WO2010086588A2

WO2010086588A2 - Sensing inside and outside tubing

Info

Publication number: WO2010086588A2
Application number: PCT/GB2010/000105
Authority: WO
Inventors: Mladen Todorov; Neal Carter
Original assignee: Sensornet Limited
Priority date: 2009-01-27
Filing date: 2010-01-22
Publication date: 2010-08-05
Also published as: GB2479087B; GB2467177A8; GB0901288D0; GB2479087A; WO2010086588A3; GB201110941D0; GB2467177A

Abstract

A sensing system for sensing in a bore having tubing (30) inserted, has a sensing fiber (55) extending along the bore at least predominantly in an annular space (250) outside the tubing to sense a condition, such as pressure conditions, in the annular space in a distributed manner, parts of the fiber being arranged to sense a condition, such as pressure conditions inside the tubing. The system also includes a transmitter and a receiver (42) coupled to receive light from the sensing fiber for sensing pressure conditions inside the tubing and in the annular space.

Description

SENSING INSIDE AND OUTSIDE TUBING

Field of the Invention

The present invention relates to sensing systems, to methods of installing such systems and to methods of using such systems.

Background

There is a requirement in industry for the measurement of conditions such as strain or temperature and other conditions at all points over long distances. Typical uses are for monitoring oil and gas wells, long cables and pipelines. The measurements can be displayed or analysed and used to infer the condition of the structures. Distributed temperature sensors (DTS) often use Raman or Brillouin components of scattered light in optical fibers as the means to determine the temperature. Here, light from an optical source is launched into a fiber and the small amount of light that is scattered back towards the source is analysed. By using pulsed light and measuring the returning signal as a function of time, the backscattered light can be correlated to distance along the fiber. This backscattered light contains a component which is elastically scattered (Rayleigh light) and components that are up- and down-shifted in frequency from the source light (Raman and Brillouin anti-Stokes and Stokes light respectively, also known as inelastic scattered light). The powers of the returning Raman components are temperature dependent and so analysis of these components yields the temperature. The powers and frequency of the returning Brillouin components are strain and temperature dependent and so analysis of both components can yield temperature and strain independently. Such systems have been known for many years. Raman back scattering analysis is discussed, for example, in U.K. Patent Application 2,140,554, published November, 1984, which is hereby incorporated by reference in its entirety.

It is known to monitor temperature and pressure using fiber optic Distributed Temperature Sensors (DTS) which have the ability to take measurements every 1m with a resolution of less than 1°C. The cable carrying the sensing fiber can be attached to the outside of a production tube as it is inserted into the bore. The cable can be affixed to the production tube by adhesives, by strapping, by wrapping, or by physical integration with the tube. The cable can be installed by clamping the cable to the pipe as it is placed in the well, e. g. using well-known collar protectors used in the oil and gas industry. Calibration point sensors may each be installed within splice sleeves. This is accomplished by cutting the cable, fusion splicing optical calibration sensor (e.g., fiber Bragg gratings FBGs) to either end of the calibration fiber protruding from the ends of cut cable using well-known fiber optic cable splicing techniques, splicing any other fibers or wires within the cable, e. g. a sensing fiber, and sealing the splice(s) and sensor within the splice sleeve. The splice sleeve may comprise a 3/8 inch metal tube welded onto a 1/4 inch metal sheath of the cable. Where the cable does not have a metal sheath, the sleeve may comprise a snap sleeve or heat shrink sleeve capable of providing a hermetic seal.

It is known from US Patent 5845033 to provide a sensor apparatus for flowline of a hydrocarbon production system, which utilizes an optical fiber or optical fiber cable which is engaged along and preferably spirally wound around and along the flowline. Spaced apart strain gauges which are advantageously in the form of Bragg grating sensors, are engaged with the optical fiber at spaced locations along the flowline. The strain gauges are also engaged with the flowline to measure strain in the flowline. The hoop strain in particular indicates a change in pressure which can be measured by light signals supplied to and received from the optical fiber. Differences in strain along the flowline indicate pressure gradients in the flowline which in turn identify restrictions in the flowline.

It is known from an article "World's first multiple fiber-optic intelligent well" in World Oil, March, 2003 by Sigurd Erlandsen, Gisle Void, Graham D. Makin, to use separate fibers dedicated to each of the two single-point pressure gauges, one for monitoring tubing pressure, the other monitoring annular pressure. Two of the three fibers in the downhole cable are used to communicate with these single-point sensors. The third fiber is a multimode fiber used for DTS measurements.

Summary of the Invention

A first aspect of the present invention relates to a sensing system for sensing in a bore, the bore having tubing inserted along the bore, the system having a sensing fiber extending along the bore at least predominantly in an annular space outside the tubing to sense a condition in the annular space in a distributed manner, one or more parts of the fiber being arranged to sense a condition inside the tubing at one or more selected locations in the bore, the system also having a transmitter for transmitting light along the fiber, and a receiver coupled to receive light from at least one of the part or parts of the sensing fiber for sensing a condition in the tubing and the other parts of the fiber for sensing a condition in the annular space. The system may be configured for sensing pressure conditions in a bore. The system may be configured for sensing temperature conditions within a bore.

The system may be configured for sensing multiple conditions within a bore.

A second aspect of the present invention relates to a sensing system for sensing pressure conditions in a bore, the bore having tubing inserted along the bore, the system having a sensing fiber extending along the bore at least predominantly in an annular space outside the tubing to sense pressure conditions in the annular space in a distributed manner, one or more parts of the fiber being arranged to sense pressure conditions inside the tubing at one or more selected locations in the bore, the system also having a transmitter for transmitting light along the fiber, and a receiver coupled to receive light from the part or parts of the sensing fiber for sensing pressure conditions in the tubing and to receive light from the other parts of the fiber for sensing pressure conditions in the annular space.

Being able to sense the pressure conditions inside the tubing with the same sensing fiber used for distributed sensing outside the tubing can enable a reduction in the amount of equipment in the borehole. This can reduce the cost of the hardware, and reduce the time needed for installation, which in turn can lead to reduced costs. Furthermore the risk of faults and therefore of time needed for repairs may also be reduced if there is less equipment down the borehole. Another aspect provides a corresponding method of installing a sensing system according to the first or second aspects.

Another aspect provides a corresponding method of sensing using a sensing system according to the first or second aspects.

Another aspect provides a part of a system according to the first or second aspects, such as an adapted section of tubing arranged to expose the sensing fiber to the a condition, such as pressure conditions inside the tubing.

Another aspect of the present invention relates to a sensing system for sensing in a bore having tubing received therein, the system comprising a sensing fiber, wherein part of the sensing fiber is exposed to a condition externally of the tubing and arranged for distributed sensing such condition, and part of the sensing fiber is exposed to a condition internally of the tubing to sense said condition.

Aspects of the present invention may have application in well bores, such as used in the oil and gas industry. In such arrangements aspects may be used in combination with, or as including, oilfield tubulars, such as productions tubing, drilling tubing, casing, liner or the like. Any additional features can be added to any of the aspects. Other advantages will be apparent to those skilled in the art, especially in relation to other prior art not known to the inventors. Any of the additional features can be combined together and combined with any of the aspects, as would be apparent to those skilled in the art.

Brief description of the drawings

These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig 1 shows a schematic view of a sensing system according to an embodiment of the invention in a bore and showing a rig floor at a well head,

Figs 2 to 7 show cross section views of embodiments of adapted sections of tubing in a bore,

Fig 8 shows a cross section view of a port for passing the sensing fiber into or out of the adapted section of tubing and showing an example of how to provide a seal against the fiber,

Fig 9 shows a cross section view of a sealable port of an adapted section of tubing according to an embodiment,

Figure 10 shows a cross section view of a cable involving an outer conduit and an inner braided cable,

Figure 11 shows a schematic view of parts of a sensing system coupled to the sensing fiber for transmitting and receiving, for distinguishing between light from different parts of the fiber, and for processing the received signals, and

Figures 12 and 13 show steps in methods according to embodiments.

Detailed Description Definitions:

References to distributed sensing are intended to encompass sensing along an essentially continuous line rather than sensing at one or many individual points. References to pressure conditions are intended to encompass direct or indirect indications of pressure, and relative or absolute measures of pressure.

References to a chamber can encompass chambers of any size or shape, chambers formed of a main body, a lid and a part for attaching, or other parts, or being a one-piece item, with the part for attaching being integral with the main body. The chamber can be one-time sealable, or resealable many times. They can be of metal or other materials. References to an assembly can encompass parts preassembled, or parts assembled on site.

References to a part for attaching the chamber can encompass any type of fixing, including straps for strapping, clamps of any sort, clamps for clamping the chamber to tubing using well-known collar protectors, as the tubing is placed in a well. Attaching can be by bolts, by wrapping, by welding, by adhesives, or by physical integration with the tubing, or any other way. If the attaching part is a bolt and a corresponding hole in the tubing for example, then the main body part can follow the shape of the tube to make a good fit. References to ports can encompass sealable holes, resealable holes, holes of any shape, sealable in any way, or recesses in a joint between parts of a chamber for example.

References metals is intended to encompass metals such as steel and non corrosive metals such as alloys, stainless steels such as SS 316 and others, lncoloy 825 and others.

Introduction to embodiments

Various embodiments are described below, to exemplify one or more of the claimed features. The sensing system for sensing pressure conditions in a bore, can encompass any type of sensing system. The system can be regarded as including signal processing once the optical signals have been received, or can exclude such signal processing, as such processing can in principle be carried out remotely. The tubing inserted along the bore can be any kind or size of tubing for any purpose, not limited to production tubing for oil or gas production. The sensing fiber extending along the bore at least predominantly in an annular space outside the tubing, can be any size or length or type of fiber suitable for sensing pressure conditions in the annular space directly or indirectly in a distributed manner. It can be permanently or temporarily installed for example. It can be one piece of fiber or made up of many sections spliced or connected together. The sections need not be of the same type of fiber. The one or more parts of the fiber arranged to sense pressure conditions inside the tubing at one or more selected locations in the bore can be parts of the one piece of fiber, or can be spliced into the rest of the fiber. The transmitter for transmitting light along the fiber can be of any type suitable for distributed sensing. The receiver coupled to receive light for sensing pressure conditions in the tubing and in the annular space can be any kind of receiver and can be a single receiver or made up of several individual receivers for different wavelengths or polarisations or timings for example.

Additional features Embodiments of the invention can have any features added to those mentioned above. Some notable additional features will be described and some are the subject of dependent claims. Many others can be envisaged.

The system can have a means to distinguish light received from the different parts of the fiber, to determine the conditions inside and outside the tubing. This can encompass for example a timing gate following established practice in DTS systems as referenced above, or can involve wavelength demultiplexing as described below with reference to figure 10. Alternatively or aswell, the signals can be time division multiplexed, so that signals for a point sensor are not in the fiber at the same time as signals for distributed sensing. The system can be arranged to sense the pressure conditions in the tubing in a distributed manner. This can be instead of or as well as using a point sensor. If no point sensor is used, then there is less equipment at the well head, as no interrogator is needed. If one or more point sensors are provided on the sensing fiber arranged to sense pressure conditions inside the tubing, there is less need for a long length of sensing fiber to be used for the sensing inside the tubing, and the arrangement can be more compact.

The system can have an adapted section of tubing arranged to expose the fiber to the conditions in the tubing. The adapted portion of the tubing can have a chamber offset from a main channel of the tubing and connected so as to have the same pressure conditions as the main channel, the chamber having at least one port to allow the fiber to pass from the annular space into the chamber, the port being sealable to isolate the pressure conditions in the chamber from those in the annular space. Various arrangements of this adapted section of tubing can be envisaged, and some are described in more detail below. The chamber can be arranged to retain a number of coils of the sensing fiber.

The chamber can have at least two of the ports, to allow the fiber to pass through. This can enable the conditions in the annular space to be sensed both before and after the chamber. The system can be arranged with the sensing fiber passing through the adapted section intact without a splice or connector. This can reduce installation time, reduce optical losses and reduce risk of failures. The adapted section can be arranged in sealable parts so that an intermediate part of the fiber can be laid in the adapted portion and sealed without a need to cut the fiber or without a need to thread an end of the fiber through an aperture. The fiber can be protected within a metal conduit. The part of the fiber for sensing pressure conditions inside the tubing can be attached to the tubing so as to sense a hoop strain of the tubing, to represent pressure conditions inside the tubing. This can ease the requirements for sealing since the fiber need no longer pass into a chamber having the same pressure conditions as are present inside the tubing.

Fig 1. fiber splicing equipment at a rig floor at a well head

Figure 1 shows a schematic view of a system according to an embodiment. It shows a sensing system for sensing in a bore, the bore having tubing 30 inserted along the bore, the system having a sensing fiber extending along the bore at least predominantly in an annular space outside the tubing to sense conditions in the annular space. Also shown is equipment at a rig floor 60 at a well head. Other applications are of course conceivable. The bore and casing 70 are shown, with the rig floor 60 above, which may be part of an offshore or onshore rig for example. Other parts such as a derrick and insertion mechanisms are not shown for the sake of clarity. Infrastructure along which the cable runs is shown in the form of tubing 30, though other such infrastructure can be envisaged. The tubing may be for example production tubing, and can be inserted or extracted from the borehole. The sensing fiber or fibers can be contained in a cable and can be attached or detached from the production tubing as it is being lowered or raised respectively. It can be taken from or be spooled onto a fiber coil 50. Fiber splicing equipment can be provided nearby on the rig floor. When the fiber is installed and used for sensing, the fiber is coupled to transmitter and receiver equipment 42 provided for launching and receiving optical signals into or from the fiber. There is received light from the part or parts of the sensing fiber for sensing conditions in the tubing and received light from the other parts of the fiber for sensing conditions in the annular space. This receiver can be locatedon or near the rig floor. This equipment can involve devices such as DTS equipment following established practice which need not be described in more detail here. The received light can be processed by any means to distinguish light received from the different parts of the fiber, to determine the conditions inside and outside the tubing. Examples can include means for processing in the optical domain or in the electrical domain for example, to distinguish which light has come from which part of the sensing fiber. In the optical domain, different wavelengths can be used and distinguished by a wavelength demultiplexer. Or a timing gate can be used to distinguish pulsed light based on its time of travel along the fiber. The sensing can be distributed or use discrete or point sensors or a combination. Point sensors can be for example Fiber Bragg Gratings (FBG), which reflect a given wavelength.

The fiber splicing equipment on the rig floor can be used for example to couple a new section of cable, or to insert sensors or anything else into the optical path in mid cable, or to terminate the cable. A U-bend may be inserted at the lower end of the cable for example. Figure 1 also shows an adapted section of the tubing where the fiber is passed into the tubing to be exposed to the conditions inside the tubing. The adapted section can optionally have a bulge or side chamber at this selected location, to accommodate the fiber without impeding a main channel of the tubing. Many different variations can be envisaged, and a number will be described in more detail below. The section exposed to the conditions can be of any suitable length. If a meter or less, it may be easier to construct and install, but if distributed sensing is used, then the sensing may be less accurate, unless other measures are taken. For example, coils of fiber may be retained in this adapted section so that the effective length for sensing is much longer. Or a point sensor may be added for example. These are examples of a part of the fiber being arranged to sense conditions inside the tubing at the one or more selected locations in the bore.

A seal 80 may be provided where the fiber passes into or out of the tubing. There may be many such adapted sections along the tubing. It may be particularly useful to sense the conditions before and after junctions of a mother path with laterals, or before and after packers used for isolating sections of the annular space.

Figure 2, embodiment having chamber

Figure 2 shows a cross section view of the part of the bore where the sensing fiber is arranged to sense the conditions in the tubing. The fiber is contained in a cable 220. The tubing is in sections connected by joints 210. and the cable runs alongside the tubing. Between the tubing joints is an adapted section of tubing arranged to expose the fiber to the conditions in the tubing. The adapted section of the tubing has a chamber 85 offset from a main channel of the tubing and connected by aperture 230 so as to have the same pressure conditions as the main channel. The chamber has ports to allow the cable carrying the fiber to pass from the annular space 250 into the chamber, the ports being sealable by seals 80 to isolate the pressure conditions in the chamber from those in the annular space. This enables the part of the fiber in the chamber to be exposed to conditions in the tubing. The length of the chamber can be anything from a few tens of centimetres up to a meter, or longer if it is desired to have a longer length of fiber for distributed sensing inside the tubing. In this case a length or 1m to 5m or even up to 10m might be considered. For such longer lengths, it might be awkward or time consuming to provide a seal along the entire length. So embodiments having apertures rather than full length seals might be more practical.

In some cases, the chamber can be formed in parts so that the chamber can be opened to enable the cable to be laid in the chamber when installing the tubing with this adapted section. The parts would need to be closed and sealed on the rig floor during the installation. The parts could be halves bolted together around the tubing for example, or a lid bolted onto a suitable opening in the chamber, so as to close the ports around the cable. Such arrangements would be more suitable for shorter chambers such as those of less than 1m length or less than 0.5m length for example.

Or the chamber could be formed in one piece and have apertures through which the cable would be threaded, then these apertures could be sealed before the adapted section is inserted in the bore. This threading could involve cutting the cable and rejoining it by splices or connectors, or the entire length of the cable up to the desired location could be threaded through the apertures to avoid needing to cut the cable.

Figure 3. chamber with coiled fiber. Figure 3 shows another example, similar to that of figure 2 and using similar reference numerals as appropriate. In this case the cable in the chamber has been stripped back to the fiber to enable a number of coils of fiber to be contained in the chamber. By having coils of the fiber exposed to the conditions in the tubing, a greater length can be exposed. This can be useful to enable more accurate sensing using distributed sensing techniques for example. It means that for example any time gating used to determine which received light is from this part of the fiber, can have larger tolerances, and be less susceptible to noise.

A splice 88 is shown, on the basis that it may be easier to strip back the cable if a cut is made. But alternatively in some cases the cable can be stripped back without making a cut, depending on the nature of the cable. This would avoid the optical loss inherent in a splice. References to a splice can also encompass alternatives such as fiber connectors.

In this example, the seal 80 may need to be made against the metal conduit of the cable and also against the stripped fiber, so that the pressure conditions of the chamber do not extend into the cable inside the metal conduit. The stripped fiber may need some additional coating or other protection against the fluids in the chamber, depending on the application and the conditions. A splice is shown either side of the point sensor.

Figure 4, point sensor in chamber

Figure 4 shows another example, similar to that of figure 3 and using similar reference numerals as appropriate. In this case the cable in the chamber has been stripped back to the fiber to enable a point sensor 83 to be contained in the chamber. This could be an FBG for example, or other optical device. In principle, multiple point sensors can be provided to make measurements of different characteristics, or to improve accuracy by enabling averaging, or to improve reliability. A splice is provided either side of the point sensor. In some cases, the point sensor can be provided by writing a grating onto the fiber without needing to splice the point sensor into the line. If desired, coils of fiber and point sensors can be combined in the same chamber.

Figure 5 chamber as a cylindrical sleeve

Figure 5 shows another example, similar to that of figure 4 and using similar reference numerals as appropriate. In this case the chamber is shown as a cylindrical sleeve 105. This may be easier to manufacture to withstand high pressures than other shapes. It is connected to the main channel of the tubing by hollow mountings 260. These can be threaded or welded and reinforced as necessary. Although shown with a cable running through, clearly the cable could be stripped to the fiber and optical components could be spliced in such as point sensors as described above. The sleeve can be a single piece or can be in parts such as halves suitable to be bolted together once the cable is laid in one half for example.

Figure 6. chamber with removable lid

Figure 6 shows another example, similar to that of figure 4 and using similar reference numerals as appropriate. In this case the chamber has a removable lid 330, fastened for example by bolts or other fixings. The seals around the cable are split seals 180 so part of the seal is on the lid, and so the seals are closed by closing the Nd. This means the cable can be laid in the chamber when the adapted section is on the rig floor, the lid can be closed and sealed and the adapted section with the cable can be inserted into the bore without the need for a splicing operation. This can save time on the rig floor and thus save costs, and reduce optical losses, and reduce risk of failure.

Figure 7 hoop strain sensor

Figure 7 shows another example, similar to that of figure 4 and using similar reference numerals as appropriate. In this case the part of the fiber arranged to sense the conditions in the tubing is exemplified by using part of the fiber as a hoop strain sensor. This part of the fiber is looped around the tubing, or laid around at least part of the circumference, so that pressure differences between the inside and outside of the tubing which cause circumferential strain, or hoop strain in the tubing, cause strain in the fiber which can be sensed. There may be many loops so as to increase the signal to noise ratio. The looped fiber can be attached or embedded to the tubing in various ways as would be apparent to those skilled in the art. This could in some cases be carried out during installation, or carried out beforehand. Splices are shown for connecting the looped fiber to the fiber in the cable.

Figs 8 and 9, port and its seals

Fig 8 shows a cross section view of a port for passing the sensing fiber into or out of the adapted section of tubing and showing an example of how to provide a seal against the fiber. Figure 9 shows an example of a compression seal for sealing between the cable and the aperture in the wall of the adapted section, or between the sealant housing of figure 8 and the aperture in the wall of the adapted section.

In figure 8, the port comprises an aperture in a wall of chamber 85 of the adapted section of tubing. In the aperture is shown a sealant housing 320, for containing sealant 310 such as resin which can be injected through sealant injection port 300 and left to harden. Also shown is a cable fitting 330 for attaching and sealing the sealant housing to the cable protecting the sensing fiber. The sealant can be a resin having suitable thermal characteristics to provide a resilient bond to both fiber and steel.

An example of a sequence of operations for sealing is as follows:

1. Slide cable fitting and sealant housing on to end of the sensing cable, 2. Carefully cut back sensing cable leaving required length of sensing fiber, 3. Run the fibers through the sealant housing,

4. Introduce fibre to the metal sealing substance through the sealant injection port,

5. Check for leakages of the sealant on the fiber exits, 6. Seal the injection port 300,

7. Using sealant fitting, seal cable fitting to the sealant housing,

8. Test the fibre for optical continuity and eventual insertion loss from the seal.

9. Attach the assembly to the aperture in the wall of the chamber, for example as shown in figure 9. Either the sealant housing or the cable fitting could be attached and sealed to the aperture.

Fig 9 shows a cross section view of a port for use in sealing cable carrying the fiber, for passing the cable into the chamber of the adapted section. Many other sealing arrangements can be envisaged, either resealable or permanent, such as welding metal to metal, or using adhesive for example. The cable 220 carrying the fiber 140 enters an aperture in the adapted section as shown, to enable the fiber to reach the space inside the adapted section, shown at the bottom of this view. The seal has a threaded main part 820, typically cylindrical and sealed to the cable by a ring seal 810. During installation this main part and its ring seals are slid along the cable into position. The thread 850 of this main part engages with thread 860 of the aperture and is tightened to force the main part downwards against tapered part 870. This forces the tapered part downwards into the corresponding tapered part of the aperture to form a seal between the aperture of the wall and the outer face of the cable such as a metal conduit for protecting the fiber. A further ring seal 830 is provided to seal the gap above the thread, as a precaution. This seal can be compressed by the head of the main part, or be arranged to fit in a recess in parts 1 and or 820, and be compressed by external pressure to create a seal for example.

A passage 840 can be provided for testing the seal. This passage can extend into both gaps above the tapered part as shown. Testing the seals can involve injecting fluid into this passage and inspecting for leakage at the exterior of the chamber of the adapted section at rings 830 and 810, and optionally checking for leakage into the chamber past the tapered part. In some cases, the sensing will be affected by leakage and so can be used to detect failed seals, either during testing or later in operation. As shown, there is no separate seal against the fiber. This can optionally be added using the arrangement shown in figure 8 for example if needed, to reduce risk of leakage along the cable between the fiber and metal conduit.

Cable

Cables incorporating the sensing fiber for use in harsh environments often found in boreholes will be described briefly. The useful life of the fiber in such environments depends on countering three major causes of deterioration: glass oxidation or other glass deterioration at high temperature, hydrogen ingress and physical damage during installation. The protective fibre coating can have a simultaneous effect on some or all of these in that the coating prevents exposure of the surface of the glass of the fiber to oxidation or other deterioration in such high temperatures. Various protective coatings can substantially prevent hydrogen ingress and also protect the glass from physical damage. Increasing the useful life can help reduce costs of replacing fibers in locations such as bore holes. Different protective coatings are used depending on the temperature and environmental conditions of a particular installation.

Cables to protect the sensing fibers can take a number of configurations. For example a fiber or fibers can be surrounded by a metal conduit. This may be a stainless steel sheet wrapped around the fiber and welded. Another example is an Al tube, which can provide good hydrogen ingress resistance at high temperatures. It may have insulating and or hydrogen protective coatings on inside and/or outside surfaces of the conduit, and may be filled with a hydrogen scavenging gel. The metal may be surrounded by a second metal conduit, with a filler material in between. This can help avoid transfer of stresses to the fiber which could interfere with measurements. A further outer encapsulation layer of for example HDPE can be provided to give abrasion protection. Cables can have other components such as electrical supply lines (FOC+EL),

It is often good practice to install two fibres within a cable for both redundancy, and to enable different calibration techniques. It will often be necessary to connect the two fibres together at the far end of the cable. It is common to do this using a fusion splice. Splices are often regarded as a source of failures. A high-quality fusion splice is often measured by two parameters: i. Splice loss and ii. Tensile strength For graded-index multimode fibers, the fiber related factors include core diameter mismatch, numerical aperture (NA) mismatch, index profile mismatch, core/cladding concentricity error and cladding diameter mismatch. Splice process- related, factors are those induced by the splicing methods and procedures. Splice process factors include lateral and angular misalignment, contamination and core deformation. Fiber preparation includes fiber stripping, surface cleaning and fiber-end angle.

The cable is intended to protect the optical fiber or fibers, and enable a good seal with the housing. An example is a multi-layer cable, to provide the protection required for downhole installations in oil wells. Other examples can be envisaged. A basic cable might have a single stainless steel tube surrounding carbon or polyimide coated fibers in a filler. For more protection a double wall or tube in tube type can be used. An example specified for operation within the -40° C to 150° C temperature range, will be described in more detail. Two fibres are included in the cable allowing for double ended measurements. An inner tube of 304 SS or other metal, has a diameter of approximately 3.2mm, and wall thickness of 0.2mm. This can be filled with a hydrogen scavenging gel for example. This is surrounded by a belting of polypropylene of thickness 0.2mm to provide separation from an outer tube of 316 SS or other metal. This outer tube has a diameter of 6.3mm and wall thickness of 0.7mm approx. An outer encapsulation can be of Santoprene or other materials such as HDPE of outer diameter 11mm and thickness of 3.9mm approx.

To create a good seal the outer encapsulation can be stripped back to enable a metal on metal compression seal for example.

Figure 10, cable for two stage installation

Figure 10 shows a cross section view of a cable for use in a particular installation method, which can involve using an adapted section of tubing according to some of the embodiments described above. The cable has an outer conduit 350, which retains a filler 360 which surrounds a braided cable 370 having many strands of steel wire. The filler can be any material, fluid or otherwise, which is suitable to transfer the pressure on the outer conduit onto the sensing fiber which is located inside this braided cable is a fiber or fibers embedded in polymer 390 or other buffer material, and having a metal coating 380.

By providing an outer conduit typically of metal or similar material, perhaps of standard quarter inch diameter or similar, this can be installed first and the braided cable can then be fed into the outer conduit by a conventional method such as fiber blowing or fibre pushing This may involve providing a return path by providing a U- bend at the bottom of the outer conduit and a return outer conduit back to the surface. This provides a complete flow path for the fluid used for pumping the sensing fiber. Optionally the sensing fiber can be passed all the way back along the return path to provide the possibility of double ended distributed sensing following established practice which need not be described here.

Optionally the sensing fiber can be installed without the braided cable protection if desired. Optionally optical components can be spliced into the fiber or fibers such as different types of fiber or point sensors such as FBGs or other known sensors.

Once the braided cable is deployed to the required depth or length, the filler can be pumped in and additional pressure can be applied to the filler as desired if the filler is a fluid. The filler can serve to transmit pressure on the outside of the outer conduit to the sensing fiber or fibers inside the braided cable. Pressure inside the outer conduit can be made greater than pressure outside the tube. An alternative would be to pump the filler at the same time as the fiber is being inserted, if the filler is a suitable material. The filler may be for example a fluid such as a hydraulic oil, or a material which becomes more viscous or solid once in situ, such as a silicone based fluid. The installation of the outer conduit can be executed in the same way that conventional hydraulic control lines are installed in the wells. At locations where jointing and terminating is needed, standard fittings can be used. As described above, the outer conduit can be made to pass through an adapted section of tubing to expose the outer conduit to pressure conditions inside the tubing. The embodiments of figures 2, 5 or 6 would be suitable as they show the cable passing through without cutting or peeling back to the fiber during installation.

Once the outer conduit is deployed a continuity test can take place. This can be done by pumping air and checking for a drop of the pressure during a predetermined period of time. Once the continuity test is successfully completed, the braided sensing cable can be inserted to the required depth and the outer conduit can be filled by pumping the filler in. A gradual increase of the filler pressure can then be carried out so that the filler pressure is greater than the outside pressure. Alternatively the filler can be of a material which hardens or becomes more viscous once pumped in, so that it can better transmit outside conduit pressure onto the sensing fiber in the braided cable inside the outer conduit. Also the outer conduit can be made of a material which is somewhat compliant or responsive to the outside pressure. In these ways there is less or no need for the cable to be perforated, to be exposed to the outside pressure. Also, installation can be easier and more reliable if the outer conduit is installed first without the sensing cable inside. Advantages for the outer conduit include at least the following:

Easy, technically unchallenging installation,

Better protection from debris in the well, standard hydraulic connectors can be used, hydraulic disconnect can be used to enable the system to be deployed in bores having multiple stage completions.

Easy installation through different well components such as packers ICV (Inflow Control Valve), and ICD (Inflow control Device), etc.

An advantage for the braided cable is increased strength compared with only optical fiber deployment without braiding;

Figure 11 head end with wavelength mux/demux

Figure 11 shows a schematic view of an example of head end equipment, other examples can be envisaged. This example uses multiple optical wavelengths to distinguish optical signals from different parts of the fiber. Transmitters are provided for transmitting at wavelengths λ1 and λ2. One wavelength can be used for a point sensor such as an FBG, the other wavelength for distributed sensing for example.

Many more wavelengths can be used. These signals are multiplexed by multiplexer

400 and coupled into the sensing fiber by directional coupler 420. Received signals are passed from the directional coupler to wavelength demultiplexer 410 and the separated wavelengths can be converted to electrical signals by separate receivers.

A signal processing part 430, located locally or remotely can deduce the conditions from the signals, and deduce the locations from the timing of the signals, using a pulse timing input from the transmitter. An output can be fed to a user interface 440, or processed further as desired.

Figure 12 steps for installing without fiber cut

Figure 12 shows some of the principal steps involved in installing a sensing fiber for such a sensing system, without involving cutting the fiber. At step 500, tubing such as steel tubing is inserted in the bore, typically in sections coupled by joints. The cable having the sensing fiber is attached to the outside of the tubing as the tubing is assembled and inserted. At step 510, at a selected location in the run of tubing, an adapted section of tubing is inserted into the run of tubing. The cable is now laid through that adapted section without cutting the cable. This can be achieved either by having the adapted section in parts or with a lid which can be closed over the cable, or if the cable must be passed through an aperture in the adapted section, and through any seals, then this can be done before the installation is started. The entire length of cable down to the selected location can be pulled through the aperture, either on the rig floor or elsewhere. If desired, the metal conduit can be removed or peeled back from part of the sensing fiber inside the adapted section, to make the fiber more sensitive to the pressure conditions, or to enable coils of fiber to be retained in the adapted section. This can enable a longer length of fiber to be used for sensing inside the adapted section. This can make the measurements more accurate for distributed sensing.

At step 520, seals for the ports of the adapted section can then be closed around the cable. This enables the pressure conditions inside and outside the tubing to be isolated. Once sealed, the seals can be tested if desired, and the fiber tested for integrity or damage, and the insertion of the tubing can continue until the adapted section is at its desired position down the bore, as shown by step 530.

These steps can equally apply when deploying the outer conduit of the cable of figure 10. In such a case, there would be the additional steps of inserting the sensing fiber into the deployed outer conduit, and providing the filler in the outer conduit suitable to transmit the pressure outside the outer conduit to the sensing fiber.

Figure 13 steps for installing with fiber cut Figure 13 shows some of the principal steps involved in installing a sensing fiber for such a sensing system, similar to figure 10 but involving cutting the fiber. At step 600, tubing such as steel tubing is inserted in the bore, typically in sections coupled by joints. The cable having the sensing fiber is attached to the outside of the tubing as the tubing is assembled and inserted. At step 610, at a selected location in the run of tubing, an adapted section of tubing is inserted into the run of tubing. At step 615, the cable is cut, and is stripped back to the fiber. The ends are inserted through apertures forming the ports in the adapted section, and passed through any seals for those apertures. Then an optical component can be spliced into the sensing fiber, such as a point sensor, a coil of sensing fiber, or a hoop strain sensor for example. This splicing can be carried out using equipment on the rig floor for example. At step 620, seals for the ports of the adapted section can then be closed around the cable. This enables the pressure conditions inside and outside the tubing to be isolated. Once sealed, the seals can be tested if desired, and the fiber tested for integrity or damage, and the insertion of the tubing can continue until the adapted section is at its desired position down the bore, as shown by step 630.

Concluding remarks

Some or all of the measures or features described can be combined. Other variations within the claims can be conceived.

Claims

1. A sensing system for sensing in a bore, the bore having tubing inserted along the bore, the system having a sensing fiber extending along the bore at least predominantly in an annular space outside the tubing to sense a condition in the annular space in a distributed manner, one or more parts of the fiber being arranged to sense a condition inside the tubing at one or more selected locations in the bore, the system also having a transmitter for transmitting light along the fiber, and a receiver coupled to receive light from at least one of the part or parts of the sensing fiber for sensing a condition in the tubing and from the other parts of the fiber for sensing a condition in the annular space.

2. The system of claim 1 having a means to distinguish light received from the different parts of the fiber, to determine the conditions inside and outside the tubing.

3. The system of claim 1 or 2, arranged to sense a condition in the tubing in a distributed manner.

4. The system of claim 1 or 2 or 3 and having one or more point sensors on the sensing fiber arranged to sense a condition inside the tubing.

5. The system of any preceding claim having an adapted section of tubing arranged to expose the fiber to a condition in the tubing.

6. The system of claim 5, the adapted portion of the tubing having a chamber offset from a main channel of the tubing and connected so as to have the a same condition as the main channel, the chamber having at least one port to allow the fiber to pass from the annular space into the chamber, the port being sealable to isolate a condition in the chamber from a condition in the annular space.

7. The system of claim 6, the chamber being arranged to retain a number of coils of the fiber.

8. The system of claim 6 or 7, the chamber having at least two of the ports, to allow the fiber to pass through.

9. The system of claim 5 or any preceding claim when dependent on claim 5, the fiber being arranged to pass through the adapted section intact without a splice or connector.

10. The system of claim 5 or any claim when dependent on claim 5, the adapted section being arranged in sealable parts so that an intermediate part of the fiber can be laid in the adapted portion and sealed without a need to cut the fiber or without a need to thread an end of the fiber through an aperture.

11. The system of any preceding claim, the fiber being protected within a metal conduit.

12. The system of any preceding claim, the part of the fiber for sensing a condition inside the tubing being attached to the tubing so as to sense a hoop strain of the tubing, to represent the condition inside the tubing.

13. The system of any preceding claim, wherein the sensing fiber is configured to sense pressure conditions in the annular space.

14. The system of any preceding claim, wherein one or more parts of the fiber are configured to sense pressure conditions inside the tubing.

15. A method of installing a sensing system in a bore, the bore having tubing inserted along the bore, the method having the steps of installing a sensing fiber along the bore at least predominantly in an annular space outside the tubing to sense a condition in the annular space in a distributed manner, installing one or more parts of the fiber so as to sense a condition inside the tubing at one or more selected locations in the bore, coupling the fiber to transmitting means or a transmitter for transmitting light along the fiber, and coupling the fiber to receiving means or a receiver to receive light from at least one of the part or parts of the sensing fiber for sensing a condition in the tubing and from the other parts of the fiber for sensing a condition in the annular space.

16. The method of claim 13, comprising installing an outer conduit of the cable first, then inserting the sensing fiber into the outer conduit and providing a filler within the outer conduit suitable to transmit a condition outside the outer conduit to the sensing fiber.

17. A method for sensing in a bore having tubing inserted along the bore, using a sensing system having a sensing fiber extending along the bore at least predominantly in an annular space outside the tubing to sense a condition in the annular space in a distributed manner, one or more parts of the fiber being arranged to sense a condition inside the tubing at one or more selected locations in the bore, the system also having a transmitter for transmitting light along the fiber, and a receiver coupled to receive light from the fiber, the method having steps for transmitting light along the fiber, receiving light from different parts of the fiber, determining a condition from the received light, and determining which of the received light has come from the part or parts of the sensing fiber for sensing a condition in the tubing and determining which of the received light has come from the other parts of the fiber for sensing a condition in the annular space.

18. An adapted section of tubing for use in a sensing system as set out in any of claims 1 to 14, and arranged to expose a part of the sensing fiber to a condition in the tubing, the adapted portion of the tubing having a chamber offset from a main channel of the tubing and connected so as to have a same condition as the main channel, the chamber having at least one port to allow the sensing fiber to pass from the annular space into the chamber, the port being sealable to isolate the chamber from the annular space.

19. A sensing system for sensing pressure conditions in a bore, the bore having tubing inserted along the bore, the system having a sensing fiber extending along the bore at least predominantly in an annular space outside the tubing to sense pressure conditions in the annular space in a distributed manner, one or more parts of the fiber being arranged to sense pressure conditions inside the tubing at one or more selected locations in the bore, the system also having a transmitter for transmitting light along the fiber, and a receiver coupled to receive light from the part or parts of the sensing fiber for sensing pressure conditions in the tubing and to receive light from the other parts of the fiber for sensing pressure conditions in the annular space.

20. A sensing system for sensing in a bore having tubing received therein, the system comprising a sensing fiber, wherein part of the sensing fiber is exposed to a condition externally of the tubing and arranged for distributed sensing such condition, and part of the sensing fiber is exposed to a condition internally of the tubing to sense said condition .