Depth compensated actuator and use of same in association with a transportable heave compensator

Abstract

The publication relates to a depth compensated actuator, for a transportable inline depth compensated heave com-pensator for subsea lifting operations. The actuator comprises a cylinder shaped body and a piston with a piston rod, the piston rod being intended for exposure to external water pressure, a first and second connection means associated with the actuator. Further, the actuator comprises a depth compensator comprising a cylinder, a piston and a piston rod, the end of which being exposed to surrounding water; and conduit means between at least one volume in the actuator and one volume in the depth compensator. The pistons and piston rods are shaped as any of: hollow piston rod, ring shaped piston, ring piston rod. The depth compensated actuator solves the problem if improving depth compensation performance regarding size, weight, required fluid consumption, internal/inherent friction and adaptability. Further, use of a depth compensated actuator is claimed.

Claims

1. A hydraulic depth compensated actuator, suitable to form a part, for example, of a transportable inline depth compensated heave compensator for subsea lifting or loading operations, the hydraulic depth compensated actuator comprising: an actuator, the actuator comprising a first cylinder, a second cylinder mounted concentric with the first cylinder, and connected to the first cylinder at an upper end of the first cylinder, a ring piston configured to slide on the outer diameter of the second cylinder, a hollow piston rod connected to the ring piston and configured to reciprocate inside the first cylinder, wherein the hollow piston rod comprises a tail surface configured to be exposed to external water pressure, a first connection means and second connection means associated with the actuator such that first connection means is positioned at an axial end of the actuator and second connection means is positioned at an opposing axial end of the actuator, and a plurality of actuator volumes configured to contain a gas or a liquid, the plurality of actuator volumes including a first volume formed by an outer diameter surface of the hollow piston rod, a lower end surface of the first cylinder, an inner diameter surface of the first cylinder, and a lower end surface of the ring piston, wherein the first volume is configured to be filled with oil or gas, a second volume formed by an outer diameter surface of the second cylinder, an upper end surface of the first cylinder, the inner diameter surface of the first cylinder, and an upper end surface of the ring piston, wherein the second volume is configured to be filled with oil or gas or under vacuum, and a third volume formed by an inner diameter surface of the second cylinder, the upper end surface of the first cylinder, an inner diameter surface of the hollow piston rod, and a lower end surface of the hollow piston rod, wherein the third volume is configured to be filled with oil or gas or under vacuum; and a depth compensator, the depth compensator comprising a third cylinder, a second piston, a second piston rod extending out through an end closure of the depth compensator, wherein an end of the second piston rod is configured to be exposed to a surrounding water, and a plurality of depth compensator volumes, the plurality of depth compensator volumes including a fifth volume formed by a lower end surface of the third cylinder, an inner diameter surface of the third cylinder, a lower end surface of the second piston, and an outer diameter surface of the second piston rod, wherein the fifth volume is configured to be filled with gas or under vacuum, and a sixth volume formed by an upper end surface of the third cylinder, the inner diameter surface of the third cylinder, and an upper end surface of the second piston, wherein the sixth volume is configured to be filled with oil; and a conduit configured to connect the second volume in the actuator and the sixth volume in the depth compensator.

2. The hydraulic depth compensated actuator according to claim 1, wherein the hydraulic depth compensated actuator further comprises a depth compensator, the depth compensator comprising: a first accumulator cylinder, a second accumulator cylinder having a smaller diameter than the first accumulator cylinder, and concentrically arranged inside the first accumulator cylinder at one end of the first accumulator cylinder, an accumulator piston configured to reciprocate inside the first accumulator cylinder, and dividing the first accumulator cylinder into a ninth volume at a side of the first accumulator piston and a tenth volume at an opposite side of the first accumulator piston, and an accumulator piston rod fixed to and projecting out from the accumulator piston, a second accumulator piston connected to an opposite end of the accumulator piston rod, wherein the second accumulator piston is positioned inside the second accumulator cylinder, and a first conduit configured to establish fluid communication between the ninth volume and at least one of the plurality of actuator volumes, a second conduit configured to establish fluid communication between the tenth volume and a separate one of the plurality of actuator volumes, and a reversible pump configured to be positioned along the second conduit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, schematic embodiments of the invention shall be described in further detail, showing only the major components involved, referring to the drawings, wherein:

(2) FIG. 1 discloses schematically an illustration of a prior art depth compensated actuator used as a heave compensator for subsea use.

(3) FIG. 2 discloses schematically an illustration of one embodiment of a depth compensated actuator according to the present invention, where it is forming a part of an active heave compensator.

(4) FIG. 3 discloses schematic an illustration of a high-pressure depth compensated actuator according to the present invention in which the major component parts of the high-pressure depth compensated actuator are specifically identified.

(5) FIG. 4 discloses schematically an illustration of an embodiment of a compensated actuator according to the present invention in which the major component parts of the actuator are specifically identified.

(6) FIGS. 5 and 6 disclose schematic illustrations of various embodiment of a depth compensated actuator according to the present invention in which only the major component parts of the depth compensated actuator are specifically identified.

DETAILED DESCRIPTION OF EMBODIMENTS DISCLOSED IN THE DRAWINGS

(7) The following description of embodiments of the invention refers to the accompanying drawings. The same reference numbers in the different drawings identify the same or similar elements. Various volumes in the actuators and depth compensators are identified with V and a digit (V1, V2, . . . Vn) and volumes have the same function or position are given the same identification, independent of fluid to be contained in such volume. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, regarding the terminology and structure of actuators and depth compensators to be used for offshore lifting operations where the actuators and the depth compensators are forming part of a transportable, inline heave compensator to follow the payload subsea.

(8) Reference throughout the specification to “one embodiment” or “an embodiment” means that a feature, structure or characteristics describing an embodiment included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further features, structures or characteristics may be combined in any suitable manner or in one or more embodiments. The same applies to whether a volume is filled with gas or a liquid.

(9) It should also be appreciated that elements commonly associated with actuators or depth compensators are from a simplicity point of view not always disclosed or indicated. Typical elements associated with a system, such as seals, accumulators, other types of pressure intensifiers, pumps, valves, control systems are not disclosed in detail.

(10) FIG. 1 discloses schematically an illustration of a prior art depth compensated actuator 0 incorporated in a heave compensator for subsea use. The parts disclosed are a conventional actuator consisting of a cylinder, a piston 2 reciprocally arranged inside the cylinder 1 and a piston rod 3 rigidly fixed to the piston 2. The piston 2 establishes two different volumes in the cylinder 1. At its upper end the actuator 1 is provided with a first connection means 8 while the free end of the piston rod 3 is provided with a second connection means 9. The first connection means 8 is configured to be connected to a crane or the like (not shown), while the second connection means 9 is configured to be fixed to a payload (not shown) to be installed on the seabed.

(11) Moreover, the actuator is fluidly communicating with a depth compensator 20, consisting of a cylinder 21 and a piston 22, reciprocally arranged inside the depth compensator 20. A piston rod 23 is fixed to the piston 22, the piston rod 23 extending out through the bottom closure of the cylinder 21, having an end surface exposed to the surrounding water pressure. The upper volume of the depth compensator is in fluid communication with the upper volume of the actuator.

(12) The system disclosed in FIG. 1 also consist of a gas accumulator consisting of a cylinder and a piston reciprocally arranged inside the cylinder, splitting the cylinder into an upper and lower volume. The lower volume of the accumulator is in fluid communication with the lower volume of the actuator.

(13) FIG. 2 discloses schematically an illustration of one embodiment of a depth compensated actuator 0 according to the present invention wherein the depth compensated actuator 0 is forming a part of an active heave compensator, disclosing how to implement active control of the actuator rod, i.e. the active heave compensation. The depth compensated actuator 0 disclosed in FIG. 2 corresponds to the embodiment disclosed in FIG. 5, and will be described in further detail below. The following components are added compared to depth compensated actuator 0 disclosed in FIG. 5: A reversible pump 37 A gas accumulator 38 with a first 39 and second piston 40.

(14) An active heave compensator (AHC) comprises an actuator connected to one or more accumulators, which further may be connected to one or more gas tanks. The accumulator shown allows for very efficient use of commercially available hydraulic motors, used to gain active control the hydraulic actuator. Automatic control of the hydraulic actuator is used to compensate for heave motion. The automatic control is controlled by a computer that calculates the control signal based on measurements from several sensors, where the most important ones are the piston position sensor, the accelerometer and the wire rope speed sensor. Information about the wire rope speed is transferred to the compensator with wireless signals while the compensator is in air and with acoustic transmission while it is submerged. The compensator can operate in several different modes with variable stiffness and damping with or without active control of the hydraulic actuator and with or without active control of the pressure levels in the various gas volumes. The compensator is energy efficient, since the passive part of the compensator carries the entire load of the payload weight and the actively controlled hydraulic pumps only must compensate for gas compression effects and friction, which typically is about 15% of the force compared to static force. Energy regeneration is also used so that only friction and oil leakage and mechanical losses in the hydraulic pump contributes to the energy consumption. Further, acoustic communication subsea and wireless communication topside allows for control and monitoring of the compensator, on-board sensors allows the user to verify performance after a lift is concluded.

(15) Such AHC has the following advantages compared to the prior art; mobile construction, lower cost for same capacity, as good performance for long wave periods and better performance for short wave periods, excellent splash zone crossing performance, well-suited for resonance protection, reduced wear of the steel wire rope, low energy consumption.

(16) The following equations are used to design the accumulator, actuator and depth compensator (strength calculations not included and will influence the design somewhat, but those calculations depend on which design standard is being used to calculate strength). These equations are based on one pump, one actuator and one accumulator, but can easily be modified for multiple components of either type.

(17) The main design criterions are: Capacity of compensator (F.sub.phc) Actuator stroke length and compression ratio (S.sub.act, c) Speed of AHC system (v.sub.ahc) Actuator volume criterion Force balance for depth compensator Depth compensator volume criterion

(18) The capacity of the compensator determines the size of the actuator piston and the actuator rod outer diameter (rod size indirectly calculated by strength calculations and actuator rod inner diameter) based on a design pressure.

(19) $F_{\mathrm{phc}}=p_{\mathrm{phc}}\frac{\pi }{4}\left(d_{\mathrm{act}}^2-d_{\mathrm{rod},o}^2\right)$

(20) where F.sub.phc—Compensator capacity (max force) p.sub.phc—Actuator design pressure

(21) d.sub.act—Inner diameter of actuator

(22) d.sub.rod,o—Outer diameter of actuator rod

(23) The compression ratio determines the change in force as the actuator rod is extended due to compression of the gas in the system.

(24) $C=\frac{V_{\max }}{V_{\min }}=\frac{V_{\mathrm{tank}}+V_{\mathrm{acc}}}{V_{\mathrm{tank}}+V_{\mathrm{acc}}-\frac{\pi S_{\mathrm{act}}}{4}\left(d_{\mathrm{act}}^2-d_{\mathrm{rod},o}^2\right)}$

(25) where C—Compression ratio V.sub.max—Gas volume at zero actuator stroke V.sub.min—Gas volume at maximum actuator stroke V.sub.tank—Gas volume of tanks V.sub.acc—Gas volume of accumulator S.sub.act—Actuator stroke length (max)

(26) The minimum required force in the active part of the system to be able to compensate for gas compression effects is (factor 2 is due to that the AHC system can influence the actuator piston in two directions):

(27) $F_{\mathrm{ahc}}=F_{\mathrm{phc}}\left(\frac{C^{\kappa }-1}{2}\right)=p_{\mathrm{ahc}}\frac{\pi }{4}{d}_{\mathrm{rod},i}^2$ F.sub.ahc—Force that AHC can exert on the actuator piston (single direction) p.sub.ahc—Design pressure in AHC system d.sub.rod,i—Inner diameter of actuator rod K—Adiabatic compression coefficient

(28) The size of the pump is related to the required actuator speed.

(29) $v_{\mathrm{ahc}}=\frac{4Q_{\mathrm{pump}}}{\pi d_{\mathrm{rod},i}^2}$

(30) where v.sub.ahc—Speed of actuator rod under active control Q—Pump volume flow

(31) The oil volume of the passive part of the actuator must fit inside the accumulator.
(d.sub.act.sup.2−d.sub.rod,o.sup.2)S.sub.act<d.sub.acc.sup.2S.sub.acc

(32) where d.sub.acc—Accumulator diameter S.sub.acc—Accumulator stroke length (max)

(33) To balance flow of oil through the oil pump the following equation must be true:

(34) $\frac{\left(d_{\mathrm{act}}^2-d_{\mathrm{rod},o}^2\right)}{d_{\mathrm{acc}}^2}=\frac{d_{\mathrm{acc},\mathrm{ahc}}^2}{d_{\mathrm{rod},i}^2}$

(35) where d.sub.acc,ahc—Accumulator smaller piston diameter

(36) To balance the pressure from the seawater the following equation must be fulfilled:

(37) $\frac{d_{\mathrm{rod},o}^2}{d_{\mathrm{act}}^2-d_{\mathrm{rod},i}^2}=\frac{d_{\mathrm{rod},dc}^2}{d_{dc}^2}$

(38) where d.sub.rod,dc—Diameter of depth compensator rod d.sub.dc—Diameter of depth compensator cylinder

(39) The final criterion is to make sure that the depth compensator has enough oil available to compensate the full actuator stroke:
(d.sub.act.sup.2−d.sub.rod,i.sup.2)S.sub.act<d.sub.dc.sup.2S.sub.dc
where S.sub.dc—Stroke of depth compensator

(40) The gas accumulator 38 consists of up to four volumes; two pistons 39,40, interconnected by means of a common piston rod 41. According to the embodiment disclosed, the second piston 40 has a larger diameter than the first piston 39. The second piston 40 is reciprocally arranged in a cylinder 42 with a corresponding inner diameter as the second piston 40, the second piston 40 separating the cylinder 42 into a lower volume, the ninth volume, V9 and an upper volume V10 above the larger piston 40. Volume V9 is located between the lower end of the gas accumulator 38 and the large piston 40 and is filled with oil. The upper volume V10 is located between the upper surface of the large piston 40 and the upper end of the gas accumulator 38, and is filled with gas. Both larger and/or the smaller pistons may be provided with sealing devices (not shown).

(41) A second cylinder 45 with a smaller diameter is concentrically arranged inside the larger cylinder 42, at the upper end of volume V10. The smaller piston 39 is intended to reciprocate inside the smaller cylinder 45. The inner diameter of the smaller cylinder 45 corresponds to the outer diameter of the smaller piston 39. The smaller piston 39 divides the volume of the smaller cylinder 45 into an upper, eleventh volume V11, placed between the upper surface of the piston 39 and the upper end of the gas accumulator 38, and a smaller, twelfth volume V12 below the lower surface of the smaller piston 39 and the bottom closure of the smaller cylinder 45. The lower closure or end of the smaller cylinder 45 is provided with a sealed opening in which the interconnecting piston rod 41 is reciprocating with the pistons 39,40. The eleventh volume V11 is filled with oil, while the twelfth volume V12 is normally under low pressure. The twelfth volume V12 is ring-shaped due to the volume of the interconnecting piston rod 41, and thus smaller than the volume V11.

(42) According to the embodiment disclosed, volume V1 is connected to volume V9 through a conduit 43 providing the main passive force in the actuator 10. Volume V3 is connected to volume V11 via a conduit 44 with the reversible pump 37, providing active force on the actuator rod 13 in two directions.

(43) The ring-shaped volume V12 formed between the outer surface of the common piston rod 41 and the inner surface of the smaller cylinder 45 may be separated or sealed from both volume V11 above the smaller piston 39 and the surrounding volume V10, forming a vacuum. Alternatively, as a first option, the volume V12 may be in fluid communication with the volume V11 inside the smaller cylinder 45 above the piston. In such case, the piston 39 may be removed, leaving only the piston rod 41 to reciprocate inside the smaller cylinder 45, the pressure exposed area then being reduced to the end surface of the piston rod 41. The volume V11 is then filled with oil. Yet second may be to allow the volume V12 to be in fluid communication with the surrounding volume V12, filled with gas. In such case the seals around the common piston rod 41 may be omitted.

(44) A transportable heave compensator of this configuration may be substantially more simple, lighter, less cost related to construction, and a more robust and safer solution. Compared with the prior art solutions with same capacity, the overall weight may be decreased by around 10%, the cost by 10 to 15%, the risk for jamming of the piston is at least substantially reduced, if not eliminated. Moreover, and of importance: it may be possible to actively drive the actuator piston rod, due to the proposed configuration and the reversible pump.

(45) FIG. 3 relates to a high-pressure depth compensated actuator (HPDCA) is an actuator design intended for subsea usage. It compensates for water pressure effects that often are problematic.

(46) The HPDCA uses a hollow rod actuator to significantly reduce the required size and weight of the depth compensator as only the volume of the inner tube must be filled with oil, comparted to the prior art solutions. Friction is also smaller as the seal size is significantly reduced (from full actuator diameter to rod inner diameter).

(47) The main features of the present invention are given in the independent claim. Additional features of the invention are given in the dependent claims.

(48) The novel design of the HPDCA is use of a hollow rod actuator combined with a high-pressure depth compensator cylinder, to provide a light design with a minimum amount of friction while adding an extra pressure surface.

(49) As previously mentioned, FIG. 3 illustrates the HPDCA 0 with all the major sub-components numbered 1 through 25, as well as all volumes indicated by V1 through V5. In Table 1, the component description is identified. The HPDCA 0 can be used vertically, horizontally or at an angle. One application can be as an actuator for subsea valves operated at low pressure; another is as an actuator used at different water depths, typically as part of a heave compensator.

(50) FIG. 3 shows the invention, details explained below: a hollow rod actuator 10, consisting of a first cylinder 11, a first piston 12, a first hollow piston rod 13, connection means 14 at each axial end of the hollow rod actuator 10, a second cylinder 15 mounted concentric with the first cylinder 11 and fastened to the upper end of the first cylinder 11 and a second, stationary piston 16 fixed to the lower end of the second cylinder 15 a first volume V1 is formed between the outer diameter of the hollow rod 13, the lower end of the first cylinder 11, the inner diameter of the first cylinder 11 and the lower end of the first piston 12, and may be filled with oil, gas or be under vacuum a second volume V2 is formed by the outer diameter of the second cylinder 15, the upper end of the first cylinder 11, the inner diameter of the first cylinder 11, the upper end of the first piston 12, the inner diameter of the first hollow rod 13 and the upper end of the second piston 16 and may be filled with oil, gas or be under vacuum a third volume V3 is formed by the inner diameter of the second cylinder 15, the upper end of the first cylinder 11, the inner volume of the hollow rod 13, the lower end of the second piston 15 and the lower end of the hollow rod 13, and may be filled with oil, gas or be under vacuum. Normally, volume V3 is however, nearly always filled with oil and connected to volume V5. If the volume of oil in V5 is smaller than that of the volume V3 vacuum may arise. a depth compensator 20, consisting of a third cylinder 21, a second hollow rod 22, a fourth cylinder 23 mounted concentrically within the third cylinder 21 and fastened to the upper end of the third cylinder 21, a third, stationary piston 24 mounted at the lower end of the fourth cylinder 23 and a mechanical stroke limiter 25 mounted at the upper end of the second hollow rod 22, preventing the second hollow rod 22 from stroking too much, co-acting with the upper surface of the stationary piston 24 a fourth volume V4 is formed between the upper surface of the stationary third piston 24 at the lower end of the third cylinder 21, the inner diameter of the third cylinder 21, the outer diameter of the fourth cylinder 23, and is displaced by the second hollow rod 22 as well as the mechanical stroke limiter 25, which may be filled with gas or be under vacuum a fifth volume V5 is formed between the lower end of the second hollow rod 22, the inner diameter of the fourth cylinder 23, the lower end of the second hollow rod 22, the upper end of the third cylinder 23 and the lower end of the third piston 24, which may be filled with oil conduit means 17 between the fifth volume V5 and the third volume V3.

(51) The invention shown in FIG. 3 works in the following way: Both hollow rods are exposed to external pressure. The third volume and the fifth volume is connected through a conduit and will have the same pressure (internal pressure). To negate external pressure influence on the first hollow rod, the internal pressure needs to be equal to the external pressure times the square of the ratio between the outer and inner diameter of the first hollow rod

(52) $\left(p_{V3}=p_{\mathrm{ext}}.Math.{\left(\frac{d_o}{d_i}\right)}^2\right).$ A force balance gives:

(53) $F_{\mathrm{net}}=p_{\mathrm{ext}}.Math.\frac{\pi }{4}{d}_0^2-p_{\mathrm{ext}}.Math.{\left(\frac{d_o}{d_i}\right)}^2.Math.\frac{\pi }{4}.Math.d_i^2=0$ A second requirement is that the volume of the fifth volume is large enough to provide oil to the third volume for the entire usable stroke length. To achieve these requirements, the diameter ratio between the outer and inner diameter of the second hollow rod needs to be the same as the ratio between the outer and inner diameter of the first hollow rod a

(54) $\left(\frac{d_{o,1}}{d_{i,1}}=\frac{d_{o,2}}{d_{i,2}}\right),$
second requirement is then that the inner diameter of the second hollow rod needs to be equal to the inner diameter of the first hollow rod times the square root of the ratio of the stroke length of the first hollow rod and the stroke length of the second hollow rod

(55) 0$\left(d_{i,2}=d_{i,1}.Math.\sqrt{\frac{s_1}{s_2}}\right).$ The first volume V1 can be used for passive heave compensation means by connecting it to a gas accumulator. The second volume V2 is left unused and can be utilized as an extra pressure surface for active heave compensation purposes by connecting it to e.g. a pump.

(56) The fourth volume V4 should normally be without pressure.

(57) FIG. 4 discloses an embodiment of a hydraulically compensated actuator (HCA) is an actuator design intended for subsea usage. It compensates for water pressure effects that often are problematic.

(58) The novel design of the HCA is use of a traditional actuator combined with a ring based compensation cylinder, all in one compact, symmetrical assembly. The ring based compensation cylinder ensures that the water pressure effect is negated.

(59) As previously mentioned, FIG. 4 illustrates the HCA (0) with all the major sub-components numbered as listed in the table below. The HCA 0 can be used vertically, horizontally or at an angle. One application can be as an actuator for subsea valves operated at low pressure; another is as an actuator used at different water depths, typically as part of a heave compensator. When used as a valve actuator the first connection means 14 and the second connection means 14 are connected to either a fixed or movable point. When used as part of a heave compensator, the first connection means 14 and the second connection means 14 are usually connected to the payload and/or the crane. The connection means 14 can be at least one of: a padeye and a clevis, but not limited only thereto. Further the HCA 0 consists of a cylinder 1, with piston 12 and piston rod 3. The piston 12 divides the cylinder into two volumes, V1, which is the volume below the piston 12 and housing the piston rod 3. A second cylinder 31 is mounted concentrically on the upper part, the volume having a general shape of an annulus (the top side with the first connection means 14, the second cylinder 31 has a larger diameter than the first cylinder 1, but shorter length. The second cylinder 31 features a ring-shaped piston 32 connected to a ring-shaped piston rod 33. The area ratio between the ring shaped piston 32 and the ring-shaped piston rod 33 is equal to or smaller than the area ration between the piston 12 and the piston rod 3. A conduit means 17 connects the oil side of the ring-shaped cylinder 31 and volume V2 in the cylinder 1 together, effectively cancelling the effect of the external pressure. The HP side of the cylinder 1 is connected to other hydraulics means, such as a piston accumulator or a HPU (not shown). The LP side of the ring-shaped cylinder 31 can be connected to other hydraulic means, such as a hydraulic pump in an active heave compensator, or be gas filled with low pressure gas.

(60) The piston 32 divides the ring-shaped cylinder 31 into a ring shaped volume or annulus V3, while the ring-shaped piston rod 33 divides the volume below the piston into two concentrically arranged ring-formed volumes V4 and V5, where volume V4 is positioned between the outer wall surface of the centrally arranged volume V2 of the actuator cylinder 1, while volume V5 is arranged between the outer surface of the ring-shaped piston rod 33 and the inner surface of the outer concentrically arranged wall of the ring-shape cylinder 31. The area ratio between the ring-shaped piston 32 and the ring-shaped piston rod 33 is equal to or smaller than the area ratio between the piston 12 and the piston rod 3. Volume V1 contains a high-pressure fluid, while the volumes V4 and V5 contain a low-pressure fluid. Moreover, the various volumes have a cylindrical cross sectional. The high-pressure fluid may be oil, although also gas may be used instead.

(61) To eliminate the effect of the surrounding water pressure, the correlation between the various volumes may be defined by the following equation:

(62) ${\left(\frac{f}{e}\right)}^2=\frac{c^2-d^2}{a^2-b^2}$

(63) where a=the internal diameter of the outer ring-shaped cylinder 31 b=the inner diameter of the ring-shaped piston 32 c=the outer diameter of the ring-shaped piston rod 33 d=The inner diameter of the ring-shaped piston rod 33 f=the diameter of the piston rod 3 e=the diameter of the actuator cylinder 1, corresponding more or less to the diameter of the piston 12.

(64) Moreover, in such case in order to use the full stroke of the actuator, the oil volume in the main cylinder and the ring-shaped cylinder must be equal.

(65) A conduit means 17 connects the oil side of the ring-shaped cylinder 31 and volume V2 at the top of the cylinder 1 together, effectively cancelling the effect of external pressure. The HP side of the cylinder 1 is connected to other hydraulics, such as a piston accumulator or a HPU (not shown). The LP side of the ring-shaped cylinder 31 can be connected to other hydraulics, such as a hydraulic pump in an active heave compensator, or be gas filled with low pressure gas. The low-pressure volumes may not be exposed to any significant pressure, but a pressure may be used if it is desirable to active controlling the piston rod 3. In such case the volumes may be connected to a hydraulic pressure unit (HPU).

(66) FIGS. 5 and 6 relate to a hydraulically depth compensated actuator (HDCA) is an actuator design intended for subsea usage. It compensates for water pressure effects that often are problematic.

(67) The prior art compensation is performed utilizing an external cylinder to compensate the effect of the water pressure acting on the piston rod, thus requiring at least one large second hydraulic cylinder connected to the main hydraulic cylinder, while the present HCDA uses a hollow rod actuator to significantly reduce the required size and weight of the depth compensator as only the volume of the inner tube must be filled with oil. Friction is also much smaller as the seal size is significantly reduced (from full actuator diameter to inner tube diameter).

(68) The novel design of the HDCA is use of a hollow rod actuator combined with various depth compensators cylinder, to provide a light design with a minimum amount of friction while adding an extra pressure surface.

(69) As previously mentioned, FIG. 5 and FIG. 6 illustrate the HDCA 0 with all the major sub-components numbered 1 through 34, as well as all volumes indicated by V1 through V10. In Table 1, the component description is identified. If the volume V1 is fluidly connected to an accumulator, the fluid will always be oil. The hydraulically depth compensated actuator (HDCA) 0 can be used vertically, horizontally or at an angle. One application can be as an actuator for subsea valves operated at low pressure; another is as an actuator used at different water depths, typically as part of a heave compensator.

(70) The two HDCA-embodiments shown has the following in common: a hollow rod actuator 10, consisting of a first cylinder 11, a first ring piston 12, a hollow rod 13, connection means 14 at each axial end of the hollow rod actuator 10 and a second cylinder 15, mounted concentric with the first cylinder 11 and fastened to the upper end of the cylinder 11 a ring piston 12 adapted to slide on the outer diameter of the second cylinder 15 a first volume V1 is formed by the outer diameter of the hollow rod 13, the lower end of the first cylinder 11, the inner diameter of the first cylinder 11 and the ring piston 12 and may be filled with oil or gas a second volume V2 is formed by the outer diameter of the second cylinder 15, the upper end of the first cylinder 11, the inner diameter of the first cylinder 11 and the ring piston 12, and may be filled with oil, gas or be under vacuum a third volume V3 is formed by the inner diameter of the second cylinder 15, the upper end of the first cylinder 11, the inner diameter of the hollow rod 13 and the lower end of the hollow rod 13, and may be filled with oil, gas or be under vacuum a depth compensation means is connected to either the second volume V2 via conduit means.

(71) FIG. 5 shows the first embodiment which in addition to the common parts contain: a third cylinder 21 a piston rod 23 connected to the piston 22, where the piston 22 is exposed to external pressure, and both are adapted for reciprocation inside the third cylinder 21 a fourth cylinder 24, mounted concentrically with the third cylinder 24 at the lower end of the third cylinder 21 a fourth volume V4 is formed between the lower end of the fourth cylinder 24, the inner diameter of the fourth cylinder 24, the lower end of the third cylinder 21 and displaced by the piston rod 23, which may be filled with oil a fifth V5 volume is formed between the lower end of the third cylinder 21, the inner diameter of the third cylinder 21, lower end of the piston 22 and the outer diameter of the piston rod 23, which may be filled with gas or be under vacuum conduit means between the fourth volume V4 and the third volume V3.

(72) FIG. 6 shows the second embodiment which in addition to the common parts contains: a fifth cylinder (31) a second ring piston (32), adapted for sliding motion of the outside diameter of any cylinder (shown with dashed line in FIG. 4) adapted for reciprocation inside the fifth cylinder (31) a ring piston rod (33) connected to the ring piston (32), exposed to external pressure and adapted 20 for reciprocation inside the fifth cylinder (31) an eighth volume (V8) is formed between the lower end of the fifth cylinder (31), the inner diameter of the ring piston rod (33) and the second ring piston (32), which may be filled with gas or be under vacuum a ninth volume (V9) is formed between the lower end of the fifth cylinder (31), the outer diameter 25 of the ring piston rod (33), the inner diameter of the fifth cylinder (31) and the second ring piston (32), which may be filled with gas or be under vacuum a tenth (V10) volume is formed between the upper end of the fifth cylinder (35), the upper end of the second ring piston (32), the inner diameter of the fifth cylinder (31), which may be filled with oil 30 conduit means between the tenth volume (V10) and the second volume (V2)

(73) Many possible combinations exist when it comes to various fluids in the various volumes shown in the four embodiments.

(74) TABLE-US-00001 TABLE 1 Component Description  0 Depth compensated actuator  1 Cylinder-prior art  2 Piston-prior art  3 Piston rod-prior art  7 Conduit means  7′ Conduit means between actuator and accumulator  8 First connection means-prior art  9 Second connection means-prior art 10 Hollow rod actuator 11 First cylinder 12 Piston 13 Hollow piston rod 14 Connection means 15 Second cylinder 17 Conduit 20 Depth compensator 21 Third cylinder 22 Piston 23 Piston rod 24 Fourth cylinder 30 Ring based depth compensator 31 Fifth cylinder 32 Second ring piston 33 Second ring piston rod 34 Sixth cylinder 35 Gas accumulator 36 Gas accumulator piston 37 Reversible pump 38 Gas accumulator 39 First piston 40 Second piston 41 Common piston rod 42 Large cylinder 43 Conduit between V9 and V1 44 Conduit between volume V3 and V11, incorporating the pump 37 45 The smaller cylinder V1 First volume V2 Second volume V3 Third volume V4 Fourth volume V5 Fifth volume V6 Sixth volume V7 Seventh volume V8 Eighth Volume V9 Ninth volume V10 Tenth volume V11 Eleventh volume V12 Twelfth volume