WO2018160071A1

WO2018160071A1 - Hydraulic system

Info

Publication number: WO2018160071A1
Application number: PCT/NO2018/050052
Authority: WO
Inventors: Nils Terje Ottestad
Original assignee: Obs Technology As
Priority date: 2017-02-28
Filing date: 2018-02-27
Publication date: 2018-09-07
Also published as: NO343020B1; NO20170285A1

Abstract

A subsea-based hydraulic system including a pump device (1) which is arranged to produce hydraulic power by extracting pressure energy from liquid that is being transferred from a reservoir (120) at approximately ambient pressure into a low-pressure tank (301) at an internal pressure which is lower than the ambient pressure, the pump device (1) being provided with a piston arrangement (24) which is reciprocated under the influence of a pressure difference between the reservoir (120) and the low-pressure tank (301 ), and the pump device (1) being arranged to produce a desired pressure in liquid that is supplied to the pump device (1) from the reservoir (120) or from another reservoir that has approximately the same pressure as a water mass surrounding the pump device (1).

Description

HYDRAULIC SYSTEM

The invention relates to a hydraulic system which is arranged to produce hydraulic energy for power-demanding subsea operations at any ocean depth, especially at depths in excess of roughly estimated 300 metres. The hydraulic system is based on a pump device which efficiently converts energy that can be obtained by utilizing the pressure difference between the ambient water pressure and a volume not filled with liquid in a tank at low pressure, so that even at moderate depths, hydraulic energy and pressure levels on a level with what is obtained with conventional subsea hydraulic systems can be produced.

US 6192680 B1 and NO 333477 B1 are both based on utilizing the difference between the internal pressure of a tank or container at low pressure and the pressure of the surrounding water mass to directly operate hydraulic equipment suitable therefor. In the existing design, the prior art is suitable for operational use at ocean depths in excess of roughly estimated 6,000 feet (about 1 ,825 metres), it being considered that the pressure generated at shallower ocean depths will be too low (cf. the description in US 6192680 B1 ) Background of the invention

The oil industry is constantly seeking simpler and more cost-effective ways of carrying out oil recovery at great depths. One of the great challenges is ensuring a sufficient supply of energy to make it possible at all times to carry out the operations necessary to prevent a potentially dangerous situation from escalating, for example shutting in a well by operating one or more valves on a wellhead.

Until today, it is, in the main, hydraulic power which is used for this purpose, as it is relatively easy to produce such hydraulic power by the use of accumulators in which the pressure of compressed gas is transmitted to liquid in bladder or piston accumulators. This is a solution which is considered to work satisfactorily at shallow and moderate depths. The hydraulic system is to provide energy for operating various types of actuators.

In 1999, a U.S. patent (US 6192680 B1) was granted for a new system for producing hydraulic energy at great ocean depths. The idea was to utilize the pressure difference between liquid which has ambient pressure and flexible volumes which achieve an approximately atmospheric pressure by means of a vent line up to the surface. It appears from the document that this solution was intended for depths in excess of roughly estimated 6,000 feet (about 1 ,825 metres). At a depth of 6,000 feet, the difference between ambient water pressure and atmospheric pressure will represent a driving pressure of 183 bars which may be utilized to operate hydraulic equipment suitable there- for. This equipment would have to be designed for a large external positive pressure, whereas conventional subsea-based hydraulic equipment is, in the main, designed for a hydraulic overpressure in relation to the ambient water pressure.

NO 333477 B1 discloses an alternative method for generating a low reference pressure in the form of an approximately pressure-less, liquid-free volume in a pressure-resistant tank. This is achieved by the tank in question first being filled completely with liquid which is then pumped out by means of a displacement pump suitable therefor. The most important advantage of this method is that a low reference pressure may be established down in the water mass without a vent line to the surface being required. Otherwise, the technology according to this document has, in the main, the same limitations as US 6192680 B1 , and, in the described embodiment, the solution is only suit- able for relatively great ocean depths.

Even at relatively shallow depths, considerable amounts of energy can be produced via actuators that are influenced by the difference between the ambient water pressure (PAmt>) and the pressure (Piank) in an approximately pressure-less tank. Potentially utilizable energy E is given by the formula

E = (P_Amb - PTank) * V = P_Amb * V,

in which V is the volume that can be delivered to the tank.

Hydraulic material used in subsea-based oil activity is, to a great degree, arranged to be operated by hydraulic pressures typically lying in the range of 200-350 bars above the ambient water pressure. This means that the above-mentioned potential energy will have to be converted to enable the operation of conventional hydraulic equipment.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art.

The object is achieved through the features that are specified in the description below and in the claims that follow. The invention concerned solves this by means of a new pump technology which is arranged to liberate and convert the potential energy in one and the same operation. Pump devices based on this technology can be scaled for quick production of large amounts of energy even at modest ocean depths. In the description of exemplary embodiments that follows, it is shown how hydraulic systems based on this pump technology may be able to operate large blowout preventers (BOPs) even at, as far as it goes, shallow ocean depths, such as 300 metres. The oil industry wishes to move more of the subsea-based oil activity onto the seabed. This new technology makes it possible to go in for subsea installations where most operations, possibly all, can be performed with the necessary power supply from surface-controlled subsea robots. The subsea robots can transmit electrical and/or hydraulic energy that can be used directly for operat- ing the various valve arrangements. The surface vessel may provide all the necessary logistics between the subsea installation and the vessel. This may involve components that need replacing, chemicals that are to be injected in the well etc. In a system like that, the hydraulic backup capacity that is required for relevant safety operations could lie more or less permanently stored on the seabed in the form of empty, leak-proof, low-pressure tanks. It is possible to compensate for the hy- draulic capacity spent, by empty liquid reservoirs and filled low-pressure tanks being raised to the surface and being returned after the contents of the low-pressure tanks have been pumped over to the liquid reservoirs. In extreme consequence, it is possible completely to avoid the use of an umbilical, and control functions and safety operations may be controlled by acoustic signals from the surface in cooperation with a suitable battery pack down on the installation. By utilizing these possibilities, it will also be possible to achieve substantial cost reductions related to the start-up as well as the operation of future subsea wells.

The invention is defined by the independent claim. The dependent claims define advantageous embodiments of the invention.

The invention relates more specifically to a subsea-based hydraulic system including a pump de- vice which is arranged to produce hydraulic power by extracting pressure energy from liquid that is being transferred from a reservoir at approximately ambient pressure to a low-pressure tank having an internal pressure which is lower than the ambient pressure, characterized by the pump device being provided with a piston arrangement with is reciprocated under the influence of a pressure difference between the reservoir and the low-pressure tank, the pump device being arranged to produce a desired pressure in liquid which is supplied to the pump device from the reservoir or from another reservoir which has approximately the same pressure as a water mass surrounding the pump device.

The piston arrangement may extend from at least two pump chambers into several drive chambers, a piston arranged in the piston arrangement forming a displaceable interface between the pump chambers and having an effective first piston area, and said pressure energy being extracted by the liquid being routed via the drive chambers of the pump device.

The drive chambers may be formed of a first chamber and a fourth chamber, the liquid being routed alternately into the first chamber and the fourth chamber.

The pump chambers may be formed of a second chamber and a third chamber, separated by the piston. The piston arrangement may be provided with two piston stems extending from the piston in axially opposite directions through fluid-sealing guides into a first chamber and a fourth chamber, respectively, and being provided with first piston areas which form the effective drive-pressure areas of the piston arrangement. The pump device may be provided with a change-over mechanism which is connected to the piston arrangement via an actuation rod and causes the piston arrangement to be reciprocated between defined end positions by causing the first chamber and the fourth chamber alternately to be in open communication with said reservoir and said low-pressure tank, respectively.

The change-over mechanism may include a first hydraulically controlled change-over valve, a sec- ond hydraulically controlled change-over valve and an actuation valve.

The first hydraulically controlled change-over valve may be connected to an inlet port, an outlet port and a first valve port in communication with the first chamber via a channel, the first hydraulically controlled change-over valve being formed of a first change-over-valve body and an annular first change-over-valve element which are displaceable in a cut-out in a housing and are arranged to close the first valve port.

The first change-over-valve body may be provided with a first projection which is arranged to be able rest sealingly against a first change-over-valve seat to be able thereby to shut off the communication between the inlet port and the first chamber, and a second projection which is arranged to be able to rest sealingly against a first valve-element seat which forms an annular mouth in the first valve element, to be able thereby to shut off the communication between the first chamber and the outlet port.

A fifth chamber may form a first actuation chamber, it being defined by the first change-over-valve element, and the first change-over-valve element, by alternating pressurization of the fifth chamber, producing a displacement of the first change-over-valve element, whereby the first change-over- valve body alternates between resting sealingly against the first change-over-valve seat and the first valve-element seat, so that an approximately leakage-free shifting between a first state, in which the communication between the inlet port and the first chamber is open and the communication between the first chamber and the outlet port is shut off, and a second state, in which the communication between the first chamber and the inlet port is shut off and the communication be- tween the first chamber and the outlet port is open, is provided.

The second hydraulically controlled change-over valve may be connected to the inlet port, the outlet port and a second valve port in communication with the fourth chamber, the second hydraulically controlled change-over valve being formed of a second change-over-valve body and a second change-over-valve element which are displaceable in a cut-out in the housing and are arranged to close the second valve port. The second change-over-valve body may be provided with a first projection which is arranged to be able to rest sealingly against a second change-over-valve seat to be able thereby to shut off the communication between the inlet port and the fourth chamber, and a second projection which is arranged to be able to rest sealingly against a second valve-element seat, which forms an annular mouth in the second change-over-valve element, to be able thereby to shut off the communication between the fourth chamber and the outlet port.

A sixth chamber may form a second actuation chamber which is defined by the second changeover-valve element, the sixth chamber being connected to the first chamber via a channel, and alternating pressure in the first chamber producing a displacement of the second change-over- valve element, which is the reverse of that of the first changeover-valve element, so that an approximately leakage-free shifting of the state of the second hydraulically controlled change-over valve to the reverse state of the one that the first hydraulically controlled change-over valve is in is provided.

The actuation valve may be connected to the inlet port, the outlet port and a third valve port which forms a connection to the first actuation chamber connected to the first change-over valve, the actuation valve being arranged to cooperate with the piston arrangement via the actuation rod by a displacement of the actuation rod being brought about every time the piston arrangement approaches a turning position, whereby the actuation valve shifts between a first state, in which a communication between the third valve port and the inlet port is open and the communication be- tween the third valve port and the outlet port is shut off, and a second state, in which the communication between the third valve port and the inlet port is shut off and the communication between the third valve port and the outlet port is open.

In what follows, an example of a preferred embodiment is described, which is visualized in the accompanying drawings, in which: Figure 1 a and figure 1 b show principle drawings in sections of the prior art, in which the difference between the ambient water pressure and a chamber or tank at low internal pressure is utilized to apply a working pressure to a working member;

Figures 2a and 2b show sections through a pump device according to the invention;

Figure 3 shows a section through a change-over mechanism for the pump device; and Figure 4 shows a principle drawing in section of a hydraulic system according to the invention.

The description that follows is simplified by the surface pressure being counted as 1 bara, and by the pressure in a water mass increasing by 1 bar for every 10 metres of increased depth. Example: Ambient pressure at a water depth of 300 metres = 1 + 300/10 bara = 31 bara. Further, liquid that has approximately the same pressure as a surrounding water mass will be termed a "pressure-equalized liquid".

A reservoir containing pressure-equalized liquid is termed a "pressure-equalized reservoir", and a tank containing a liquid-free volume at low pressure is termed a "low-pressure tank". Figures 1 a and 1 b show principle drawings of a hydraulic system which utilizes the pressure difference between a pressure-equalized liquid and a prior-art low-pressure tank 301 . In figure 1 a, a liquid-free volume is achieved in the low-pressure tank 301 , which is made approximately pressure- less at the same time, by a displacement pump 10 removing liquid from the low-pressure tank 301 . This is in accordance with the technology disclosed in NO 333477 B1 . In the set-up shown, the liquid is delivered to an elastic liquid reservoir 120.

Figure 1 b correspondingly shows an embodiment in accordance with US 6192680 B1 . Here, a liquid-free volume and approximately atmospheric pressure are achieved in the low-pressure tank 301 by means of a vent line 40 to the surface. Gas and liquid are kept separated by a leakage-free barrier in the form of a piston 20. Hydraulic consumers are represented by an actuator 80 accord- ing to figure 1 a. The direction of stroke of the actuator 80 may be changed by switching a directional control valve 100 according to figure 1 a. In the position shown, opening a valve 60 according to figure 1 a will lead to a port 90 according to figure 1 a being supplied with liquid from the reservoir 120 according to figure 1 a via a pipe connection 1 10 according to figure 1 a, and the actuator 80 delivering a corresponding amount of liquid to the low-pressure tank 301 via a port 70 according to figure 1 a. Consequently, the actuator 80 will contract.

In the prior art according to figure 1 a, the actuator 80 is given a defined driving pressure corresponding to the ambient water pressure. The practical consequence of this is that the hydraulic material must be adapted for the relevant depth, and a considerable ambient pressure is required before sufficient power is achieved to achieve a sufficiently quick actuation of, for example, a blow- out preventer (BOP).

A hydraulic system according to the invention is based on the use of at least a pump device which is arranged to liberate and convert the energy from liquid which is received from a reservoir of pressure-equalized liquid and delivered to a low-pressure tank. The pump device includes a piston arrangement which is reciprocated in cooperation with chambers that are in contact with respective pressure-sensing surfaces in the pump device.

Reference is now made to figure 2a which shows a section through a pump device 1 according to the invention. The pump device 1 has a housing 2 which is made up of five sections 22, 23, 30, 34, 36 that are preferably held together by external, longitudinal rods 2'. The pump device 1 includes a change-over mechanism 360 which is arranged in a first end section 36 of the housing 2. This co- operates with a piston arrangement 24 via an actuator rod 15 and will be described later with refer- ence to figure 3 which shows an enlarged view of the end section 36.

The piston arrangement 24 comprises a piston 39 and first and second piston stems 24a, 24b extending axially in opposite directions out from the piston 39.

In figure 2a, the piston arrangement 24 is shown as a compact element, but is made up, in a pre- ferred embodiment, of replaceable components as shown in figure 2b. The piston 39 is provided with a sliding seal 28, and the piston stems 24a, 24b are formed as two sleeves 38 and 40, respectively, which are held in position by a rod 36 arranged axially and two nuts 35, 37.

The piston arrangement 24 is arranged in a middle section 30 of the housing 2, the piston 39 forming a fluid-tight interface between a second chamber I I and a third chamber II I in the middle section 30. The second and third chambers II , III form pump chambers in the pump device 1 . The piston stems 38, 40 are displaceable in a leakage-free manner in sliding seals 25 and 26, respectively, which are arranged in guides 32 and 33, respectively. The guides 32, 33 separate the second chamber I I from an adjacent first chamber I, and the third chamber I II from an adjacent fourth chamber IV into which the respective piston stems 24a, 24b extend (see figure 2a) . Each of the piston stems 24a, 24b have first piston areas AD which form the effective drive-pressure areas of the piston arrangement 24 in the first and fourth chambers I , IV, respectively. The piston 39 is displaceable in a leakage-free manner in the middle section 30 and is provided with second piston areas Ap which form the effective pump-pressure areas of the piston arrangement 24 in the second and third chambers II , II I, respectively. Said change-over mechanism 360 provides for the first chamber I and the fourth chamber IV alternately to be in open communication with said reservoir 120 and with the low-pressure tank 301 , respectively. The communication between the fourth chamber IV and said change-over mechanism 360 runs via a pipe connection (not shown) between a port 35 in the first end section 36 and a port 21 in a second end section 22. The first chamber I and the fourth chamber IV are termed drive chambers in that the pressure changes generate force of alternating directions, and these pressure alternations cause the piston arrangement 24 to be reciprocated and transmit axially acting forces to liquid in the second chamber I I and the third chamber II I. The second chamber II and the third chamber I II are supplied with pressure-equalized liquid via a supply port 1 9, and the pressure- equalized liquid is directionally guided towards an outlet gate 29 by four check valves 1 7, 20, 17, 31 . The second chamber II and the third chamber I II thereby function as pump chambers. Said axially acting forces are transmitted to the liquid in the one of the second and third chambers I I, I II that is in compression mode. The force is transmitted via the pressure area Ap of the piston 39 and produces a defined pressure increase in this liquid.

The friction forces in the three sliding seals 26, 28, 33 are at a modest level in relation to the forces producing the displacement of the piston 39. These friction forces are disregarded in the considerations that follow concerning the connection between the ambient water pressure PAmb and the hy- draulic pressure PH produced by the pump device 1 .

The starting point for the considerations is the situation of the first chamber I being in open communication with a pressure-balanced reservoir 120 at a pressure PAmt>, and the fourth chamber IV thereby being in open communication with the low-pressure tank 301 at a pressure PTank (see fig- ure 4).

The piston arrangement 24 will then be affected by a right-hand directed force from the second and third chambers II, III (the drive chambers) of the magnitude

FD = (P_Amb - PTank) * AD = P_Amb * AD (PTank put at = 0)

As the inlet port 1 9 of the drive chambers II, III is in open communication with the pressure- balanced reservoir 120, the pressure of the second chamber II will be approximately equal to PAmb. This pressure produces a right-hand directed force against the piston arrangement 24 in a magni¬

Accordingly, the total right-hand force against the piston arrangement will be

FT = FD + FK = P_Amb * (AD + AP) This force is transmitted to the liquid in the third chamber III via a pressure area of a size Ap. This results in the liquid in the third chamber III being pressurized to

Pi = FT/AP = PAmb * (1 + AD/AP)

As the effective hydraulic pressure PH is the overpressure of the liquid in relation to the ambient pressure, it is found that:

Putting AD = Ap, it is found that PH = PAmb. This means that the pump device 1 according to the invention utilizes the pressure difference between the surrounding water mass and a zero pressure, that is to say the pressure in the low-pressure tank 301 , to raise the pressure in liquid which has already been pressure-equalized with the surroundings. The pressure on the liquid is raised from PAmb to 2 * PAmb, that is to say to a level lying PAmb above the ambient pressure. This conversion happens with a minimum of energy loss.

If correspondingly choosing AD = 4 * Ap, it is found that PH = 4 * PAmb.

So, if a pump device with this ratio of areas is operated at a depth of 500 metres, it will produce a hydraulic pressure PH = (1 + 500/1 0) bara * 4 = 51 bara * 4 = 204 bar above the ambient pressure. In this situation, the amount of liquid that the drive chambers II, III will have to be supplied with and deliver to the low-pressure tank 301 is 4 times larger than the amount of hydraulic fluid supplied to the hydraulic consumers. In addition to this, the change-over mechanism 360 has a consumption which may typically be 3-4 % of the consumption of the drive chambers II , II I. The hydraulic fluid which is pumped up to the hydraulic consumers will normally result in a simultaneous return of a corresponding amount of liquid. Consequently, this liquid is circulated in a way that does not affect the liquid contents of the reservoir or the low-pressure tank.

When a pump frequency is going up towards an upper value of roughly estimated 0.5 Hz, the viscosity of the liquid and friction in the sealing rings will lead to the hydraulic pressure falling somewhat in relation to the calculated value. This pressure fall will, in practice, be of little importance as the pump device 1 will be dimensioned for the liquid flowrate through the change-over mechanism 360 and associated fluid channels being at a modest level.

By means of the pump device 1 in question , it will thus, in principle, be possible to operate any type of hydraulically operated subsea equipment. At large depths, such as 3,000 metres, PAmb^— Plank will be 301 bara. Accordingly, Ap will have to be 50 % larger than AD if the pump device is to serve hydraulic consumers that are designed for a supply pressure of 200 bars. In a situation like that, the pump will deliver approximately 50 % more hydraulic fluid than the amount that will have to be delivered to the low-pressure tank.

Figure 3 shows a section through the first end section 36 which contains the change-over mechanism 360 which makes the piston arrangement 24 be reciprocated and able to transmit energy. The description that follows takes for a starting point that the pump device 1 is standing vertically.

The change-over mechanism 360 cooperates with the piston arrangement 24 via the actuation rod 15. The change-over mechanism 360 consists of an actuation valve 53 provided with an actuation- valve body 531 and an actuation-valve element 56, a first hydraulically controlled change-over valve 41 provided with a first change-over-valve element 41 1 and a first change-over-valve body 44, and a second hydraulically controlled change-over valve 47 provided with a second changeover valve body 471 and a second change-over-valve element 52. The actuation valve 53 initiates a switching of the direction of motion of the piston arrangement 24 when the actuation rod 15 is displaced axially between two defined positions. The actuator rod 15 is slidable relative to a channel 152 arranged axially in the piston arrangement 24 and is forced to be displaced together with the piston arrangement 24 when a projecting end portion 151 of the actuation rod 15 meets an abutment surface 16, 18 in the channel 152 (see figure 2a).

When the actuation rod 1 5 is displaced, the change-over mechanism 360 shifts between two states:

a first state which involves the first chamber I being in open communication with the inlet port 13 via a channel 59 and being shut off from the outlet port 14, and the fourth chamber IV simultaneously being in open communication with the outlet port 14 via the port 21 and a pipe connection, not shown, to the port 35 in the first end section 36 and being shut off from the inlet port 13; and

a second state which involves the first chamber I being in open communication with the outlet port 14 via the channel 59 and being shut off from the inlet port 13, and the fourth chamber IV simultaneously being in open communication with the inlet port 13 and being shut off from the outlet port 14.

In figure 3, the change-over mechanism 360 is shown in the first state.

The first hydraulically controlled change-over valve 41 directionally guides the liquid in and out of the first chamber I via the channel 59. The first change-over-valve element 41 1 is displaceable between an upper position and a lower position corresponding to the first and second states of the change-over mechanism 360. The first change-over-valve element 41 1 is displaced to the upper position by the actuation valve 53 pressurizing an actuation chamber V, and the second position is brought about correspondingly by the actuation valve 53 venting the same actuation chamber V. The second change-over-valve element 52 in the second change-over valve 47 may also be dis- placed between upper and lower positions. The position of the second change-over-valve element 52 is controlled by the pressure in sixth, seventh and eighth chambers VI, VII and VIII. The eighth chamber VIII is in permanently open communication with the inlet port 13 via a channel 49 and consequently has the pressure PAmt>. The seventh chamber VII is in permanently open communication with the outlet port 14 via a channel 51 and is consequently approximately pressure-less. By choosing a suitable ratio between the respective pressure surfaces of the second change-over valve element 52 in the second change-over valve 47, this is displaceable between the lower and upper positions by the sixth chamber VI being pressurized and vented, respectively. The two above-mentioned states are brought about by the sixth chamber VI having a permanently open channel 57 to the first chamber I, so that the second change-over-valve element 52 of the second change-over valve 47 immediately goes to its lower position when the first change-over-valve element 41 1 of the first change-over-valve 41 is displaced to its upper position, and immediately goes to its lower position when said first change-over-valve element 41 1 is displaced to its lower position. This means, then, that the second change-over-valve element 52 of the second change-over valve 47 is a slave under the first change-over-valve element 41 1 of the first change-over valve 41 . When the actuation valve 53 pressurizes the fifth chamber V, the first change-over-valve element 41 1 is pushed upwards. The first thing that happens then is that an annular first valve-element seat 46 of the first change-over-valve element 41 1 comes into contact with a first (lower) projection 44a on the first change-over-valve body 44. Thereby the opening between the channel 59 and the outlet port 14 is closed completely, so that the piston arrangement 24 is slowed down until at rest. The fifth chamber V is still supplied with liquid, and the first change-over-valve element 41 1 is moved further upwards, lifting a second (upper) projection 44b on the first change-over-valve body 44 up from a first change-over-valve seat 43. Thereby the communication between the inlet port 13 and a first valve port 45 is opened so that the first chamber I is pressurized via the channel 59. This pres- surization produces an immediate pressurization of the sixth chamber VI via the channel 57, so that the second change-over-valve element 52 is pushed downwards. This leads to a first (upper) projection 471 a on the second change-over-valve body 471 coming to rest against a second changeover-valve seat 48 so that the second change-over-valve body 471 cannot follow the continued downward movement of the second change-over-valve element 52. Thereby the fourth chamber IV is vented through the outlet port 14 via the opening created between a second (lower) projection 471 b on the second change-over-valve body 471 and an annular second valve-element seat 50 on the second change-over-valve element 52. This means that the change-over mechanism 360 has shifted from the second to the first state. The actuation valve 53 has as its function to initiate a change-over operation every time the piston arrangement 24 is near a turning position. Such an initiation involves the implementation of a quick pressurization or venting of the fifth chamber V. Figure 3 shows a preferred embodiment of an actuation valve consisting of an actuation-valve body 53 and an actuation-valve element 56. The actuation-valve element 56 has an upper position in which the actuation-valve body 53 has been pushed upwards and has opened to liquid supply from the inlet port 13 to the fifth chamber V via an outlet port 62. Correspondingly, the actuation-valve element 56 has a lower position in which the fifth chamber V is vented to the outlet port 14 via a channel 55.

On the underside of the actuation-valve element 56, an actuation-valve seat 61 is arranged, which is arranged to seal against a conical, projecting end portion 60 of the actuation rod 15. In the situa- tion shown, the change-over mechanism 360 is in the first state. This means that the first chamber I is pressurized and the piston arrangement 24 is moving towards the first end section 36. This state was initiated while the piston arrangement 24 was moving upwards, and the actuation-valve element 56 had been pushed so far upwards that the actuation-valve body 53 opened to a supply of liquid to the fifth chamber V. The first change-over-valve element 41 was then quickly pushed up- wards, and the first chamber I was pressurized in the moment when the upper projection of the first change-over-valve body 44 was lifted up from the first change-over-valve seat 43. This immediately stopped the upward movement of the piston arrangement 24. The pressurization of the first chamber I resulted in the second change-over-valve element 52 being pushed quickly downwards, and at the same time, the underside of the projecting end portion 60 of the actuation rod 15 was sup- plied with liquid via a channel 58 so that the actuation-valve element 56 was quickly pushed up to the upper position. This operation ensures that the change-over mechanism 360 will remain in the first state until the piston arrangement 24 is approaching its bottom turning position, pushing the actuation rod 15 downwards. The first thing that happens now is that the liquid supply to the fifth chamber V is shut off, and immediately afterwards, the venting of the fifth chamber V via the chan- nel 55 starts.

The channels in the change-over mechanism 360 are preferably so dimensioned that the time from when a change-over has been initiated until the piston arrangement 24 has reached full speed in the opposite direction is in the order of 1/10 second. The change-over mechanism 360 in question is specially developed for this pump device 1 . It has especially three important characteristics:

• It has no dead point. This means that the piston arrangement 24 starts from any position as soon as a certain minimum pressure difference has been established between the inlet port 13 and the outlet port 14.

• The change-over process involves a controlled slowing-down of the piston arrangement 24 until at rest and a corresponding controlled acceleration of reverse direction a few milliseconds after this. This gives a smooth flow and low mechanical strain on the system as a whole.

· The change-over mechanism 360 is easily scalable and may, for example, be dimensioned for the flow area of the liquid flow to correspond to a circular opening with a diameter of 40 mm.

• The change-over mechanism 360 is driven by energy which is taken from the liquid supplied to the drive chambers (the second and third chambers II, III) and is independent of electric control signals or any other form of external energy. It will typically consume 3-4 % of the amount of liquid that is supplied to the drive chambers (the second and third chambers II, III).

Figure 4 shows a principle drawing in section of how such a pump device 1 may be implemented in a hydraulic system according to the invention. The hydraulic consumer is represented by the actua- tor 80. Here, the low-pressure tank 301 may be isolated completely from all other parts of the system by a shut-off valve 60 at the low-pressure tank 301 being closed. In that case, the entire system, with the exception of the low-pressure tank 301 itself, will, in principle, have an internal pressure on a level with the ambient water pressure.

A subsea hydraulic system should preferably be able to operate various hydraulic consumers, and may therefore contain a number of pump devices producing different pressure levels.

In many connections, it will be blowout preventers (BOPs) and other capacity-demanding types of hydraulic equipment that will be dimensioning for the hydraulic system. While smaller valves can be operated by means of small pump devices, it will be natural to take larger pump devices as a basis for operating equipment requiring large hydraulic capacity. Below, examples of what is required of a system according to the invention to achieve a hydraulic capacity corresponding to 150 litres of liquid at a positive pressure of 245 bars at different depths are described. This is on a level with what is required for the operation of a BOP of a moderate size. Here, a pump device in which the piston guide in the middle section 30 has a diameter of 200 mm and the piston arrangement 24 has a stroke of 350 mm has been taken as a starting point. A pump like that will have a total length of about 100 cm and a total weight of roughly estimated 150 kg. In the calculations that follow, a pump device 1 in which the cylinder guide in the middle section 30 has a diameter = 200 mm is taken as a starting point. This means that AD + Ap = 314 cm². It is desirable to produce a hydraulic pressure of 245 bars at the following three depths: 300, 800 and 2,000 metres.

It is further taken as a basis that the pump device 1 is to have a stroke of 350 mm and a pump frequency of 0.5 Hz. It is chosen to dimension the change-over mechanism 360 for a flow area corresponding to a light opening of 0 = 40 mm.

Further it is taken as a basis that the hydraulic system is to deliver 1 50 litres of liquid at a pressure of 245 bars in 30 seconds (= 5 litres/second), and it is looked at how many pumps will be needed for this to be achieved .

Example 1

Depth = 300 metres

Ambient pressure 31 bara. P_H = PAmb * AD/AP = 31 * AD/AP = 245 bar

From this: AD/AP = 7.9. From this: AD =7.9 * (314 cm² - AD)

This gives AD = 278.7 cm² - corresponding to a diameter 0 = 1 88 mm.

This means that the drive chambers have a total volume of stroke V = 278.7 cm² * 35 cm = 9,755 cm³ = 9.76 litres.

As the pump device 1 is a double-acting one, 1 9.52 litres of liquid will pass through the pump chambers for every pump cycle (the consumption of the change-over mechanism 360 is not included in this).

With a pump frequency of 0.5 Hz, the pump will thus receive energy from 9.76 litres of liquid per second.

Under these conditions, the pump chambers will have a hydraulic capacity corresponding to 9.75/7.9 = 1 .23 l/sec of liquid at a pressure of 245 bars.

This corresponds to a power transmission W = 1 ,230 cm³/sec * 245 kp/cm² = 29.6 kW.

A total liquid supply of 5 l/sec is required. Each pump device 1 delivers 1 .23 l/sec. This means that 4-5 pump devices 1 (unit weight 1 50 kg) will be necessary.

The system requires low-pressure tanks of a total volume V = 1 50 litres * AD/AP * 1 .05 = 1 ,244 litres. This corresponds to 1 1 low-pressure tanks of 1 20 litres each. Weight 120 kg each - in total 1 ,320 kg. Weights of a pressure-balanced reservoir + liquid + 5 pump devices 1 are estimated at 0.5 tonnes,

1 .25 tonnes and 0.75 tonnes, respectively. Total weight < 3 tonnes.

Example 2 Depth = 800 metres Ambient pressure 81 bara. P_H = PAmb * AD/AP = 81 * AD/AP = 245 bar From this: AD/AP = 3.02. From this: AD =3.02 * (314 cm² - AD) This gives AD = 235.9 cm² - corresponding to a diameter 0 = 173.3 mm.

This means that the drive chambers have a volume of stroke V = 235.9 cm² * 35 cm = 8,257 cm³ =

8.26 litres. As the pump device 1 is a double-acting one, 16.52 litres of liquid will pass through the pump chambers for every pump cycle (the consumption of the change-over mechanism 360 not included).

With a pump frequency of 0.5 Hz, the drive chambers will thus receive energy from 8.26 litres of liquid per second. Under these conditions, the pump chambers will have a hydraulic capacity corresponding to 8.26/3.02 = 2.74 l/sec of liquid at 245 bars.

This corresponds to a power transmission W = 2,740 cm³/sec * 245 kp/cm² = 65.9 kW.

A total liquid supply of 5 l/sec is required. Each pump device delivers 2.74 l/sec. Thus, 2 pump devices (unit weight about 150 kg) will be necessary. A total liquid supply of 5 l/sec is required. Each pump device delivers 1 .23 l/sec. This means that 4- 5 pump devices (unit weight 150 kg) will be necessary.

The system requires low-pressure tanks of a total volume V = 150 litres * AD/AP * 1 .05 = 433 litres. This corresponds to 4 low-pressure tanks of 120 litres each. Weight 120 kg each - in total 840 kg.

Weights of a pressure-balanced reservoir + liquid + 2 pump devices are estimated at 0.5 tonnes, 0.44 tonnes and 0.3 tonnes, respectively. Total weight < 2.2 tonnes.

Example 3

Depth = 2,000 metres

Ambient pressure 201 bara. P_H = PAmb * AD/AP = 201 bar * AD/AP = 245 bar From this: AD/AP = 1 .22. AD + AP = 31 4 cm². From this: AD = 1 .22 * (314 cm² - AD)

This gives AD = 1 72.4 cm² - corresponding to a diameter 0 = 148 mm.

This means that the drive chambers have a volume of stroke V = 1 72.4 cm² * 35 cm = 6,034 cm³ = 6.03 litres.

As the pump device is a double-acting one, 1 2.06 litres of liquid will pass through the pump chambers for every pump cycle (the consumption of the change-over mechanism not included).

With a pump frequency of 0.5 Hz, the drive chambers will thus receive energy from 6.03 litres of liquid per second. Under these conditions, the pump chambers will have a hydraulic capacity corresponding to 6.03/1 .22 = 4.95 litres/sec of liquid at a pressure of 245 bars.

This corresponds to a power transmission W = 4,950 cm³/sec * 245 kp/cm² = 1 1 9.1 kW.

A total liquid supply of 5 l/sec is required. Each pump device 1 delivers 4.95 l/sec. This means that 1 -2 pump devices (unit weight about 1 70 kg) will be necessary.

The system requires low-pressure tanks of a total volume V = 1 50 litres * AD/AP * 1 .05 = 1 92.2 litres. This corresponds to 2 low-pressure tanks of 120 litres each. Weight 360 kg each - in total 720 kg.

Weights of a pressure-balanced reservoir + liquid + 2 pump devices are estimated at 0.5 tonnes, 0.2 tonnes and 0.3 tonnes, respectively. Total weight < 1 .8 tonnes.

The pump device 1 is adapted for the different depths by replacing the sleeves 38, 40 and the guides 32, 33.

As appears from the calculations, the energy conversion in a pump device 1 like that will increase with increasing depth. With a pump frequency of 0.5 Hz as a starting point, the pump will convert power in magnitudes of 30 kW at a depth of 300 metres, 66 kW at a depth of 800 metres and 1 1 9 kW at a depth of 2,000 metres.

It is found that at a depth of 300 metres, four to five pump devices will be necessary to deliver 1 50 litres of liquid at 245 bars in 30 seconds. At a depth of 200 metres, one pump device will almost be sufficient to do the same.

It is difficult to make a direct comparison with corresponding use of conventional gas-based accumulators. The dimensioning of a system like that may vary a great deal on the basis of a number of factors: thermal effects connected to quick expansion of the gas; how large pressure variations are accepted in liquid supplied; whether bladder or piston accumulators are used, and so on.

What can be ascertained is that at depths that large, a hydraulic system according to the invention will have a weight-/volume-saving of at least a factor of 10 in relation to gas-based accumulators with the same yield. This advantage is reduced when the depth decreases, with an assumed point of intersection at a depth of about 300 metres.

The set-up that is shown in figure 4 comprises a displacement pump 10. A pump like that is not a necessary component in a hydraulic system according to the invention. As mentioned earlier, it may be taken as a basis that the hydraulic system in its entirety is to be independent of surface connections in the form of an umbilical, etc. The hydraulic system may be based on low-pressure tanks 301 and filled liquid reservoirs 120 being taken down to the seabed by means of an ROV or the crane system on a surface vessel. They can be recycled via the surface when the low-pressure tank 301 has been filled up. Alternatively, the ROV may be provided with a pump device which can connect to the circuit and recover consumed energy by pumping liquid from the low-pressure tank 301 over to a pressure-balanced liquid reservoir 120.

Different types of hydraulic consumers have nominal operational pressures that the hydraulic system must be able to produce to ensure the completion of an operation cycle. However, parts of such a cycle may normally be carried out at a considerably lower pressure. A shear ram in a BOP may, for example, require a hydraulic pressure of 200 bars to perform a cutting operation. However, a substantial part of the movement of the hydraulic cylinders of the shear ram will be used to get the knives into position around the drill string. This part of the operation normally requires a hydraulic pressure of 20-30 bars at the most, but as much hydraulic capacity as if the entire sequence required 200 bars is still absorbed. In situations like that, it may be beneficial to connect a first pump device arranged to produce a hydraulic pressure of 40 bars, for example. The driving cylinders of this pump device will have, roughly estimated, 25 % of the liquid consumption of the other pump devices when delivering the same amount of hydraulics. The parts of the operational sequence that are not very power-demanding, may thereby be quicker and in addition be far less energy-demanding.

To prevent disturbing pressure waves by quick reciprocation of the pump device 1 described above, it may be an advantage to increase the dimensions of the piston arrangement 24 so that a desired pumping capacity is achieved at a lower pump frequency.

Even though a four-chambered pump device is described here, other solutions may be used as well. Norwegian patent No. 340558 discloses a pump type which, by moderate modification, can be used for this purpose.

It should be noted that all the above-mentioned embodiments illustrate the invention, but do not limit it, and persons skilled in the art may construct many alternative embodiments without departing from the scope of the attached claims. In the claims, reference numbers in brackets are not to be regarded as restrictive. The use of the verb "to comprise" and its different forms does not exclude the presence of elements or steps that are not mentioned in the claims. The indefinite article "a" or "an" before an element does not exclude the presence of several such elements.

Claims

C l a i m s

1 . A subsea-based hydraulic system comprising a pump device (1) which is arranged to produce hydraulic power by extracting pressure energy from liquid that is being transferred from a reservoir (120) at approximately ambient pressure to a low-pressure tank (301) at an internal pressure which is lower than the ambient pressure, c h a r a c t e r i z e d i n that the pump device (1) is provided with a piston arrangement (24) which is reciprocated under the influence of a pressure difference between the reservoir (120) and the low-pressure tank (301), the pump device (1) being arranged to produce a desired pressure in liquid that is supplied to the pump device (1) from the reservoir (120) or from another reservoir that has approximately the same pressure as a water mass surrounding the pump device (1).

2. A subsea-based hydraulic system according to claim 1 , c h a r a c t e r i z e d i n that the piston arrangement (24) extends from at least two pump chambers (II, III) into several drive chambers (I, IV), a piston (39) arranged in the piston arrangement (24) forming a displaceable interface between the pump chambers (II, III) and having an effective first piston area (Ap), and said pressure energy being extracted by the liquid being routed via the drive chambers (I, IV) of the pump device (1).

3. A subsea-based hydraulic system according to claim 2, wherein the drive chambers are formed of a first chamber (I) and a fourth chamber (IV), and the liquid is routed alternately into the first chamber (I) and the fourth chamber (IV).

4. A subsea-based hydraulic system according to claim 2, wherein the pump chambers are formed of a second chamber (II) and a third chamber (III) which are separated by the piston (39).

5. A subsea-based hydraulic system according to claim 2, wherein the piston arrangement (24) is provided with two piston stems (24a, 24b) extending from the piston (39) in axial- ly opposite directions through fluid-sealing guides (32, 33) into first and fourth chambers (I, IV), respectively and being provided with first piston areas (AD) forming the effective drive-pressure areas of the piston device (24).

6. A subsea-based hydraulic system according to claim 2, wherein the pump device (1) is provided with a change-over mechanism (360) which is connected to the piston arrangement (24) via an actuation rod (15) and causes the piston arrangement (24) to be reciprocated between defined end positions by causing a first chamber (I) and a fourth chamber (IV) alternately to be in open communication with said reservoir (120) and said low-pressure tank (301), respectively.

7. A subsea-based hydraulic system according to claim 6, wherein the change-over mechanism (360) includes a first hydraulically controlled change-over valve (41), a second hydraulically controlled change-over valve (47) and an actuation valve (53).

8. A subsea-based hydraulic system according to claim 7, wherein the first hydraulically controlled change-over valve (41) is connected to an inlet port (13), an outlet port (14) and a first valve port (45) in communication with the first chamber (I) via a channel (59), the first hydraulically controlled change-over valve (41) being formed of a first changeover-valve body (44) and an annular first change-over-valve element (41 1) which are displaceable in a cut-out in the housing (2) and are arranged to close the first valve port

(45) .

9. A subsea-based hydraulic system according to claim 8, wherein the first change-over- valve body (44) is provided with a first projection (44a) which is arranged to rest sealing- ly against a first change-over-valve seat (43) to be able thereby to shut off the communication between the inlet port (13) and the first chamber (I), and a second projection (44b) which is arranged to be able to rest sealingly against a first valve-element seat

(46) which forms an annular mouth in the first valve element (41 1), to be able thereby to shut off the communication between the first chamber (I) and the outlet port (14).

10. A subsea-based hydraulic system according to claim 8, wherein a fifth chamber (V) forms a first actuation chamber and is defined by the first change-over valve element (41 1), the first change-over-valve element (41 1), on alternating pressurization of the fifth chamber (V), producing a displacement of the first change-over-valve element (41 1), whereby the first change-over-valve body (44) alternates between resting sealingly against the first change-over-valve seat (43) and the first valve-element seat (46), so that an approximately leakage-free shifting between a first state, in which the communication between the inlet port (13) and the first chamber (I) is open and the communication between the first chamber (I) and the outlet port (14) is shut off, and a second state, in which the communication between the first chamber (I) and the inlet port (13) is shut off and the communication between the first chamber (I) and the outlet port (14) is open, is provided.

1 1 . A subsea-based hydraulic system according to claim 7, wherein the second hydraulically controlled change-over valve (47) is connected to the inlet port (13), the outlet port (14) and a second valve port (35) in communication with the fourth chamber (IV), the second hydraulically controlled change-over valve (47) being formed of a second change-over-valve body (471) and a second change-over-valve element (52) which are displaceable in a cut-out in the housing (2) and are arranged to close the second valve port (35).

12. A subsea-based hydraulic system according to claim 1 1 , wherein the second changeover-valve body (471) is provided with a first projection (471 a) which is arranged to rest sealingly against a second change-over-valve seat (48) to be able thereby to shut off the communication between the inlet port (13) and the fourth chamber (IV), and a second projection (471 b) which is arranged to be able to rest sealingly against a second valve-element seat (50) which forms an annular mouth in the second change-over-valve element (52), to be able thereby to shut off the communication between the fourth chamber (IV) and the outlet port (14).

13. A subsea-based hydraulic system according to claim 1 1 , wherein a sixth chamber (VI) forms a second actuation chamber and is defined by the second change-over-valve element (52), the sixth chamber (VI) being connected to the first chamber (I) via a channel (57), and alternating pressure in the first chamber (I) produces a displacement of the second change-over-valve element (52) which is the reverse of that of the first changeover-valve element (41 1), so that an approximately leakage-free shifting of the state of the second hydraulically controlled change-over valve (47) to the reverse state of the one that the first hydraulically controlled change-over valve (41) is in is provided.

14. A subsea-based hydraulic system according to claim 7, wherein the actuation valve (53) is connected to the inlet port (13), the outlet port (14) and a third valve port (62) which forms a connection to a first actuation chamber (V) connected to the first change-over valve (41), the actuation valve (53) being arranged to be able to cooperate with the piston arrangement (24) via the actuation rod (15) by a displacement of the actuation rod (15) being produced every time the piston arrangement (24) approaches a turning position, whereby the actuation valve (53) shifts between a first state, in which a communication between the third valve port (62) and the inlet port (13) is open and the communication between the third valve port (62) and the outlet port (14) is shut off, and a second state, in which the communication between the third valve port (62) and the inlet port (13) is shut off and the communication between the third valve port (62) and the outlet port (14) is open.