wells on an oil field. Overhauling a single well can typically take place over a couple of weeks and requires access to hydraulic power to operate a "Workover/completion system" that includes various types of hydraulic tools. The various tools will normally be designed for different operating pressures, and the individual accumulators to be used will have to be configured accordingly. Between each individual overhaul, it is common to pull the maintenance material up to the surface and prepare it for the next well. For the field as a whole, the operation must be able to be carried out without having to carry out in-depth interventions on any part of the equipment. The invention in question can have significant operational advantages in that it is possible to recover used hydraulic capacity down on the seabed, in that the accumulator system can be moved to other sea depths without readjustments being required, and in that the maintenance equipment has such a large hydraulic capacity that relatively extensive operations can be carried out without the need to recover hydraulic capacity along the way. Already at 400 meters of sea depth, accumulator units according to the invention will be able to provide a significantly more favorable ratio between hydraulic capacity and weight/dimensioning than traditional gas-based accumulators.

Tables 1 and 2 show the calculated relationship between minimum depth and hydraulic capacity for accumulator units with the following specifications;

• Total length of approx. 2300 mm.

• The piston device 4) must have a stroke of 1000 mm, and piston diameter =300 mm.

• Hydraulic pressure PH= 345 bar

• Estimated weight is based on accepted standards for strength calculations, and takes into account that the weight of the piston rod varies with the ratio A1/A2.

These tables show that 28 accumulator units with a total weight of 5.8 tonnes, dimensioned based on the given specifications, will have the capacity to release 200 liters of liquid with 345 bar hydraulic pressure at a depth of 400 meters before recharging. At a depth of 3,000 metres, equivalent hydraulic capacity can be achieved with 7 accumulator units with a total weight of 3.2 tonnes.

Table 1

Accumulator for 300 – 1000 msw (max depth 1000 msw) / hydraulic pressure 345 bar

Table 2

Accumulator for 1000 – 3000 msw (max depth 3000 msw) / hydraulic pressure 345 bar

According to table 2, an accumulator unit configured for a minimum depth of 1000 msw will be able to deliver 15.9 liters of liquid with PH=345 bar. Provided the depth does not exceed the accumulator unit's pressure rating, it can deliver 345 bar at any depth > 1000 msw, but the hydraulic capacity will be limited to 15.9 litres. If the operational depth is increased to 3,000 msw, it is beneficial to use accumulator units that are configured for a minimum depth of, for example, 3,000 metres. The table shows that the hydraulic capacity is thereby increased to 32.9 litres, i.e. almost double. One must therefore weigh up the practical advantages of using the accumulator unit over a wide depth interval versus weight savings. In any case, it will be possible to achieve large weight savings even with the use of traditional gas-based accumulators.

Preferred embodiments of an accumulator system according to the invention.

In the following description, we will first deal with the main elements in a preferred structure of an accumulator system according to the invention. We will later, with reference to fig.7, give an example of how such an accumulator system can be configured to have the possibility of maintaining/recovering hydraulic capacity while it is in operational use.

Fig. 5 shows a sketch of a simplified setup for an embodiment where the accumulator system is intended for the hydraulic capacity to be recovered on the surface. The accumulator system comprises 5 accumulator units which are arranged within the dotted frame marked a) as well as two pressure converters if arranged within the dotted frame b). All accumulator units are configured to deliver the same hydraulic pressure, which is chosen here to be 50,000 psi. In general, it is beneficial to base yourself on a pressure level that will serve many consumers, and this pressure level should be relatively high, as an accumulator unit with a large amplification factor will extract more energy from the drive medium. In this simplified setup, the number of delivery pressures in excess of 5,000 psi is limited to 10,000 psi and 3,000 psi respectively. The propellant is preferably circulated in a closed circuit via an elastic bellows 21) as indicated in the figure.

Pressure converters according to the invention are designed to have the ambient pressure PAMB as a reference, and convert the pressure according to a factor that is uniquely determined based on the chosen configuration. A pressure transducer configured for an amplification factor of 2 will produce a hydraulic pressure that, in relation to the PAMB, is twice as high regardless of depth.

The function of the pressure converter is explained with reference to fig.6. Like an accumulator unit, this consists of a housing 4) which is composed of two cylindrical bushings and has a piston arrangement which is axially displaceable in these bushings. The piston arrangement separates the housing into three separate chambers I-III), and consists of a piston rod with cross-section A2 which is connected to a piston whose pressure surface A1 faces out towards the largest chamber I. Consequently, the chambers II,III have effective pressure areas against the piston arrangement of respectively A2 and (A1-A2).

The piston device cooperates with two sliding seals 2.5) which prevent leakage between the chambers. Chamber II has an open connection with liquid that maintains ambient pressure (PAMB) via port 9), as chambers I and III are assumed to contain liquid pressurized to, respectively. PI and PIII. The pressure balance between these three chambers will be given by the formula;

PAMB*(A1-A2)+ PI*A2= PIII*A1 ,

which can be transformed into (PI-PAMB)/ (PIII-PAMB) = A2/A1

If, for example, you choose A2 = A1/2, by supplying liquid with pressure PH= 345 bar to chamber III, you will get liquid with pressure PH= 690 bar from chamber I. Similarly, supplying liquid with pressure PH= 345 bar to chamber I result in a delivery pressure of pressure PH= 172.5 bar from chamber III.

Tables 1 and 2 (pages 8 and 9) illustrate how much hydraulic capacity can be achieved using accumulator units that have a total length of approx. 230 cm, and which have a piston arrangement with a stroke length of 100 cm and a piston diameter of 300 mm. The pressure converters will have the task of converting the pressure of liquid which initially has high pressure, and this conversion takes place with little energy loss. A pressure converter will consequently have a significantly greater hydraulic capacity than an accumulator unit with similar dimensions. For example, a pressure converter with A2 = A1/2 will be able to convert 70 liters of liquid supplied from the accumulator units with PH=345 bar into 35 liters of liquid with PH= 690 bar. Alternatively, 35 liters of liquid with PH=345 bar can be converted into 70 liters of liquid with PH=172.5 bar.

Fig 7 outlines how an accumulator system composed of accumulator units and pressure converters can be interconnected so that it becomes possible to maintain/recover hydraulic capacity while the system is located on the seabed. In the following, this process will be referred to as charging the components.

The main components of this simplified scheme are two accumulator units arranged within the dashed frame marked with a), and two converters arranged within the dotted frame marked with b).

Charging of accumulator units and converters is preferably carried out using an electrically driven pump. The system is designed so that both the accumulator units and the pressure converters can be charged by one and the same pump, and that this charging operation can be carried out without requiring high pump pressures. The setup in fig.7 is based on the fact that the drive medium and the hydraulic fluid are the same type of fluid and can therefore use the same storage bellows 21). It is easy to change this by connecting separate storage pods.

In order to be able to maintain a moderate pump pressure during the charging process, the units in question are isolated from lines with high pressure, and the chambers that are to have liquid returned are connected to lines that can supply liquid with pressure PAMB from the storage bellows. The chambers to be emptied of liquid are simultaneously connected to the pump's suction side and pressure gradients are created which enable the charging operation to be carried out. The desired connection is achieved by using one-way valves and directional valves as shown in Figure 7A. Fig.7B shows how the liquid flows through or an accumulator unit and a pressure transducer during the charging process.

Example 1. We look at the converter which is arranged on the right in fig.7B), and which is designed to reduce the hydraulic pressure PH from 5000 psi to 3000 psi. This means that the ratio between the piston rod's cross-section A2 and the piston's area A1 is given by A2/A1 = 0.6. This means that equilibrium between upwardly directed and downwardly directed pressure forces against the piston device is given by PAMB*A1 = PAMB*(A1 -A2) P2*A2 From this; PAMB*A2 = P2*A2 which gives P2 = PAMB

Consequently, minimal energy is required to recover hydraulic capacity on a converter.

Example 2. We look at the accumulator unit which is arranged on the left in fig.6B), and take as a starting point that this is configured to be able to deliver a hydraulic pressure PH= 5000 psi (345 bar) at a minimum depth of 1000 msw. This means that the ratio between the piston rod's cross-section A2 and the piston's area A1 is given by A2/A1 = 0.226. Equilibrium between upwardly directed and downwardly directed pressure forces against the piston device is now given by;

P1*A1 = PAMB*A2 which gives P1= 0.226* PAMB

Consequently, the suction side of the pump must be lower than 0.226* PAMB for the charging process to be carried out. This means that the pump must operate with a real pump pressure > 0.774* PAMB to carry out the charging operation.

An alternative embodiment of the accumulator unit is outlined in fig.8. The essential difference between this embodiment and the previously described accumulator unit is the piston device 3) consists of a piston rod connected to a piston at each end. Like the previously described accumulator unit, this comprises a housing 4) with an inlet 13) for supplying the driving medium, and an outlet 8) for releasing pressurized hydraulic fluid. The piston device is axially displaceable in two cylindrical guides in the housing, and four pressure surfaces on the piston device thereby form a displaceable barrier between four separate chambers in the housing, of which;

- A first chamber I is filled with hydraulic fluid which is in contact with a first upwardly directed pressure surface on the piston arrangement, and when the piston device is displaced, this chamber will be able to absorb and release hydraulic fluid via the outlet 8)

- A second chamber II is made virtually depressurized, is in contact with a second upwardly directed pressure surface and is without any external connection other than a possible closed connection to a depth compensation valve.

- A third and fourth chamber (III,IV) each faces a downwardly directed surface of the piston device, and is connected to each other and to the inlet 13)

The connection between chamber II and chamber IV is provided by a channel 22) through said piston rod. The piston device cooperates with three sliding seals 2,2a,3) which prevent unwanted leakage between the chambers.

When assembling the accumulator, it is assumed that chamber II is open to the atmosphere via port 9) when the piston device is pushed into place. Further procedure is the same as for a first embodiment of the accumulator, so that chamber II is isolated in a compressed position, and thereby becomes virtually depressurized when it is expanded. This embodiment also includes a depth compensation valve (12) which is preferably arranged in the end cap 11) in the accumulator unit. The depth compensation can, like previously described versions, be self-regulating or manually adjustable. The dashed line between port 10) of the depth compensation valve and port 9) indicates the pressure connection that must be established between chamber II and for the self-regulating design of the depth compensation valve to function.

The propellant has pressed PS after it has passed through the depth compensation valve. This is the current pressure both in chamber III and in chamber IV, and will exert an upwardly directed force FU against the piston arrangement given by;

FU = PS*A3+ PS*(A1-A2) = PS*(A1+A3-A2).

Chamber II faces an oppositely directed pressure surface, but as this chamber is virtually pressureless, the counterforce from this chamber is negligible. This means that the force FU is transferred in its entirety to the hydraulic fluid in chamber I, which generates a counter force FD of magnitude

FD= PI*(A3-A2) , where PI is the absolute pressure produced in chamber I. Force and counterforce will be in equilibrium, and consequently applies; PS*(A1+A3-A2) = PI*(A3-A2)

The hydraulic pressure PH is defined as the excess pressure of the hydraulic fluid in relation to the ambient pressure. That is PH= PI-PAMB. Hence PH = PS*(1+A1/(A3-A2))-PAMB

In order to make a comparison between hydraulic capacity in this design of the accumulator unit in relation to the previously described design, A3=2*A2 is selected.

From this we find PH = PS*((A1+A2)/A2)-PAMB. Effective cross-section in chamber I is A3-A2 = A2. We set the piston device's stroke length L, and thereby the hydraulic capacity for this accumulator unit is given by ;

6. E = PH*A2*L = PS*(A1+A2)/A2)-PAMB)*A2*L

For the first described accumulator design, formula 2 (page 3) applies;

PH = PS*A1/A2 - PAMB

Also in this embodiment, chamber I has a cross-section of A2. Consequently, the earth hydraulic capacity for this design is given by; 7.

7. E = PH*A2*L = (PS*A1/A2 -PAMB)*A2*L

We see from formula 6 and formula 7 that the last described design has a greater capacity than the first when the designs are based on the same piston cross-section A1. Both versions are relevant, but the benefit of increased capacity must be weighed against the complexity.

Claims (4)

Claim 1. An underwater-based accumulator system comprising one or more accumulator units which are arranged to extract energy from a liquid received in an inlet (13) with a pressure approximately equal to the surrounding water pressure, and to utilize the energy extracted to produce a the desired excess pressure in a hydraulic fluid which is stored in the accumulator unit and is released via an outlet (8), characterized in that - the inlet (13) and the outlet (8) are arranged in a housing (4) which comprises two cylindrical guides for a piston device (3), the energy being extracted from the supplied liquid by the piston device (3) being displaced in these guides by an axially acting force which is produced by the downwardly directed pressure surfaces on the piston device (3) being affected by a pressure PS which arises from the the received liquid is routed through a valve device (12) which is arranged downstream of the inlet (13), the displacement of the piston arrangement being directed towards two chambers (I,II) which are in contact with the sealed pressure surfaces on the piston arrangement, and by the fact that one of these chambers (II) is made almost depressurised, said axial force is almost fully transferred to the other chamber (I) and produces a pressure increase on the hydraulic fluid stored in this chamber - the piston arrangement (3), which consists of a piston rod connected to one or two pistons, will, with a given choice of dimensions, cause a clear relationship between the pressure PS and the pressure produced in the hydraulic fluid, as the valve arrangement (12) can be arranged so that the pressure drop over the valve device is regulated so that the hydraulic fluid will be able to maintain the desired excess pressure in relation to the surrounding water pressure at any depth beyond a defined minimum depth 2 An underwater-based accumulator system according to claim 1, characterized in that the valve device (12) comprises a valve body (14) which cooperates with an annular seat (18) and is arranged in a housing (11) to be able to receive liquid from the inlet (13) and deliver this to chambers that are in contact with the upwardly directed pressure surfaces of the accumulator unit's piston device (3) via an opening that appears when the valve body's sealing surface (19) is displaced in relation to the seat (18), whereas, - the valve body (14) is displaceable along an axis running through the center of the seat and perpendicular to the seat plane, and has a cylindrical protrusion (16) with a sliding seal (15) which forms a leak-free barrier between a first chamber IV which is open to the supply line ( 13), and a second chamber V which is connected to the depressurized chamber II in the accumulator unit via a port (10), - a spring (17) or similar force-generating device exerts an adjustable downward force against the valve body (14), - the ratio between the cross-section of the seat (18) and the cross-section of the cylindrical protrusion (16) is adapted to the dimensions of the piston device (3) so that the pressure (PS) downstream of the seat (18) automatically adjusts so that a spring force set to zero will result in the pressure in chamber I and the inlet (13) becomes approximately the same 3. An underwater-based accumulator system according to claim 1, characterized in that - the valve device (12) comprises a valve body (14) which interacts with an annular seat (18) and is arranged in a housing (11) to be able to receive liquid from the inlet (13) and deliver this to chambers which are in contact with the upwards aligned the pressure surfaces of the accumulator unit's piston device (3), - release of liquid takes place via an opening that occurs when the valve body's sealing surface (19) is displaced in relation to the seat (18), - a spring (17) is arranged which is arranged to be able to exert an adjustable force which seeks to press a sealing surface (19) on the valve body against the seat (18) 4 An underwater-based accumulator system according to claim 1, characterized in that the hydraulic capacity for accumulator units and pressure converters is recovered by the relevant units being isolated from pressurized lines, as chambers to be refilled with liquid are connected to a supply line for liquid that has approximately the same pressure as surrounding water, and that chambers that are to be emptied of liquid are connected to the suction side of a pump which preferably discharges the liquid that is absorbed into a reservoir that is pressure equalized with surrounding water..