Heave compensator and method for reducing the risk of snap-loads during the splash-zone phase

Abstract

A method and heave compensator for eliminating snap-load and heave effects at offshore deposition of a load into or onto the sea or seabed involves a heave compensator suspended between the load and the lifting device having a relatively stiff stroke response at small to moderate stroke lengths and then a softer stroke response at larger stroke lengths to avoid exceeding the dynamical amplification factor (DAF)-limitations of the crane/lifting device or on the load.

Claims

1. A heave compensator intended to be suspended between a lifting device and a load to be lifted, wherein the heave compensator comprises: a main piston housing comprising: an upper end having a hook for releasable attachment of the lifting device, a lower end having an opening adapted to provide a fluid tight enclosure around a piston rod, an inner space divided by a slide-able first piston into an upper chamber and a lower first liquid filled chamber, and the piston rod comprising a first end having a hook for releasable attachment of the load, and a second end attached to the piston, and which stretches through the liquid filled chamber and further a distance out of the opening at the lower end, a first accumulator comprising: an upper end, a lower end, and an inner space divided by a slide-able piston into an upper chamber filled with a gas and a lower chamber filled with a liquid, a second accumulator comprising: an upper end, a lower end, and an inner space divided by a slide-able piston into an upper chamber filled with a gas and a lower chamber filled with a liquid, and a liquid distribution circuit comprising: a liquid manifold, a first liquid conduit which in a first end is fluidly connected to the liquid filled chamber of the main piston housing and in a second opposite end is fluidly connected to the liquid manifold, a second liquid conduit which in a first end is fluidly connected to the liquid filled chamber of the first accumulator and in a second opposite end is fluidly connected to the liquid manifold, a third liquid conduit which in a first end is fluidly connected to the liquid filled chamber of the second accumulator and in a second opposite end is fluidly connected to the liquid manifold], and wherein the total volume of liquid in the heave compensator is adapted such that when the piston of the main piston housing is in an initial position, which is the closest obtainable proximity of the upper end of the cylindrical main piston housing, and the piston of the second accumulator is an initial position, which is the closest obtainable proximity of the lower end of the second accumulator, the piston of the first accumulator becomes positioned substantially in the middle of the inner space of the first accumulator, and the amount of preloaded gas in the upper chamber of the first accumulator is adapted to give a gas pressure of p.sub.1 and the amount of preloaded gas in the upper chamber of the second accumulator is adapted to give a gas pressure of p.sub.2, where p.sub.2>p.sub.1, when the piston of the first accumulator and the piston of the second accumulator are in initial working positions.

2. The heave compensator according to claim 1, wherein at least one of the first, second and third liquid conduits is equipped with a valve for regulating the flow of liquid through the at least one of the liquid conduits.

3. The heave compensator according to claim 2, wherein the valve being applied on the at least one of the first, second and third liquid conduits is able to regulate the flow of liquid into and out of the liquid filled chamber of the main piston housing.

4. The heave compensator according to claim 1, further comprising a gas distribution circuit comprising: a first gas conduit fluidly connected to the gas filled upper chamber of the first accumulator, the first gas conduit has a valve for regulating a flow of gas in the first gas conduit, a second gas conduit fluidly connected to the gas filled upper chamber of the second accumulator, the second gas conduit has a valve for regulating a flow of gas in the second gas conduit, a third gas conduit fluidly connected to an environment exterior of the heave compensator, the third gas conduit has a valve for regulating a flow of gas in the third gas conduit, and a gas manifold fluidly interconnecting the first, the second, and the third gas conduits.

5. The heave compensator according to claim 4, further comprising: a pressure and temperature sensor located in the gas-filled upper chamber of the first accumulator, a pressure and temperature sensor located in the gas-filled upper chamber of the second accumulator, a position sensor located on the piston of the main piston housing, a position sensor located on the piston of the first accumulator, a position sensor located on the piston of the second accumulator, a pressure and temperature sensor located in the lower chamber of the main piston housing, a pressure and temperature sensor located on the outside of the heave compensator for measuring the pressure and temperature of the environment exterior of the heave compensator, and a control unit having computer software which enables determining the equilibrium position of the piston of the main piston housing as from the sensor data, and then determine whether to vent off an amount of gas from the first accumulator or to extract an amount of gas from the second accumulator into the first accumulator as described above, and which enables engaging and controlling actuators of the valves of the gas distribution circuit to obtain the intended venting off or injection of gas out off/into the first accumulator.

6. The heave compensator according to claim 1, wherein the total amount of liquid in the heave compensator is adapted, when the piston of the main piston housing and the piston of the second accumulator are in initial positions, to make the piston of the first accumulator to be in an initial position in the range of from to , where the position is defined to be zero when the piston is at the upper end of the inner space of the main piston housing or first or second accumulator, respectively, and increasing linearly towards the lower end where the position per definition is set to 1.

7. The heave compensator according to claim 1, wherein: a pre-loaded amount, n.sub.1, of gas in the upper chamber of the first gas accumulator is determined by applying a gas equation of state with a gas pressure of: p.sub.1=(DAF(t).Math.m.sub.load.Math.gp.sub.atm.Math.A.sub.m)/A.sub.1 and a gas volume V.sub.1 equal to the volume of the upper chamber of the first accumulator when the piston is in an initial working position, where: A.sub.1 is the upper surface area of the piston of the first accumulator, m.sub.load is the mass of the load, g is the gravity of Earth, p.sub.atm is the atmospheric pressure, and A.sub.m is the surface area of the upper side of the piston of the main piston housing, and a pre-loaded amount, n.sub.2, of gas in the upper chamber of the second gas accumulator is determined by applying a gas equation of state with a gas pressure of: p.sub.2=.sub.1p.sub.1, where .sub.1 is a constant in the range of [1.1, 0.95.Math.DAF.sub.max], [1.15, 0.90.Math.DAF.sub.max], [1.20, 0.85.Math.DAF.sub.max], or (DAF.sub.stat+DAF.sub.max)/2, and where DAF.sub.max1.5.

8. The heave compensator according to claim 1, further comprising: a third accumulator comprising: an upper end, a lower end, and an inner space divided by a slide-able piston into an upper chamber filled with a gas and a lower chamber filled with a liquid, a fourth accumulator comprising: an upper end, a lower end, and an inner space divided by a slide-able piston into an upper chamber filled with a gas and a lower chamber filled with a liquid, and where the liquid distribution circuit further comprises: a fourth liquid conduit having a valve for regulating the flow of liquid in the liquid conduit, and which in a first end is fluidly connected to the liquid filled chamber and in a second opposite end is fluidly connected to the liquid manifold, and a fifth liquid conduit having a valve for regulating the flow of liquid in the liquid conduit, and which in a first end is fluidly connected to the liquid filled chamber and in a second opposite end is fluidly connected to the liquid manifold, and wherein the total volume of liquid in the heave compensator is adapted such that when: the piston of the main piston housing is in its initial position, which is 0, the piston of the first accumulator becomes positioned in its initial position, which is from 0 to , the piston of the second accumulator is in its initial position, which is 1, where the position is defined to be zero when the piston is at the upper end of the inner space of the main piston housing or first or second accumulator, respectively, and increasing linearly towards the lower end where the position per definition is set to 1, and wherein the amount of gas in the upper chamber of the first accumulator is adapted to give a gas pressure of p.sub.1 and the amount of gas in the upper chamber of the second accumulator is adapted to give a gas pressure of p.sub.2, when the piston of the first accumulator, the piston of the second accumulator are in initial working positions, and where p.sub.2>p.sub.1.

9. The heave compensator according to claim 8, further comprising a gas distribution circuit comprising: a fourth gas conduit having a valve for regulating a flow of gas in the fourth gas conduit, and which in a first end is fluidly connected to the gas filled upper chamber of the third accumulator and in a second opposite end is fluidly connected to the gas manifold, and a fifth gas conduit having a valve for regulating a flow of gas in the fifth gas conduit, and which in a first end is fluidly connected to the gas filled upper chamber of the fourth accumulator and in a second opposite end is fluidly connected to the gas manifold.

10. The heave compensator according to claim 8, further comprising: a pressure and temperature sensor located in the gas-filled upper chamber of the third accumulator, a pressure and temperature sensor located in the gas-filled upper chamber of the fourth accumulator, a position sensor located on the piston of the third, and a position sensor located on the piston of the fourth accumulator.

11. The heave compensator according to claim 8, wherein: the initial working position of the piston of the first accumulator is substantially , the initial working position of piston of the second accumulator is substantially 1, and the initial working position of piston of the third accumulator is substantially 1, where the position is defined to be zero when the piston is at the upper end of the inner space of the main piston housing or first or second accumulator, respectively, and increasing linearly towards the lower end where the position per definition is set to 1, and a pre-loaded amount, n.sub.1, of gas in the upper chamber of the first gas accumulator is determined by using a gas equation of state with a gas volume V.sub.1 equal to the volume of the upper chamber of the first accumulator when piston is in an initial working position, and a gas pressure of:
p.sub.1=(DAF.sub.stat.Math.m.sub.load.Math.gp.sub.atm.Math.A.sub.m)/A.sub.1, where: m.sub.load is the mass of the load, g is the gravity of Earth, p.sub.atm is the atmospheric pressure, A.sub.m is the surface area of the upper side of piston of the main piston housing, and A.sub.1 is the surface area of the upper side of piston of the first accumulator, a pre-loaded amount, n.sub.2, of gas in the upper chamber of the second gas accumulator is determined by applying a gas equation of state with a gas volume V.sub.2 equal to the volume of the upper chamber of the second accumulator when piston is in an initial working position, and a gas pressure of: p.sub.2=.sub.2.Math.p.sub.1, where .sub.2 is a real number constant having a value in the range of [1.2, 0.97.Math.DAF.sub.max], preferably of [1.3, 0.95.Math.DAF.sub.max], and most preferably (DAF.sub.stat+DAF.sub.max)/1.9, and where DAF.sub.max1.5, and the amount, n.sub.3, of gas being pre-loaded into chamber of the third gas accumulator is determined by applying a gas equation of state with a gas volume V.sub.3 equal to the volume of the upper chamber when piston is in an initial working position, and a gas pressure of: p.sub.3=.sub.3.Math.p.sub.1, where .sub.3 is a real number constant having a value in the range of [1.3, 0.98.Math.DAF.sub.max], preferably of [1.4, 0.96.Math.DAF.sub.max] and most preferably of (DAF.sub.stat+DAF.sub.max)/1.8, and where DAF.sub.max1.5 and p.sub.3>p.sub.2>p.sub.1.

12. The heave compensator according claim 8, where: the main piston housing is a cylindrical piston cylinder having a slide-able piston dividing an inner space thereof into an upper vacuum chamber and a lower oil-filled chamber, and where the piston rod has a hook for releasable attachment of a load and the upper end of the main piston cylinder has a hook for releasable attachment of a crane/lifting device, the cylindrical piston cylinder is located in the centre of the first, second, third, and fourth accumulators being arranged in a quadratic configuration of one accumulator in each corner, the cylindrical piston cylinder and the first, second, third, and fourth accumulators are arranged in parallel and mechanically attached in an intended configuration by an upper and a lower assembly plate located at the upper and lower ends of the main piston cylinder and accumulators, respectively, and wherein: the gas distribution manifold is integrated into the upper assembly plate by a set of linear bores in the assembly plate, where: bore is made from a side of the upper assembly plate and extends inward in the upper assembly plate until passing through the centre axis of the first accumulator and the centre axis of the second accumulator, where entrance of the bore into the upper assembly plate is gas tight blocked by a welded plug, and where the bore is fluidly connected to the upper chamber of the first accumulator by the first gas conduit and the upper chamber of the second accumulator by the second gas conduit, the bore is made from a side of the upper assembly plate and extends inward in the upper assembly plate until passing through the centre axis of the third accumulator and the second accumulator, where entrance of the bore into the upper assembly plate is gas tight blocked by a welded plug, and where the bore is fluidly connected to the upper chamber of the third accumulator by the third gas conduit and the upper chamber of the second accumulator by the second gas conduit, and the bore enters from a side of the upper assembly plate and extends inward in the upper assembly plate until passing through the centre axis of the fourth accumulator and the first accumulator, where the entrance of the bore into the upper assembly plate is gas tight blocked by a welded plug, and where the bore is fluidly connected to the upper chamber of the fourth accumulator by the fourth gas conduit and the upper chamber of the first accumulator by the first gas conduit, and the liquid distribution manifold is integrated into the lower assembly plate by a set of linear bores in the lower assembly plate, where: bore is made from a side of the lower assembly plate and extends inward in the lower assembly plate until passing through the centre axis of the first accumulator and the centre axis of the second accumulator, where the entrance of the bore into the lower assembly plate is fluid tight blocked by a welded plug, and is fluidly connected to the second liquid conduit of the first accumulator and the third liquid conduit of the second accumulator, bore enters from a side of the lower assembly plate and extends inward in the lower assembly plate until passing through the centre axis of the third accumulator and the centre axis of the second accumulator, where: the entrance of the bore into the lower assembly plate is fluid tight blocked by a welded plug, and is fluidly connected to the fourth liquid conduit of the third accumulator and the third liquid conduit of the second accumulator, bore enters from a side of the lower assembly plate and extends inward in the lower assembly plate until passing through the centre axis of the fourth accumulator and the centre axis of the first accumulator, where: the entrance of the bore into the lower assembly plate is fluid tight blocked by a welded plug, and is fluidly connected to the fifth liquid conduit of the fourth accumulator and the second liquid conduit of the first accumulator, bore is fluidly connected to the first liquid conduit and bore, bore is fluidly connected to the first liquid conduit and bore, and bore is fluidly connected to the first liquid conduit and bore.

13. The heave compensator according to claim 12, wherein: the upper assembly plate has a circular throughgoing cut forming a circular opening centred at the longitudinal centre axis at each of the four accumulators, where each of the four cuts has a recess in the assembly plate running all the way around the circular cutting edge at a height corresponding to the location of the bores, a circular insert being fitted to gas-tightly close the opening formed by the circular throughgoing cut is inserted in each of the circular openings, and where each of the circular inserts has a recess running along a lateral edge which corresponds to the recess in the assembly plate such that an annular channel running around the circular insert is formed inside the upper assembly plate, and wherein each of the first, second, fourth, and fifth gas conduit is integrated in a circular insert by comprising a horizontal bore into the circular insert, a first vertical bore fluidly connected in one end to the horizontal bore and in the other end to an inner space of a solenoid valve, and a second vertical bore fluidly connecting the inner space of the solenoid valve with the upper chamber of the accumulator being located below the circular insert.

14. The heave compensator according to claim 12, wherein: the second liquid conduit is formed by a vertical bore in the bottom of the first accumulator fluidly connecting the lower chamber of the accumulator with bores, and which has a solenoid valve located in the conduit able to close and open the liquid conduit, the third liquid conduit is formed by a vertical bore in the bottom of the second accumulator fluidly connecting the lower chamber of the accumulator with bores, and which has a solenoid valve located in the conduit able to close and open the liquid conduit, the fourth liquid conduit is formed by a vertical bore in the lower end of the third accumulator fluidly connecting the lower chamber of the accumulator with bore, and which has a solenoid valve located in the conduit able to close and open the liquid conduit, the fifth liquid conduit is formed by a vertical bore in the bottom of the fourth accumulator fluidly connecting the lower chamber of the accumulator with bore, and which has a solenoid valve located in the conduit able to close and open the liquid conduit, and the first liquid conduit consists of three equal conduits, each comprising a horizontally oriented bore fluidly connected to the lower chamber of the main piston cylinder and a vertically oriented bore fluidly connected to one of bores, respectively, and where each of the vertically oriented bores has a solenoid valve able to close and open the conduit.

15. The heave compensator according to claim 7, wherein the gas equation of state being applied to determine the gas amounts to be preloaded into the first and the second accumulator is either; the ideal gas law, Van der Waal's equation for real gases, or the Peng-Robinson gas equation of state.

16. A method for reducing the risk of snap-loads during the splash-zone phase when a load is deployed into the sea/water from a floating deployment vessel having a lifting device, wherein the method comprises the following steps: A) applying a heave compensator according to claim 4, B) preparing the heave compensator before commencing the lifting operation by: placing the piston of the second accumulator in a lower position in the closest possible proximity to the second end, and loading the upper chamber of the second accumulator with an amount of a gas sufficient to obtain gas pressure p.sub.2, and loading the upper chamber of the first accumulator with an amount of a gas sufficient to obtain gas pressure p.sub.1, wherein p.sub.1<p.sub.2, and releasable attaching a lifting device to the attachment means and releasable attaching the load to the attachment means, C) executing the dry-zone phase of the lifting operation by lifting of the load from the basement of the deployment vessel by the lifting device, G) executing the splash-zone phase of the lifting operation by lowering the load to make contact with the water/sea by use of the lifting device.

17. The method according to claim 16, wherein: step A) applies a heave compensator wherein at least one of the first, second and third liquid conduits is equipped with a valve for regulating the flow of liquid through the liquid conduits and which at least comprises a valve on the first liquid conduit, step B) further comprises opening valve of the first liquid conduit, and if present, opening valve the second liquid conduit, after loading the upper chamber of the second accumulator with gas and before loading the upper chamber of the first accumulator with gas, and wherein the method further comprises the following step to be performed after step C) and before step G): D) if valve of the first liquid conduit is closed, open valve after the load is lifted to a safe distance above a deployment vessel thereof in order to engage the heave compensation effect of the heave compensator.

18. The method according to claim 17, further comprising the following process step which is to be applied after step D) and before step G): E) determining the equilibrium position of piston of the main piston housing by: 1) measuring the position of piston of the main piston housing by the position sensor and employing the measured positions to determine the measured equilibrium position, S.sub.k, of the piston, 2) comparing the measured equilibrium position, S.sub.k, with a predetermined intended equilibrium position S.sub.0, of the piston, and 3) determine the difference |S.sub.0S.sub.k|, F) executing the following sub-process steps: 1) if |S.sub.0S.sub.ki<K.sub.1, where K.sub.1 is a predetermined adjustment threshold criteria, then abort the dry phase adjustment of the equilibrium position and go directly to step G) below, or else continue to sub-step 2) below: 2) if S.sub.0S.sub.k>0, then: i) open valve on the first gas conduit and valve on the third gas conduit, ii) continuously measuring the position of piston of the main piston housing by the position sensor and employing the measured positions to continuously determine the measured equilibrium position, S.sub.k, of the piston, and then continuously determining the difference |S.sub.0S.sub.k| and comparing the continuously determined difference with the predetermined adjustment threshold criteria, and if |S.sub.0S.sub.k|<K.sub.1, close valve on the first gas conduit and valve on the third gas conduit, and go directly to step G), or: 3) if S.sub.0S.sub.k<0, then: j) open valve on the first gas conduit and valve on the second gas conduit, jj) continuously measuring the position of piston of the main piston housing by the position sensor and employing the measured positions to continuously determine the measured equilibrium position, S.sub.k, of the piston, and then continuously determining the difference |S.sub.0S.sub.k| and comparing the continuously determined difference with the predetermined adjustment threshold criteria, and if |S.sub.0S.sub.k|<K.sub.1, close valve on the first gas conduit and valve on the second gas conduit, and go directly to step G).

19. The method according to claim 18, further comprising the following process step to be performed after step G): H) equalising the pressures of the first and second gas accumulator by opening valve on the first gas conduit and valve on the second gas conduit.

20. The method according to claim 19, further comprising the following process steps to be performed after step H): I) determining the equilibrium position of piston of the main piston housing by: 1) measuring the position of piston of the main piston housing by the position sensor and employing the measured positions to determine the measured equilibrium position, S.sub.k, of the piston, 2) comparing the measured equilibrium position, S.sub.k, with a predetermined intended equilibrium position S.sub.0, of the piston, and 3) determine the difference |S.sub.0S.sub.k|, and J) executing the following sub-process steps: 1) if |S.sub.0S.sub.k|<K.sub.1, where K.sub.1 is a predetermined adjustment threshold criteria, then stop the wet phase adjustment of the equilibrium position, or else continue to sub-step 2) below: 2) i) open valve on the third gas conduit, and ii) continuously measuring the position of piston of the main piston housing by the position sensor and employing the measured positions to continuously determine the measured equilibrium position, S.sub.k, of the piston, and then continuously determining the difference |S.sub.0S.sub.k| and comparing the continuously determined difference with the predetermined adjustment threshold criteria, and if |S.sub.0S.sub.k|<K.sub.1, close valve on the third gas conduit, and stop the wet phase adjustment of the equilibrium position of piston of the main piston housing.

21. The method according to claim 16, wherein the adjustment threshold criteria K.sub.1 is less than 5% of the intended equilibrium position S.sub.0.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1a) is a schematic view as seen from the side of an example embodiment of the heave compensator according to the first and second aspect of one or more embodiments of the present invention when the pistons are in the initial positions.

(2) FIG. 1b) is a schematic view as seen from the side of the same heave compensator as shown in FIG. 1a), when the pistons are in the initial working positions.

(3) FIG. 1c) is a schematic view as seen from the side of the same heave compensator as shown in FIGS. 1a) and 1b), when the pistons are in a typical working position at relatively large strokes.

(4) FIG. 1d) is a graph illustrating the stroke response curve of the example embodiment of the heave compensator shown in FIGS. 1a) to 1c).

(5) FIG. 2 is a schematic view as seen from the side of another example embodiment of a heave compensator according to the first and second aspect of one or more embodiments of the present invention which includes a gas distribution circuit.

(6) FIG. 3 is a schematic view as seen from the side of another example embodiment of a heave compensator according to the third and fourth aspect of one or more embodiments of the present invention by including sensor automatics for automatic regulation of the heave compensator.

(7) FIG. 4a) is a schematic view as seen from the side of another example embodiment of a heave compensator according to the first and second aspect of one or more embodiments of the present invention which includes four gas accumulators, but no gas distribution circuit.

(8) FIG. 4b) is a schematic view as seen from the side of another example embodiment of a heave compensator according to the first and second aspect of one or more embodiments of the present invention which includes a gas distribution circuit.

(9) FIG. 4c) is a schematic view as seen from the side of another example embodiment having four accumulators of a heave compensator according to the third and fourth aspect of one or more embodiments of the present invention by including sensor automatics for automatic regulation of the heave compensator.

(10) FIGS. 5a) to c) are schematic views of an example embodiment of the present invention having the gas manifold integrated in an upper assembly plate and the liquid manifold integrated in a lower assembly plate.

(11) FIGS. 6a) and b) are cut views showing details of the integration of the gas manifold and the gas conduits into the upper assembly plate of the example embodiment shown in FIGS. 5a) to c).

(12) FIGS. 7a) and b) are cut views showing details of the integration of the liquid manifold and the liquid conduits into the lower assembly plate of the example embodiment shown in FIGS. 5a) to c).

DETAILED DESCRIPTION OF THE INVENTION

(13) The present invention will be described in greater detail by way of example embodiments.

(14) The design of an example embodiment of the heave compensator according to one or more embodiments of the present invention is illustrated in FIGS. 5a) to 5c). As seen on the figures, the example embodiment of the heave compensator has a compact construction by having a centre located main piston housing (100) surrounded by four accumulators (110), (120), (130), and (140) in a quadratic configuration by an upper (150) and a lower (180) assembly plate fastened to main piston housing and the four accumulators at their upper and lower ends.

(15) The main piston housing (100) of the example embodiment is a vertically oriented piston cylinder having a similar configuration as the main piston housing according to any aspect of one or more embodiments of the present invention by having a slide-able piston (not shown) with a piston rod (101). The slide-able piston divides the inner space of the main piston cylinder into an upper vacuum chamber (not shown) and a lower oil-filled chamber (not shown). The piston rod (101) is shown in retracted position and sticking out of the lower end (102) of the main piston cylinder. The piston rod has in its lower end a hook (104) for releasable attachment of a load. A similar hook (105) is attached to the upper end (103) of the main piston cylinder for releasable attachment of a crane hook etc. Each of the four accumulators have a configuration equal to the accumulators according to any aspect of one or more embodiments of the present invention by having a slide-able piston (not shown) dividing the inner space of the accumulators into an upper gas filled chamber (not shown) and a lower oil filled chamber (not shown).

(16) The compactness of the configuration of the example embodiment is further strengthened by integrating the gas distribution manifold into the upper assembly plate (150). This feature is shown schematically in FIG. 6a), which is a schematically exploded view showing the upper part of the four accumulators (110, 120, 130, and 140) attached in their upper ends to the assembly plate (150), see also FIG. 5c). The main piston cylinder is omitted in FIG. 6a) for the cause of clarity. The gas distribution manifold comprises three bores (151, 152, and 153) forming a conduit fluidly connecting together the four accumulators (110, 120, 130, and 140). The bores are formed by boring a linear bore from the side and into the bulk mass of the upper assembly plate (150) and positioned such that it intersects the centre axis of one accumulator and extends further until it intersects the centre axis of the adjacent accumulator at the opposite side of the upper assembly plate (150). That is, the bore (151) is located such and has a length such that it enters from a first side of the upper assembly plate (150) and passes through the centre axis of the first accumulator (110) and the centre axis of the second accumulator (120), the bore (152) is located such and has a length such that it enters from a second side perpendicular to the first side of the upper assembly plate (150) and passes through the centre axis of the third accumulator (130) and the second accumulator (120), and the bore (153) is located such and has a length such that it enters from the second side perpendicular to the first side of the upper assembly plate (150) and passes through the centre axis of the fourth accumulator (140) and the first accumulator (110). In addition there is made a bore (154) from a third side opposite the second side of the upper assembly plate (150) which extends to and fluidly connects to the bore (151). The latter bore (154) constitutes the third gas conduit and is open at its entrance in the sidewall of the upper assembly plate (150). The bores (151, 152, and 153) constitute the gas manifold fluidly interconnecting the first, second, third, fourth and fifth gas conduits. The bores (151, 152, and 153) are gas-tightly closed at their entrances into the upper assembly plate (150) by welded plugs (155, 156, and 157), respectively.

(17) The fluidly interconnection of e.g. the second gas conduit to the gas manifold is obtained as illustrated on FIG. 6b), which is, as seen from the side, a vertical cross-sectional cut taken along the dotted line marked A-A in FIG. 6a). Thus FIG. 6b) shows a cut-out section of the upper part of the second accumulator (120) and how it is integrated with the upper assembly plate (150). As seen from FIG. 6b), the cylinder wall of the second accumulator (120) is attached at its upper end to the assembly plate (150). The interior of the second accumulator (120) shown in FIG. 6b) is the upper gas-filled chamber (121) of the second accumulator. Thus the upper assembly plate (150) functions as the upper end closure of the second accumulator (and similarly for the three other accumulators).

(18) Furthermore, as seen from FIGS. 6a) and 6b), there is made a circular throughgoing cut (160) in the upper assembly plate (150) removing a circular section of the assembly plate at the centre position of the longitudinal centre axis of the second accumulator. Likewise, similar circular throughgoing cuts are also made at the centre position of the longitudinal centre axis of the three other accumulators. Along the cut (160) in the upper assembly plate (150) there is located a centre located recess (161) running all the way around the circular cutting edge at a height corresponding to the location of the bores (151, 152, and/or 153) such that the bores becomes fluidly connected to the recesses (161) as illustrated in FIG. 6b) where bore (152) ends at the recess (161). The circular cut-out section of the assembly plate (150) is plugged by a circular insert (162) being fitted to gas-tightly close the opening formed by the circular throughgoing cut (160) in the upper assembly plate (150). The circular insert (162) has a recess (163) running along its lateral edge which corresponds to the recess (161) in the assembly plate (150), such that an annular channel (164) running around the circular insert (162) is formed inside the upper assembly plate (150) which allows gas to flow freely around the circular insert (162). As seen from FIG. 6a), the bores (151) and (152) are fluidly connected to each other by having one end ending in the same annular channel (164). Similarly, the other end of bore (152) is fluidly connected to the annular channel inside the assembly plate (150) formed above the third accumulator (130). The other end of bore (151) is fluidly connected to the annular channel above the first accumulator (110), and the bore 152 is fluidly connected annular channel above the first accumulator (110) and the annular channel above the fourth accumulator (140).

(19) As seen from FIG. 6b) the gas in the annular channel (164) gains access to the inner space of the second accumulator (120) via the second gas conduit consisting of a horizontal bore (165) into the circular insert (162) and a first vertical bore (166) fluidly connected in one end to the horizontal bore (165) and in the other end to the inner space (167) of a solenoid valve (168), and a second vertical bore (170) fluidly connecting the inner space (167) of the solenoid valve (168) with the upper chamber (121) of the second accumulator. The solenoid valve (168) opens and closes the second gas conduit by an electromagnetically controlled magnetic body (169) able to slide from one side of the inner space (167) to the other. As seen on FIG. 6b), the magnetic body (169) will close the second gas conduit by blocking the openings of the vertical bores (166) and (170) when located at one side of the inner space (167). When the magnetic body is located at the other side of the inner space (167), gas is free to flow to and from the vertical bores (166) and (170) via the inner space (167) of the solenoid valve. In this manner, it is obtained a very quick opening and closing of the gas conduit at the very high pressure gradients which may arise in heave compensators utilising gas to provide the spring of the stroke. As seen on FIG. 6a), similar solenoid valves are applied on the gas conduits of the other three accumulators.

(20) FIGS. 7a) and 7b) are cross-sectional cut out views of the lower assembly plate (180) illustrating the integration of the liquid manifold and liquid conduits at the lower end of the main piston cylinder (100) and the four accumulators (110, 120, 130, and 140).

(21) FIG. 7a) illustrates two of the three bores (181, 182) which constitute the part of the liquid manifold fluidly connecting the lower liquid-filled chambers of the four accumulators. As seen on the figure, bore (181) fluidly connects the first (110) and the second (120) accumulator and the bore (182) fluidly connects the third (130) and the second (120) accumulator. A third bore (not shown) runs in parallel with the bore (182) and fluidly connects the fourth (140) and the first (110) accumulator.

(22) The first liquid conduit (113) is formed in the bottom end (114) of the accumulator and fluidly connects the lower chamber (112) of the first accumulator (110) with the bores (181) and (183), the latter connection is not shown. At the lower part of the first liquid conduit there is located a solenoid valve (115) able to close and open the liquid conduit. FIG. 7a) illustrates the closed position of the valve. Similarly, the second liquid conduit (123) is formed in the lower end (124) of the second accumulator (120), fluidly connecting the lower chamber (122) of the second accumulator with bores (181) and (182) of the liquid manifold. A solenoid valve (125) is located in the lower part of the second liquid conduit (123). A similar solution, by a liquid conduit (133) in the lower end (134) and solenoid valve (135), is applied for fluidly connecting the lower chamber (132) of the third accumulator (130) with bore (182) and the lower chamber fourth accumulator (140) with bore (183), the latter connection is not shown on the figure.

(23) The fluid connections between the bores (181, 182, and 183) of the liquid manifold and the first liquid conduit (107) in fluid communication with the lower chamber (106) of the main piston cylinder (100) are obtained by three additional bores (184, 185, and 186) oriented perpendicular to the bores (181, 182, and 183). FIG. 7a) shows the opening of bore (185) into bore (182) and the opening of bore (184) into bore (181).

(24) FIG. 7b) shows further details of the fluid connection between the lower chamber (106) of the main piston cylinder (100) and the bores (181, 182, and 183) of the liquid manifold. This figure illustrates, as seen form the side, a cross-sectional cut taken along a vertically oriented plane dividing the lower assembly plate (180) in two equal parts. As seen on the figure, the first liquid conduit of this example embodiment consists of three equal conduits, each comprising a horizontally oriented bore (107) and a vertically oriented bore (108) having a solenoid valve (109) at the lower part of the vertically oriented bore (108) able to close of open the first liquid conduit. The use of more than one liquid conduit has the advantage of enabling a more rapid volume flow of the liquid/oil in and out of the lower chamber 106).