Conduit monitoring

Abstract

A method for monitoring a fluid carrying conduit by introducing an acoustic pulse into the conduit, and interrogating an optic fibre positioned along the path of said conduit to provide distributed acoustic sensing. By measuring the response at each of a plurality of locations, a conduit condition profile can be derived. A condition profile can be obtained quickly and easily with minimum disruption to the pipeline infrastructure and contained flow. Existing optic fibres running along the path of a pipe can be employed for sensing purposes, allowing relatively long spans of pipeline to be monitored with only limited access to the pipe.

Claims

1. A method for monitoring a fluid carrying conduit comprising a pipeline the method comprising the steps of: interrogating an optic fibre positioned along the path of said conduit to provide distributed acoustic sensing; passing a pig through the pipeline, the pig generating (inducing) at least one acoustic pulse as it passes through the pipeline; measuring by distributed acoustic sensing the response to said at least one acoustic pulse at each of a plurality of discrete longitudinal sensing portions; and deriving from said plurality of measurements a conduit condition profile.

2. A method according to claim 1, comprising deriving one or more further conduit profiles and comparing said profiles to determine a change in conduit characteristics.

3. A method according to claim 2, comprising determining the longitudinal location of a change in conduit characteristics.

4. A method according to claim 1, wherein the amplitude of response to said at least one acoustic pulse is measured.

5. A method according to claim 1, wherein the spectral content of the response to said at least one acoustic pulse is measured.

6. A method according to claim 1, wherein the distributed acoustic fibre is located inside said conduit.

7. A method according to claim 1, wherein the distributed acoustic fibre is located adjacent to said conduit.

8. A method according to claim 1, wherein the spatial resolution of said distributed fibre optic sensor is less than or equal to 25 m.

9. A method according to claim 1, wherein the length of said distributed fibre optic sensor is greater than or equal to 20 km.

10. Pipeline monitoring apparatus comprising: an optic fibre interrogator adapted to interrogate an optic fibre and provide distributed acoustic sensing; a processor adapted to receive sensed data from said interrogator in response to acoustic pulses generated by a pig passing through the pipeline and to derive a conduit condition profile from said sensed data.

Description

DESCRIPTION OF THE DRAWINGS

(1) Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1 illustrates the basic component of a distributed fibre optic sensor

(3) FIG. 2 shows a fibre sensor arranged along a length of pipeline

(4) FIG. 3 is a cross section of a pipeline and sensing fibres

(5) FIGS. 4 and 5 show pipeline monitoring data outputs.

DESCRIPTION OF THE INVENTION

(6) FIG. 1 shows a schematic of a distributed fibre optic sensing arrangement. A length of sensing fibre 104 is connected at one end to an interrogator 106. The output from interrogator 106 is passed to a signal processor 108 and optionally a user interface, which in practice may be realised by an appropriately specified PC. The sensing fibre can be many kilometres in length, and in this example is approximately 40 km long.

(7) The interrogator launches an interrogating optical signal, which may for example comprise a series of pulses having a selected frequency pattern, into the sensing fibre. The phenomenon of Rayleigh backscattering results in some fraction of the light input into the fibre being reflected back to the interrogator, where it is detected to provide an output signal which is representative of acoustic disturbances in the vicinity of the fibre. The form of the optical input and the method of detection allow a single continuous fibre to be spatially resolved into discrete sensing lengths. That is, the acoustic signal sensed at one sensing length can be provided substantially independently of the sensed signal at an adjacent length. The spatial resolution in the present example is approximately 10 m, resulting in the output of the interrogator taking the form of 4000 independent data channels.

(8) In this way, the single sensing fibre can provide sensed data which is analogous to a multiplexed array of adjacent sensors, arranged in a linear path.

(9) FIG. 2 shows an arrangement employing a method according to the present inventiOn, whereby a sensing fibre 202 (and associated interrogator and/or processor 204) is arranged along the path of a pipeline 206. An impulser 208 is arranged at a point along the pipeline, and adapted to introduce a pressure pulse into the fluid in the pipe. Impulser 208 can take a variety of forms, but in this example comprises a hydraulic ram. The pressure pulse generated travels in both directions down the pipe, away from the impulser. The pipe acts as a waveguide and it has been found that the pulse can travel for tens of kilometres without being unduly attenuated.

(10) As the pulse passes through any particular length of pipe, it creates an acoustic disturbance which can be detected by the distributed fibre sensor 202. FIG. 3 shows a cross section of a pipe 302 with possible locations of a sensing fibre able to detect the response of the pulse in the pipe.

(11) The pipe in the present example has an internal diameter of 1200 mm and 50 mm carbon steel walls, carrying natural gas at approximately 80 bar. The pipe may be buried approximately 1-2 m below the surface which may be ground level or the seabed in certain situations. Fibre 304 is located inside the interior bore of the pipe 302, resting on the bottom of the pipe. Fibre 306 is bonded to the exterior of the pipe, while fibre 308 is located in a separate cable carrying conduit 310, located approximately 1.5 m from the centreline of the gas transmission pipeline. Conduit 310 is typically laid at the time of installing the pipeline to carry communication and/or SCADA lines. Fibre 312 is directly buried in the ground alongside the pipeline, at approximately 1 m from the pipe centreline.

(12) It will be understood that for each different fibre placement, the measured response to the pressure pulse in the pipe will be different, and will depend on different factors. The signal sensed by fibre 308 will depend on the transfer characteristics of the ground between pipe 302 and conduit 310, for example, while sensing fibres 304 and 306 will be less affected. As will be explained below however, this does not adversely affect the present invention, and any fibre placement which produces a reliable response to the pressure pulse can be used.

(13) FIG. 4 shows a histogram and associated waterfall plot illustrating a distributed fibre sensor output in response to a series of pressure pulses introduced into an adjacent pipeline. Data in FIG. 4 was produced by a sensing fibre in a conduit. The x-axis of the histogram and waterfall is the length of the sensing fibre which is this case is approximately 40 km. The histogram shows, at an instant in time the amplitude of the sensed acoustic signal returned from the sensing fibre. In order that all 4000 channels can be viewed, each bar in the diagram represents the peak amplitude from a group of 10 m sections. Individual 10 m could be viewed if desired. The lower plot is a waterfall with an update rate of 0.05 seconds showing sound intensity against distance and time, time plotted along the y-axis of the waterfall, most recent data plotted at the top.

(14) Two main features can be seen from the waterfall plot. The first is an area of constant activity towards the left of the plot at 402, corresponding to a length of approximately 4000 m of the sensing fibre. This is attributable to an industrial unit located over that section of fibre, producing a steady vibrational noise. Secondly distinct chevron patterns can be seen, most clearly in region 404, away from the constant noise of the industrial unit.

(15) The vertex of each chevron is located at point 406 along the fibre, corresponding to the location of an impulser. The V shape of the plot corresponds to the pressure pulse moving along the pipe in both directions away from the source of the pulse, and the slope of the V shape corresponds to the speed of sound in the pressurised gas contained within the pipe which in this case is approximately 400 ms.sup.1. It can be seen that a series of pressure pulses are introduced into the gas, and multiple traces are formed. On the top histogram plot, the individual pulses appear in their respective positions at that instant, spaced along the fibre.

(16) FIG. 5 shows data in a similar form to that of FIG. 4, but with the axes of both the histogram and the lower waterfall plot similarly rescaled. In FIG. 5, the x-axis of the waterfall plot corresponds to a section of the sensing cable approximately 4 km long (as opposed to 40 km in FIG. 4) and the update rate of FIG. 5 is set to 2 seconds (as opposed to 0.05 sec in FIG. 4).

(17) Data for FIG. 5 comes from the same pipe and fibre arrangement as in FIG. 4, but taken during a pigging run, and the path of the pig is clearly visible as a diagonal trace 502 in the waterfall plot. Also visible in the waterfall plot of FIG. 5 are a series of vertical lines having various intensities. The lines correspond to various locations along the length of the pipe, and can be considered as a fingerprint or barcode of the pipe, the pattern of lines corresponding to the physical characteristics or condition of the pipe, and to a certain extent its immediately surrounding environment (in this case the ground in which it is buried.

(18) Considering the condition profile provided by this barcode effect, it will be understood that this corresponds to the chevron effect of FIG. 4, but viewed with a compressed time axis. The pressure pulses passing through the pipe can be thought of as acoustically illuminating each portion of the pipe they pass through, eliciting a response from the pipe and its environment, whereby the response is detected by the distributed sensing fibre. By averaging over time, it can be seen that some sections of the pipe have a different response to the pulses than others. Possible causes of these differences include a local hydrocarbon build up on the pipe wall, a weakness in the pipe wall or variation in the wall profile, or variation in the ground composition in the vicinity of the pipe for example. In this way the plot provides a condition profile of the pipe at a given time or date.

(19) It is noted that while the pressure pulses seen in FIG. 4 are produced by a dedicated impulser, the pulses in FIG. 5, which give rise to the condition profile of the pipe are created as the pig passes each girth weld in the pipe, as explained above.

(20) Although not illustrated the spectral content of the sensed data can be extracted and provided. This would add an extra dimension to the plots of FIGS. 4 and 5, and would enable enhanced condition monitoring capability. Seismic signals are typically dominant at frequencies below 500 Hz due to the high attenuation of higher frequencies through the ground.

(21) For example, by looking at a selected frequency band or bands, the noise from the industrial plant in region 402 of FIG. 4 could be filtered out. A pipe profile or barcode as explained above, additionally decomposed by frequency provides more detail to a user and allows more sophisticated analysis. For example different types of physical phenomena may be associated with particular frequency bands. For instance, changes in the higher frequency bands may be indicative of turbulent flow in the pipe caused by the build-up of wax deposits whereas changes in the lower frequency band may be indicative of changes to the ground condition in which the pipe is laid. The interpreted results may therefore provide a greater quantity and quality of information to a user.

(22) It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention.

(23) Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.