WO2012041528A1

WO2012041528A1 - Arrangement for determining a physical quantity for monitoring a device and system for monitoring a physical quantity

Info

Publication number: WO2012041528A1
Application number: PCT/EP2011/051043
Authority: WO
Inventors: Endre Brekke; Fredrik Dessen
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-09-29
Filing date: 2011-01-26
Publication date: 2012-04-05

Abstract

Arrangement for determining a physical quantity for monitoring a device and system for monitoring a physical quantity. It is described an arrangement for determining a physical quantity being indicative of a condition of an electrical or/and mechanical device, the arrangement comprising: a sealed container (105, 205) for accommodating the device (103); an optical fiber (113, 213) for guiding light, the optical fiber comprising a fiber Bragg grating sensor (111a- 111g, 211a-211g) for changing a characteristics of the light in dependence of the physical quantity, the fiber Bragg grating sensor being located within the container; and an optical connector (119, 219) for optically connecting the optical fiber (113, 213) to an external optical fiber (117, 217) external to the container for guiding the light into the optical fiber and guiding the light having passed through the fiber Bragg grating sensor out of the optical fiber for determining the physical quantity based on the changed characteristics of the light. Further a corresponding method is provided.

Description

DESCRIPTION

Arrangement for determining a physical quantity for monitor^¬ ing a device and system for monitoring a physical quantity

Field of invention

The present invention relates to an arrangement for determin- ing a physical quantity being indicative of a condition of an electrical and/or mechanical device, to a system for monitor^¬ ing a physical quantity, and to a method for determining a physical quantity in a sealed container accompanying a de^¬ vice. In particular, the present invention relates to an arrangement for determining a physical quantity being indica^¬ tive of a condition of an electrical and/or mechanical de^¬ vice, to a system for monitoring a physical quantity and to a method for determining a physical quantity in a sealed con^¬ tainer accompanying a device, wherein the physical quantity is measured using a fiber Bragg grating sensor.

Art Background It has been observed that it may be difficult to monitor a condition of equipment, in particular a condition of an electrical and/or mechanical device using a conventional method. In particular, it has been observed that it is diffi^¬ cult to monitor an operation condition of a device located in a large depth below the sea surface level. Conventionally, multiple singular temperature, pressure, stress, vibration and similar sensors are used with singular electrical inter^¬ faces that in some way interfaces to a data collection system arranged above the sea surface level. Thereby, many problems during operation may occur. There may be a need for an arrangement for determining a physical quantity being indicative of a condition of an electrical and/or mechanical device and for a system for monitoring a physical quantity as well as for a method for determining a physical quantity associated with the device.

Summary of the Invention This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

Embodiments of the present invention are in particular suit- able for surveillance of oil-submerged, pressurized power electrical components and process equipment. In particular, embodiments of the present invention are suitable for subsea power grid condition monitoring. According to an embodiment at least one fiber Bragg grating (FBG) sensor is used for condition monitoring of a transformer, a switch gear, a power drive, an electromotor, a pump, a compressor and similar equipment .

Embodiments of the present invention may enable to measure several physical quantities within an electrical and/or mechanical device. These physical quantities may help to determine the operational condition of the device. Opera^¬ tional condition in this context may mean how well the device is working, how close it is to failing, how soon it will need service and the like.

According to an embodiment an arrangement for determining (which may in particular comprise detecting, measuring, calculating, and/or transforming) a physical quantity (such as a physical parameter, a physical property reflecting in particular a condition of the device, wherein the physical quantity may be indicative of an operational condition of the device) being indicative of a condition (thus in particular allowing to derive the condition based on the physical quan^¬ tity) of an electrical and/or mechanical device (which may be in particular purely mechanical or may be in particular purely electrical or a combination of a mechanical and elec^¬ trical device) , wherein the arrangement comprises a sealed (in particular air sealed or sealable and/or liquid sealed or sealable) container (wherein the sealed container may be an airtight sealed container or a water-tight sealed container withstanding a water pressure, the container in particular being an enclosure enclosing the device or a casing of the device belonging to the device, wherein the container may not be a separate unit) for accommodating (or housing) the device (thereby providing enough volume within the sealed container such that the device may be placed within the sealed con^¬ tainer entirely) ; an optical fiber (such as a glass fiber having a diameter between 10 ym and 2 mm or between 1 ym and 5 mm) for guiding light (having a wavelength in particular in between 500 nm and 2000 nm) , the optical fiber comprising a fiber Bragg grating sensor (the fiber Bragg grating sensor being formed in particular by periodic perturbations, such as dotation, of the optical fiber thereby creating a regular modulation of the optical properties of the optical fiber resulting in interference effects when the light interacts with the fiber Bragg grating in transmission or in reflection) for changing a characteristics of the light (for exam^¬ ple a maximum of the reflectivity or the transmissivity after interaction with the fiber Bragg grating sensor, wherein the fiber Bragg grating may be integrated into the optical fiber having a periodic variation in the refractive index in the core of the optical fiber, wherein the Bragg grating may act as a light reflector for a certain wavelength, while other wavelengths are transmitted or which may act as a device for transmitting light of different wavelengths with a different degree or amplitude) in dependence of the physical quantity (in particular, the fiber Bragg grating sensor may influence the light or the characteristics of the light in a different manner when the physical quantity changes such that depending on the physical quantity a particular wavelength is reflected or transmitted which is changed when the physical quantity changes) , wherein the fiber Bragg grating sensor is located within the container (in particular, also at least a portion of the remaining optical fiber may be located within the container, which may still allow that another portion of the optical fiber may be located external to the container) ; and an optical connector for optically connecting the optical fiber to an external optical fiber external to the container for guiding the light into the optical fiber (wherein the light may be generated external to the container by a light source, such as a laser diode or a laser) and guiding the light having interacted with or passed through the fiber Bragg grating sensor out of the optical fiber for determining the physical quantity based on the changed characteristics of the light.

Thereby, the fiber Bragg grating sensor may exhibit an influ- ence of the characteristics of light which has interacted with the fiber Bragg grating sensor such that the characteristics of light having interacted with the fiber Bragg grat^¬ ing sensor is different than the characteristics of light not having interacted with the fiber Bragg grating sensor. The kind of change or the degree of change of the characteristics of the light having interacted with the fiber Bragg grating sensor may depend on the physical quantity. For example, if the physical quantity has a first value the fiber Bragg grating sensor changes the characteristics of the light to a first characteristics and if the physical quantity has a second value the fiber Bragg grating sensor changes the characteristics of the light to a second characteristics different from the first characteristics. In particular, the physical quantity may affect the fiber Bragg grating sensor regarding a mechanical configuration. In particular, the fiber Bragg grating sensor may be compressed, expanded, stressed, stretched, pressurized or temporized in dependence of the physical quantity.

The sealed or sealable container may comprise a container wall for enclosing an interior of the container. The optical connector may in particular be arranged at a container wall of the container. In an alternative embodiment the optical connector may be located external to the container at an end of the optical fiber which is located outside the container. In particular, the optical connector may be connected to an external optical fiber using a remote operation, such as a remotely controlled robot. Thereby, the arrangement for determining the physical quantity may be employed at a loca^¬ tion not accessible by humans.

In particular, the light having interacted with the fiber Bragg grating sensor carries information regarding one or more physical quantities related to the condition of the electrical and/or mechanical device, in particular when a plurality of fiber Bragg grating sensors is comprised in the optical fiber, wherein each of the fiber Bragg grating sensors is located within the container and is associated or interacts with the electrical and/or mechanical device. In particular, the one or more fiber Bragg grating sensors may be attached to the device, such as by gluing, using bolts, and/or other connection means.

According to an embodiment the container is adapted to with^¬ stand a water pressure in a depth of between 50 m and 5000 m, in particular between 500 m and 3000 m. Thereby, the arrange^¬ ment is adapted to be used for monitoring of subsea power grid equipment. In particular it is not necessary to use electrical interfaces to connect to a measuring system as in a conventional system or method.

According to an embodiment the electrical and/or mechanical device is selected from the group consisting of a switch gear (in particular a combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment from each other, wherein the switch gear may be used to de- energize equipment to allow work to be done and to clear falls downstream) , a transformer (a device that transfers electrical energy from one circuit to another through induc^¬ tively coupled conductors, wherein a varying current in the first or primary winding creates a varying magnetic flux in the transformer' s core and thus a varying magnetic field through the secondary winding, wherein it is enabled to transform an input voltage to a higher or to a lower output voltage at the secondary winding) , a power drive, an electric motor, a pump (for pumping water and/or for pumping oil and/or for air) and a compressor (for compressing air, for compressing hydrocarbon gas or any other gas) . In particular a power drive in the context of the present application may refer to a device that converts power from an electric (AC or DC) power source, to the voltages and currents needed at the input terminals of an electric motor. The motor, and hence the input terminals might need AC voltage of variable voltage and frequency or DC voltage of variable voltage. Other names for the power drive could be variable speed drive (VSD) , motor drive, or simply drive. In particular, the sealed container provides enough space in the interior of the container to accommodate the device.

According to an embodiment a plurality of arrangements is provided each arrangement comprising a sealed container, the optical fiber and the optical connector, as explained above. Thereby, it is enabled to monitor a number of units each having a particular equipment portion or device accompanied in the respective container. Thereby, a condition of the equipment comprising a plurality of units may be monitored regarding its operational condition. According to an embodiment the sensor is attached to the device, for example by bolting or gluing. In particular, the kind of attachment may depend on the particular physical quantity to be measured. For example, when a stress or strain is to be measured, the sensor may be attached at two differ^¬ ent locations of the device, such that the sensor may detect a relative displacement of the two positions at the device.

According to an embodiment the sensor is adapted to detect (in particular change its physical constitution) at least one of a temperature, a stress, a volume, a pressure, a strain, a displacement of elements, and a vibration within the con^¬ tainer, wherein these physical parameters in particular relate to the device and are indicative of the operational condition of the device. Thus, a large number of physical properties or parameters of the device which may be important for its operation may be measured by the arrangement accord^¬ ing to an embodiment. According to an embodiment the optical fiber is arranged in a ring-type configuration. Thereby, the ring-type configuration not necessarily means a circular arrangement, but may com^¬ prise a configuration wherein the optical fiber comprises for example a number of fiber Bragg grating sensors which are arranged in a chain along the optical fiber, wherein the chain is curved such that the optical fiber may form a loop comprising portions which are adjacent to each other and comprising another portion forming one or more protrusions in the loop or ring-type configuration. Thereby, the portions being adjacent to each other of the optical fiber may com^¬ prise a portion for supplying light into the optical fiber and may comprise another portion for leading out the light having interacted with the sensor (s) out of the optical fiber .

According to an embodiment the optical fiber comprises an optical fiber entry portion for guiding the light into the optical fiber and comprises an optical fiber exit portion for guiding light out of the optical fiber which light has inter^¬ acted with the fiber Bragg grating sensor, wherein the connector is adapted to connect the optical fiber entry portion to an exit portion of the external fiber and to connect the optical fiber exit portion to an entry portion of the external fiber. Thus, the optical fiber may comprise an ingoing portion and an outgoing portion. Thereby, it may be simplified to connect a supply and processing system to the ingoing portion and the outgoing portion.

According to an embodiment the optical fiber entry portion is spaced apart from the optical fiber exit portion by a dis^¬ tance which amounts between one times and 50 times of a diameter of the optical fiber. Thus, the fiber entry portion and the fiber exit portion may be adjacent to each other such that the optical connector may be adapted or configured as small as possible. Thus, sealing the container may be simpli^¬ fied and adaptation of the optical connector may be facili- tated.

According to an embodiment the optical fiber entry portion and the optical fiber exit portion may be optically connected to a common optical fiber using a splitter or light divider. The common optical fiber may be adapted to supply light to the optical fiber via the optical fiber entry portion and to guide light out of the optical fiber via the optical fiber exit portion. According to an embodiment the optical fiber does not com^¬ prise an optical fiber entry portion and a separate optical fiber exit portion but comprises an optical fiber entry/exit portion (i.e. a single portion serving two functions, namely allowing entry of light into the optical fiber and allowing also exit of light out of the optical fiber) for guiding the light into the optical fiber and for guiding the light having passed through (or having interacted with) the fiber Bragg grating sensor out of the optical fiber, wherein the connector is adapted to optically connect the optical fiber en^¬ try/exit portion to an exit/entry portion of the external fiber. Thus, also the external fiber may be a single fiber serving two functions, namely guiding light from an external light source into the optical fiber and receiving the light out of the optical fiber to guide the light to a supply and processing system located external to the container. According to an embodiment the optical fiber comprises a reflection-type fiber Bragg grating sensor. Other fiber Bragg grating sensors comprised in the optical fiber may be trans^¬ mission-type fiber Bragg grating sensors. If at least one reflection-type fiber Bragg grating sensor is comprised in the arrangement this sensor may give rise to reflections, such that it may not be required that the fiber 113 is con^¬ figured in a closed loop. If exclusively transmission-type fiber Bragg grating sensors are comprised in the arrangement they may give rise to transmitted light (light passing through the sensor) but no (or only to a small degree) re^¬ flected light. In this case the fiber may be configured in a closed loop, to lead the light back (close the loop) .

According to an embodiment a system for monitoring a physical quantity is provided, wherein the system comprises at least one arrangement for measuring a physical quantity according to one of the embodiments explained above; an external opti^¬ cal fiber external to the container, wherein the connector optically connects the optical fiber of the arrangement for measuring a physical quantity to the external optical fiber; and a supply and processing system (in particular located outside of or external to the container) adapted to generate the light (in particular using a diode or a laser) having a wavelength, to introduce the light into the external fiber (in particular involving shaping the light using a shaping optics) , to receive the light (in particular comprising a light sensitive detector being sensitive to an intensity of received light, in particular wavelength resolved) having passed through (or interacted with) the optical fiber and having passed through or interacted with the fiber Bragg grating sensor via the external optical fiber, to convert the received light to an electrical signal (in particular using a detector, such as photodetector) , and to process the electrical signal to monitor the physical quantity (in particular using one or more calibration curves relating the physical quantity to the detected characteristics of the light having interacted with the fiber Bragg grating sensor) .

In particular, the supply and processing system may comprise one optical to electrical interface (115, 215) for each external fiber (117, 127) . One or more electrically based communication device (127, 227) may serve all the optical to electrical interfaces (115, 215) within a single enclosure (109, 209) .

In particular, the supply and processing system may be the only component within the system for monitoring a physical quantity which transforms the measurement data into electri^¬ cal signals. In particular, the supply and processing system may be located in an enclosure, in particular remote from the location of the arrangement ( s ) for determining the physical quantity. In particular, the supply and processing system may be located onshore or above sea surface level, while the arrangement ( s ) for determining the physical quantity may be located offshore. According to an embodiment the supply and processing system is adapted for generating further light having a further wavelength different from the wavelength (of the light) , wherein the optical fiber further comprises a further fiber Bragg grating sensor for sensing a characteristics of the further light in dependence of the further physical quantity. Thereby, it is enabled to measure different physical quanti^¬ ties of the device located within the container by using only a single optical fiber having several fiber Bragg grating sensors comprised within the single optical fiber. Thereby, the operation condition of the device may be monitored in more detail without additional optical fibers, connectors or signal conditioning units.

According to an embodiment a method for determining a physi^¬ cal quantity in a sealed container accompanying a device is provided, wherein the method comprises guiding light from an external optical fiber external to the container via an optical connector into an optical fiber; passing the light through a fiber Bragg grating sensor comprised in the optical fiber, the fiber Bragg grating sensor being located within the container; changing a characteristics of the light in dependence of the physical quantity; guiding the light out of the optical fiber; and determining the physical quantity based on the (changed) characteristics of the light.

According to an embodiment the method is performed at a location in the sea, in particular at a depth between 50 m and 5000 m, further in particular between 100 m and 3000 m, below a surface level of the sea. Thereby, it is enabled to monitor operational conditions of equipment or devices lo^¬ cated at a larger depth in the sea.

According to an embodiment the method for determining a physical quantity further comprises guiding the light guided out of the optical fiber to another location above the sur^¬ face level of the sea, wherein in particular at the other location the supply and processing system is located. There^¬ by, it is not required to protect the supply and processing system against potential harsh conditions at the actual measurement side in a large depth in the sea. Thereby, the supply and processing system may be easier to manufacture, easier to maintain and easier to operate. According to an embodiment the method for determining a physical quantity further comprises guiding the light guided out of the optical fiber to another location under the surface level of the sea, wherein in particular at the other location the supply and processing system is located. There^¬ by, it is not required to protect the supply and processing system against potential harsh conditions, such as extreme pressure, voltage level, vibration, temperature or chemical environment, at the actual measurement side. Thereby, the supply and processing system may be easier to manufacture, easier to maintain and easier to operate.

According to an embodiment the method for determining a physical quantity further comprises guiding the light from more than one piece of monitored equipment to a single other location above or under the surface level of the sea, wherein in particular at this other location the supply and process^¬ ing system for more than one piece of monitored equipment is located. Thereby, it is not required to provide separate units and enclosures for monitoring each piece of equipment. Thereby, the supply and processing system may be easier to manufacture, easier to maintain and easier to operate.

According to an embodiment FBG arrangement is used for a) condition monitoring of b) above mentioned equipment, if necessary restricted to c) subsea applications.

It should be understood that features (individually or in any combination) disclosed, described, mentioned or explained with respect to the arrangement for determining a physical quantity being indicative of a condition of an electrical and/or mechanical device may also applied to, used for, or employed (individually or in any combination) to the method for determining a physical quantity in a sealed container accompanying a device and vice versa. It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method type claims whereas other embodiments have been described with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless other noti^¬ fied, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the method type claims and features of the apparatus type claims is considered as to be disclosed with this document. The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi- ment but to which the invention is not limited.

Brief Description of the Drawings

Embodiments of the present invention are now described with reference to the accompanying drawings to which the invention is not restricted.

Fig. 1 schematically illustrates a system for monitoring a physical quantity according to an embodiment;

Fig. 2 schematically illustrates another embodiment of a system for monitoring a physical quantity;

Fig. 3 schematically illustrates still another embodiment of a system for monitoring a physical quantity; and Fig. 4 schematically illustrates yet another embodiment of a system for monitoring a physical quantity.

Detailed Description

The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit.

Fig. 1 schematically illustrates a system for monitoring a physical quantity according to an embodiment. The system 100 comprises an arrangement 101 for measuring a physical quan- tity being indicative of a condition of a device 103 located within a sealed container 105 and further comprises a supply and processing system 107 which is located in another container 109. The arrangement 101 can be considered as equip^¬ ment in harsh environment. The processing system 107 provides an enclosure in better protected environment. In the illus^¬ trated example the device 103 is a transformer used for transforming a medium voltage to a high voltage. Details of the transformer 103 are not illustrated in Fig. 1. In particular, arrangement 101 may also be considered to represent the monitored device or unit, which is enclosed by the enclo^¬ sure 105 to protect the unit against a harsh environment, such as pressure or salt water. Unit 115 may also considered to be a signal conditioning unit. The device may be a motor, an electric drive, a switch gear, a transformer or any single unit of that type. Sensors 111 may measure different (or same) physical quantities in dif^¬ ferent places in order to assess the operational condition of the unit.

The arrangement 101 for determining a physical quantity comprises the sealed container 105 which seals an interior of the container from an external environment. In particular the container 105 may be a casing or the device 103. In particu^¬ lar, the container 105 may be located at a sea ground, such as more than 1000 m below a surface level of the sea. The transformer 103 is used to supply a load, such as a pump, with electric energy in order to operate subsea equipment. For monitoring an operational condition of the transformer 103 the arrangement 101 for determining a physical quantity comprises a plurality of fiber Bragg grating sensors 111a, 111b, 111c, llld, llle, lllf and lllg which are all comprised in a single optical fiber 113 and which are all located within the container 105.

The fiber Bragg grating sensors 111a, 111b, 111c, llld, llle, lllf, lllg may be configured in a same manner or in different manners, such as to be adapted to measure a same physical quantity or different physical quantities. In particular, the fiber Bragg grating sensors 111 may be adapted to detect a temperature, a stress, a volume, a pressure, a strain, a displacement of elements and/or a vibration within the con^¬ tainer, wherein the corresponding physical quantity is indicative of a condition of the device 103. For measuring the one or more physical quantities the fiber Bragg grating sensors comprise a periodic variation in the refractive index leading to an interference phenomenon, when light interacts with the one or more fiber Bragg grating sensors.

In the illustrated example light is generated by the supply and processing system 107 which harbors a light source in the optical to electrical interface.

In particular, the optical to electrical interface 115 will send light into the fiber 117, which leads the light to all sensor llla-lllg. The light will be transmitted (compare Fig 1) or reflected (compare Fig2 described below) and lead back to unit 115 which can interpret the received light and con^¬ vert it to an electrical voltage or a digital value. This electric quantity is sent to unit 127 which is an electri^¬ cally based communication device usually in contact with a control system. The generated light has a particular wavelength (or a spec^¬ trum of wavelengths) which are guided through the external fiber 117 out of the container 109 enclosing the supply and processing system 107. The external fiber 117 is a bidirectional fiber allowing to supply light to the arrangement 101 and to receive the light returning from the arrangement

101, wherein the returned light carries information regarding the physical quantity indicative of the condition of the device 103.

The bi-directional external fiber 117 is optically connected to the fiber 113 by an optical connector 119. The connector 119 either couples the bi-directional fiber 117 to an entry portion of the optical fiber 113 and an exit portion of the optical fiber 113 or couples the bi-directional external optical fiber 117 to a bi-directional bridging optical fiber leading to a bifurcation point 121 or splitter device 121. The splitter device 121 or the optical fiber entry portion then couples the light in the direction as indicated with the arrow 123 into the optical fiber 113. The light guided by the optical fiber 113 interacts with the fiber Bragg grating sensor 111a such that the characteristics of the light is changed in dependence of the physical quantity to be meas^¬ ured, for example the temperature. After that, the light interacts with the remaining fiber Bragg grating sensors 111b, 111c, llld, llle, lllf, lllg, wherein upon the multiple interactions the characteristics of the light is further modulated or changed.

As part of, or close to, the splitter device 121 it may or may not be necessary to install a device preventing that both the transmitted and reflected light from each individual sensor 111 enters optical fiber 117. The light having the changed or modified characteristics is led out of the arrangement 101 along the arrow 125 and is optically coupled to the bi-directional external optical fiber 117 by the optical connector 119. In other embodiments the optical connector 119 may not be fixed at the container wall of the container 105 but may be attached to the optical fiber 113 at a portion located outside of the container 105. The light having the changed characteristics is received by the optical to electrical interface 115 of the supply and processing system 107, wherein based on the light having the changed characteristics (in particular a spectrum of the light having the changed characteristics) electrical signals are generated representing for example raw measuring data relating to the one or more physical quantities measured within the arrangement 101. The electrical signals are sup^¬ plied to an electrically based communication device 127 which may also be adapted to process the raw measurement data to derive or determine the one or more physical quantities being indicative of a condition of the device 103.

According to an embodiment a plurality of arrangements 101 harboring each a particular device 103 are optically con- nected to the external optical fiber 117 or to a plurality of external optical fibers which is or are in turn connected to the single system 107, to enable monitoring of one or more physical quantities of one or more devices comprised in an equipment to be monitored.

Fig. 2 schematically illustrates another embodiment of a system 200 for monitoring a physical quantity, wherein the system 200 comprises an arrangement 201 for determining a physical quantity being indicative of a condition of a device 203 harbored within a container 205 and further comprises a supply and processing system 207 in a container 209. The arrangement 201 can be considered as equipment in harsh environment. The processing system 207 provides an enclosure in better protected environment.

While the fiber Bragg grating sensors llla-lllg illustrated in Fig. 1 are at least partially transmission-type fiber

Bragg grating sensors (a characteristics of transmitted light is changed upon interaction with the transmission-type sen^¬ sors) , the fiber Bragg grating sensors 211a-211g are reflec^¬ tion-type fiber Bragg grating sensors.

As is indicated by the double arrow 229 the light introduced into the optical fiber 213 propagates along the optical fiber 213 and partially reflected by each of the fiber Bragg grat^¬ ing sensors 211a-211g. In the embodiment illustrated in Fig. 2 also an alternative arrangement is shown, wherein the optical connector 219 is arranged external to the container 205 and is not fixed at the container 205. The supply and processing system 207 illustrated in Fig. 2 is similar to the supply and processing system 107 illustrated in Fig. 1.

The fiber Bragg grating sensor (FBG sensors) may be passive sensors that modulate light. In particular, FBG sensors may function by modulating light of a certain color (frequency, signal channel, wavelength) which is guided through it, but without affecting other colors also passing through (or being reflected) . Thereby, the FBG sensors may be very simple and may be not disturbed for example by electromagnetic fields. Thereby, it is enabled, to string several sensors, such as sensors llla-lllg, (each modulating light of a distinct color or wavelength) along a single optical fiber 113 or 213, requiring only a single interface module or optical connector 119, 219 or only a single optical to electrical interface 115 or 215 to be placed in a protected closure. Further, the physical interface 115, 215 to the arrangements 101 or 201 is only required to consist of a single optical fiber. In the examples shown in Figures 1 and 2, seven different physical quantities are measured, alternatively a single quantity (such as temperature) can be measured in more than one of the seven places. Other combinations apply too.

Fig. 3 schematically illustrates an embodiment of a system 300 for monitoring a physical quantity. The system 300 com^¬ prises five arrangements 301 for measuring a physical quan^¬ tity, such as for example arrangements 101 or 201 described above. Not illustrated connectors optically connect each optical fiber 313 of the arrangements 301 to a corresponding external optical fiber 317. The arrangements 301 may be monitored equipment such as transformer, switchgear, motor drive, motor, or a pump, to give examples.

Each external optical fiber 317 is connected via an optical- to-electrical interface 315 to a corresponding supply and processing system 307 (each having an electrical based communication device 327) adapted to generate the light having a wavelength, to introduce the light into each external fiber 317, to receive the light having interacted with the fiber Bragg grating sensor via each external optical fiber 317, to convert the received light to an electrical signal, and to process the electrical signal to monitor the physical quan- tity.

The data obtained from all five supply and processing systems 307 are fed to a central subsea node 303, which transmits the data to a topside unit 304 located above the water surface 306. Subsea node 303 comprises an electrically based communi^¬ cation device 308 interfacing to undersea units 307 and an electrically based communication device 310 interfacing to topside unit 304 via fiber optic and/or electrical base communication line 312. Topside unit 304 comprises an elec- trically based communication device 314 and a computer and data storage 316. Fig. 4 schematically illustrates yet another embodiment of a system 400 for monitoring a physical quantity. The arrange^¬ ments 401 may be monitored equipment such as transformer, switchgear, motor drive, motor, or a pump, to give examples. Arrangement 401 is similar to the embodiment illustrated in Fig. 3. However, there is only a single supply and processing systems 407 having for each external optical fiber 417 an optical-to-electrical interface 415 and having only a single electrical based communication device 427 serving all ar- rangements 401.

It should be noted that the term "comprising" does not ex^¬ clude other elements or steps and "a" or "an" does not ex^¬ clude a plurality. Also elements described in association with different embodiments may be combined. It should also noted that reference signs in the claims should not be con strued as limiting the scope of the claims.

Claims

CLAIMS :

1. Arrangement for determining a physical quantity being indicative of a condition of an electrical or/and mechanical device,

the arrangement comprising:

• a sealed container (105, 205) for accommodating the device (103) ;

• an optical fiber (113, 213) for guiding light, the

optical fiber comprising a fiber Bragg grating sensor

(llla-lllg, 211a-211g) for changing a characteristics of the light in dependence of the physical quantity, the fiber Bragg grating sensor being located within the container; and

· an optical connector (119, 219) for optically connecting the optical fiber (113, 213) to an external optical fiber (117, 217) external to the container for guiding the light into the optical fiber and guiding the light having passed through the fiber Bragg grating sensor out of the optical fiber for determining the physical quantity based on the changed characteristics of the light.

2. Arrangement according to claim 1, wherein the container is adapted to withstand a water pressure in a depth of between 50 m and 5000 m, in particular between 500 m and 3000 m.

3. Arrangement according to claim 1 or 2, wherein the device is selected from the group consisting of a switch gear, a transformer, a power drive, an electric motor, a pump and a compressor.

4. Arrangement according to one of claims 1 to 3, wherein the sensor is attached to the device.

5. Arrangement according to one of the preceding claims, wherein the sensor is adapted to detect at least one of a temperature, a stress, a volume, a pressure, a strain, a displacement of elements, and a vibration within the con^¬ tainer .

6. Arrangement according to one of the preceding claims, wherein the optical fiber (113) is arranged in a ring-type configuration, in particular a closed-loop configuration or wherein the optical fiber (113) is arranged in a linear-type configuration, in particular an unclosed-loop configuration.

7. Arrangement according to one of the preceding claims, wherein the optical fiber comprises an optical fiber entry portion for guiding the light into the optical fiber and comprises an optical fiber exit portion for guiding light out of the optical fiber, wherein the connector is adapted to connect the optical fiber entry portion to an exit portion of the external fiber (117, 217), and to connect the optical fiber exit portion to an entry portion of the external fiber (117, 217) .

8. Arrangement according to one of claims 1 to 6, wherein the optical fiber (213) comprises an optical fiber entry/exit portion for guiding the light into the optical fiber and for guiding light having passed through the fiber Bragg grating sensor out of the optical fiber, wherein the connector is adapted to optically connect the optical fiber entry/exit portion to an exit/entry portion of the external fiber (117, 217) .

9. System for monitoring a physical quantity, the system comprising:

• at least one arrangement (101, 201) for measuring a physical quantity according to one of the preceding claims;

• at least one external optical fiber (117, 217) external to the container, wherein the connector optically connects each optical fiber (113, 213) of the at least one arrange^¬ ment for measuring a physical quantity to the at least one external optical fiber (117, 217); and • a supply and processing system (107, 207) adapted to generate the light having a wavelength, to introduce the light into the at least one external fiber, to receive the light having interacted with the fiber Bragg grating sensor via the at least one external optical fiber, to convert the received light to an electrical signal, and to process the electrical signal to monitor the physical quantity.

10. System according to claim 9, wherein the supply and processing system (107, 207) comprises for each of the at least one external fiber (117, 217) an optical-to-electrical interface for converting an optical signal to an electrical signal and converting an electrical signal to an optical signal .

11. System according to claim 9 or 10, wherein the supply and processing system is adapted for generating further light having a further wavelength different from the wavelength, wherein the optical fiber further comprises a further fiber Bragg grating sensor for changing a characteristics of the further light in dependence of the further physical quantity.

12. Method for determining a physical quantity in a sealed container accompanying a device, the method comprising:

· guiding light from an external optical fiber (113, 213) external to the container (105, 205) via an optical connec^¬ tor (119, 219) into an optical fiber;

• passing the light through a fiber Bragg grating sensor (llla-lllg, 211a-211g) comprised in the optical fiber, the fiber Bragg grating sensor being located within the container;

• changing a characteristics of the light in dependence of the physical quantity;

• guiding the light out of the optical fiber; and

· determining the physical quantity based on the changed characteristics of the light.

13. Method according to claim 12, wherein the method is performed at a location in the sea, in particular at a depth between 50 m and 5000 m, further in particular between 100 m and 3000 m, below a surface level of the sea.

14. Method according to claim 13, further comprising guiding the light guided out of the optical fiber to another location above the surface level of the sea.

15. Method according to claim 13, further comprising guiding the light guided out of the optical fiber to another location under the surface level of the sea where the optical signal is further conditioned.