Fail-safe subsea pressure vessel comprising a vacuum interrupter

Abstract

 The present disclosure relates to a subsea pressure vessel (1). The subsea pressure vessel (1) comprises a housing (3) arranged to maintain a pressure within an interior of the subsea pressure vessel (1) which differs from ambient pressure, a vacuum interrupter (5) arranged within the housing (3), which vacuum interrupter (5) has an enclosure (7) for maintaining a vacuum within the vacuum interrupter (5), and which vacuum interrupter (5) has a fixed terminal (9) with a fixed contact (9a) and a movable terminal (11) with a movable contact (11a), wherein the movable terminal (11) is operable between an open state in which the fixed contact (9a) and the movable contact (11a) are electrically separated and a closed state in which the fixed contact (9a) and the movable contact (11a) are electrically connected, and a movement counteracting arrangement (14) comprising a first counteracting member (15) arranged to move concurrently with the movable terminal (11) between the first state and the second state, and a second counteracting member (17) arranged to apply a counteracting force to the first counteracting member (15) when the movable terminal (11) is in the open state and the interior of the subsea pressure vessel (1) is subjected to a pressure increase compared to a normal operating pressure in the subsea pressure vessel (1) to counteract movement of the movable terminal (11) from the open state to the closed state. A subsea unit comprising a subsea pressure vessel (1) is also presented herein.

Description

TECHNICAL FIELD

[0001] The present disclosure generally relates to subsea installations and in particular to a subsea pressure vessel for use in a subsea unit arranged to be pressurised to the hydrostatic pressure on the sea floor, and to such a subsea unit.

BACKGROUND

[0002] In recent years, there has been a growing interest in installing electrical installations on the sea floor in depths from a few tens of meters to even kilometres. Oil and gas production subsea employs electric equipment like drilling motors, pumps, and compressors driven by frequency converters.

[0003] In bringing power electronics subsea, two general concepts exist: (1) the equipment stays at or near atmospheric pressure in a pressure vessel; and (2) the equipment is pressurized to the hydrostatic pressure level on the seabed. The two concepts can be differentiated as follows. Concept (1) has the advantage that standard electric/electronic components, known from onshore installations, can be used, while disadvantages include thick walls needed for the enclosure to withstand the pressure difference between inside and outside. Thick walls make the equipment heavy and costly. Concept (2) has the advantage that no thick walls are needed for the enclosure since the pressure difference between inside and outside the containment is much less than for concept (1). Disadvantages of concept (2) are that all the components must be free of gas inclusions and compressible voids; otherwise they implode during pressurization and are destroyed.

[0004] In order to provide protection of any electrical or electronic equipment, such as semiconductor power electronics, from overload and short circuit, circuit breakers are typically utilised. One type of circuit breaker is the vacuum interrupter which employs a fixed terminal with a fixed contact, and a movable terminal having a movable contact. The fixed contact and movable contact are arranged within an enclosure which is subject to very low pressure, thus essentially defining a vacuum. The movable contact and the fixed contact are in electrical connection when the device which the vacuum interrupter protects is in operation. In case the circuit breaker trips, or if the equipment which the vacuum interrupter protects is set in standby mode, the movable contact and the fixed contract are set in an open state in which they are electrically insulated from each other by means of vacuum.

[0005] EP1942514 discloses switchgear for high pressure environments such as subsea or deep sea environments. In particular a vacuum circuit breaker assembly is disclosed having a housing in which a vacuum circuit breaker is accommodated. The housing is fluid tight and arranged to maintain a pressure inside the housing at a predetermined level which is independent of a pressure outside the housing. The vacuum circuit breaker assembly may be used in a switchgear assembly with an external housing. The external housing has an inner space sealed from an external environment, in which the inner space is filled with a substantially non-compressible material such that, in operation, the pressure in the inner space is substantially equal to the pressure of the external environment.

SUMMARY

[0006] In existing solutions, the movable terminal would be brought from the open state to the closed state in case of a pressure increase in the housing in which the vacuum interrupter is accommodated, for example due to leakage.

[0007] In view of the above, a general object of the present disclosure is to provide a subsea pressure vessel comprising a vacuum interrupter which when in an open state is subjected to a pressure increase can maintain the open state.

[0008] Hence, according to a first aspect of the present disclosure there is provided a subsea pressure vessel comprising: a housing arranged to maintain a pressure within an interior of the subsea pressure vessel which differs from ambient pressure; a vacuum interrupter arranged within the housing, which vacuum interrupter has an enclosure for maintaining a vacuum within the vacuum interrupter, and which vacuum interrupter has a fixed terminal with a fixed contact and a movable terminal with a movable contact, wherein the movable terminal is operable between an open state in which the fixed contact and the movable contact are electrically separated and a closed state in which the fixed contact and the movable contact are electrically connected; and a movement counteracting arrangement comprising a first counteracting member arranged to move concurrently with the movable terminal between the first state and the second state, and a second counteracting member arranged to apply a counteracting force to the first counteracting member when the movable terminal is in the open state and the interior of the subsea pressure vessel is subjected to a pressure increase compared to a normal operating pressure in the subsea pressure vessel to counteract movement of the movable terminal from the open state to the closed state.

[0009] Thus, in case the vacuum interrupter is arranged to protect equipment that is set in standby mode, i.e. the equipment is set off, and thus the movable terminal is set in the open state, this open state may be maintained in case of a pressure increase in the subsea pressure vessel. Thus, the subsea pressure vessel ensures a fail-safe vacuum interrupter as long as the pressure increase in the subsea pressure vessel is below a critical pressure at which the vacuum interrupter implodes.

[0010] According to one embodiment the second counteracting member is arranged to move in a direction opposite to a direction defined from the open state to the closed state when the interior of the subsea pressure vessel is subjected to a pressure increase compared to the normal operating pressure.

[0011] According to one embodiment the second counteracting member is essentially motionless when subjected to normal operating pressure.

[0012] According to one embodiment the second counteracting member is arranged to apply a force to the first counteracting member greater than a force applied by the first counteracting member to the second counteracting member when the interior of the subsea pressure vessel is subjected to a pressure increase compared to the operating normal pressure.

[0013] According to one embodiment the second counteracting member comprises a first end portion arranged to interact with the first counteracting member to counteract movement of the movable terminal when the interior of the subsea pressure vessel is subjected to a pressure increase compared to a normal operating pressure.

[0014] According to one embodiment the second counteracting member comprises a second end portion which defines a surface perpendicular to an axis defined by the movable terminal, wherein the second end portion has a dimension that is greater than a cross-sectional dimension of the movable terminal.

[0015] One embodiment comprises an extension shaft that is mechanically coupled to and electrically insulated from the movable terminal.

[0016] According to one embodiment the extension shaft is provided with the first counteracting member.

[0017] According to one embodiment the first counteracting member defines a shoulder arranged to abut the second counteracting member when the subsea pressure vessel is subjected to a pressure increase compared to a normal operating pressure.

[0018] According to one embodiment the second end portion of the second counteracting member is arranged between the first counteracting member and the vacuum interrupter.

[0019] According to one embodiment the movement counteracting arrangement comprises chamber, wherein the second end portion of the second counteracting member seals the chamber and is arranged to move in parallel with the axis defined by the movable terminal, in a direction from the closed state to the open state, into the chamber when the interior of the subsea pressure vessel is subjected to a pressure increase compared to a normal operating pressure.

[0020] According to one embodiment the chamber is pressurised to the normal operating pressure of the subsea pressure vessel when the interior of the subsea pressure vessel is subjected to normal operating pressure.

[0021] According to one embodiment the first end portion of the second counteracting member is arranged at a distance from the first counteracting member when the interior of the subsea pressure vessel is subjected to normal operating pressure.

[0022] According to one embodiment the first end portion of the second counteracting member is slidingly arranged around the extension shaft.

[0023] According to a second aspect of the present disclosure there is provided a subsea unit comprising an external housing; a passive pressure compensator arranged to reduce a pressure difference between ambient subsea pressure and pressure inside the external housing; a dielectric liquid for counteracting deformation of the external housing; and a subsea pressure vessel according to the first aspect presented herein.

[0024] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a longitudinal section of a subsea pressure vessel;

Figs 2a-b depict a movement counteracting arrangement of the subsea pressure vessel in Fig. 1; and

Fig. 3 is a side view, with the external housing on the side cut-away, of a subsea unit comprising the subsea pressure vessel in Fig. 1.

DETAILED DESCRIPTION

[0026] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description.

[0027] Fig. 1 schematically illustrates an example of a subsea pressure vessel 1. The subsea pressure vessel 1 comprises a housing 3. The housing 3 is arranged to maintain a pressure within an interior of the subsea pressure vessel 1 which pressure differs from ambient pressure when the subsea pressure vessel 1 is installed subsea. An example of a suitable pressure within the subsea pressure vessel 1 is 1 bar for normal operating purposes. The subsea pressure vessel 1 may advantageously be made of material with high mechanical withstand strength, for example metal such as steel. The subsea pressure vessel 1 is typically filled with a dielectric fluid, for example air, helium, nitrogen or a mixture of gasses.

[0028] The subsea pressure vessel 1 comprises a vacuum interrupter 5 arranged within the housing 3. The vacuum interrupter 5 has an enclosure 7 for maintaining a vacuum within the enclosure 7. The vacuum interrupter 5 comprises a fixed terminal 9 having a fixed contact 9a, and a movable terminal 11 having a movable contact 11a. The moveable terminal 11 and the movable contact 11a are however fixed relative to each other, and are thus arranged to move simultaneously. The fixed terminal 9 and the movable terminal 11 are aligned and extend longitudinally along a common axis A. The movable terminal 11 extends from the enclosure 7, and is operable to move with a rectilinear motion along the common axis A between an open state in which the fixed contact 9a and the movable contact 11a are electrically insulated from each other and a closed state in which the fixed contact 9a and the movable contact 11a are electrically connected. When the movable terminal 11 is in the open state, the fixed contact 9a and the movable contact 11a are separated by vacuum. Thus the vacuum interrupter 5 does not conduct current in the open state. The vacuum interrupter 5 is arranged to conduct current when the movable terminal 11 is in the closed state. The movable terminal 11 is operable by means of an actuator 21.

[0029] The subsea pressure vessel 1 further comprises a movement counteracting arrangement 14. The movement counteracting arrangement 14 comprises a first counteracting member 15 and a second counteracting member 17 arranged to interact with each other to counteract movement of the movable terminal 11 from the open state to the closed state when the interior of the subsea pressure vessel 1 is subjected to a pressure increase compared to a normal operating pressure in the subsea pressure vessel 1. Thus, when the pressure increases inside the subsea pressure vessel 1 and the movable terminal 11 is in the open state, the movement counteracting arrangement restricts movement of the movable terminal 11 towards the closed state. The open state may thereby be maintained even if a pressure increase has occurred, for example due to leakage in the housing 3 of the subsea pressure vessel 1.

[0030] According to one variation, the movable terminal 11 is provided with an extension shaft 13. The extension shaft 13 is mechanically coupled to and electrically insulated from the movable terminal 11. The extension shaft 13 extends along the common axis A, and forms an axial extension of the movable terminal 11. According to one variation, the extension shaft 13 is provided with the first counteracting member 15. In particular, the first counteracting member 15 defines a shoulder of the extension shaft 13, which shoulder is arranged to abut the second counteracting member 17 when the subsea pressure vessel 1 is subjected to a pressure increase compared to a normal operating pressure. The first counteracting member 15 hence protrudes from the extension shaft 13 in a direction transverse to the common axis A. The first counteracting member 15 may be integral with the extension shaft 13, or it may be mounted to the extension shaft 13. In the latter case, the first counteracting member may for example be a flange.

[0031] The movement counteracting arrangement 14 and its operation will now be described in more detail with reference to Figs 2a and 2b. Fig. 2a depicts a close-up view of the movement counteracting arrangement 14 in Fig. 1. The second counteracting member 17 has a first end portion 17a and a second end portion 17b. The first end portion 17a and the second end portion 17b are fixedly arranged relative to each other. The first end portion 17a is arranged to interact with the first counteracting member 15 to counteract movement of the movable terminal 11 when the interior of the subsea pressure vessel 1 is subjected to a pressure increase compared to a normal operating pressure. According to one variation, the first counteracting member 15 defines a surface 15a which faces the vacuum interrupter 5, and which surface 15a is arranged to abut the first end portion 17a of the second counteracting member 17 when the interior of the subsea pressure vessel 1 is subjected to a pressure increase compared to a normal operating pressure. The second counteracting member 17 comprises a second end portion 17b which defines a surface 17c which is perpendicular or essentially perpendicular to the common axis A. The second end portion 17b has a dimension that is greater than a cross-sectional dimension of the movable terminal 11 and/or the movable contact 11a, whichever of the latter two has the largest cross-sectional dimension.

[0032] The first end portion 17a of the second counteracting member 17 is arranged between the first counteracting member 15 and the vacuum interrupter 5 such that movement of the first counteracting member 15, and thus the movable terminal 11, towards the vacuum interrupter 5 may be restricted when the interior of the subsea pressure vessel 1 is subjected to a pressure increase compared to a normal operating pressure.

[0033] The first end portion 17a may be slidably arranged relative to the extension shaft 13. The first end portion 17a may for example be arranged to enclose or partially enclose the extension shaft in a slidable manner. The first end portion may thus be slidably arranged around the extension shaft. Under normal operating pressure, the second counteracting member 17 lies essentially still and motionless, and the first end portion 17a of the second counteracting member 17 is axially displaced relative to the first counteracting member 15. The first end portion 17a is thus arranged at a distance from the first counteracting member 15. The first counteracting member 15 and thus the movable terminal 11 may thereby move rectilinearly along the common axis A a distance corresponding to the axial distance between the first end portion 17a and the first counteracting member 15 in case the movable terminal 11 is to be set from the open state to the closed state by means of the actuator 21 when the interior of the subsea pressure vessel 1 is subjected to normal operating pressure.

[0034] According to one realisation of the second counteracting member, the second counteracting member may be generally L-shaped with the first end portion forming a leg of the L directed towards the extension shaft, perpendicular to the common axis A, and the second end portion forms the other leg of the L, which extends parallel with the common axis A in a direction away from the vacuum interrupter. However, other shapes of the second counteracting member are also envisaged as would be apparent to the skilled person reading this disclosure. In general, the surface defined by the second end portion of the second counteracting member should be essentially perpendicular to the common axis, and the first end portion should be able to abut the first counteracting portion to prevent movement of the extension shaft from the open state to the closed state when the internal pressure in the subsea pressure vessel increases compared to the normal operating pressure.

[0035] The movement counteracting arrangement 14 comprises a chamber 19 which has an opening that is sealed by the second end portion 17b of the second counteracting member 17. The chamber 19 is thus sealed off from the interior of the subsea pressure vessel 1. The chamber 19 is pressurised to the normal operating pressure of the subsea pressure vessel 1 when the interior of the subsea pressure vessel 1 is subjected to normal operating pressure. The chamber 19 may for example be filled with a gas, such as air. The second end portion 17b of the second counteracting member 17 is arranged to move in parallel with the common axis A in a direction from the closed state to the open state, into the chamber 19 when the interior of the subsea pressure vessel 1 is subjected to a pressure increase compared to a normal operating pressure. The second end portion 17b may thus be seen as a piston which is movable in the chamber 19. Under normal operating pressure, the chamber 19 has a first volume V1 defined by the walls of the chamber 19 and the second end portion 17b of the second counteracting member 17 which seals the chamber 19.

[0036] Fig. 2a shows a situation in which the pressure inside the subsea pressure vessel 1 is increased compared to the normal operating pressure. The movable terminal 11 is initially in the open state. A pressure increase may for example occur due to leakage in the housing 3 of the subsea pressure vessel 1, which will result in ambient pressure fluid entering the subsea pressure vessel 1. Such fluid may for example be dielectric fluid of a subsea unit pressurised to the hydrostatic level of the sea floor, and in which the subsea pressure vessel 1 may be arranged. The subsea pressure vessel 1 has three enclosed spaces which may be subjected to different pressures. The interior of the subsea pressure vessel 5 has a first pressure P1. The interior of the chamber 19 has a second pressure P2, which under normal operating pressure in the subsea pressure vessel is equal to the first pressure P1. The interior of the vacuum interrupter 5 is subjected to a third pressure P3, a very low vacuum-creating pressure.

[0037] Due to the increase in the first pressure P1 as a result of the leakage, while at the same time the third pressure P3 is maintained, the movable terminal 11 and thus the extension shaft 13 will be subject to a first force F1 which tries to bring the movable terminal 11 into its closed state. The second counteracting member 17 is arranged to move in a direction opposite to the direction defined from the open state to the closed state. In particular, since the first pressure P1 now is greater than the second pressure P2, and since the surface area, i.e. the dimension, of the second end portion 17b is greater than that of the cross-section of the movable terminal 11 and/or the movable contact 11a, the second end portion 17b will be subjected to a second force F2 which initially is greater than the first force F1. The second end portion 17b will thus be moved further into the chamber 19, compressing the fluid therein, and thus reducing the volume in the chamber 19 to a second volume V2. The second end portion 17b will move into the chamber 19 corresponding to a distance at which the second volume V2 has been reduced by such an amount that the second pressure P2, i.e. the pressure in the chamber 19 equals the first pressure P1, as shown in Fig. 2b. Since the first end portion 17a and the second end portion 17b are fixedly arranged relative to each other, the first end portion 17a is moved in a direction away from the vacuum interrupter 5, to abut the first counteracting member 15 and thus restricting the movement of the movable terminal 11 from the open state to the closed state.

[0038] The subsea pressure vessel 1 may beneficially be utilised as a circuit breaker in a subsea unit comprising electronic and or electrical devices and which is pressurised to the hydrostatic pressure level on the sea floor. Fig. 3 schematically depicts a side view, with the external housing on the side cut-away, of a subsea unit 23 for installation on the seabed. Subsea unit 23 has an external housing 25, and comprises a passive pressure compensator 27, a dielectric liquid 29 and an electric or electronic device 31. The external housing is made of a material which has high thermal conductivity, preferably metal such as steel e.g. stainless steel. The dielectric liquid 29 fills the entire interior space of the subsea unit 23 so as to prevent the occurrence of any air gaps between the internal surface of the external housing 25 and any internal component, such as electric or electronic device 31, contained in the subsea unit 23. The dielectric liquid 29 counteracts deformation of the external housing 25 when the subsea unit 23 is subjected to an ambient subsea pressure higher than a pressure that the external housing 25 can withstand without deformation. The dielectric liquid 29 may for example be oil or an ester, and prevents short circuit of any electronic or electric device contained in the subsea unit 23. The subsea unit 23 further comprises a subsea pressure vessel 1, as previously described. The subsea pressure vessel 1 is electrically connectable to the electric or electronic device 31.

[0039] The passive pressure compensator 27 is arranged to transmit ambient subsea pressure to the inside of the external housing 25. Thereby a pressure difference between ambient subsea pressure and pressure inside the external housing 25 may be reduced. The passive pressure compensator 27 may for example be defined by a mechanically flexible portion of the external housing 25, or a membrane such as an impermeable membrane. The interior volume of the subsea unit 23 and thus the pressure inside the external housing 25 is hence dependent of the ambient subsea pressure.

[0040] Examples of electronic devices are capacitors, for example capacitors based on metalized film technology or on oil-soaked film-foil technology, and power electronic devices such as insulated gate bipolar transistor (IGBT) modules, integrated gate-commutated thyristors (IGCT), diodes, and thyristors, while examples of electric devices are frequency converters and transformers, which in the former case include capacitors and power electronic devices such as IGBT modules.

[0041] It is envisaged that the subsea pressure vessel and subsea unit presented herein find applications within the oil and gas industry for example for subsea HVDC/HVAC power provision systems, i.e. power transmission and power distribution systems, as well as offshore power generation such as wind energy, tidal energy, wave energy, and ocean current energy.

[0042] The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims

1. A subsea pressure vessel (1) comprising:

a housing (3) arranged to maintain a pressure within an interior of the subsea pressure vessel (1) which differs from ambient pressure,

a vacuum interrupter (5) arranged within the housing (3), which vacuum interrupter (5) has an enclosure (7) for maintaining a vacuum within the vacuum interrupter (5), and which vacuum interrupter (5) has a fixed terminal (9) with a fixed contact (9a) and a movable terminal (11) with a movable contact (11a), wherein the movable terminal (11) is operable between an open state in which the fixed contact (9a) and the movable contact (11a) are electrically separated and a closed state in which the fixed contact (9a) and the movable contact (11a) are electrically connected, and

a movement counteracting arrangement (14) comprising a first counteracting member (15) arranged to move concurrently with the movable terminal (11) between the first state and the second state, and a second counteracting member (17) arranged to apply a counteracting force to the first counteracting member (15) when the movable terminal (11) is in the open state and the interior of the subsea pressure vessel (1) is subjected to a pressure increase compared to a normal operating pressure in the subsea pressure vessel (1) to counteract movement of the movable terminal (11) from the open state to the closed state.

2. The subsea pressure vessel (1) as claimed in claim 1, wherein the second counteracting member (17) is arranged to move in a direction opposite to a direction defined from the open state to the closed state when the interior of the subsea pressure vessel (1) is subjected to a pressure increase compared to the normal operating pressure.

3. The subsea pressure vessel (1) as claimed in claim 1 or 2, wherein the second counteracting member (17) is essentially motionless when subjected to normal operating pressure.

4. The subsea pressure vessel (1) as claimed in any of the preceding claims, wherein the second counteracting member (17) is arranged to apply a force to the first counteracting member (15) greater than a force applied by the first counteracting member (15) to the second counteracting member (17) when the interior of the subsea pressure vessel (1) is subjected to a pressure increase compared to the operating normal pressure.

5. The subsea pressure vessel (1) as claimed in any of the preceding claims, wherein the second counteracting member (17) comprises a first end portion (17a) arranged to interact with the first counteracting member (15) to counteract movement of the movable terminal (11) when the interior of the subsea pressure vessel (1) is subjected to a pressure increase compared to a normal operating pressure.

6. The subsea pressure vessel (1) as claimed in claim 5, wherein the second counteracting member (17) comprises a second end portion (17b) which defines a surface (17c) perpendicular to an axis (A) defined by the movable terminal (11), wherein the second end portion (17b) has a dimension that is greater than a cross-sectional dimension of the movable terminal (11).

7. The subsea pressure vessel (1) as claimed in any of the preceding claims, comprising an extension shaft (13) that is mechanically coupled to and electrically insulated from the movable terminal (11).

8. The subsea pressure vessel (11) as claimed in claim 6, wherein the extension shaft (13) is provided with the first counteracting member (15).

9. The subsea pressure vessel (1) as claimed in claim 7, wherein the first counteracting member (15) defines a shoulder arranged to abut the second counteracting member (17) when the subsea pressure vessel (1) is subjected to a pressure increase compared to a normal operating pressure.

10. The subsea pressure vessel (1) as claimed in any of claims 5-9, wherein the second end portion (17b) of the second counteracting member (17) is arranged between the first counteracting member (15) and the vacuum interrupter (5).

11. The subsea pressure vessel (1) as claimed in any of claims 6-10, wherein the movement counteracting arrangement (14) comprises a chamber (19), wherein the second end portion (17b) of the second counteracting member (17) seals the chamber (19) and is arranged to move in parallel with the axis (A) defined by the movable terminal (11), in a direction from the closed state to the open state, into the chamber (19) when the interior of the subsea pressure vessel (11) is subjected to a pressure increase compared to a normal operating pressure.

12. The subsea pressure vessel (1) as claimed in claim 11, wherein the chamber (19) is pressurised to the normal operating pressure of the subsea pressure vessel (1) when the interior of the subsea pressure vessel is subjected to normal operating pressure.

13. The subsea pressure vessel (1) as claimed in any of claims 6-12, wherein the first end portion (17a) of the second counteracting member (17) is arranged at a distance from the first counteracting member (15) when the interior of the subsea pressure vessel (1) is subjected to normal operating pressure.

14. The subsea pressure vessel (1) as claimed in any of claims 7-13, wherein the first end portion (17a) of the second counteracting member (17) is slidingly arranged around the extension shaft (13).

15. A subsea unit (23) comprising:

an external housing (25),

a passive pressure compensator (27) arranged to reduce a pressure difference between ambient subsea pressure and pressure inside the external housing (25),

a dielectric liquid (29) for counteracting deformation of the external housing (25), and

a subsea pressure vessel (1) as claimed in any of claims 1-14.

Drawing