GB1601341A

GB1601341A - Examining structures for the formation of cracks

Info

Publication number: GB1601341A
Application number: GB3547577A
Authority: GB
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1978-05-24
Filing date: 1978-05-24
Publication date: 1981-10-28

Description

(54) EXAMINING STRUCTURES FOR THE FORMATION OF CRACKS (71) We, E.M.I. LIMITED, a British company, of Blyth Road, Hayes, Middlesex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the examination of structures for the formation of cracks.

Structures such as oil extraction platforms for submarine oil fields are frequently formed from steel tubes welded together.

Many of the welds are below the sea surface and the checking of them for the formation of cracks requires the use of a diver. Typical techniques used hitherto have involved, at each examination, applying a magnetic record material to the structure after cleaning off weed growth etc. This is a lengthy and tedious process.

It is an object of the present invention to provide an alternative examination technique to detect the formation of cracks in a structure.

According to one aspect of the invention there is provided apparatus for detecting the formation of a crack in a structure, comprising a plurality of frangible members of electrically insulative material, a protective member of flexible material in which the frangible members are embedded to protect them from a severe environment, means by which a break in any one of the frangible members is detectable outside the protective member, and means for attaching the protective member to the structure.

According to another aspect of the invention there is provided a structure having attached to its a plurality of apparatus according to said one aspect.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which; Figure la shows a pair of detector elements and Figure lb shows one of the elements applied to a portion of a structure and Figure 2 is a schematic diagram of a structure having a monitoring system, and detector elements attached to it.

Each of the pair of detector elements in Figure la is a strip 10 of an electrically insulative frangible material which also carries a layer of electrically conductive material 15. The frangible material is a weak material such as ceramic or glass, formed so that it can be engaged at two points with a structure to be fractured if the points move relative to one another. For example enlarged ends 11 and 12 maybe formed to fit hooks 13, 14 which can be stuck or otherwise secured to the structure. The layer 15 may be a thick film deposit with connection pads at 16 and 17.

The detector element is applied by securing it to a selected zone of a structure. Figure lb shows one of the detector elements of Figure la secured across a weld 3 between tube portions 1 and 2.

The tube portions are prepared, eg by grinding, by cutting grooves 4 and 5 either side of the weld 3 spaced to receive the hooks 13, 14 and by cleaning the surface to receive adhesive, 6, by which the hooks are secured.

Leads 18, 19 are connected to pads 16, 17.

Generally many detector elements are required to bridge a single weld at many points along the weld. Furthermore a structure such as an oil-rig comprises very many welds. To simplify assembly of the numerous detector elements and to protect them from the severe environment to which they will be subject groups of the detector elements are embedded (as shown in Fig. lb) in a pretensioned form in a flexible strip 7, of rubber or plastics along which they are spaced as shown in Fig. la, which shows the two detector elements extending in parallel across the strip 7. The whole strip is then applied as described above. By using this "pre-fabricated" technique correct spacing and speedy assembly is achieved. Also the interconnections are simplified and can be checked before the arrangement is applied to the structure.Typically all the conductive layers of the group of detectors at one weld are connected in series. The strip may be up to two to three metres long.

While it is possible for each group of detector elements at each weld to be examined by applying a suitable instrument, as described below, this would require the services of a diver for submarine structures.

If the structure is an oil-rig for example with many groups of detectors, examining all of them is a difficult and expensive task for a diver to perform. Accordingly the invention also provides a system by which the elements can be monitored centrally, eg on the platform itself, above the surface of the water.

This is indicated in Figure 2 in which strips 7 are shown wrapped around each weld on the platform legs. In one form of the system multi-core conductors such as 40, 41 extended from the groups of detector elements 21, 22 etc at the weld to the platform and the resistance of the elements at each weld can be determined. A significant change is detected as indicating a fault, for example a fractured element will produce an open circuit and therefore a higher resistance when measured from the central monitor, 30.

To reduce the number of cores required a version of the "data-bus" technique could be used. A group of two or three wires extends to all the groups 21, 22 of detector elements in common, the groups being connected in parallel to the wires. Each detector group 21, 22 etc has an associated semiconductor chip which is embedded in the flexible strip and by which that individual group can be addressed with appropriate address signals on the common conductors. Power for the chips can also pass along the common conductors. By appropriate selective signalling and processing techniques well known in the art the groups can be individually addressed with address signals systematically by the central monitor. Each group of detectors is tested by its chip, which transmits to the monitor a signal indicative of the conductivity of the group.

If it is desired to examine individual groups without using a central monitor eg by use of a diver then a transducer can be included in each group. For example a pair of coils may be built into the moulded strip, the coils and the detector elements of the group being connected in series. By bringing up an instrument with a corresponding pair of coils one of the built-in coils could be excited and the presence or absence of current in the other detected. The built-in coils could have axes in the plane of the bridges and at right angles to each other.

Ferrite cores could be provided. Alternatively, for example, a light emitting diode could be energised by the induced current in an unbroken series circuit.

In a further form of the invention the individual groups could be arranged to provide a report when a failure occurred, or at the next occurrence of a selected time interval which was set aside for the purpose.

A long life battery, eg isotope powered, could drive a clock based on an LSI circuit for several years so that the reporting time interval would be known to the unit with adequate accuracy. To convey the information an acoustic or mechanical signal could be used. Thus on failure of a detector element at the next occurrence of the interval a series of explosions could be detonated in a coded signal indicating the location of the failed element. The explosions could be detected by sensors on the structure near to water level. Alternatively a "sea-battery" could be activated to power a sonar beacon supplying a characteristic signal detectable by hydrophones beneath the sea surface and connected to a monitor unit.

The techniques described above are not limited to steel structures, as are the known magnetic techniques, and can be applied to where ever a relative movement of parts is to be checked. The structures could be of metal such as steel, non-metal such as concrete, or a mixture of metal and non-metal such as reinforced concrete.

In the foregoing the frangible members and the strip 7 are attached to the structure as described by hooks. However, they could instead be attached by use of adhesive to stick the strip 7 to the structure. Other alternatives include bolting the strip to the structure or attaching it to a peg welded onto the structure.

In an alternative arrangement of the invention, the or each frangible strip 10 with the conductive layer 15 is replaced by an optical fibre held in tension in the strip 7. A light source such as an LED is provided for shining a light beam along the fibre, and a light detector is provided for detecting whether the light beam is transmitted to it by the fibre. If not light is detected, it is assumed that the fibre has broken.

In one form of the alternative a multiplicity of such fibres are arranged on the oil platform. A corresponding multiplicity of light sources and detectors may be arranged at the central monitoring station, each fibre extending from that station to the weld to be monitored and back again. Alternatively, a single source and detector may be provided centrally and the fibres tested sequentially. In another form of the alternative identical to the aforementioned "data-bus" technique a light source and detector is provided for each fibre, or commonly for all fibres, in each protective strip 7 and means provided in each strip for signalling the results of testing the fibres in each strip to the central station.

These means would be the semiconductor chip mentioned before.

WHAT WE CLAIM IS: 1. Apparatus for detecting the formation

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. While it is possible for each group of detector elements at each weld to be examined by applying a suitable instrument, as described below, this would require the services of a diver for submarine structures. If the structure is an oil-rig for example with many groups of detectors, examining all of them is a difficult and expensive task for a diver to perform. Accordingly the invention also provides a system by which the elements can be monitored centrally, eg on the platform itself, above the surface of the water. This is indicated in Figure 2 in which strips 7 are shown wrapped around each weld on the platform legs. In one form of the system multi-core conductors such as 40, 41 extended from the groups of detector elements 21, 22 etc at the weld to the platform and the resistance of the elements at each weld can be determined. A significant change is detected as indicating a fault, for example a fractured element will produce an open circuit and therefore a higher resistance when measured from the central monitor, 30. To reduce the number of cores required a version of the "data-bus" technique could be used. A group of two or three wires extends to all the groups 21, 22 of detector elements in common, the groups being connected in parallel to the wires. Each detector group 21, 22 etc has an associated semiconductor chip which is embedded in the flexible strip and by which that individual group can be addressed with appropriate address signals on the common conductors. Power for the chips can also pass along the common conductors. By appropriate selective signalling and processing techniques well known in the art the groups can be individually addressed with address signals systematically by the central monitor. Each group of detectors is tested by its chip, which transmits to the monitor a signal indicative of the conductivity of the group. If it is desired to examine individual groups without using a central monitor eg by use of a diver then a transducer can be included in each group. For example a pair of coils may be built into the moulded strip, the coils and the detector elements of the group being connected in series. By bringing up an instrument with a corresponding pair of coils one of the built-in coils could be excited and the presence or absence of current in the other detected. The built-in coils could have axes in the plane of the bridges and at right angles to each other. Ferrite cores could be provided. Alternatively, for example, a light emitting diode could be energised by the induced current in an unbroken series circuit. In a further form of the invention the individual groups could be arranged to provide a report when a failure occurred, or at the next occurrence of a selected time interval which was set aside for the purpose. A long life battery, eg isotope powered, could drive a clock based on an LSI circuit for several years so that the reporting time interval would be known to the unit with adequate accuracy. To convey the information an acoustic or mechanical signal could be used. Thus on failure of a detector element at the next occurrence of the interval a series of explosions could be detonated in a coded signal indicating the location of the failed element. The explosions could be detected by sensors on the structure near to water level. Alternatively a "sea-battery" could be activated to power a sonar beacon supplying a characteristic signal detectable by hydrophones beneath the sea surface and connected to a monitor unit. The techniques described above are not limited to steel structures, as are the known magnetic techniques, and can be applied to where ever a relative movement of parts is to be checked. The structures could be of metal such as steel, non-metal such as concrete, or a mixture of metal and non-metal such as reinforced concrete. In the foregoing the frangible members and the strip 7 are attached to the structure as described by hooks. However, they could instead be attached by use of adhesive to stick the strip 7 to the structure. Other alternatives include bolting the strip to the structure or attaching it to a peg welded onto the structure. In an alternative arrangement of the invention, the or each frangible strip 10 with the conductive layer 15 is replaced by an optical fibre held in tension in the strip 7. A light source such as an LED is provided for shining a light beam along the fibre, and a light detector is provided for detecting whether the light beam is transmitted to it by the fibre. If not light is detected, it is assumed that the fibre has broken. In one form of the alternative a multiplicity of such fibres are arranged on the oil platform. A corresponding multiplicity of light sources and detectors may be arranged at the central monitoring station, each fibre extending from that station to the weld to be monitored and back again. Alternatively, a single source and detector may be provided centrally and the fibres tested sequentially. In another form of the alternative identical to the aforementioned "data-bus" technique a light source and detector is provided for each fibre, or commonly for all fibres, in each protective strip 7 and means provided in each strip for signalling the results of testing the fibres in each strip to the central station. These means would be the semiconductor chip mentioned before. WHAT WE CLAIM IS:

1. Apparatus for detecting the formation

of a crack in a structure, comprising a plurality of frangible members of electrically insulative material, a protective member of flexible material in which the frangible members are embedded to protect them from a severe environment, means by which a break in any one of the frangible members is detectable outside the protective member, and means for attaching the protective member to the structure.

2. Apparatus according to claim 1, wherein each frangible member is an optical fibre and the means by which a break in any one of the frangible members is detectable includes light source means for transmitting light along the fibres and lightdetector means for receiving light transmitted along the fibres from the source means.

3. Apparatus according to claim 1, wherein the means by which a break is detectable includes an electrically conductive layer on each frangible member to be broken with the member, whereby a crack can be detected by testing the electrical conductivity of the layer.

4. Apparatus according to claim 3, wherein the break detecting means includes a pair of induction coils embedded in the flexible member and connected in series via the said conductive layers which are also connected in series.

5. Apparatus according to claim 4, further comprising means for energising one of the coils and means for sensing current flow in the other coil.

6. Apparatus according to claim 3, wherein the break detecting means comprises an induction coil embedded in the flexible member and connected in series with a light source the light of which is visible outside the flexible member via the conductive layers which are also connected in series.

7. Apparatus according to claim 6, further comprising means for energising the induction coil.

8. Apparatus according to claim 3, wherein the break detecting means comprises testing means combined with the flexible member for responding to a preset address signal to test the conductivity of the said layers in the flexible member and to cause the production of a signal indicative of the conductivity of the said layers.

9. Apparatus according to claim 3, wherein the break detecting means comprises testing means combined with the flexible member for testing the conductivity of the said layers and for producing a signal indicative of the conductivity at preset intervals.

10. Apparatus according to claim 8 or 9, wherein the testing means is embedded in the flexible member.

11. Apparatus according to any one of claims 3 to 10, wherein each of the said conductive layers is a thick film of conductive material.

12. Apparatus according to any one of clauses 3 to 11, wherein each frangible member is of ceramics material.

13. Apparatus according to any one of claims 3 to 11, wherein each frangible member is of glass.

14. Apparatus according to any one of claims 1 to 13 wherein the attaching means comprises hooks engaged with opposite portions of the frangible members, the hooks being provided to engage in spaced recesses in the structure to hold the frangible member in tension on the structure.

15. Apparatus according to any one of claims 1 to 14 wherein the flexible member is of resilient material in which the frangible members are embedded, the flexible member holding the frangible members in tension.

16. Apparatus according to any preceding claim, wherein the attaching means comprises adhesive.

17. Apparatus according to claim 15, wherein the attaching means comprises provision for bolting the flexible member to the structure.

18. Apparatus according to any preceding claim, wherein the flexible member is in the form of a strip and the frangible members are spaced apart along the strip and extend substantially parallel to each other across the strip.

19. Apparatus for detecting the formation of a crack in a structure substantially as hereinbefore described with reference to Figures la and lb of the accompanying drawings.

20. A structure having attached to it a plurality of apparatus according to any preceding claim.

21. A structure having attached to it a plurality of apparatus according to claim 8, 9 or 10 or any one of claims 11 to 18 when appended to claim 8, 9 or 10, and further comprising a common monitor for monitoring the production of a said signal indicative of the conductivity of the said layers.

22. A structure having attached to it a plurality of apparatus according to claim 2 or any one of claims 14 to 18 when appended to claim 2, and a common monitor connected to all the apparatus to monitor whether the optical fibres transmit light from their associated light source means to their associated light detector means.

23. A structure according to claim 20, 21 or 22, which is an oil drilling or extraction platform.

24. A structure according to claim 23, and substantially as hereinbefore described with reference to Figs. la, lb and 2 of the accompanying drawings.