APPARATUS AND METHODS FOR DETECTING STRAIN IN STRUCTURES

Technical Field

[0001] This invention relates to apparatus and methods for detecting strain in structures. In particular, it relates to strain detection in structures such as oil and gas pipes and supporting structures in oil and gas installations or the like.

Background Art

[0002] The area of concern for this invention is the monitoring of structures by measuring strain. More specifically, the measurement of strain in order to infer the perimeter of a structure as it changes with time, temperature, pressure or any other parameter.

[0003] The monitoring of structures is of great importance in many areas, in particular in the oil and gas industry, even more important in sub sea environment where access to the structures is difficult. For example, a pipeline running at the sea bed between an offshore production location to a transportation hub may need to be monitored to provide information on the perimeter of the pipe in order to estimate internal pressure.

Disclosure of Invention

[0004] A first aspect of the invention provides an apparatus for monitoring physical parameters of a structure, comprising:

- a clamp that in use is placed to embrace the structure to be monitored; and

- a strain sensor attached to the clamp an arranged so as to detect strain in the direction of the perimeter of the structure, preferably around the periphery of the structure.

[0005] The clamp can be a strap or belt or a compliant material, and may be shaped to the outside shape of the structure. In a preferred embodiment, the clamp is made using composite material. The strain sensor can embedded into the clamp in one preferred embodiment.

[0006] The sensor may comprise an optical sensor such as an optical fibre sensor, for example a fibre Bragg grating based sensor, an interferometric sensor, or the like.

[0007] The structure to which the apparatus is applied is typically a pipe or tube, such an oil and/or gas pipe, or a water pipe. The structure may also be a sub sea structure such as a sub sea riser. [0008] The structure may be located below the surface and may be totally or partially buried. [0009] A data collection unit may also be attached to the structure or the clamp.

The data collection unit can include an optical interrogation unit operating, for example, by means of spectrum analysis. [0010] A second aspect of the invention provides a method of monitoring a structure, comprising:

- placing a clamp to embrace the structure to be monitored, the clamp having a strain sensor attached thereto; and

- collecting information from the strain sensor attached to the clamp placed in order to detect strain in the direction of the perimeter of the structure.

[0011] The strain information may be used to calculate the perimeter dimensions of the structure, the temperature of the structure, the outer pressure of the structure, or the inner pressure of the structure.

[0012] Further aspects of the invention will be apparent from the following description.

Brief Description of Figures in the Drawings

[0013] Figure 1 shows one embodiment of an apparatus according to the invention;

Figure 2 shows an embodiment of the invention installed on a structure to be monitored; and '

Figure 3 shows a flow chart of a method according to an embodiment of the invention.

Mode(s) for Carrying Out the Invention

[0014] Referring now to Figure 1 , the embodiment of the invention shown therein comprises a ring-type clamp 11 , typically made out of a composite material. In this embodiment, the clamp 11 is formed in two semicircular halves secured together using a securing system 12. In this case, the securing system comprises two nut and bolt arrangements on opposite sides of the clamp. Other releasable securing systems can also be used

and it is also possible to replace one securing system with a hinge. The shape of the claim 11 in Figure 1 is circular, although other shapes can be used depending on the shape of the structure to be monitored.

[0015] A strain sensor 13 is located on the clamp 11 or imbedded into the clamp material (as is shown in Figure 1). In one possible embodiment, the clamp 11 is made of composite material and the sensor 13 is an optical fibre Bragg grating sensor. By providing the clamp 11 with a sensor that measures strain, securing the clamp 11 to a structure to be monitored, for example a pipe or tube, such an oil and/or gas pipe, or a water pipe, or a sub sea structure such as a sub sea riser, means that strain imposed on the clamp 11 by the structure can in turn be measured by the sensor 13. The direction of the strain measured will depend on the configuration of the clamp. In the embodiment of Figure 1, the sensor 13 will measure the tangential strain in the clamp 11 which in turn is created by the behaviour of the structure at its periphery or perimeter where the clamp is located.

[0016] Figure 2 shows a clamp 31 attached to a structure 32 such as a pipe. A data collection unit 33 is also be attached to the structure 32 by means of further clamps or other locating devices and connected to the sensor in the clamp 31 by a cable 34. The data collection unit can include a battery or other power source or can be connected to a power source by means of a cable. Likewise, data stored in the unit 33 can be delivered to a processing system directly via a cable, or by periodically downloading the data via a wireless link in response to interrogation by a reader.

[0017] Figure 3 shows a flow diagram of the steps in a method according to one embodiment of the invention to calculate internal pressure of a structure, such as a pipe line, using the clamped system of Figure 2 using a fibre Bragg grating sensor. In a first step 40, data is collected from the sensor referenced to a centre wavelength of the fibre Bragg grating mounted in the clamp. This approach has been used in other fibre Bragg grating devices.

[0018] The data is then translated from the centre-wavelength referenced data into a strain measurement (42). This can be done in one of a number of known ways, dependent on the exact form and orientation of sensor used.

The strain measurement can then in turn be used to calculate the hoop strain on the clamp (44) and from this the perimeter dimensions of the structure beneath the clamp can be determined (46), knowing the dimensions of the clamp and using the known physical properties of the clamp material and structure. Finally, by knowing the physical structure of the pipe and its material properties, and its environment, the internal pressure of the pipe can be calculated (48). Other properties such as the temperature of the structure, the outer pressure of the structure can also be calculated from this data.

[0019] Steps 42-48 can be performed in a processing unit remote from the structure and sensor if desired. Also, a series of measurement over time can be made to determine time-varying properties of the pipe or other structure being monitored.

[0020] Changes can be made while remaining within the scope of the invention. For example, multiple sensors can be provided in a single clamp. Also, multiple clamps can be provided spaced along the structure of interest.