Displacement sensor, in particular for use in a subsea device

A displacement sensor for sensing the displacement of a movable component of a device is provided. A flexible element of the displacement sensor includes a mounting portion mounted to the device and a coupling portion spaced apart from the mounting portion. A displacement conversion mechanism is coupled to the movable component and is further coupled to the coupling portion of the flexible element. The displacement conversion mechanism is configured to convert a larger displacement of the movable component into a smaller displacement of the coupling portion of the flexible element. The flexible element is arranged such that a displacement at the coupling portion causes the flexible element to bend.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to EP Patent Application No. 12180901 filed Aug. 17, 2012. The contents of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to a displacement sensor for sensing the displacement of a movable component of a device, in particular of a subsea device such as a pressure compensator. The disclosure further relates to a method of sensing the displacement of a movable component of a device.

BACKGROUND

Oil platforms are often used in offshore oil and gas production. More recently, processing facilities are being relocated to the ocean floor. Such subsea installations may be located in considerable water depths, for example in a depth of more than 1.000, 2.000 or even more than 3.000 meters. At such water depths, corresponding pressures of about 100, 200 or 300 bar, respectively, prevail. The devices forming part of such subsea installation do accordingly need to be capable of handling such high ambient pressures.

One possibility of handling high pressures is the use of a pressure resistant enclosure which maintains a close to atmospheric pressure inside, thus enabling the use of standard topside components. A further possibility is the use of a pressure compensated enclosure. Such enclosure generally comprises a pressure compensation system, or pressure compensator, which balances the pressure inside the enclosure to the pressure prevailing in the ambient seawater. Due to the large volume changes experienced by gases when increasing the pressure, the pressure compensated enclosure is generally filled with a dielectric liquid, thus keeping the volume changes which the liquid experiences and which the pressure compensator needs to compensate relatively low.

When using such pressure compensated enclosure, it is desirable to know which status the pressure compensator has at any point in time and whether the pressure compensator operates correctly. In conventional subsea devices, only pressure sensors are available for determining the pressure inside the subsea device, which may indicate the functioning of the pressure compensator. A particular problem is that due to the pressure and temperature differences between the topside and the ocean floor, relatively large volume differences can occur, resulting in large variations of the pressure compensator. Large variations are difficult to measure with high precision, and furthermore, the sensors available are generally not operable in the environment prevailing inside a pressure compensated subsea device, i.e. within a dielectric liquid at high pressures. It is thus desirable to measure relatively large displacements with high accuracy in such difficult environment.

SUMMARY

One embodiment provides a displacement sensor for sensing the displacement of a movable component of a device, the displacement sensor comprising a flexible element mounted to the device at a mounting portion, the flexible element further having a coupling portion spaced apart from the mounting portion, a displacement conversion mechanism coupled to the movable component and further coupled to the flexible element at the coupling portion, wherein the displacement conversion mechanism is configured to convert a larger displacement of the movable component into a smaller displacement of the flexible element at the coupling portion, the flexible element being arranged such that a displacement at the coupling portion causes the flexible element to bend, and a fiber optic strain sensor attached to the flexible element in such way that a bending of the flexible element can be detected as strain by the fiber optic strain sensor.

In a further embodiment, the flexible element comprises a plate, preferably a metal plate, more preferably an aluminium plate.

In a further embodiment, the flexible element is an elongated plate having two ends in longitudinal direction, the mounting portion being adjacent to one of said ends, the coupling portion being adjacent to the other of said ends.

In a further embodiment, the movable component is movable in a first direction, and wherein the flexible element extends in a second direction that is substantially perpendicular to the first direction.

In a further embodiment, the displacement conversion mechanism is configured such that a movement of the movable component in the first direction is at least partially converted into a movement of the coupling portion into the second direction.

In a further embodiment, the movable component is movable in a first direction, and wherein the displacement conversion mechanism comprises an elongated member attached to the movable component and extending substantially parallel to the first direction.

In a further embodiment, the displacement conversion mechanism comprises a support member mounted to the device and having a guiding element which contacts the elongated member to guide a movement of the elongated member together with the movable component in the first direction.

In a further embodiment, the movable component is movable in a first direction, wherein the displacement conversion mechanism comprises a elongated member attached to the movable component, the elongated member having an elongated engagement portion extending at an angle, in particular at an acute angle, to the first direction, the flexible element having an engagement member at the coupling portion which is in engagement with the engagement portion of the elongated member.

In a further embodiment, the engagement portion comprises at least one of a slit, a groove, a recess, or one or more ridges.

In a further embodiment, the engagement member comprises at least one of a protrusion, a bar, or a knob.

In a further embodiment, the engagement portion is a slit extending at an acute angle with respect to the first direction from a first position to a second position, and wherein the engagement member is a bar that reaches through the slit, the bar sliding in the slit when the elongated member is moved together with the movable component, the slit being arranged such that when the engagement member is located at the first position, the flexible element is unbent and when the engagement member is located at the second position, the flexible element is bent.

In a further embodiment, the fibre optic strain sensor comprises an optical fiber having at least one fiber Bragg grating, a length of the optical fiber which includes the at least one fiber Bragg grating being attached to the flexible element in a direction substantially parallel to the longitudinal extension of the flexible element, in particular between the mounting portion and the coupling portion.

In a further embodiment, the fibre optic strain sensor comprises an optical fiber having plural fiber Bragg gratings, the optical fiber being attached to the flexible element such that the fiber Bragg gratings are distributed along the flexible element between the mounting portion and the coupling portion.

Another embodiment provides a pressure compensator for a subsea device comprising a displacement sensor as disclosed above, wherein the movable component forms part of the pressure compensator and is movable to change an internal volume of the pressure compensator, wherein the displacement sensor is configured to detect changes of the internal volume of the pressure compensator by detecting a displacement of the movable component.

Another embodiment provides a method of sensing the displacement of a movable component of a device, in particular of a pressure compensator of a subsea device, comprising the steps of providing a flexible element mounted to the device at a mounting portion, the flexible element having a coupling portion spaced apart from the mounting portion, providing a displacement conversion mechanism coupled to the movable component and further coupled to the flexible element at the coupling portion, converting a larger displacement of the movable component of the device to a smaller displacement and applying the smaller displacement to the flexible element at the coupling portion so as to bend the flexible element, and detecting the bending of the flexible element using a fiber optic strain sensor.

DETAILED DESCRIPTION

Some embodiments provide a displacement sensor that is capable of detecting relatively large displacements with high precession, and which is operable in a pressure compensated subsea device.

One embodiment provides a displacement sensor for sensing the displacement of a movable component of a device. The displacement sensor comprises a flexible element mounted to the device at a mounting portion, the flexible element further having a coupling portion spaced apart from the mounting portion. The displacement sensor comprises a displacement conversion mechanism coupled to the movable component and further coupled to the flexible element at the coupling portion. The displacement conversion mechanism is configured to convert a larger displacement of the movable component into a smaller displacement of the flexible element at the coupling portion. The flexible element is arranged such that the displacement at the coupling portion causes the flexible element to bend. The displacement sensor further includes a fiber optic strain sensor attached to the flexible element in such way that a bending of the flexible element can be detected as strain by the fiber optic strain sensor.

By means of the displacement conversion mechanism, a relatively large movement may be converted into a smaller movement which is detectable with high precession. In particular, the use of the fiber optic strain sensor enables a detection of the bending of the flexible element and thus of the displacement at the coupling portion with enhanced precision. Since the displacement conversion mechanism may only comprise mechanical elements, and furthermore, since an optical fiber is generally relatively insensitive to its environment, the displacement sensor can be employed in a pressurized, dielectric liquid filled environment, in particular inside a pressure compensated enclosure of a subsea device, for example inside a pressure compensator.

In one embodiment, the flexible element comprises a plate. The plate may for example be a metal plate, in particular, it may be an aluminum plate. A light weight but robust displacement sensor may thus be achieved.

The flexible element may be an elongated plate having two ends in longitudinal direction. The mounting portion may be adjacent to one of the ends, and the coupling portion may be adjacent to the other of the ends. In particular, the elongated plate may be mounted at one of its ends to the device, and in proximity to the other of its ends, it may be coupled to the displacement conversion mechanism.

In one embodiment, the movable component may be movable in a first direction, and the flexible element may extend in a second direction that is substantially perpendicular to the first direction. Application of a force to the flexible element at the coupling portion is thus facilitated.

The displacement conversion mechanism may be configured such that a movement of the movable component is at least partially converted into a movement of the coupling portion of the flexible element towards the mounting portion of the flexible element, so as to cause the flexible element to bend.

The displacement conversion mechanism may be configured such that a movement of the movable component in the first direction may at least partially be converted into a movement of the coupling portion into the second direction. By such movement, the flexible element may be caused to bend.

In one embodiment, the movable component may be movable in a first direction, and the displacement conversion mechanism may comprise an elongated member attached to the movable component and extending substantially parallel to the first direction.

The displacement conversion mechanism may comprise a support member mounted to the device and having a guiding element which contacts the elongated member to guide a movement of the elongated member together with the movable component in the first direction. The displacement conversion mechanism may thus be stabilized and made more robust.

The guiding element may for example comprise at least two rollers mounted to a frame of the support member. The frame may be attached to the device, and the rollers may contact the elongated member on opposite sides thereof, for example to prevent a flexing of the elongated member. The rollers may roll along the longitudinal direction of the elongated member.

In one embodiment, the movable component is movable in a first direction, and the displacement conversion mechanism comprises an elongated member attached to the movable component, the elongated member having an elongated engagement portion extending at an angle, in particular at an acute angle, to the first direction. The flexible element may have an engagement member at the coupling portion which is in engagement with the engagement portion of the elongated member. In such way, a relatively simple conversion of a larger displacement into a smaller displacement may be achieved, while at the same time, a simple coupling between the displacement conversion mechanism and the flexible element may be provided.

The elongated engagement portion may for example extend at an acute angle in such way that the elongated engagement portion is angled towards or away from the mounting portion. In particular, the elongated engagement portion may lie within a plane defined by the first direction and by the direction in which the flexible element extends.

By means of the angle of the engagement portion, the ratio of the displacement of the movable component to the displacement of the flexible element at the coupling portion may be adjustable. An effective means for adjusting the displacement ratio may thus be provided.

The engagement portion may comprise at least one of a slit, a groove, a recess, or one or more ridges. As an example, a slit may be provided in which the engagement member can move, or parallel ridges may be provided between which the engagement member is received.

The engagement member may for example comprise at least one of a protrusion, a bar, or a knob. It may for example comprise a bar in transverse direction of an elongated flexible element, which may be in engagement with an engagement portion in form of a slit, or in another example, it may include one or more conical or cylindrical protrusions, which may for example be in engagement with one or two pairs of parallel ridges.

In one embodiment, the engagement portion is a slit extending at an acute angle with respect to the first direction from a first position to a second position, and the engagement member is a bar that reaches through the slit (in particular perpendicular to the slit), wherein the bar is slidable within the slit when the elongated member is moved together with the movable component. The slit may be arranged such that when the engagement member is located at the first position, the flexible element is not bent, i.e. it may be in an equilibrium position, and when the engagement member is located at the second position, the flexible element is bent. A relatively simple mechanical mechanism may thus be provided, which is robust and applicable in a dielectric liquid filled and pressure compensated subsea device. At the same time, an effective conversion of a larger displacement of the movable component into a smaller displacement of the coupling portion of the flexible element can be achieved.

In one embodiment, the fiber optic strain sensor comprises an optical fiber having at least one fiber Bragg grating (FBG). A length of the optical fiber which includes the fiber Bragg grating is attached to the flexible element in a direction parallel to the longitudinal extension of the flexible element, in particular between the mounting portion and the coupling portion. With such type and arrangement of the optical fiber, a bending of the flexible element may be measured with high precision. Accordingly, the displacement of the movable component can be determined precisely in an effective way. The fiber Bragg grating can be highly sensitive to a strain variations (in particular the Bragg wavelength at which the fiber Bragg grating reflects light), and by means of the attachment of the optical fiber, strain is effectively transferred from the flexible element to the optical fiber. A bending of the flexible element may thus result in stain within the optical fiber, with corresponding changes in the optical properties of the fiber Bragg grating, which are detectable with high sensitivity by light transported within the optical fiber.

In one embodiment, the fiber optic strain sensor may comprise an optical fiber having plural fiber Bragg gratings, and the optical fiber may be attached to the flexible element such that the fiber Bragg gratings are distributed along the flexible element between the mounting portion and the coupling portion. Strain and thus the bending of the flexible element may thus be measured at different positions along the flexible element, giving a precise measurement of the displacement of the flexible element at the coupling portion. The precision with which the displacement of the movable component is detected may thus be improved.

The maximum displacement of the movable component may be at least 100 mm, preferably at least 250 mm, and more preferably at least 500 mm. The maximum displacement of the movable component of the device may for example lie within a range of about 250 to about 2.000 mm, preferably within a range of about 500 to about 1.500 mm.

The displacement conversion mechanism may be configured such that the maximum displacement of the flexible element at the coupling portion may lie within a range of about 1 to about 100 mm, preferably within a range of about 10 to about 50 mm.

A further embodiment provides a pressure compensator for a subsea device, comprising a displacement sensor according to any of the above described configurations. The movable component forms part of the pressure compensator and is movable to change an internal volume of the pressure compensator. The displacement sensor is configured to detect changes of the internal volume of the pressure compensator by detecting a displacement of the movable component.

Accordingly, by means of such pressure compensator including the displacement sensor, the status of the pressure compensator, in particular of the current size of its internal (compensation) volume, can be measured on request, continuously or at time intervals. Measurements are possible even if the subsea device towards which the pressure compensator is mounted is deployed subsea. The pressure compensator may have a relatively large internal volume, for example when being used for providing pressure compensation for a subsea transformer, and the displacement sensor allows variations of such large internal volume to be measured with high precision.

In one embodiment, the pressure compensator may be configured to provide pressure compensation for the subsea device up to a deployment water depths of at least 1.000 preferably 2.000, more preferably at least 3.000 meters.

In one embodiment, the movable component is a lid of a bellow or a cylinder of the pressure compensator. The displacement conversion mechanism may be mounted to the lid. In the above mentioned application, the lid may make relatively large movements, for example within a range of about 100 to about 2.000 mm, which are still detectable by means of the displacement sensor comprising the displacement conversion mechanism.

It should be clear that other configurations of the pressure compensator are also conceivable. As an example, the movable component may be a membrane of the pressure compensator, and the displacement conversion mechanism may be mounted to the membrane in order to make larger movements of the membrane detectable in any of the above described ways.

A further embodiment provides a method of sensing the displacement of a movable component of a device, in particular the displacement of a movable component of a pressure compensator of a subsea device. The method comprises the steps of providing a flexible element mounted to the device at a mounting portion, wherein the flexible element has a coupling portion spaced apart from the mounting portion; providing a displacement conversion mechanism coupled to the movable component and further coupled to the flexible element at the coupling portion; converting a larger displacement of the movable component of the device into a smaller displacement and applying the smaller displacement to the flexible element at the coupling portion so as to bend the flexible element; and detecting the bending of the flexible element by means of a fiber optic strain sensor. By means of the method, similar advantages as the ones outlined further above with respect to the displacement sensor may be achieved.

In one embodiment, the method may be performed by the displacement sensor in any of the above outlined configurations.

The features of the embodiments of the invention mentioned above and those yet to be explained below can be combined with each other unless noted to the contrary.

In the following, example embodiments of the invention will be 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 furthermore 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 presentation of the various elements is chosen such that their function and general purpose become apparent to a person skilled in the art.

FIG. 1is a schematic drawing showing a subsea device10, which may for example be or comprise a pressure compensator, and which has an enclosure11and a movable component12which is movable with respect to the enclosure11. In the example ofFIG. 1, the enclosure11has a cylindrical shape, and the movable component12moves in axial direction of the cylindrical shape. Movable component12may thus correspond to a lid or piston which moves inside the cylindrical enclosure11to change the inner volume thereof.

The subsea device10comprises a displacement sensor30according to an embodiment of the invention. The displacement sensor30comprises a displacement conversion mechanism31which is connected to or mounted to the movable component12. The displacement sensor30further comprises a flexible element32which is at a mounting portion51mounted to the enclosure11of the device10. The displacement conversion mechanism31is coupled to the flexible element32at a coupling portion52. The coupling portion52is spaced apart from the mounting portion51of the flexible element32. In operation, the displacement conversion mechanism31converts a larger displacement of the movable component12into a smaller displacement applied to the flexible element32at the coupling portion52. The displacement of the coupling portion52causes the flexible element32to bend, which is detected optically by means of an fiber optic strain sensor31attached to the flexible element32. The fiber optic strain sensor33is for example interrogated optically by means of a fiber optic connection34. The flexible element32and the fiber optic strain sensor33may form a fiber optic displacement sensor.

The displacement conversion mechanism31is configured to convert a larger displacement into a smaller displacement. It can be implemented in a variety of ways. In some embodiments, it can be mounted or attached to the movable component12and may thus mostly move together with the movable component12. In other embodiments, it may be mounted to the enclosure11of the device10, and may have a moving element which is coupled to the movable component12. In some embodiments, it may comprise a gear box which converts a larger movement of movable component12into a smaller displacement which is applied to flexible element32. In other embodiments, it may comprise a simple mechanical guiding mechanism which coupling portion52of the flexible element32engages in such way that the flexible element32bends, as explained hereinafter with respect toFIG. 2.

In the example ofFIG. 1, an embodiment is illustrated in which a displacement of the movable component12along the arrows illustrated inFIG. 1is converted into a smaller displacement in the same direction which is applied to the movable component32in order to bend the movable component32. This means that the flexible element32experiences at the coupling point52a force that is substantially parallel to the direction of displacement of movable component12. In other embodiments, the displacement conversion mechanism31may be configured so that the force is applied in another direction, for example substantially perpendicular to this first direction defined by the motion of movable component12(as indicated by arrows). If a force is applied at coupling portion52of flexible element32towards the mounting portion51, the flexible element32will also be forced to bend (if the magnitude of the force is high enough). In other embodiments, for example the embodiment depicted inFIG. 2, a mixture of such forces can be applied to the flexible element32at coupling portion52.

The conversion ratio of the displacement conversion mechanism will be chosen in accordance with the particular application, i.e. with the maximum displacement of the movable component12and the displacement required at coupling portion in order to achieve a bending of the flexible element32that is detectable with the required precision. As an example, the maximum displacement of movable component12may lie within a range about 500 to about 1.500 mm, whereas the displacement at coupling portion52for this maximum displacement of the movable component12may lie within a range of about 5 mm to about 50 mm. The conversion ratio may then be a ratio according to a combination of any two values out of these two ranges, it may for example lie between about 10:1 and about 300:1. It should be clear that these are only some specific examples given for the purpose of illustration, and that embodiments of the displacement sensor may be realized with different than the above mentioned maximum displacements and conversion values.

For detecting the bending of the flexible element32, a fiber optic strain sensor33is attached to the flexible element32. As an example, the fiber optic strain sensor33may be provided by a length of optical fiber that is over this length firmly attached to the flexible element32. The length of the optical fiber is oriented substantially parallel to the longitudinal direction of a flexible element32. The length of optical fiber attached to flexible element32may comprise one or more fiber Bragg gratings (FBGs), which are distributed along the length of optical fiber. Due to the firm attachment to flexible element32, a bending of the flexible element32causes strain within the firmly attached length of optical fiber, thereby changing the optical characteristics of the fiber Bragg gratings. Each fiber Bragg grating can be configured to have a wavelength (Bragg wavelength) at which it reflects light, the light being transmitted at most of the other wavelengths. Such fiber Bragg grating can be generated inside the optical fiber by a variation of the refractive index of the fiber core. The Bragg wavelength is sensitive to strain, so that applied strain shifts the Bragg wavelength. This can be detected optically since the wavelength of light which is reflected by the fiber Bragg grating is shifted. This can be can be measured (in transmission or in reflection) by a spectroscopic measurement. Plural strain sensors can be realized within the same optical fiber by using fiber Bragg gratings having different Bragg wavelengths, wherein each Bragg grating can be interrogated by monitoring reflection or transmission at a different optical wavelength, which corresponds to optical multiplexing.

Besides being sensitive to strain, the Bragg wavelength of fiber Bragg gratings is generally also sensitive to temperature. To make a temperature independent measurement of strain, the length of optical fiber used as fiber optic strain sensor33may extend around the flexible element32in such way that strain is induced and measured both at the upper and the lower side of flexible element32. The compression experienced at the lower side of flexible element32can similarly be measured by means of the fiber Bragg gratings (i.e. the optical fiber may also be firmly attached to the lower side of the flexible element32). From the strain profile of the upper and the lower side of flexible element32, a temperature independent determination of the deflection of flexible element32at the coupling portion52is possible. On the other hand, the temperature dependent of strain measurement by means of FBG sensors may be avoided by making an additional temperature measurement, for example by an additional temperature sensor, or by a decoupled fiber Bragg grating which is not exposed to strain (e.g. in a loop of optical fiber). The temperature effect on the FBG strain sensors in the fiber optic strain sensor33can then be eliminated. Another possibility is the assumption of a relatively constant temperature and a corresponding gauging of the displacement sensor, which might be feasible for subsea applications in which the environmental temperature remains relatively constant (e.g. between 1 and 5° C.).

For measuring actual displacement at the coupling portion52of flexible element32, a gauge measurement may for example be performed, in which strain curves for a particular displacement of movable component12are identified. After gauging the fiber optic strain sensor33, precise optical measurements of the displacement of movable component12become possible. The fiber Bragg gratings may for example be interrogated by means of the fiber optic connection34, through which light of different wavelengths can be transmitted.

It should be clear that the above mentioned types of measurements for determining displacements of flexible element32at coupling portion52are only examples, and that other types of arrangements of the fiber optic strain sensor33are also conceivable. Also, other types of fiber optic strain sensors may be used, for example polarization dependent sensors or the like. The type and arrangement of fiber optic strain sensor used within the displacement sensor30will depend on the particular type of application and environment.

FIG. 2is a schematic drawing showing a subsea device10comprising a particular embodiment of the displacement sensor30. The fiber optic strain sensor33can be configured as described above. The flexible element32is provided in form of an elongated plate, which is affixed at the mounting portion51to the enclosure11of device10. The elongated plate of flexible element32extends essentially perpendicular to the direction of movement of the movable component12(indicated by arrows). At coupling portion52, the flexible element32has an engagement member in form of a bar35. The bar35extends substantially in transverse direction compared to the longitudinal direction of flexible element32. The elongated plate has further an opening36. As can be seen, a length of optical fiber is attached to the upper side of the elongated plate, and it may also be attached to the lower side. The elongated plate may be a metal plate, it may for example be an aluminum plate.

The displacement sensor30of the embodiment ofFIG. 2comprises a displacement conversion mechanism41. The displacement conversion mechanism41includes an elongated member42, which extends substantially parallel to the direction of movement of the movable component12, and which is attached to the movable component12. The elongated element42comprises an engagement portion43. Engagement portion43is elongated and extends at an acute angle to the direction of movement of movable component12(indicated by arrows). As can be seen inFIG. 2, elongated member42and engagement portion43may be realized by means of a elongated plate having a slit provided therein. The engagement member35of the flexible element32is in engagement with the engagement portion33of the elongated member42. In particular, the bar35reaches through the slit43and is movable along the slit.

The displacement conversion mechanism41furthermore comprises a support member44, which is mounted to the enclosure11of device10. The support member44comprises a frame45affixed to enclosure11, and furthermore comprises rollers46which contact the elongated member42at opposite faces thereof. The support member44accordingly provides guidance when the elongated member42moves together with the moveable component12in direction of the arrows. In particular, a flexing of the elongated member42due to a force applied by the engagement member35of the flexible element32may be prevented, thereby ensuring that a movement of movable component12will lead to a proportional displacement and bending of the flexible element32.

As can be seen fromFIG. 2, the engagement portion43is angled towards the mounting point51. InFIG. 1, the displacement sensor30is shown in a situation in which the engagement member35is located at a fist position within the engagement portion43which the flexible element32is essentially unbent, i.e. no force is applied to the flexible element32at its coupling portion52. Staring from this position, if the movable component12together with the elongated member42is moved upwards, the bar35is forced to move within the slit43. The angled slit43will force the bar to move towards the mounting portion51, thereby bending the flexible element32. The larger the displacement in upward direction of the moveable component12, the more the flexible element32will be bent. This is detected by means of the fiber optic strain sensor33. The support member44prevents that instead of the flexible element32, the elongated member42is bent. In the example ofFIG. 2, the slit43ends at a second position at which the flexible element32is bent at its maximum. The usable measurement range of the displacement sensor30may lie between the first and the second position.

It should be clear that the displacement sensor30shown inFIG. 2is only a particular example and can be modified in various ways. For example, the engagement portion43may be angled the other way, so that the equilibrium position of flexible element32is reached when the movable component12is moved to its maximum upward position. Instead of a slit, engagement portion43may comprise a groove or parallel ridges, in which for example an engagement member35in from of a knob or a protrusion is engaged. Also, the elongated member42may be made sturdy enough so that no support member is required, or the support member44may be configured differently, e.g. mounted to a different position, comprising no rollers or more rollers or the like. Also, the flexible element32does not need to extend perpendicular to the direction of motion of the movable component12, it may in other embodiments extend substantially parallel thereto, with the displacement conversion mechanism41comprising corresponding means for changing the direction of displacement, such as a gear affixed to the enclosure11, a bent lever, a rocker or the like.

The subsea device10illustrated inFIGS. 1 and 2may for example be a pressure compensator. As such, the subsea device10may be mounted to another subsea device, in particular its enclosure, which requires pressure compensation.

FIG. 3shows a subsea device10comprising a pressure compensator20in accordance with an embodiment of the invention. Inside the pressure compensator20, a displacement sensor30comprising the displacement conversion mechanism31, the flexible element32and the fiber optic strain sensor33is provided. The subsea device10has a movable component in form of the lid22of the pressure compensator20. Pressure compensator20is implemented by a bellow comprising an enclosure in form of corrugated side walls21and the lid22. The bellow is mounted onto the housing11the subsea device10.

The housing11is filled with a dielectric liquid and is pressure compensated to the surrounding ambient pressure by means of the pressure compensator20. This means that if the volume of the dielectric liquid inside the housing11changes, e.g. due to a temperature change or a pressure change, the volume change is taken up by pressure compensator20, in particular by a movement of the lid22. For this purpose, an opening15is provided between the interior of housing11and the interior of the pressure compensator20, so that dielectric liquid can flow between the pressure compensator and the housing. Accordingly, no significant overpressure or under pressure compared to the ambient pressure can build up inside the housing11.

The displacement conversion mechanism31is coupled to the lid22of the pressure compensator20. In the example illustrated inFIG. 3, a vertical movement of the lid22is converted into a horizontal displacement applied to the flexible element32. In other embodiments, the displacement sensor30, in particular the displacement conversion mechanism31can be configured as described above with respect toFIGS. 1 and 2, so the explanations given above apply correspondingly.

The lid22of pressure compensator20may for example have a maximum allowable movement of about one meter. Displacement conversion mechanism31may reduce this maximum movement to a maximum displacement of about 10-25 mm applied to flexible element32, which is detected by fiber optic strain sensor33.

A measuring unit40located in the subsea device10may further be provided and may be part of the displacement sensor30. The measuring unit40is connection via fiber optic connection34to the fiber optic strain sensor33. Measuring unit40may for example include a light source for providing white light or light of a particular wavelength to the fiber optic strain sensor33. It should be clear that in some embodiments, a single optical fiber may provide both the fiber optic strain sensor33and the fiber optic connection34, while in other embodiments, plural optical fibers and fiber optic connectors may be used. A light source within measuring unit40may for example supply via the fiber optic connection light of different wavelengths to the fiber optic strain sensor33for interrogating one or more fiber Bragg gratings disposed therein. The light reflected or transmitted through the fiber Bragg gratings is guided via the fiber optic connection34to the measuring unit40, where it is analyzed. Measuring unit40may for example employ spectroscopic measuring techniques for determining the wavelengths of the light received from the fiber optic strain sensor33. Measuring unit40may furthermore evaluate the detected signal, e.g. for determining the position of the movable component22of pressure compensator20, or for determining whether pressure compensator20operates according to specifications. Measuring unit40may communicate such evaluation results to a topside installation, or it may communicate the measured signal to a topside installation for being evaluated.

Note that although a pressure compensator20which has the form of a bellow is illustrated inFIG. 3, it should be clear that the invention is not restricted to such pressure compensator, but different types of pressure compensators may be employed (e.g. a double bellow compensator, a piston-type compensator, a membrane-type compensator or the like). Also, it should be clear that the above described displacement sensor30may be used for measuring displaced in devices other than pressure compensators. The above described displacement sensor has the particular benefit that it can be used under harsh conditions, such as in a pressure compensated enclosure of a subsea device. Large displacements of a movable component can distinctly or continuously be measured with high resolution. The implementation by means of the displacement conversion mechanism is comparatively simple and robust. Furthermore, the use of a fiber optic sensing technique makes any electronics within the displacement sensor obsolete, the displacement sensor thus being essentially immune to electromagnetic interference, radio frequency interference (RFI), electric discharges and the like.

Features of the above outlined embodiments can be combined with each other. The skilled person will appreciate that the above described embodiments are only examples given for the purpose of illustration, and that modifications may be made without departing from the scope of the invention.