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Title:
A HERMETICALLY SEALED APPARATUS, PARTICULARLY FOR USE IN A SUBSEA ENVIRONMENT
Document Type and Number:
WIPO Patent Application WO/2014/042547
Kind Code:
A1
Abstract:
The invention refers to a hermetically sealed apparatus, particularly for use in a subsea environment, comprising a casing (1) filled with a medium generating a pressure (p1) inside the casing (1) and one or more differential pressure sensing devices measuring the differential pressure between inside and outside the casing (1). The hermetically sealed apparatus is characterized in that at least one differential pressure sensing device comprises an optical sensing means (3) for providing at least one optical signal, where the at least one differential pressure sensing device is adapted to interact with a wall (2) of the casing (1) such that the optical signal is indicative of the bending of the wall (2) caused by the differential pressure, where the at least one differential pressure sensing device is further adapted to determine and output a differential pressure value associated with the bending of the wall (2) based on the at least one optical signal.

Inventors:
NIKOLIN IVAN VLADIMIROVICH (RU)
Application Number:
PCT/RU2012/000754
Publication Date:
March 20, 2014
Filing Date:
September 12, 2012
Export Citation:
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Assignee:
SIEMENS AG (DE)
NIKOLIN IVAN VLADIMIROVICH (RU)
International Classes:
G01M3/32; G01L1/24
Domestic Patent References:
WO2007143369A12007-12-13
Foreign References:
US6233746B12001-05-22
US6218661B12001-04-17
US5870632A1999-02-09
GB2456831A2009-07-29
Attorney, Agent or Firm:
MITS, Alexander Vladimirovich (ul. В. Spasskaya 25/, Moscow 0, RU)
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Claims:
PATENT CLAIMS

1. A hermetically sealed apparatus, particularly for use in a subsea environment, comprising a casing (1) filled with a medium generating a pressure (pi) inside the casing (1 ) and one or more differential pressure sensing devices measuring the differential pressure between inside and outside the casing (1),

characterized in that

at least one differential pressure sensing device comprises an optical sensing means (3) for providing at least one optical signal, where the at least one differential pressure sensing device is adapted to interact with a wall (2) of the casing (1) such that the optical signal is indicative of the bending of the wall (2) caused by the differential pressure, where the at least one differential pressure sensing device is further adapted to determine and output a differential pressure value associated with the bending of the wall (2) based on the at least one optical signal.

2. The apparatus according to claim 1 , characterized in that the optical sensing means (3) includes at least one optical fibre (4, 4') comprising a fibre Bragg grating (5,

5') with a predetermined period where light fed into the at least one optical fibre (4, 4') and having a spectrum including the Bragg wavelength of the fibre Bragg grating (5, 5') is reflected in the at least one optical fibre (4, 4') at the Bragg wavelength, where the bending of the wall (2) causes a change of the period of the fibre Bragg grating (5, 5') resulting in a reflection of the light at another Bragg wavelength, where the at least one optical signal provided by the optical sensing means (3) depends on the reflection of the light.

3. The apparatus according to claim 2, characterized in that the optical sensing means (3) includes a pair of optical fibres (4, 4') where the bending of the wall (2) changes the period of the fibre Bragg grating (5, 5') in one optical fibre (4, 4') of the pair differently from the period of the fibre Bragg grating (5, 5') in the other optical fibre (4, 4') of the pair, where two optical signals are provided each being dependent on the reflection of light in the respective fibre (4, 4') of the pair, the differential pressure value being determined based on the difference between the Bragg wavelengths of the fibre Bragg gratings (5, 5') in the optical fibres (4, 4') of the pair.

4. The apparatus according to one of the preceding claims, characterized in that the wall (2) of the casing (1) has a rod (7) attached thereto extending to the inside of the casing (1), where the displacement of the rod (7) caused by the bending of the wall (1) changes the at least one optical signal.

5. The apparatus according to claim 4 in combination with claim 2, characterized in that the rod (7) and the at least one optical fibre (4, 4') are arranged such that the displacement of the rod (7) due to the bending of the wall (2) causes a strain in the longitudinal direction of the at least one optical fibre (4, 4') causing a change in the period of the fibre Bragg grating (5, 5').

6. The apparatus according to claim 5, where the displacement of the rod (7) is converted in a strain by an elastic element (6), particularly a spring.

7. The apparatus according to claim 4 in combination with claim 2, characterized in that a bending plate (8) is arranged inside the casing (1) such that the displacement of the rod due to the bending of the wall (2) causes a bending of the bending plate where a first optical fibre (4) with a fibre Bragg grating (5) is attached to one side of the bending plate (8), the period of the fibre Bragg grating (5) of the first optical fibre (4) being changed due to the bending of the bending plate (8).

8. The apparatus according to claim 7, characterized in that that a contact element (9) and/or an elastic element is arranged between the rod (7) and the bending plate (8).

9. The apparatus according to claim 7 or 8, characterized in that a second optical fibre (4') with a fibre Bragg grating (5') is attached to the other side of the bending plate

(8), the period of the fibre Bragg grating (5') of the second fibre (4') being changed due to the bending of the bending plate (8) differently from the first optical fibre (4), where the at least one differential pressure sensing device is adapted to determine the differential pressure value based on the difference between the Bragg wavelengths of the fibre Bragg gratings of the first and second optical fibres (4, 4').

10. The apparatus according to one of the preceding claims in combination with claim 2, characterized in that one or more optical fibres (4, 4') comprising fibre Bragg gratings (5, 5') are attached to the wall (2) of the casing (1) or included therein, where the bending of the wall (2) causes a bending of the one or more optical fibres (4, 4') resulting in a change of the Bragg wavelength of the fibre Bragg gratings (5, 5') in the optical fibre or fibres (4, 4').

1 1. The apparatus according to claim 10, characterized in that one optical fibre (4) is arranged on or adjacent to one side of the wall (2) and another optical fibre (4') is arranged on or adjacent to the other side of the wall (2), where the at least one differential pressure sensing device is adapted to determine the differential pressure value based on the difference between the Bragg wavelengths of the fibre Bragg gratings (5, 5') in the optical fibres (4, 4').

12. The apparatus according to one of the preceding claims, characterized in that the optical sensing means (3) comprises an interferometric sensing means for generating an interference pattern as the at least one optical signal, where a change of the interference pattern caused by the bending of the wall (2) is associated with a differential pressure value.

13. The apparatus according to one of the preceding claims, characterized in that one or more electric components are arranged inside the casing (1), particularly one or more electric components for electric power distribution.

14. The apparatus according to one of the preceding claims, characterized in that the medium inside the casing (1) is a fluid and particularly a dielectric fluid.

15. A method for measuring the differential pressure between inside and outside a casing of a hermetically sealed apparatus according to one of the preceding claims, the casing (1) being filled with a medium generating the pressure (pi) inside the casing (1), characterized in that,

an optical sensing means (3) of at least one differential pressure sensing device provides at least one optical signal, where the at least one differential pressure sensing device interacts with a wall (2) of the casing (1) such that the optical signal is indicative of the bending of the wall (2) caused by the differential pressure, where a differential pressure value associated with the bending of the wall (2) is determined and output by the at least one differential pressure sensing device based on the at least one optical signal.

Description:
A HERMETICALLY SEALED APPARATUS, PARTICULARLY FOR USE

IN A SUBSEA ENVIRONMENT

DESCRIPTION

The invention refers to a hermetically sealed apparatus, particularly for use in a subsea environment, as well as to a method for measuring the differential pressure between inside and outside a casing of a hermetically sealed apparatus.

In many industrial fields, there is a need to protect equipment from environmental influences. Particularly in a subsea environment, equipment is to be protected against water. To do so, hermetically sealed apparatuses are used comprising a casing filled with medium generating a pressure inside the casing. The casing and the medium prohibit the penetration of water from outside.

In order to monitor the hermeticity of a hermetically sealed apparatus, differential pressure sensors are used which measure the differential pressure between inside and outside the casing of the apparatus. A substantial change of the differential pressure is an indication of lacking hermeticity of the apparatus. Currently, separate components in the form of electronic sensors are used for differential pressure measuring. The measurement principle of those sensors is usually based on a membrane inside the sensor which moves due to the differential pressure. This movement can be detected e.g. based on electromagnetic induction. One disadvantage of electronic sensors is the need of a penetration in a casing wall of the apparatus for installing the sensor because the sensor needs an access to both the internal and external volume for measuring a differential pressure. Furthermore, electronic sensors are often sensitive to electromagnetic noise.

Document GB 2 456 831 A describes a method for monitoring of pipes before and during installation based on a distributed strain gauge sensor. The strain gauge sensor uses fibre Bragg gratings installed along the pipe in order to detect strain applied to the pipe.

It is an object of the invention to enable an easy and reliable measurement of the differential pressure between inside and outside a casing of a hermetically sealed apparatus.

This object is solved by the independent patent claims. Preferred embodiments of the invention are defined in the dependent claims. The hermetically sealed apparatus according to the invention comprises a casing filled with a medium generating a pressure inside the casing and one or more differential pressure sensing devices measuring the differential pressure between inside and outside the casing. Depending on the circumstances, the medium inside may be a gas or a fluid. In a preferred embodiment, the medium inside is a dielectric fluid and particularly an oil. This variant is particularly advantageous for an apparatus used in a subsea environment because the fluid provides sufficient pressure in order to prohibit the collapse of the casing due to the outside pressure of the water.

At least one differential pressure sensing device of the hermetically sealed apparatus comprises an optical sensing means for providing at least one optical signal, where the at least one differential pressure sensing device is adapted to interact with a wall of the casing such that the optical signal is indicative of the bending of the wall caused by the differential pressure. Furthermore, the at least one differential pressure sensing device is adapted to determine and output a differential pressure value associated with the bending of the wall based on the at least one optical signal. Here and in the following, the term optical signal particularly refers to light comprising wavelengths between infrared and ultraviolet. Furthermore, the term "wall" is to be interpreted broadly and may also refer to a partial section of a wall.

The invention has the advantage that the differential pressure measurement is based directly on the bending of a wall of the casing without the need to penetrate the wall in order to install sensors. In other words, a wall of the casing forms a membrane in order to detect differential pressure. Furthermore, the use of an optical sensing means ensures a reliable and precise pressure measurement with high resolution. Moreover, an optical sensing means is not sensitive to electromagnetic noise.

In a particularly preferred embodiment, the optical sensing means includes at least one optical fibre comprising a fibre Bragg grating with a predetermined period. Fibre Bragg gratings are well-known in the prior art and provide a periodic modulation of the refractive index in an optical fibre causing a reflection of light at the Bragg wavelength. In this variant of the invention, light fed into the at least one optical fibre and having a spectrum including the Bragg wavelength of the fibre Bragg grating is reflected in the at least one optical fibre at the Bragg wavelength, where the bending of the wall causes a change of the period of the fibre Bragg grating resulting in a reflection of the light at another Bragg wavelength, where the at least one optical signal provided by the optical sensing means depends on the reflection of the light. Particularly, it is determined at which wavelength an intensity peak of the reflected light occurs in order to determine the Bragg wavelength. This Bragg wavelength can be associated with the period of the fibre Bragg grating and, thus, with the bending of the wall and the differential pressure. For feeding light into the fibre, one or more appropriate light sources can be used, e.g. LEDs and/or laser light sources.

In a particularly preferred embodiment, the optical sensing means includes a pair of optical fibres where the bending of the wall changes the period of the fibre Bragg grating in one optical fibre of the pair differently from the period of the fibre Bragg grating in the other optical fibre of the pair. As a consequence, two optical signals are provided each being dependent on the reflection of light in the respective optical fibre of the pair, where the differential pressure value is determined based on the difference between the Bragg wavelengths of the fibre Bragg gratings in the optical fibres of the pair. This embodiment has the advantage that external influences changing the Bragg wavelength, e.g. a change in temperature, are eliminated because the determination of the differential pressure value is based on the difference of two Bragg wavelengths. Preferably, without applying strain to the optical fibres of the pair, the periods of the fibre Bragg gratings of the optical fibres are different.

In another embodiment of the invention, the wall of the casing the bending of which is used for a differential pressure measurement has a rod attached thereto extending to the inside of the casing, where the displacement of the rod caused by the bending of the wall changes the at least one optical signal.

The above embodiment is preferably combined with an optical sensing means comprising a fibre Bragg grating. In a preferred variant, the rod and the at least one optical fibre are arranged such that the displacement of the rod due to the bending of the wall causes a strain in the longitudinal direction of the at least one optical fibre causing a change in the period of fibre Bragg grating. Preferably, the displacement of the rod is converted in a strain by an elastic element, e.g. a spring.

In another embodiment of the invention comprising a rod and one or more optical fibre Bragg gratings, a bending plate is arranged inside the casing such that the displacement of the rod due to the bending of the wall causes a bending of the bending plate where a first optical fibre with a fibre Bragg grating is attached to one side of the bending plate, the period of the fibre Bragg grating of the first optical fibre being changed due to the bending of the bending plate. Preferably, this bending plate extends in parallel to the wall of the casing the bending of which is used for differential pressure measurement. Furthermore, the bending plate is preferably mounted at the bottom of the casing and extends in an upward direction. Moreover, the rod preferably extends in a direction perpendicular to the bending plate and/or in a horizontal direction. Preferably, a contact element and/or an elastic element is arranged between the rod and the bending plate ensuring a good contact between those components.

In the above embodiment comprising a bending plate, a second optical fibre additionally to the first optical fibre may be used. The second optical fibre includes a fibre Bragg grating and is attached to the other side of the bending plate where the period of the fibre Bragg grating of the second optical fibre changes due to the bending of the bending plate differently from the first optical fibre. The at least one differential pressure sensing device is adapted to determine the differential pressure value based on the difference between the Bragg wavelengths of the fibre Bragg gratings of the first and second optical fibres. Preferably, the period of the fibre Bragg grating of the first optical fibre is different from the period of the fibre Bragg grating of the second optical fibre when both fibres are not subjected to strain. In this embodiment, external influences, such as temperature changes, manipulating the measurement are eliminated.

In another embodiment of the invention, one or more optical fibres comprising fibre Bragg gratings are attached to the wall of the casing or included therein, where the bending of the wall causes a bending of the one or more optical fibres resulting in a change of the Bragg wavelength of the fibre Bragg gratings in the optical fibre or fibres. This embodiment provides a direct measurement of the bending of the wall and thus of the differential pressure.

In a preferred variant of the above described embodiment, one optical fibre is arranged on or adjacent to one side of the wall and another optical fibre is arranged on or adjacent to the other side of the wall, where the at least one differential pressure sensing device is adapted to determine the differential pressure value based on the difference between the Bragg wavelengths of the fibre Bragg gratings in the optical fibres. Analogously to other embodiments descried above, this embodiment eliminates external influences manipulating the measurement.

In another embodiment of the invention, the optical sensing means comprises an interferometric sensing means for generating an interference pattern as the at least one optical signal, where a change of the interference pattern caused by the bending of the wall is associated with a differential pressure value.

In a preferred embodiment, one or more electric components are arranged inside the casing of the hermetically sealed apparatus. Particularly, those electric components are components for electric power distribution. I.e, the hermetically sealed apparatus may be used for distributing power in an electric power grid, e.g. in a subsea power grid.

As mentioned above, the medium inside the casing is particularly a fluid and preferably a dielectric fluid, e.g. oil. Besides providing a pressure inside the casing, the fluid usually has the function to cool electric components inside the casing and to provide an electric isolation between the electrical components.

Besides the above hermetically sealed apparatus, the invention also refers to a method for measuring the differential pressure between inside and outside a casing of such a hermetically sealed apparatus. The casing of the apparatus is filled with a medium generating the pressure inside the casing. In this method, an optical sensing means of at least one differential pressure sensing device provides at least one optical signal, where the at least one differential pressure sensing device interacts with a wall of the casing such that the optical signal is indicative of the bending of the wall caused by the differential pressure, where the differential pressure value associated with the bending of the wall is determined and output by the at least one differential pressure sensing device based on the at least one optical signal.

Embodiments of the invention will be described in the following with respect to the accompanying drawings, wherein:

Fig. l is a schematic illustration of a first embodiment according to the invention;

Fig.2 is a schematic illustration of a second embodiment according to the invention; and

Fig.3 is a schematic illustration of a third embodiment according to the invention.

In the embodiments described in the following, the pressure difference between the inside and outside pressure of a hermetically sealed subsea apparatus is determined by use of an optical sensor comprising one or more fibre Bragg gratings. The subsea apparatus comprises a casing preferably being made of stainless steel, inside the casing, one or more electric components are disposed. In a preferred embodiment, the electric components are power distribution elements used in a subsea electric power grid which provides electricity to electric equipment used in subsea environments, e.g. for oil mining motors. The casing of the hermetically sealed apparatus is filled with dielectric oil which provides the pressure inside the casing and ensures that no water from outside penetrates inside the casing.

In order to monitor the hermeticity of the casing, a differential pressure sensing device is used which derives the pressure difference from the bending of a wall of the casing. Hence, the wall or part of the wall works as a membrane for measuring the differential pressure between inside and outside the casing. In case that this differential pressure changes significantly, this is a sign that the casing is no longer hermetic and water penetrates inside it. Hence, in a preferred embodiment, a corresponding alarm is generated and transmitted to a central monitoring station in case that a significant change in the pressure difference is detected.

In the embodiment shown in Fig. 1 , the casing 1 of the apparatus is shown schematically by a dashed line. The wall of this casing which is used for pressure sensing is designated by reference numeral 2. Inside the casing, only the differential pressure sensing device is shown, though more components are inside the casing. Due to the schematic illustration, the dimensions of the components shown in Fig. 1 are not to scale. Particularly, the components 4 to 7 forming the differential pressure sensing device are usually much smaller in comparison to the dimension of the casing.

According to Fig. 1 , an optical sensor 3 in the form of an optical fibre 4 comprising a fibre Bragg grating 5 is disposed at a fixed position inside the casing. Fibre Bragg gratings are well-known in the art and provide a region of a periodic modulation of the refractive index in the fibre. The period of this modulation is associated with the well-known Bragg wavelength. As a consequence, light fed into the fibre 4 having a spectrum comprising the Bragg wavelength results in a reflection of the part of the light having the Bragg wavelength.

In order to detect the bending of the wall 2 which is associated with the differential pressure, a rod 7 is attached to the wall, e.g. by welding. Due to the bending of the wall, the rod 7 is displaced as indicated by the arrow AR in Fig. 1. The rod 7 extends perpendicularly to the wall 2 and at the end of the rod remote from the wall 2, an elastic element 6 in the form of a spring is arranged. This spring extends between the rod and an end of the fibre 4. The displacement of the rod 7 is converted via the spring 6 in tension or compression in the longitudinal direction of the fibre depending on the movement of the rod 7. I.e., if the rod 7 moves to the left in Fig. 1 , tension is applied to the fibre 4 whereas, in case that the rod moves to the right, compression is applied to the fibre 4. Compression causes a reduction of a period of the fibre Bragg grating 5 whereas the period of the grating gets bigger in case of a tensile force.

According to the measurement principle of Fig. 1 , light having a spectrum including the Bragg wavelength of the fibre Bragg grating 5 is fed into the fibre 4. As a consequence, light is reflected at the Bragg wavelength of the fibre Bragg grating. This reflected light is detected by a corresponding detector (not shown). By the displacement of the rod, the period of the fibre Bragg grating 5 changes resulting in a different Bragg wavelength so that the light is reflected at another wavelength at the grating 5. Hence, the reflected light forms an optical signal in the meaning of the patent claims. The wavelength of this signal is determined by the detector and correlates with the bending of the wall 2 and, thus, with a differential pressure value output by the differential pressure sensing device. The dependency between the wavelength of the signal and the differential pressure is determined beforehand in a corresponding calibration process.

Fig. 2 shows another embodiment of a device for measuring the differential pressure in a hermetically sealed apparatus. Analogously to Fig. 1 , the bending of the casing wall 2 is measured with the aid of a rod 7 extending perpendicularly to the wall 2. At the end of the rod remote from the wall 2, a contact element 9 is disposed between the rod and a bending plate 8 which is essentially parallel to the wall 2 and mounted at a fixed position at the bottom of the casing. The contact element 9 which may be formed as a ball or cylinder provides proper contact between the rod 7 and the bending plate 8.

Contrary to the embodiment of Fig. 1 , two fibres 4 and 4' each including a fibre Bragg grating 5 and 5' are attached to each side of the plate 8 and extend in a direction upwards and parallel to the wall 2. Light having a spectrum including the Bragg wavelength of the corresponding fibre Bragg gratings 5 and 5' is fed into each of the fibres 4 and 4'. The wavelength of the light reflected at the gratings 5 and 5' is detected by corresponding detectors (not shown). Hence, the reflected light in each of the fibres forms an optical signal in the meaning of the patent claims.

The bending of the wall 2 results in a movement of the rod 7 which causes a bending of the plate 8. This bending results in strain in the fibres 4 and 4' changing the Bragg wavelength. Particularly, a displacement of the rod to the right causes tension in fibre 4' and compression in fibre 4 changing the period and the Bragg wavelength of the corresponding fibre Bragg gratings 5 and 5' in different directions. Contrary to that, a movement of the rod to the left causes compression in the fibre 4' and tension in the fibre 4 also changing the period and the Bragg wavelength of the fibre Bragg gratings 5 and 5' in different directions.

The change of the Bragg wavelengths of the fibre Bragg gratings is illustrated in diagram D. This diagram shows the intensity I of the reflected light in the corresponding fibres in dependency on the wavelength λ . In case that no strain is applied to the fibres 4 and 4', the intensity peak PI corresponds to the reflected light in fibre 4 whereas the intensity peak P2 corresponds to the reflected light in fibre 4'. The wavelength λ at each peak represents the Bragg wavelength. Evidently, the Bragg wavelengths and thus the periods of the fibre Bragg gratings are different for fibres 4 and 4'. In case that the rod 7 is displaced by the wall 2, this results in a change of the Bragg wavelengths in the fibres in different directions. In case that the rod 7 moves to the left, peak PI will be converted to peak P I ' and peak P2 to peak P2'. The difference between the wavelengths of the peaks is measured. This difference corresponds to a bending of the wall and, thus, can be associated with a differential pressure value. Due to the detection of a wavelength difference, environmental influences changing the Bragg wavelengths of the fibres do not change the measured differential pressure. Such environmental influences particularly refer to temperature variations which cause a change of the grating periods.

Fig. 3 shows another embodiment of a differential pressure sensing device according to the invention. In this variant, the wall 2 directly includes corresponding fibres 4 and 4' including fibre Bragg gratings 5 and 5'. In the illustration of Fig. 3, the inner part of the casing is shown schematically as volume VI and the outer part of the casing is schematically shown as volume V2. The corresponding inside pressure is designated as pi and the corresponding outside pressure as p2. The bending of the wall 2 due to a pressure difference is directly sensed by the fibre Bragg gratings 5 and 5'. Fibre Bragg grating 5 is arranged near the inner side of the wall 2 whereas fibre Bragg grating 5' is arranged near the outer side of the wall 2. As a consequence, the Bragg wavelengths change differently due to a bending of the wall 2. Analogously to the embodiment of Fig. 2, the reflected light with the respective wavelength is detected for each of the fibres 4 and 4'. The difference between the wavelengths is associated with the bending of the wall and, thus, a differential pressure difference. Depending on the circumstances, the fibres 4 and 4' need not be included inside the wall. In another embodiment, the fibres 4 and 4' may also be attached on the surfaces of the wall.

The embodiments as described in the foregoing have a number of advantages. A cost effective, safe and reliable method for differential pressure monitoring is provided, particularly for subsea applications. This is achieved by measuring the bending of a wall of the casing of the hermetically sealed apparatus with an optical sensing means. Hence, the wall serves as a membrane which is used in order to determine the differential pressure between inside and outside volumes of the apparatus. For a precise differential pressure measurement, special materials for the apparatus wall or part of the wall with known elastic properties may be used to provide a predictable bending of the wall in dependency on the pressure difference. The differential pressure measurement according to the invention avoids penetration of the apparatus walls for disposing electronic sensors. Such penetrations have the risk that the apparatus is no longer hermetic. Due to the differential pressure measurement based on an optical sensor, the measurement is resistant to electromagnetic noise. Furthermore, the installation costs of optical sensors are very low. Moreover, conventional hermetically sealed apparatus can be easily adapted in order to include a differential pressure sensing device according to the invention.