WO2020148667A1

WO2020148667A1 - Accumulator

Info

Publication number: WO2020148667A1
Application number: PCT/IB2020/050297
Authority: WO
Inventors: Nils Terje Ottestad
Original assignee: Obs Technology As
Priority date: 2018-11-12
Filing date: 2020-01-15
Publication date: 2020-07-23

Abstract

The present invention provides an underwater-based accumulator system comprising at least one accumulator unit adapted to extract energy from a driving medium in the form of a liquid received in an inlet (13) at a pressure approximately equal to ambient water pressure, and to utilize it. the energy to pressurize hydraulic fluid stored in the accumulator unit so that this hydraulic fluid, at any depth beyond a defined minimum depth, can be delivered via an outlet (8) with a desired overpressure relative to ambient pressure.

Description

ACCUMULATOR

Technical field of the invention

The invention relates to an underwater-based accumulator system which generates hydraulic power by utilizing the pressure difference between an approximate void and liquid supplied from a reservoir of pressure approximately equal to or lower than the ambient water pressure.

Background of the invention

There is an increasing need for providing better technology for generating hydraulic energy at great depths. Future oil recovery will occur from installations at increasingly larger water depth, and a prerequisite for the safe operation of such installations is that there is at all time an almost immediate access to a consi derable amount of hydraulic power. In today's situation, there is considered a great need for better and more compact solutions, especially at depths greater than 1500 meter sea water (msw). Gas-free accumulators designed to exploit the pressure difference between an ambient water pressure and an approximately pressure-free void in a pressure-resistant tank have great potential, especially as this technology becomes more efficient with increasing depth. The object of the invention is thus to provide an accumulator solution that utilizes this potential as best as possible in terms of good usability and optimal hydraulic capacity in relation to weight and size.

Prior art

Operation of equipment used in subsea oil operations is mainly based on exploitation of hydraulic power, the equipment being adapted to utilize hydraulic fluid overpressure relative to ambient pressure. Traditional hydraulic actuators are based on the pressure of a compressed gas being transmitted via a displaceable piston which acts as a barrier between the

compressed gas and the hydraulic fluid. Gas compressed to very high pressures loses pressure rapidly upon expansion, and consequently has limited ability to transfer energy. This ability is further impaired by thermal effects which create a temperature drop which causes a reduction in the transfer pressure.

In order to get more energy out of gas accumulators, depth-compensated accumulators have been developed whereby efforts have been made to establish forces that nullify the effect of a changed ambient pressure. This is achieved by means of a piston arrangement in which two oppositely directed surfaces are sensing respectively ambient pressure and an approximate zero pressure in a gas bottle. This solution requires costly machining and will not prevent the gas pressure from having to reach a pressure level where its compressibility is substantially reduced relative 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. The solution shown in WO 2015/154314 A is considered to represent the closest known technique in that, like the present invention, it is based on generating hydraulic power by utilizing the pressure difference between a liquid which initially has the same pressure as the surrounding water and the pressure in an approximate void.

The present invention has structural features that distinguish it from the prior art. A significant difference is the said valve arrangement which is arranged so that the hydraulic system in question can deliver fluid with a preset overpressure that is maintained at any depth beyond a certain minimum depth. This is a property that gives great operational and economic advantages in that it allows the hydraulic system to be moved over large depth intervals without having to be returned to the surface for being adapted to change of water depth. Summary of the Invention

The present invention is defined by the appended claims and in the following:

In a first aspect, the present invention provides an underwater-based accumulator system comprising at least one accumulator unit adapted to extract energy from a drive medium in the form of a liquid received at an inlet at a pressure approximately equal to ambient water pressure, and to utilize this energy to pressurize hydraulic fluid stored in the accumulator unit so that this hydraulic fluid, at any depth beyond a defined minimum depth, can be delivered via an outlet with a desired overpressure relative to ambient pressure, where the accumulator unit consists of a housing having a cylindrical guide for a piston arrangement (or piston device) with at least a first, a second and a third pressure surface, the piston arrangement being axially displaceable, dividing the housing into a first chamber, a second chamber and a driving chamber, the energy being extracted by passing the driving medium with a controlled pressure into the driving chamber which is in contact with the first pressure surface of the piston arrangement, in which the first pressure surface is greater than and, conversely directed to the second and the third pressure surface, the pressure of the drive medium being controlled by a depth compensation valve arranged upstream of the drive chamber and causing the piston arrangement to be displaced in the direction of the first and second chambers which are in contact with the second and third pressure surface respecti vely, in that one of the first and the second chamber is being rendered fluid-free and approximately pressure-free, so that the force exerted by the pressure of the driving medium again st the piston arrangement can be almost fully utilized to generate a pressure increase on a hydraulic fluid stored in the second of the first and the other chamber, the piston arrangement consists of a piston rod coupled to at least one piston, in that the piston rod and the at least one piston are comprising the first, the second and the third pressure surfaces which have a chosen proportional size ratio so as to obtain a desired ratio between the pressure of the drive medium and the pressure produced in the stored hydraulic fluid, the depth compensation valve is arranged in a valve housing and is adapted to maintain the hydraulic fluid overpressure relative to ambient water pressure at the wanted level by maintaining a defined linear relationship between ambient water pressure and the pressure in the drive chamber, the valve housing comprising a third chamber open to the inlet, a fourth chamber rendered substantially pressure-free, and a valve body having an outgrowth (or protrusion) which is leak-free displaceable between said third and fourth chambers, wherein the valve body cooperates with an annular seat disposed in the valve body so that a displacement of the valve body away from the annular seat creates an opening area which allows a drive medium to flow from the third chamber and on to the drive chamber, a spring or equivalent force generating means is provided which exerts an adjustable force on the valve body in the direction of the seat, the ratio between the cross-section of the seat and the cross-section of the cylindrical outgrowth is adjusted to the dimensions of the piston arrangement so that the pressure downstream of the seat automatically adjusts so that a spring force set to zero will cause the pressure in the first or second chamber containing the hydraulic fluid and the inlet to be approximately equal. In one embodiment of the subsea-based accumulator system, the flow of drive medium through the depth compensation valve is controlled by the valve body which interacts with the seat via a soft seal disk.

In one embodiment of the subsea-based accumulator system, the hydraulic capacity of the at least one accumulator unit can be recovered by isolating the relevant unit from pressurized lines, with chambers to be replenished with liquid being connected to a supply line of liquid having approximately the same pressure as ambient water, and that chambers to be emptied of liquid are connected to the suction side of a pump which preferably delivers the liquid which is admitted to a reservoir which is pressure equalized with ambient water. In one embodiment of the subsea-based accumulator system, the fourth chamber of the depth compensation valve is interconnected with that of the first and second chambers of the accumulator unit which are rendered virtually pressure-free.

Brief description of the drawings

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

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

Fig. 2 shows a principle view of a self-regulating valve device for controlling the hydraulic pressure

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

Fig. 4 shows an embodiment of a valve arrangement for manually adjusting the hydraulic pressure

Fig. 5 shows an embodiment of an accumulator system according to the invention

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

Fig. 7 shows a method for recovering hydraulic capacity for an accumulator system in operational use Fig. 8 shows an alternative embodiment of an accumulator unit according to the invention

Detailed description of the invention

The accumulator system according to the invention consists of one or more accumulator units which are connected to a valve arrangement which is arranged to control the pressure of the supplied liquid so that the accumulator unit (s) can maintain a preset pressure relative to the ambient pressure even if the ambient pressure changes substantially. In other words, the accumulator system is depth-compensated in the sense that it is adapted to generate a fixed hydraulic pressure relative to ambient water pressure at any depth beyond a defined minimum depth.

Compared to traditional gas-based accumulator systems, an accumulator system according to the invention has significant advantages in that it is independent of fluid supply via umbilical, has high performance in relation to weight and size, and is very user-friendly.

Figure 1) shows a view of a first embodiment of an accumulator unit according to the invention. It comprises a housing 4) with an inlet 13) for receiving the driving medium, and an outlet 8) for delivering pressurized hydraulic fluid. The supply pressure of the propellant (or drive medium) towards the inlet 13) is assumed to be approximately equal to the ambient water pressure P_AMB. The housing is composed of cylindrical guides for a piston arrangement 3) (or piston device). This divides the housing 4) into three separate chambers I-III and is composed of a piston rod of 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 assembly (12) which in this description will be referred to as a depth compensation valve. Here this valve is arranged in a valve housing 11) that is fit into an end cap of the accumulator unit. Its mode of operation will be explained later with reference to fig.2. The dotted 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

Chamber III is referred to as a drive chamber and has contact with the largest pressure surface of the piston arrangement. The drive medium is supplied to this chamber with a pressure Ps and will consequently create a force Fs = Ps * A1 against the piston arrangement via the pressure surface Al. This force seeks to push the piston arrangement in a direction away from the inlet 13) and will generate correspondingly large counterforces in the chambers I and II. However, the counterforce from chamber II is negligible because this chamber is virtually a pressure-free void. This is achieved by the gate 9) being blinded after the piston device has been pushed up towards the upper end position. In this context, the friction in the sliding seals 2.5) is considered insignificant, so that almost the total of Fs can be utilized to pressurize the hydraulic fluid in chamber I. The force balance of the axial forces affecting the piston device is thus given;

1.

where P_H is the hydraulic fluid overpressure relative to the ambient pressure P_AMB. from this; 2.

In the foregoing description, we have chosen to have the hydraulic fluid stored in chamber I and leave chamber II to be pressure-free. In principle, one can change the use of these chambers. Formula 2 shows that P_H can be maintained at a defined, fixed level if Ps is increased or decreased linearly with ambient pressure. This is achieved according to the invention by means of said depth compensation valve, which is arranged to control the relationship between Ps and P_AMB SO that P_H maintains a constant level. This means that the depth compensation valve controls the pressure drop on the supplied drive medium in relation to the ambient pressure. By inserting DP = (P_AMB - Ps) into formula 2 we find;

3.

An accumulator unit will only be able to generate a desired hydraulic pressure when it is arranged at a certain minimum depth determined by the relation A1/A2. The minimum depth is defined by the wanted hydraulic pressure being achieved with Ps = P_AMB, 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 is exceeded if the DP is regulated as follows;

4.

where Pamb is ambient pressure at the minimum depth Thus, the depth compensation valve 12) is arranged to control the DP according to 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 outgrowth 16) with a sliding seal 15) which forms a leak-free barrier between a first chamber I V open to the supply line 13) and a second chamber V connected to the virtually pressure-free chamber II in the accumulator unit via the port 10). The valve body 14), in conj unction with a fixed seat 18), controls the opening between the first chamber IV and chamber III of the accumulator unit.

Fig. 2 illustrates a situation where the depth compensation valve is in the open position. The valve body 14) is pushed upwardly, thereby providing an opening between the seat 18) and a sealing disk 19) mounted on the valve body. Thereby, the fluid supplied via the inlet 13) passes through chamber IV and on to chamber III as indicated by arrows. The cross section 18 of the seat is defined by an area A3, and the cylindrical outgrowth of the valve body 16) is defined by a cross section A4. This means that the fluid in contact with chamber IV has pressure similar to P_AMB and will affect the valve body 14) with a force of size

Frv = P_AMB * (A3-A4) directed against chamber III. Since chamber V is coupled to chamber II of the accumul ator unit, chamber V is virtually pressure-free, and the force applied to the valve body 14) from chamber V is thereby limited to the downward force Fv of a mounted spring 17). The pressure in chamber III is Ps = (P_AMB - DP), and this will exert an upward force of magnitude F_III = (P_AMB - DP) * A3 towards the valve body 14).

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

I.e.

Hence;

5.

We require Formula 4 and Formula 5 to be identical.

Consequently;

We see that this will be achieved if the geometry of the accumulator unit and the depth compensation valve are matched so that

We see that Fv = 0 will result in P_H = 0, which means that the hydraulic pressure is set to be in equilibrium with the surrounding water pressure. Accordingly, a positive force influence from the spring 17) is required for a positive hydraulic pressure to be generated, i.e., the hydraulic fluid has an overpressure relative to the ambient pressure. Furthermore, we see that the hydraulic pressure will be proportionally increasing with the spring tension Fv.

Example 1;

We want an accumulator unit to generate 345 bar at sea depth ³ 1000 msw - ie 1000 msw is set as the minimum depth. Subsequent calculations are simplified by setting the atmospheric pressure equal to 1 bar and increase of water pressure per 10-meter depth set to 1 bar. By inserting PS = 101 bar into formula 2, we find;

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

By inserting this value into formula 4 we further find;

DP=P_AMB*(1 -A2/A1) - Pamb*(l-A/Al) = 0.774*P_AMB-78.2 bar

As mentioned above, correspondence between the accumulator unit and the depth

compensation valve requires that; Fv/A3=PH*A2/Al=78.2 bar. If we choose A3 = 12 cm², the accumulator unit will generate a hydrauli c pressure of 345 bar if the set spring tensi on is of size Fv = 78.2 bar * 12cm² - corresponding to approximately 930 kp. If the spring tension is lower than this, the hydraulic pressure is reduced accordingly.

When the pressure in chamber IV is lower than Pamb, the valve body 14) is pressed to its lower position by the spring tension, providing a maximum opening between chamber IV and chamber III. When the pressure exceeds Pamb, the valve body will search for the position where the supply of propellant to chamber III is just large enough to maintain the desired force 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 that produce instantaneous, yet soft corrections of the valve body position. Oscillation suppression measures may be needed if the accumulator unit must be designed to cope with large and rapid variations in the consumption pattern. Such measures can be carried out using known techniques and will not be further discussed in this description. Depth compensation valve 12) acts as a one-way valve in the sense that it does not allow liquid to be extracted from chamber III via outlet 13). Therefore, a one-way valve 20) is provided 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 force directed towards chamber III must be established which is adjusted so that P_H is maintained at the desired level.

Figures 3 A and 3B show the relationship between depth and hydraulic pressure of an accumulator unit according to the specifications of Example 1. The dashed line of Fig. 3 A shows how P_H increases as a function of depth before the depth compensation valve comes into operation. The example is based on the ability of the accumulator unit to produce a hydraulic pressure P_H = 345 bar at a minimum depth of 1000 msw. If the adjustment of the spring tension is consistent with this, the depth compensation valve will operate at 1000 msw, preventing a further increase of P_H. If the spring tension is increased to double, the hydraulic pressure will rise to P_H = 690 bar before flattening. This will only then take place at a depth of 2000 meters - which under these assumptions will then represent the minimum depth for an accumulator unit which with A1 / A2 = 4.42 will deliver P_H = 690 bar. Fig. 3B shows correspondingly how the absolute pressure on the downstream depth compensation valve pressure, i.e. Ps, changes with the depth for the three settings. If no depth compensation valve was mounted on the accumulator unit, Ps and ambient pressure would be identical - given by the dotted line.

Fig. 4 shows an alternative embodiment in which a principle spring-loaded one-way valve is used in principle to be able to achieve the same hydraulic pressure at a new depth. A depth compensation valve in such an embodiment has no self-regulating function, as the DP will be uniquely determined by the set spring tension. One can thus adjust the accumulators to new depth by adjusting the spring tension - which will be possible in the sea using an ROY.

Components which in principle have the same function as in a self-regulating embodiment are given the same position number. A major disadvantage of using a non-self-regulating depth compensation valve is that relatively modest misalignment of pressure drop versus depth can result in large deviations of the P_H relative to the wanted value. The deviation from a wanted P_H value is a factor A1/A2 times as large as the misalignment of the pressure drop (cf.

formula 2 on page 3). A self-regulating depth compensation valve acc. to fig. 2 is based on the geometry of the valve to correspond to the geometry of the accumulator unit, and any deviation from the desired P_H value remains the same even if the depth changes.

A preferred application of an accumulator system according to the invention would be to provide hydraulic power for operations of limited duration, such as, for example, maintenance (work-over) operations of wells on an oil field. Overhaul of a single well can typically take place over a coupl e 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 overhaul, it is common to pull the work-over unit up to the surface and prepare it for the next well. For the total oil field, it must be possible to carry out the operation without having to make any profound intervention on any part of the equipment. The present invention can give significant operational advantages. It makes it possible to recover consumed hydraulic capacity on the seabed, and it allows the accumulator system to be moved to other water depths without requiring re-adjustments. Furthermore, it can provide sufficient hydraulic capacity to ensure that rather extensive operations can be performed without requiring regain of hydraulic capacity along the way. Already at a depth of 400 meters, accumulator units according to the invention can provide a significantly more favorable relationship between hydraulic capacity and weight / dimensioning than traditional gas-based accumulators. Tables 1 and 2 show the estimated relationship between minimum depth and hydraulic capacity of accumulator units with the following specifications;

Total length of about 2300 mm.

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

Hydraulic pressure P_H = 345 bar

Estimated weight is based on accepted standards for strength calculations that takes into is consideration 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 tons, dimensioned according to the given specifications, will have the capacity to deliver 200 liters of fluid with 345 bar hydraulic pressure at 400 meters depth before recharging. At a depth of 3000 meters, equivalent hydraulic capacity can be achieved with 7 accumulator units with a total weight of 3.2 tons.

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

Acc. to table 2, an accumulator unit configured for a minimum depth of 1000 msw can deliver 15.9 liters of liquid with P_H = 345 bar. Provided that the depth does not exceed the pressure class of the accumulator unit, it can provide 345 bar hydraulic pressure at any depth> 1000 msw, but the hydraulic capacity is limited to 15.9 liters. If the operational depth is increased to 3000 msw, it is more profitable to use accumulator units configured for a minimum depth of for example 3000 meters. The table shows that the hydraulic capacity is thereby increased to 32.9 liters, i.e. more than doubled. Accordingly, one should consider the utility benefits of using the accumulator unit over a wide depth range versus weight savings. In any case, it will be possible to achieve great weight savings compared to the use of traditional gas-based accumulators.

Preferred embodiments of an accumulator system according to the invention. In the following description we will first consider the main elements of a preferred structure of an accumulator system according to the invention. We will later, with reference to Fig. 7, provide an example of how such an accumulator system can be configured to have the ability to maintain / recover hydraulic capacity while in operational use.

Fig. 5 is a view of a simplified layout of an embodiment in which the accumulator system is intended to recover the hydraulic capacity on the surface. The accumulator system comprises 5 accumulator units arranged within the dotted frame labeled a) as well as two pressure converters arranged within the dotted frame b). All the accumulator units are configured to deliver the same hydraulic pressure, which here is selected to be 5000 psi. In general, it is beneficial to rely on a pressure level that will serve many consumers, and this pressure level should be relatively high since an accumulator unit with a large gain factor will extract more energy from the propellant medium. In this simplified setup, the number of delivery pressures beyond 5000 psi is limited to respectively. 10000 psi and 3000 psi. The drive medium is preferably circulated in a closed circuit via an elastic bellow 21) as indicated in the figure.

Pressure converters according to the invention are arranged to have the ambient pressure P_AMB as reference, and to convert the pressure according to a factor that is uniquely determined by the selected configuration. A pressure converter configured for a gain factor of 2 will produce a hydraulic pressure which in relation to P_AMB is twice as high at any depth.

The operation of the pressure converter is explained with reference to fig. 6. Like an accumulator unit, it consists of a housing 4) composed of two cylindrical guides and having a piston arrangement which is axially displaceable in these guides. The piston arrangement separates the housing into three separate chambers I-III and consists of a piston rod of cross- section A2 coupled to a piston whose pressure surface A1 faces the largest chamber I. Accordingly, the chambers II, III have effective pressure surfaces against the piston arrangement of respectively A2 and (A1-A2). The piston arrangement cooperates with two sliding seals 2.5) which prevent leakage between the chambers. Chamber II has open connection with liquid holding ambient pressure (P_AMB) via port 9), with chambers I and III assumed to contain liquid pressurized to respectively P_I and P_III. The pressure balance between these three chambers will be given by the formula;

which can be converted to

For example, if you choose A =Al/2, then supply of liquid with pressure P_H = 345 bar to chamber III will result in liquid with pressure P_H = 690 bar from chamber I. Similarly, supply of liquid with pressure P_H = 345 bar to chamber I will result in a delivery pressure P_H = 172.5 bar from chamber III.

Tables 1 and 2 (pages 8 and 9) illustrate the amount of hydraulic capacity that can be achieved by using accumulator units having a total length of about 230 cm and having 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 is initially high pressure, and this conversion takes place with little energy loss. Accordingly, a pressure converter will have a substantially greater hydraulic capacity than an accumulator unit of similar dimensions. For example, a pressure converter with A2 = A1/2 could convert 70 liters of liquid supplied from the accumulator units with P_H = 345 bar to 35 liters of liquid with P_H = 690 bar. Alternatively, 35 liters of liquid with P_H = 345 bar can be converted to 70 liters of liquid with P_H = 172.5 bar.

Fig. 7 outlines how an accumulator system composed of accumulator units and pressure converters can be interconnected so that it is possible to maintain / regain hydraulic capacity while the system is on the seabed. This process will hereinafter be referred to as charging. The main components of this simplified arrangement are two accumulator units arranged within the dotted frame labeled with a), and two converters located within the dotted frame marked with b).

Charging of accumulator units and inverters is preferably done using an electrically powered pump. The arrangement is arranged so that both the accumulator units and the pressure converters can be charged by a single pump, and that this charging operation can be performed without requiring high pump pressure. The setup in Fig. 7 is based on the propellant and hydraulic fluid being the same type of fluid and consequently being able to use the same storage bellows 21). It is easy to change this by connecting separate storage bellows.

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

Example 1. We look at the converter arranged to the right in fig. 7B), which is

adapted to reduce the hydraulic pressure P_H from 5000 psi to 3000 psi. This means that the ratio between the cross section A2 of the piston rod and the area Al of the piston is given at A2/A1 = 0.6. This means that equilibrium between the oppositely directed compressive forces against the piston device is given by;

Hence; P_AMB*A2 = P2*A2, giving P2 = P_AMB

Consequently, minimal amount of energy is required to regain hydraulic capacity on a converter.

Example 2. We look at the accumulator unit arranged to the left in fig. 6B) and assume that it is configured to deliver a hydraulic pressure P_H = 5000 psi (345 bar) at a minimum depth of 1000 msw. This means that the ratio between the cross section A2 of the piston rod and the area A1 of the piston is given by A2/A1 = 0.226. Equilibrium between upward and downward compressive forces against the piston device is now given by;

Accordingly, the suction side of the pump must be lower than 0.226 * P_AMB for the charging process to be completed. This means that the pump must operate at a real pump pressure > 0,774 * P_AMB to perform 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 that the piston arrangement 3) consists of a piston rod coupled to a piston at each end. Like the previously described accumulator unit, it comprises a housing 4) with an inlet 13) for supply of the driving medium, and an outlet 8) for delivery of pressurized hydraulic fluid. The piston arrangement is axially displaceable in two cylindrical guides in the housing, and four pressure surfaces of the piston arrangement 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 the piston arrangement via a first pressure surface facing away from the inlet 13), and when displacing the piston arrangement, chamber I will be able to receive and deliver hydraulic fluid via the outlet 8).

A second chamber II is made virtually pressure-free and is in contact with the piston arrangement via a second pressure surface facing away from the inlet 13). Chamber II is without any external connection other than a possible closed connection to a depth compensation valve.

A third and fourth chambers (III, IV) are both in contact with the piston device via pressure surfaces facing the inlet 13) and are connected to each other and to the inlet 13) via the depth compensation valve 12).

The connection between chamber II and chamber IV is provided by a channel 22) through said piston rod. The piston arrangement cooperates with three slidable seals 2.2a, 3) which prevent unwanted leakage between the chambers. When assembling the accumulator, it is assumed that chamber II is open to atmosphere via port 9) when the piston arrangement 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, thereby becoming virtually pressure-free when expanded. This embodiment also includes a depth compensation valve (12) which here is provided in the end cap 11) of the accumulator unit. Depth compensation is preferably a self-regulating embodiment in accordance to the invention. The dotted line between the port 10) of the depth compensation valve and the port 9) towards chamber II indicates that a self-regulating pressure compensation valve must be connected to a pressure-free chamber to achieve the wanted functionality. The drive medium has pressure Ps after passing through the depth compensation valve. This pressure is transmitted both to chamber III and to chamber IV and will exert a force Fu which seeks to push the piston device in a direction away from the inlet 13). This force is given by;

Chamber II is in contact with the piston arrangement via a pressure surface directed away from the inlet 13), but as this chamber is virtually pressure-free, the force that this chamber produces in the opposite direction of the Fu is very modest. For practical purposes, we can assume the Fu to be transferred in its entirety to the pressure surfaces which are in contact with the hydraulic fluid Hence the hydraulic fluid generates a counterforce of size

FD = P_I * (A3-A2), where PI is the absolute pressure produced in chamber I. Force and counterforce will be in equilibrium, and thus;

The hydraulic pressure P_H is defined as the hydraulic fluid overpressure in relation to the ambient pressure. I.e. P_H = PI-P_AMB. Hence P_H = Ps*(l+Al / (A3-A2)) - P_AMB

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

Hence

Effective cross section in chamber I is A3-A2 = A2. The stroke length of the piston device is L, and thus the hydraulic capacity of this accumulator unit is given by;

6.

Formula 2 (page 3) applies to the first described accumulator embodiment;

In this embodiment also, chamber I has a cross section of A2. Accordingly, the hydraulic capacity of this embodiment is given by;

7.

We see from Formula 6 and Formula 7 that the last described embodiment has greater capacity than the first when the embodiments are based on the same piston cross section Al. Both designs are relevant, but the advantage of increased capacity versus complexity should be taken into consideration

Claims

1. An underwater-based accumulator system comprising at least one accumulator unit designed to extract energy from a drive medium in the form of a liquid received at an inlet (13) at pressure approximately equal to the ambient water pressure, and to utilize this energy to pressurize hydraulic fluid that is stored in the accumulator unit so that this hydraulic fluid, at any depth beyond a defined minimum depth, can be released via an outlet (8) with a desired overpressure relative to ambient pressure, wherein the accumulator unit consists of a housing (4) having a cylindrical guide for a piston device (3) with at least a first, a second and a third pressure surface, the piston device being axially displaceable, dividing the housing into a first chamber (I), a second chamber (II) and a drive chamber (III), the energy being extracted by passing the driving medium with a pressure (Ps) into the driving chamber (III) which is in contact with the first pressure surface of the piston device (3), the first pressure surface being greater than, and conversely directed to, the second and third pressure surfaces, the pressure (Ps) of the drive medium being controlled by a depth compensation valve (12) arranged upstream of the drive chamber (III) and causing the piston device (3) to be displaced in the direction of the first (I) and the second (II) chamber having contact with the second and third pressure surface respectively, and one of the first (I) and the second (II) chamber being rendered liquid-free and approximately pressure-free such that the force which the pressure (Ps) of the drive medium applies to the piston device may be substantially fully utilized to produce a pressure increase on a hydraulic fluid stored in the other of the first (I) and second (II) chambers, the piston device (3) consists of a piston rod coupled to at least one piston, the piston rod and the at least one piston comprising the first, the second and the third pressure surfaces having a selected proportional size ratio so as to obtain a desired ratio between the drive medium pressure (Ps) and the pressure generated in the stored hydraulic fluid, the depth compensation valve (12) is arranged in a valve housing (11) and is adapted to maintain the hydraulic fluid overpressure relative to ambient water pressure at the wanted level by maintaining a defined linear relationship between ambient water pressure and the pressure (PS) in the drive chamber (III), said valve housing (11) comprising a third chamber (IV) open to the inlet (13), a fourth chamber (V) rendered substantially pressure-free, and a valve body (14) having an outgrowth (16) which is leak-free displaceable between said third and fourth chambers (IV, V), and the valve body (14) cooperates with an annular seat (18) arranged in the valve housing (11), such that a displacement of the valve body away from the annular seat creates an opening area which allows the drive medium to flow from the third chamber (IV) and on to the drive chamber (III), a spring (17) or equivalent force generating means is arranged to exert an adjustable force on the valve body (14) in the direction of the seat (18), the ratio between the cross-section of the seat (18) and the cross-section of the cylindrical outgrowth (16) is adapted to the dimensions of the piston device (3) so that the pressure (Ps) downstream of the seat (18) is automatically adjusted such that a spring force set to zero will cause the pressure in the first or second chamber (I) containing the hydraulic fluid and in the inlet (13) is approximately equal.

2. An underwater-based accumulator system according to claim 1, wherein the flow of drive medium through the depth compensation valve (12) is controlled by the valve body (14) which cooperates with the seat (18) via a soft seal disc (19).

3. An underwater-based accumulator system according to claim 1, wherein the hydraulic capacity of the accumulator unit can be recovered by isolating the relevant unit from pressurized lines, with chambers to be replenished with liquid being connected to a supply line for liquid having approximately the same pressure as surrounding water, and chambers to be emptied of liquid are connected to the suction side of a pump which preferably delivers the liquid which is absorbed into a reservoir which is pressure equalized with ambient water.

4. An underwater-based accumulator system according to claim 1, wherein the fourth chamber (V) is interconnected with that of the first (I) and the second (II) chamber of the accumulator unit which has been rendered virtually non-pressurized.