Patent Publication Number: US-2016239027-A1

Title: Pressure compensator and method of manufacturing a pressure compensator

Description:
PRIORITY STATEMENT 
     The present application hereby claims priority under 35 U.S.C. §119 to European patent application number EP15155079.5 filed Feb. 13, 2015, the entire contents of which are hereby incorporated herein by reference. 
     FIELD 
     At least one embodiment of the present invention relates to a pressure compensator for providing pressure compensation for subsea device, and to a method of manufacturing a pressure compensator. 
     BACKGROUND 
     Traditionally, fixed or floating platforms, or floating production vessels, are used in the offshore oil and gas production. In the operation of offshore platforms, it can be necessary to install electrical equipment under water, e.g. for controlling functions of a subsea Christmas tree or a subsea blowout preventer. More recently, processing facilities are being relocated to the ocean floor. Installations on the ocean floor can comprise a range of components, including pumps, compressors and the like which require electric power for operation. Power supply can occur through a subsea power grid installed on the ocean floor, which may for example comprise a subsea transformer, a subsea switchgear and a subsea variable speed drive for powering the above mentioned subsea loads. It needs to be ensured that the installed equipment operates reliably even under the high pressure that prevails at the rated installation depth, which can be 3.000 m or more. 
     To protect the equipment from the corrosive environment of the surrounding seawater and to deal with the high pressures, two different solutions were proposed. A pressure resistant enclosure can be provided, which has a close to atmospheric internal pressure, enabling the use of conventional electric and electronic components therein. Such enclosures need to have relatively thick walls and are thus bulky and heavy, since they have to withstand high differential pressures. 
     Another solution is the use of pressurized (or pressure compensated) enclosures, which employ a pressure compensator that balances the pressure in the enclosure to the pressure prevailing in the ambient seawater. 
     The pressurized enclosure is generally filled with a liquid, and components operated inside the pressurized enclosure are made to be operable under high pressures. The pressure compensator balances the pressure and compensates variations in the volume of the liquid filling the enclosure, which may for example occur due to variations in outside pressure and/or temperature. Temperature changes can also be caused by internal heating, e.g. by electric losses of components provided inside the enclosure of the subsea device. The corresponding volume increase of the liquid filling the enclosure may then be taken up by the pressure compensator, which is thus also termed volume compensator. 
     Pressure compensators may include bellows, bladders, pistons, membranes or the like. Bellows can have the disadvantage that they are either expensive to produce, or their configuration is such that the stroke length of the bellows is limited. In the latter case, a pressure compensator for a large volume of liquid (i.e. for an enclosure of a large subsea device) needs to have a significant size to provide the required compensation capacity. For some types of bellows, the bellows needs to have a size of more than three times of the size of the compensated volume. This results in a low utilization factor of the volume of the compensator system. Furthermore, the liquid filling such pressure compensator needs to be compensated itself (i.e. changes of its volume due to temperature/pressure changes need to be taken up by the compensator). Such compensator systems can thus be relatively large and heavy. 
     Furthermore, the bellows of such pressure compensator is exposed to the subsea environment, in particular to the seawater. This may cause corrosion problems for the bellows and may lead to the ingress of seawater into the enclosure of the subsea device upon failure of the bellows. Such seawater ingress may lead to a complete failure of the subsea device, e.g. due to short circuit currents or the like. 
     The document WO2010/034880A1 describes a pressure compensator that has a first bellows chamber that is surrounded by a second bellows chamber, the second bellows chamber forming a closed intermediate space around the first bellows chamber. A double barrier against the ingress of seawater is thus provided. In the disclosed configuration, the compensation capacity is determined by the size of the first bellows. The whole volume inside the first bellows chamber and in addition the volume inside the second bellows chamber are dead volumes, the liquid filling these volumes additionally requiring pressure compensation. In such configuration, the pressure compensator needs to have a significant size. Further an increase in the compensation capacity results in a significant increase in the dead volume, which in turn reduces the compensation capacity that is available for a subsea device (since the dead volume of liquid needs to be compensated). 
     SUMMARY 
     The inventors have recognized that it is desirable to provide a pressure compensator for use with a subsea device that can be manufactured easily and cost efficiently. The inventors have recognized that it is further desirable that the pressure compensator is reliable during operation and has a long lifetime. The inventors have also recognized that it is desirable to reduce the size of pressure compensators, and to increase the utilization factor and compensation capacity. The inventors have also recognized that it is desirable that the pressure compensator is protected from corrosion and provides protection against seawater ingress. 
     Accordingly, the inventors have also recognized that there is a need to obviate at least some of the drawbacks mentioned above and to provide an improved pressure compensator for subsea use. 
     The claims describe embodiments of the invention. 
     According to an embodiment of the invention, a pressure compensator for providing pressure compensation for a subsea device is provided. The pressure compensator comprises a compensator wall enclosing a compensation chamber and at least one bellows assembly. The bellows assembly comprises a metal bellows that forms part of the compensator wall. The metal bellows is deformable to change the inner volume of the compensation chamber. The bellows assembly further comprises an elastomeric bellows provided on the compensator wall and covering at least the metal bellows. The elastomeric bellows has a shape that is adapted to the shape of the metal bellows. A seal is further provided that seals the space between the metal bellows and the elastomeric bellows. The space between the metal bellows and the elastomeric bellows is provided with a vacuum (i.e. it is evacuated) so that a pressure applied by a medium to the compensator wall urges the elastomeric bellows into contact with the metal bellows. 
     According to a further embodiment of the invention, a method of manufacturing a pressure compensator for providing pressure compensation for a subsea device is provided. The method comprises providing an outer wall enclosing a compensation chamber; providing at least one bellows assembly comprising a metal bellows forming part of the compensator wall, the metal bellows being deformable to change the inner volume of the compensation chamber, and an elastomeric bellows provided on the compensator wall and covering at least the metal bellows, wherein the elastomeric bellows has a shape that is adapted to the shape of the metal bellows; sealing the space between the metal bellows and the elastomeric bellows; and evacuating the space between the metal bellows and the elastomeric bellows such that a pressure applied by a medium to the compensator wall urges the elastomeric bellows into contact with the metal bellows. 
     It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation without living the scope of the present invention. In particular, the method may be performed so as to obtain a pressure compensator in any of the above outlined configurations. Furthermore, the above described pressure compensator may be configured as described with respect to the method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages of the invention will become further apparent from the following detailed description read in conjunction with the accompanying drawings. In the drawings, like reference numerals refer to like elements. 
         FIG. 1  is a schematic drawing showing a sectional side view of a pressure compensator for a subsea device according to an embodiment of the invention. 
         FIG. 2  is a schematic drawing showing a side view of a pressure compensator according to an embodiment of the invention. 
         FIG. 3  is a schematic drawing showing a top view of the pressure compensator of  FIG. 2 . 
         FIG. 4  is a schematic drawing showing a perspective view of the pressure compensator of  FIG. 2 . 
         FIGS. 5A and 5B  are schematic drawings showing a sectional view of the pressure compensator of  FIG. 2 . 
         FIG. 6  is a schematic drawing showing a perspective view of a pressure compensator according to a further embodiment of the invention. 
         FIG. 7  is a schematic drawing showing a sectional side view of a pressure compensator according to a further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
     The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof. 
     Various example embodiments will now be described more fully with reference to the accompanying drawings in which only some example embodiments are shown. Specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. The present invention, however, may be embodied in many alternate forms and should not be construed as limited to only the example embodiments set forth herein. 
     Accordingly, while example embodiments of the invention are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments of the present invention to the particular forms disclosed. On the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Like numbers refer to like elements throughout the description of the figures. 
     Before discussing example embodiments in more detail, it is noted that some example embodiments are described as processes or methods depicted as flowcharts. Although the flowcharts describe the operations as sequential processes, many of the operations may be performed in parallel, concurrently or simultaneously. In addition, the order of operations may be re-arranged. The processes may be terminated when their operations are completed, but may also have additional steps not included in the figure. The processes may correspond to methods, functions, procedures, subroutines, subprograms, etc. 
     Specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. This invention may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the present invention. As used herein, the term “and/or,” includes any and all combinations of one or more of the associated listed items. The phrase “at least one of” has the same meaning as “and/or”. 
     Further, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are used only to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present invention. 
     Spatial and functional relationships between elements (for example, between modules) are described using various terms, including “connected,” “engaged,” “interfaced,” and “coupled.” Unless explicitly described as being “direct,” when a relationship between first and second elements is described in the above disclosure, that relationship encompasses a direct relationship where no other intervening elements are present between the first and second elements, and also an indirect relationship where one or more intervening elements are present (either spatially or functionally) between the first and second elements. In contrast, when an element is referred to as being “directly” connected, engaged, interfaced, or coupled to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between,” versus “directly between,” “adjacent,” versus “directly adjacent,” etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the terms “and/or” and “at least one of” include any and all combinations of one or more of the associated listed items. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein are interpreted accordingly. 
     Portions of the example embodiments and corresponding detailed description may be presented in terms of software, or algorithms and symbolic representations of operation on data bits within a computer memory. These descriptions and representations are the ones by which those of ordinary skill in the art effectively convey the substance of their work to others of ordinary skill in the art. An algorithm, as the term is used here, and as it is used generally, is conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of optical, electrical, or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, or as is apparent from the discussion, terms such as “processing” or “computing” or “calculating” or “determining” of “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device/hardware, that manipulates and transforms data represented as physical, electronic quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     According to an embodiment of the invention, a pressure compensator for providing pressure compensation for a subsea device is provided. The pressure compensator comprises a compensator wall enclosing a compensation chamber and at least one bellows assembly. The bellows assembly comprises a metal bellows that forms part of the compensator wall. The metal bellows is deformable to change the inner volume of the compensation chamber. The bellows assembly further comprises an elastomeric bellows provided on the compensator wall and covering at least the metal bellows. The elastomeric bellows has a shape that is adapted to the shape of the metal bellows. A seal is further provided that seals the space between the metal bellows and the elastomeric bellows. The space between the metal bellows and the elastomeric bellows is provided with a vacuum (i.e. it is evacuated) so that a pressure applied by a medium to the compensator wall urges the elastomeric bellows into contact with the metal bellows. 
     When such pressure compensator is deployed subsea, the pressure may for example be applied to one side of the compensator wall by an ambient medium, in particular seawater and to the other side of the compensator wall by a compensation medium, in particular a liquid filling the compensation chamber. 
     By providing a vacuum between the metal bellows and the elastomeric bellows, a tight contact between the metal bellows and the elastomeric bellows may be ensured and the detection of an ingress of a medium, such as seawater, into the space between the metal bellows and the elastomeric bellows may be facilitated. Since the metal bellows is deformable, the material of which it is made may have a limited thickness, and may thus form a weak part of the pressure compensator. By providing the elastomeric bellows that covers the metal bellows, such part of the pressure compensator may be provided with a double barrier. 
     Accordingly, if the metal bellows fails, e.g. due to fatigue, ingress of seawater into the compensation chamber may be prevented. Furthermore, the double barrier configuration is relatively compact, since the space between the metal bellows and the elastomeric bellows is evacuated, so that they are in direct contact and thus essentially act as a single bellows. A compact two barrier configuration can thus be achieved. 
     In an embodiment, the compensator wall comprises an outer wall having an outer side wall, which is preferably cylindrically shaped. The at least one bellows assembly may include at least an outer bellows assembly, the metal bellows of which forms part of the outer side wall. 
     The compensator wall may include a top dome closing one end of the cylindrical outer side wall. The outer side wall and/or the top dome may be made from metal. 
     The pressure compensator may further comprise a flow connection from the compensation chamber to a chamber of a subsea device. A compensation medium, such as a liquid, in particular a dielectric liquid filling the compensation chamber, may thus flow between the chamber of the subsea device and the compensation chamber. Accordingly, if the medium filling the chamber of the subsea device expands, e.g. due to heating, the additional volume of liquid may flow into the compensation chamber leading to an expansion of the bellows assembly, thereby compensating the volume increase. The flow connection may for example be provided by a duct, pipe, flow channel or an opening between the compensation chamber and the chamber of the subsea device. 
     In an embodiment, the compensator wall comprises an outer wall and an inner wall, and the compensation chamber is formed between the outer wall and the inner wall. The dead volume of the compensation chamber may thus be reduced. The pressure compensator may extend along a central axis (e.g. central axis of cylindrical side wall), and an outer wall may be defined as a wall that is further away from the central axis as the inner wall. The volume confined between the outer wall and the inner wall may constitute the compensation chamber. 
     The compensator wall may furthermore comprise an end wall to which the above mentioned outer wall is mounted. In embodiments in which an outer wall and an inner wall are provided, the end wall may for example have an annular or disc shape, and both the outer wall and the inner wall may be mounted to such annular or disc shaped end wall. 
     In an embodiment, the inner wall surrounds a first inner volume, the first inner volume being at least partially filled with an ambient medium. It may for example be at least partially filled with seawater when the pressure compensator is installed subsea. By providing a flow connection between the first inner volume and the ambient medium, so that the first inner volume may at least partially be filled with the ambient medium, the dead volume of the pressure compensator may be reduced, and the inner volume does not need to be pressure compensated. 
     The inner wall may for example have an inner surface facing the compensation chamber and being in contact with the compensation medium filling the compensation chamber, and an outer surface that is in contact with the ambient medium. In this regard, inner surface and outer surface define their positioning with respect to the compensation chamber, i.e. inside the compensation chamber or outside the compensation chamber. 
     In an embodiment, the inner wall may comprise a cylindrical inner side wall and an inner top dome closing one end of the inner cylindrical side wall. The compensator wall may for example comprise the outer side wall and outer top dome and the inner side wall and inner top dome confining the compensation volume. Such configuration may achieve a pressure compensator having a relatively small dead volume. 
     The at least one bellows assembly may comprise at least an inner bellows assembly, the metal bellows of which forms part of the inner wall, in particular the inner side wall. In such configuration, the outer wall may protect the inner bellows assembly. In other embodiments, the inner wall may be a rigid inner wall that does substantially not deform during operation of the pressure compensator, i.e. it may not comprise any bellows or bellows portion. 
     In some embodiments, both an outer bellows assembly and an inner bellows assembly may be provided. The outer bellows assembly and the inner bellows assembly may be configured similarly, i.e. each may comprise a metal bellows and an elastomeric bellows, the space between which is evacuated. 
     In an embodiment, the compensator wall, the compensation chamber and the at least one bellows assembly form a first compensator assembly. The pressure compensator may further comprise at least a second compensator assembly that is arranged in a volume surrounded by an inner wall of the first compensator assembly. Furthermore, a flow connection may be provided between the compensation chambers of the first compensator assembly and the at least one second compensator assembly. A pressure compensator having an increased compensation capacity may thus be achieved, while the dead volume of the pressure compensator may be kept low. Further compensator assemblies may be provided, for example inside a second inner volume surrounded by an inner wall of the second compensator assembly. 
     In an embodiment, the pressure compensator further comprises a leak detector configured to detect ingress of a medium, such as an ambient medium or a compensation medium, into the space between the metal bellows and the elastomeric bellows. By providing such leak detector, the failure or leakage of the metal bellows or the elastomeric bellows may be detected reliably. Accordingly, if one of these bellows should fail, the pressure compensator still remains operable, and an indication is received that the pressure compensator may need to be serviced or substituted in the near future. A reliable and continued operation of the subsea device for which the pressure compensator provides pressure compensation may thus be achieved. 
     The leak detector may be implemented in several ways. As an example, the leak detector may comprise a pressure gauge for detecting a pressure in the space between the metal bellows and the elastomeric bellows of the bellows assembly. In another implementation, the leak detector may comprise a resistance or conductance measuring device for measuring resistance or conductance in the space between the metal bellows and the elastomeric bellows. By such measuring device, it may also be differentiated between an ingress of ambient medium or of compensation medium, since for example seawater as a different conductivity compared to dielectric liquid which may be used as compensation medium. 
     In an embodiment, the metal bellows has an inner surface towards the compensation chamber and an outer surface towards an ambient medium. The elastomeric bellows may be provided on the inner surface of the metal bellows or on the outer surface of the metal bellows. Accordingly, depending on the intended application, either the elastomeric bellows or the metal bellows may be in contact with ambient medium, in particular seawater, when the pressure compensator is deployed subsea. Exposing the elastomeric bellows to the ambient medium may have the advantage that the metal bellows is protected from corrosion. Exposing the metal bellows to the ambient medium may have the advantage that diffusion of ambient medium through the elastomeric bellows may be prevented. 
     The metal bellows may have a corrugated portion providing flexibility for contraction and expansion of the metal bellows. In particular, the metal bellows may only consist of such corrugated portion, i.e. the metal bellows may be defined as a corrugated portion of the compensator wall. The elastomeric bellows may at least cover such corrugated portion of the compensator wall, i.e. the metal bellows. Accordingly, since the corrugated portion of the metal bellows may more easily fail than other parts of the compensator wall, the durability and life time of the pressure compensator may be improved by protecting the corrugated portion with the elastomeric bellows. 
     In an embodiment, the elastomeric bellows is part of an elastomeric wall covering at least part of the compensator wall. The compensator wall may for example comprise an outer wall, and the elastomeric wall may cover the outer wall completely. In other embodiments, the compensator wall may comprise an inner wall including the metal bellows, and the elastomeric wall may completely cover the inner wall, partially or completely. 
     For this purpose, the elastomeric wall may for example include a cylindrical wall section which comprises the elastomeric bellows and a top dome, that is shaped similar to the outer top dome or the inner top dome, depending on where the elastomeric wall is provided. If the outer wall or the inner wall does not comprise a metal bellows, an elastomeric wall for covering such outer wall or inner wall may not be provided. In such case, the outer wall or inner wall may be provided with sufficient thickness and of suitable non corrosive material so as to avoid the need for a double barrier. 
     In an embodiment, the compensator wall includes an outer wall having a flange and the elastomeric wall may be sealed at the flange to the outer wall. A similar configuration may be provided for the inner wall. By way of such flange, the outer wall (or the inner wall) may for example be mounted to an end wall, such as a disk shaped or annular end wall. 
     In an embodiment, the seal that seals the space between the metal bellows and the elastomeric bellows may be provided by way of a flange connection, a vulcanization of elastomeric material to the compensator wall or by a combination thereof. The seal may be provided a distance away from the elastomeric bellows and the metal bellows, for example at the above mentioned flange. 
     In an embodiment in which the elastomeric bellows is part of an elastomeric wall that completely covers for example an outer wall or an inner wall, the seal may be a flange seal between the elastomeric wall and the outer/inner wall. Thus, the seal may comprise single or double sealing around the flange, for example by extending the elastomeric wall around the flange and providing a compression, for example compressing the elastomeric wall between the flange and an end wall. 
     The length of the perimeter of the metal bellows in a direction parallel to the direction of expansion and contraction may be substantially similar to the length of the perimeter of the elastomeric bellows in the same direction. In such configuration, a stretching of the elastomeric material by which the elastomeric bellows is formed may be reduced or may even be prevented, when mounted to the metal bellows. 
     The pressure compensator may furthermore comprise a protection cover for protecting the compensator wall. Furthermore, it may comprise a stop for the expansion and/or contraction of the bellows assembly. Further compression/expansion stops may be provided for further bellows assemblies. 
     In some configurations, an inner wall or an outer wall of the compensator wall may comprise one or more bellows assemblies, for example two or more bellows assemblies. 
     According to a further embodiment of the invention, a method of manufacturing a pressure compensator for providing pressure compensation for a subsea device is provided. The method comprises providing an outer wall enclosing a compensation chamber; providing at least one bellows assembly comprising a metal bellows forming part of the compensator wall, the metal bellows being deformable to change the inner volume of the compensation chamber, and an elastomeric bellows provided on the compensator wall and covering at least the metal bellows, wherein the elastomeric bellows has a shape that is adapted to the shape of the metal bellows; sealing the space between the metal bellows and the elastomeric bellows; and evacuating the space between the metal bellows and the elastomeric bellows such that a pressure applied by a medium to the compensator wall urges the elastomeric bellows into contact with the metal bellows. 
     By such method, a pressure compensator having advantages similar to the ones outlined further above may be achieved. 
     In an embodiment, the method may further comprise the step of leak testing the sealed space between the metal bellows and the elastomeric bellows. By such leak testing, it may be ensured that the space is securely sealed and that the pressure compensator remains operational for a significant amount of time. 
     In an embodiment, the space may be evacuated through an opening, and after the pressure has dropped below a threshold value, the opening may be sealed. Prior to sealing, the above mentioned pressure testing may be performed. The threshold value may for example be a value that is smaller than 10-1 mbar, 10-2 mbar or even 10-3 mbar. 
     It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation without living the scope of the present invention. In particular, the method may be performed so as to obtain a pressure compensator in any of the above outlined configurations. Furthermore, the above described pressure compensator may be configured as described with respect to the method. 
     In the following, embodiments of the invention are described in detail with reference to the accompanying drawings. It is to be understood that the following description of the embodiments is given only for the purpose of illustration and is not to be taken in a limiting sense. It should be noted, that the drawings are to be regarded as being schematic representations only, and elements in the drawings are not necessarily to scale with each other. Rather, the representation of the various elements is chosen such that their function and general purpose become apparent to a person skilled in the art. 
       FIG. 1  schematically illustrates a subsea device  300  having a subsea enclosure  301  which encloses a chamber  302 . Subsea device  300  may for example be a subsea transformer, a subsea converter, in particular a subsea variable speed drive, a subsea switchgear, a subsea motor, a subsea control and/or communication module or the like. Accordingly, respective mechanical, electric and/or electronic components may be disposed in the chamber  302 . Chamber  302  is pressure compensated via pressure compensator  100 . It should be clear that subsea device  300  may comprise one or more pressure compensated chambers, and may additionally comprise one or more chambers in which a predetermined pressure is maintained (pressure resistant chambers), for example a close to atmospheric pressure (e.g. below 10 bar or 5 bar, e.g. about 1.5 bar). Accordingly, some components which may not be operable under the high subsea pressures may be placed in a pressure resistant enclosure inside the chamber  302 , or adjacent thereto, or the like. 
       FIG. 1  illustrates an embodiment of a pressure compensator  100  for compensating volume variations of a liquid filling the chamber  302  of subsea device  300  and for balancing the pressure inside the chamber  302  to a pressure prevailing in an ambient medium, for example in seawater when subsea device  300  is installed subsea. The pressure compensator  100  includes a compensator wall  10  which encloses a compensation chamber  20 . A flow connection  40  is provided between the compensation chamber  20  and the chamber  302  of the subsea device to allow an exchange of fluid. The compensation chamber  20  is preferably filled with the compensation medium, in particular with a liquid, such as a dielectric liquid. Examples of such liquid include oil, or a synthetic ester based liquid, such as Midel. 
     The pressure compensator  100  has a bellows assembly  11  that includes a metal bellows  15  and an elastomeric bellows  16 . The metal bellows  15  forms part of the compensator wall  10 . The metal bellows  15  may be constituted by or consist of a corrugated part of the compensator wall  10 . The elastomeric bellows  16  covers at least this corrugated part of compensator wall  10 , i.e. it covers at least the metal bellows  15 . The elastomeric bellows  16  may be provided on an outer surface of the metal bellows  15 , i.e. on a surface facing the ambient medium, such as seawater, or it may be provided on an inner surface of the metal bellows  15 , i.e. on a surface that faces the compensation chamber  20 , in particular the compensation medium therein. Either the metal bellows  15  or the elastomeric bellows  16  may thus be exposed to surrounding seawater when installed subsea. 
     In the embodiment of  FIG. 1 , the compensator wall  10  has a cylindrical side wall section which almost entirely consists of the bellows assembly  11 . In other embodiments, such side wall section may further comprise non-deformable sections, e.g. cylindrical sections without corrugation. In the example of  FIG. 1 , the compensator wall  10  further comprises an upper end wall and a lower end wall  19 . The end walls and the cylindrical side wall define the compensation chamber  20 . 
     The space between the metal bellows  15  and the elastomeric bellows  16  is sealed by a seal. Accordingly, no medium can enter the space between the bellows  15 ,  16 . The space between the bellows  15 ,  16  is further evacuated, so that no medium is present between these bellows. Accordingly, the elastomeric bellows  16  sits closely on the metal bellows  15 . Due to the vacuum and the elasticity of the elastomeric bellows  16 , the elastomeric bellows  16  is pushed against the metal bellows  15  by the ambient pressure. Accordingly, in the schematic drawing of  FIG. 1 , the elastomeric bellows  16  and the metal bellows  15  are shown as a single bellows. 
     Due to the pressure compensation capabilities of pressure compensator  100 , the pressure inside chamber  20  is substantially similar to the pressure in the outside environment, so that substantially the same pressure acts from both sides on the bellows  15 ,  16  and urges them into contact. 
     As can be seen, such configuration provides a double barrier protection for at least the corrugated part of the compensator wall  10 . If the elastomeric bellows  16  is provided on the outside in contact with seawater, the metallic bellows  15  can be protected from the corrosive subsea environment. Furthermore, if one of the bellows  15 ,  16  fails, for example due to a leak, the other bellows still prevents the ingress of ambient medium, in particular seawater into the compensation chamber  20 . Reliability of the pressure compensator  100  can thus be increased. Furthermore, since the space between the bellows  15 ,  16  is evacuated, the additional barrier does not require additional space. Also, no additional dead volume is generated by the additional barrier. Pressure compensator  100  is thus relatively compact and has a relatively small dead volume compared to conventional two-barrier solutions. 
     Pressure compensator  100  may furthermore comprise a leak detector  50  which detects a leakage in one of the bellows  15 ,  16 . Leak detector  50  employs a sensor for detecting whether any compensation medium from the chamber  20  or any ambient medium such as seawater enters the space between the metal and elastomeric bellows  15 ,  16 . As an example, leak detector  50  may employ a pressure sensor or pressure gauge to detect a change in pressure in the space between the bellows  15 ,  16 . Leak detector  50  may also additionally or alternatively employ a sensor for measuring conductivity or resistance within the space between the bellows  15 ,  16 . Such sensor may for example comprise two or more electrodes (e.g. conductor plates) that are disposed in the space between the bellows  15 ,  16  and that are electrically isolated from the metal bellows  15 . If seawater enters the space, the resistance between such spaced apart conductor plates will change. The change will be different from a situation in which compensation medium, in particular a dielectric liquid enters the space, which generally has a lower conductivity. Accordingly, by such type of sensor, it may even be determined whether the bellows that is in contact with the compensation medium or that is in contact with the ambient medium has leaked. 
     Leak detector  50  may comprises a connection  51  to the respective sensor element. 
     In operation, the bellows  15 ,  16  of the bellows assembly  11  will expand and/or contract upon a change of the volume of the liquid filling the chamber  302  and the compensation chamber  20 . A difference between the outside pressure and the pressure prevailing in chambers  20 ,  302  is balanced by a corresponding expansion or contraction of the bellows assembly  11 . In some embodiments, the pressure compensator  100  may be biased, for example to generate a small overpressure within the chamber  302  of the subsea device  300 . This way, the ingress of seawater upon leakage may be prevented, and such leakage may furthermore be detected by a drift in the pressure compensator when compensation medium is leaking out. The biasing may for example be generated by adding weight to the upper end wall of the compensator wall  10 , by respective spring assemblies or the like. 
       FIG. 2  shows a side view of a particular implementation of the pressure compensator  100 , so that the above explanations are equally applicable to the pressure compensator  100  of  FIG. 2 . Pressure compensator  100  has a compensator wall that includes the outer wall  12 . Outer wall  12  has a cylindrically shaped outer side wall  17  and an outer top dome  18 . The central axis  60  of the outer side wall  17  is further indicated. 
     The compensator wall  10  further includes the end wall  19 . Outer side wall  17  includes the bellows assembly  11  comprising the metal and elastomeric bellows  15 ,  16 . In the example of  FIG. 2 , the elastomeric bellows  16  is provided on the outer surface of the outer wall  12 , and is thus in contact with seawater when pressure compensator  100  is deployed subsea. The outer wall  12  is made of metal and includes the metal bellows  15 . The elastomeric bellows  16  forms part of an outer elastomeric wall  32 , which in the example of  FIG. 2  completely covers the metallic outer wall  12 . As outlined above, in some embodiments, only part of the outer wall  12  may be covered by elastomeric material. The flow connection  40  is provided on the side wall  17 . 
       FIG. 3  is a top view of the pressure compensator  100  of  FIG. 2 . As can be seen, the pressure compensator  100  has a cylindrical shape around the central axis  60 . 
     The side wall  17  extends circumferentially around the central axis  16 , which is a symmetry axis of the pressure compensator  100 . In operation, the top dome  18  moves in the direction of the central axis  60  upon compression and expansion of the bellows assembly  11 . At the end wall  19 , the pressure compensator  100  may be fixed to the subsea device  300 . 
       FIG. 4  shows the pressure compensator  100  of  FIG. 2  in a perspective view. As can be seen, the side wall  17  comprises a cylindrical section without corrugation in which the flow connection  40  is provided. The outer wall  12  further comprises a flange  14  which is mounted to a corresponding flange of the end wall  19 . The outer elastomeric wall  32  can for example be folded around a flange  14  and compressed between the flange of the end wall  19  and the flange  14  of outer wall  12 . This way, the seal may be formed that seals the space between the elastomeric bellows  16  and the metal bellows  15 . In such configuration, in which the elastomeric wall  32  covers the outer wall  12  completely, the seal on flange  14  may be sufficient to completely seal the space between the outer elastomeric wall  32  and the outer metal wall  12 . Accordingly, by way of the above mentioned leak detector  50 , any leak in the outer elastomeric wall  32  or the outer metal wall  12  may be detected. An efficient and compact double barrier against seawater ingress as well as an effective leak detection may thus be provided. 
       FIG. 5A  shows a sectional side view of pressure compensator  100  taken along the line A-A shown in  FIG. 3 . As illustrated in  FIG. 5A , the pressure compensator  100  further comprises an inner wall  22 . In the present embodiment, inner wall  22  comprises a cylindrical inner wall section without corrugations, that is a rigid wall section and that does not deform during operation of pressure compensator  100 . It furthermore comprises an inner top dome  28  closing one end of the cylindrical inner side wall  22 . The other end of the inner cylindrical side wall  22  may be open or it may be closed by a further end wall and provided with an opening, so that ambient medium can flow into the volume  30  that is surrounded by the inner wall  22 . 
     The end wall  19  may have an annular shape that provides a connection between the inner wall  22  and the outer wall  12 . In other embodiments, it may have a disk shape thus also enclosing the inner volume  30 , and an opening may be provided in the end wall  19  to allow ambient medium to flow into the inner volume  30 . The compensator wall  10  may thus comprise the inner wall  22 , the outer wall  12  and the end wall  19 . The compensation chamber  20  is confined between the inner wall  22  and the outer wall  12 . In such configuration, the dead volume of pressure compensator  100  can be further reduced, since only the compensation chamber  20  confined between inner wall  22  and outer wall  12  is filled with compensation medium. As shown in  FIG. 5A , this volume can be relatively small. In operation, upon an expansion of the liquid filling chamber  302  of subsea device  300 , the bellows assembly  11  will expand, thus providing increased separation between outer top dome  18  and inner top dome  28 , thus leading to a significant volume increase of compensation chamber  20 . Pressure compensator  100  thus provides a relatively large compensation capacity, while the dead volume is relatively small. The flow connection  40  leads into the compensation chamber  20 . 
     In some embodiments, the inner wall  22 , in particular the cylindrical inner side wall, may comprise the bellows assembly  11 , and the outer cylindrical side wall  17  may be a rigid wall without corrugations. In other embodiments, a bellows assembly  11  may be provided both on the outer wall  12  and the inner wall  22 , as will be described further below. 
       FIG. 5B  is a schematic drawing showing an enlarged representation of the portion that is encircled in  FIG. 5A . As can be seen, the elastomeric bellows  16  is provided outside on the metal bellows  15 . Due to the evacuation of the space there between, bellows  16  sits closely on bellows  15 . Bellows  15  and  16  thus substantially act as a single bellows. 
     Furthermore, due to the required flexibility of metal bellows  15 , it may be provided with a smaller wall thickness compared to other parts of the compensator wall  10 . As illustrated, the metal bellows  15  may for example be mounted, in particular welded or soldered to the outer dome  18  having a larger wall thickness. Accordingly, the protection by the elastomeric bellows  16  is particular beneficial for the metal bellows  15 . As can be seen, the elastomeric outer wall  32  extends also over the remaining parts of the outer wall  12 , in particular also over the top dome  18 . The inner wall  22  can be a relatively thick metal wall, so that even though it is in contact with ambient medium present inside the inner volume  30 , the inner wall  22  may be sufficient to protect the compensation chamber  20  against seawater ingress. In other embodiments, the inner wall  22  may be provided with the corresponding elastomeric wall  32 . In embodiments in which inner wall  22  comprises a metal bellows, a respective elastomeric bellows  15  is provided, or an elastomeric wall (similar to  32 ) completely covering the inner wall  22  may be provided. 
     Elastomeric wall  32  may be made as a single part, or may be composed of different wall parts, which may be adhered to one another by molding techniques, adhesive, vulcanization or the like. Furthermore, the skilling of the space between the metal bellows  15  and the elastomeric bellows  16  may be achieved by different devices. As an example, a portion of the elastomeric bellows  16  or of the elastomeric wall  32  may be vulcanized to the metal bellows  15  or the respective compensator wall, in particular the outer wall  12  or the inner wall  22 . An effective sealing may thus be achieved, in particular for configurations for which the elastomeric wall  32  does not completely cover the outer wall  12  or the inner wall  22 . 
     The shape of the elastomeric bellows  16  may be adapted to the shape of the metal bellows  15 . This way, the elastomeric bellows  16  is not overly stretched or compressed when being brought into tight contact with metal bellows  15  due to the evacuation of the space there between. This may ensure long lifetime and reliable operation of the elastomeric bellows  16 . In particular, the length of a line on the surface of metal bellows  15  that is in contact with the elastomeric bellows  16  taken perpendicularly to the corrugations may be substantially the same as a corresponding line on the surface of the elastomeric bellows  16  that is in contact with the metal bellows  15  taken perpendicularly to the corrugations. Stretching of the elastomeric bellows  16  may thus be prevented. 
       FIG. 6  is a schematic drawing showing a perspective view of a further embodiment of the pressure compensator  100 . The embodiment of  FIG. 6  is a modification of the embodiment of  FIG. 2 , so that the explanations given above are equally applicable. In the example of  FIG. 6 , the compensator wall comprises two bellows assemblies  11 ,  21 . The outer side wall  17  includes these two bellows assemblies  11 ,  21  and furthermore includes a cylindrical section without corrugations there between. Again, part of the end wall  19  and the flange  14  are visible. To protect the bellows assemblies  11 ,  21 , the pressure compensator  100  may be provided with a protection cover. Furthermore, expansion stops and/or compression stops may be provided to prevent excessive expansion/compression of the bellows assemblies  11 ,  21 . The bellows assembly  21  may be configured similar to the bellows assembly  11 , i.e. as outlined further above. 
     The pressure compensator  100  of  FIG. 6  further includes a vent pipe  33 . Pressure compensator  100  also comprises an inner wall  22  surrounding an inner volume  30  that is flow communication with ambient medium. The vent pipe  33  leads into this inner volume  30 . Accordingly, when the pressure compensator  100  is deployed subsea, and air is trapped inside the inner volume  30 , it may be vented via the vent pipe  33 . 
       FIG. 7  is a schematic drawing showing a further embodiment of the pressure compensator  100 . The embodiment of  FIG. 7  is a modification of the embodiment shown in  FIG. 2 , so that the above explanations are equally applicable. The sectional side view of  FIG. 7  shows a first compensator assembly  101  which comprises the outer wall  12  and the inner wall  22 . As explained with respect to  FIG. 5 , the inner wall  22  may comprise a second bellows assembly  21 . The first bellows assembly  11  and the second bellows assembly  21  may be configured as described above. Furthermore, as mentioned above, the outer wall  12  and the inner wall  22  may both be covered, completely or partially, by an elastomeric wall  32 . In operation, upon an increase of the volume of the liquid filling the chamber  302 , the additional volume is taken up by expansion of the first bellows assembly  11  and contraction of the second bellows assembly  21 . The top dome  18  thus moves upward and the inner top dome  28  moves downward, as illustrated by arrows, thus increasing the volume of the compensation chamber  20  for taking up the additional volume of the compensation liquid. 
     Compensation chamber  20  is further closed by the end wall  19 , which may have an annular shape, and flow connection  40  provides fluid communication between the compensation chamber  20  and the chamber  302  of the subsea device. 
     In the embodiment of  FIG. 7 , the pressure compensator  100  comprises a further second compensator assembly  201 , which is configured substantially similar to the first compensator assembly  101 . The second compensator assembly  201  is arranged within the inner volume  30  that is surrounded by the inner wall  22 . The second compensator assembly  201  comprises an outer wall  212 , an inner wall  222  and an end wall  219 , which form part of a second compensator wall. Outer wall  212 , inner wall  222  and end wall  219  enclose a second compensation chamber  220 . The second compensation chamber  220  is in flow communication with chamber  302  of the subsea device via the flow connection  40 . Similar to the walls  12  and  22 , the second outer wall  212  comprises a first bellows assembly  211  and the second inner wall  220  comprises a second bellows assembly  221 . These bellows assemblies  211 ,  221  can be configured similar to the bellows assemblies  11 ,  21 , they may in particular comprise a metal bellows  15  and an elastomeric bellows  16 . 
     By such second compensator assembly  201 , the space available in the inner volume  30  may be efficiently used to further increase the compensation capacity of the pressure compensator  100 . In particular, by contraction/expansion of the bellows assemblies  211 ,  221 , the volume of the second compensation chamber  220  can be changed, thus accommodating volume changes of the liquid filling chamber  302 . It should be clear that further compensator assemblies  101 ,  201  may be provided to further increase the compensation capacity of pressure compensator  100 , for example within the volume surrounded by second inner wall  222 . 
     The end walls  19 ,  219  may be provided as a common end wall, for example in form of a disk-shaped end wall or an end wall having two concentric rings. In other configurations, they may be provided separately. 
     It should be clear that the features of the embodiments described above can be combined with each other unless noted to the contrary. As an example, one or more of the walls  11 ,  21 ,  211 ,  221  may be provided as a rigid wall without corrugations. In another example, the pressure compensator of  FIGS. 2 to 5  may comprise a second compensator assembly inside the inner volume  30  that is configured similar to the first compensator assembly illustrated in  FIG. 5A . 
     Also, some embodiments may comprise two or more bellows assemblies on an outer side wall and/or an inner side wall. 
     An embodiment of the invention further provides a method of manufacturing a pressure compensator having any of the above outlined configurations. In the method, the elastomeric bellows or wall is sealed to the outer wall or inner wall. After sealing, the space between the elastomeric bellows or elastomeric wall and the metal bellows or inner/outer wall is evacuated. This may be achieved by using a vacuum pump and a flow connection into said space. The method may comprise, after evacuation, the performing of a leak test in order to ensure that there is no leakage into the space, for example via the seal, via the elastomeric wall, or via the inner/outer wall. Such leakage test may for example be performed by way of a helium sniffer. After it has been ensured that no leak is present, the flow connection to the space between the metal bellows and the elastomeric bellows can be closed, for example by vulcanization of the elastomeric material or the like. In other embodiments, a leak detector for subsea use (as outlined above) may be attached to such fluid flow connection into that space. In other embodiments, an electric connection may be provided into the space for contacting a sensor disposed in the space. 
     The metal bellows may be made of a metal material, such as alloy 625, in particular Inconel 625. The elastomeric bellows may for example be made of a rubber material, such as natural rubber. 
     The pressure compensator  100  may be adapted to be operable in a subsea environment. In particular, it may be operable at a water depth of at least 1,000 m preferably at least 2,000 or even at least 3,000 m. A reliable and compact pressure compensator having a relatively small dead volume, a relatively large compensation capacity, and an efficient protection against seawater ingress may thus be provided for the subsea device  300 . 
     While specific embodiments are disclosed herein, various changes and modifications can be made without departing from the scope of the invention. The present embodiments are to be considered in all respects as illustrative and non-restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. 
     The aforementioned description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure. Further, although each of the embodiments is described above as having certain features, any one or more of those features described with respect to any embodiment of the disclosure can be implemented in and/or combined with features of any of the other embodiments, even if that combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with one another remain within the scope of this disclosure. 
     The patent claims filed with the application are formulation proposals without prejudice for obtaining more extensive patent protection. The applicant reserves the right to claim even further combinations of features previously disclosed only in the description and/or drawings. 
     The example embodiment or each example embodiment should not be understood as a restriction of the invention. Rather, numerous variations and modifications are possible in the context of the present disclosure, in particular those variants and combinations which can be inferred by the person skilled in the art with regard to achieving the object for example by combination or modification of individual features or elements or method steps that are described in connection with the general or specific part of the description and are contained in the claims and/or the drawings, and, by way of combinable features, lead to a new subject matter or to new method steps or sequences of method steps, including insofar as they concern production, testing and operating methods. Further, elements and/or features of different example embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
     References back that are used in dependent claims indicate the further embodiment of the subject matter of the main claim by way of the features of the respective dependent claim; they should not be understood as dispensing with obtaining independent protection of the subject matter for the combinations of features in the referred-back dependent claims. Furthermore, with regard to interpreting the claims, where a feature is concretized in more specific detail in a subordinate claim, it should be assumed that such a restriction is not present in the respective preceding claims. 
     Since the subject matter of the dependent claims in relation to the prior art on the priority date may form separate and independent inventions, the applicant reserves the right to make them the subject matter of independent claims or divisional declarations. They may furthermore also contain independent inventions which have a configuration that is independent of the subject matters of the preceding dependent claims. 
     Still further, any one of the above-described and other example features of the present invention may be embodied in the form of an apparatus, method, system, etc. For example, of the aforementioned methods may be embodied in the form of a system or device, including, but not limited to, any of the structure for performing the methodology illustrated in the drawings. 
     In this application, including the definitions below, the term ‘module’ or the term ‘controller’ may be replaced with the term ‘circuit.’ The term ‘module’ may refer to, be part of, or include processor hardware (shared, dedicated, or group) that executes code and memory hardware (shared, dedicated, or group) that stores code executed by the processor hardware. 
     The module may include one or more interface circuits. In some examples, the interface circuits may include wired or wireless interfaces that are connected to a local area network (LAN), the Internet, a wide area network (WAN), or combinations thereof. The functionality of any given module of the present disclosure may be distributed among multiple modules that are connected via interface circuits. For example, multiple modules may allow load balancing. In a further example, a server (also known as remote, or cloud) module may accomplish some functionality on behalf of a client module. 
     None of the elements recited in the claims are intended to be a means-plus-function element within the meaning of 35 U.S.C. §112(f) unless an element is expressly recited using the phrase “means for” or, in the case of a method claim, using the phrases “operation for” or “step for.” 
     Example embodiments being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.