Abstract:
A device and a method for pipeline leakage detection. A pipeline leak detector travels through the bore of a pipeline, applies a test pressure to a fluid contained within the pipeline, and measures the resultant rate of change of pressure in the pipeline by using back extrapolation of the test data to determine an initial pressure drop rate. The degree of leakage at a given position in the pipeline is then determined from the rate of change of pressure in the pipeline.

Description:
The invention relates to pipe leakage detection for use in fluid pipelines (e.g. natural gas). 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     There is a need to test pipelines for leakage and to preferably be able to do so whilst the fluid is actually flowing through the duct, so as to avoid interrupting the downstream supply or services. 
     2. Discussion of the Background 
     The present invention is concerned with providing a mechanism for testing leakage such as that which may occur in pipe joints. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a pipeline leak detector comprising means for travelling through the bore of the pipeline; means for applying a test pressure to the pipeline; means for measuring the resultant rate of change of pressure; and means for determining the degree of leakage at a given position within the pipeline from the rate of change measurement. 
     Further according to the invention there is provided a method of detecting leakage in a pipeline comprising moving a device through a pipeline to a desired location; applying a test pressure to the pipeline, measuring the resultant rate of change of pressure and determining the degree of leakage at that location from the rate of change measurement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying drawings in which: 
     FIG. 1 shows an embodiment of the leak detecting pig; 
     FIG. 2 shows a schematic view of the leak detecting pig within an existing pipeline, and 
     FIG. 3 shows a graph associated with the testing configuration. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The leak testing pig train  10  of FIG. 1 is shown within an existing pipeline  11  which incorporates a joint  12  where two sections abut. The pig train  10  includes a leak pig  13  with a flexible central body portion (e.g. of flexible plastics material) capable of 1D bend passing and connected to a valve and sensor module  27 , regulator module  20  for regulating the test gas, an electronic control module  14  and umbilical termination and power regulator module  15 , the latter being linked via a trailing umbilical cable  19  to a base station  22 . 
     The base station connects to a computer  29  (e.g. a laptop pc). The control module  20  includes a gas regulator valve  18  which receives gas for test purposes via the umbilical  19 . The umbilical also provides power to the pig as well as control and data lines (digital). The pig is towed through the pipeline  11  via tow cone  16  by means of towline  17  attached to a winch  40 . The umbilical cable  19  will be fed over an encoder wheel  21  to indicate distance travelled, this information being passed to the computer  29  via basestation  22 . The pig is automatically winched under computer control through the gas main pipeline  11  in a stepwise manner to perform pressure decay tests at each step. The system can be configured to provide some overlap at each step to ensure full checking of the pipeline. 
     A portable power generator  41  provides power for the pig, the computer  29  and basestation  22 . A gas bottle  42  (e.g. natural gas) provides the test gas for pressure tests to the pig via umbilical  19  which passes over drum  43 . 
     The pig includes four circumferential seals  23 - 26  to provide an annular test volume in the region between seals  23  and  26 . The test volume is between two annular seal volumes bounded by seals  24  and  25 . The gas supply is allowed to pass through the pipe for use, by the presence of a hollow central tube portion  28 . The flexible body portion between seals  24  and  25  allows relatively tight bends during insertion and travel to be accommodated as do the control and regulation requirements into a number of separate modules to allow 8 inch pipe testing. 
     The pig includes three sensors  30 - 32  which are spaced circumferentially around the pig to detect the location of the pipe joint  12  as the pig travels through the pipeline. These sensors can each comprise a small magnetic source with associated magnetic sensor (e.g. Hall effect). 
     The modules include microprocessors to provide a data link to and from the computer  20  via cable  19  and the on-board electronics will receive sensor information as well as control the pig operation. A battery provides the power source or an alternative source. 
     In order to carry out the leakage test operation an equalisation valve  35  is provided which, when open, under electronic control allows test and seal volume pressures to be balanced. A precision differential pressure sensor system  34  is provided to determine pressure drops during decay tests. The sensor for illustrative purposes is shown on pig  13 , but in practice it will typically reside within the appropriate module  27  and be linked to the pig  13  by small pipes to allow sensing to be achieved at the inner and outer volumes described below. 
     The mechanism associated with the leak tests is illustrated in the schematic of FIG.  2 . The schematic drawing shows the pig  13  within pipeline  11  and a leak being present in joint  12  (the leak being exaggerated for illustrative purposes). The seals  24  and  25  together with the outer wall of cylindrical pig portion  36  and the pipe wall form a first chamber  37  (when equalisation valve  35  is closed). The seals  23  and  26  together with the inner wall of cylindrical pig portion  36  and the wall of the inner cylindrical pig portion  38  form a second chamber  39 . Test gas will leak through the joint leak (Q leak). There may also be leakage (q seal) of the test gas between the chambers as these may not form a perfect seal with the pipe. However, this is dealt with in the computations. 
     The test volume is within chamber  37  and the seal volume is within chamber  39  and with the valve  35  open their pressures are balanced. For a perfect gas,               p          t       ∝     Q   V                            
     where Q is the flow out of volume V of chamber  37 . 
     Using a test pressure, leakage can be detected and in practice the degree of leakage can be measured as well. 
     A pressure decay test is performed by closing the equalisation valve and monitoring test pressure using the high resolution pressure transducer system  34 . In practice, the main valve on closure can cause disturbances downstream. In order to produce a more stable reference, a small volume reference chamber is provided with its own regulation valve in series to keep a stable reference value just during the main valve sequence. 
     Pressure decay due to a leak from the test volume will be reduced by leakage past the inner seals  24 ,  25 , from seal to test volume. However, at the instant the equalisation valve  35  is closed, there is zero differential pressure across the inner seals and therefore no leakage past them. Using back extrapolation of test data, it is possible to determine the initial pressure drop rate dp/dt at the instant the equalisation valve is closed, and since the test volume is known, leakage Q can be calculated. 
     We have determined that even though there may be leakage into the pipeline from seals  23  and  26  as well and between chambers  37  and  39  due to seals  24  and  25 , it is the slope of the leakage curve that is related to the joint leakage. 
     Hence from FIG. 3 different graphs are shown for a given joint leak (Q leak) for various leakage patterns for the seal between chambers (q seal). Thus graph (e) shows the most effective seal and graph (a) the least effective. Using calculus to determine the slope,             p          t                            
     is directly related to Q leak. The inner seal leakage is zero the instant the equalisation valve is shut. 
     By using a relatively small chamber  37 , the small test volume will give large drop rates for Q leak so detecting small leaks. Hence the pig will be winched, in steps, under computer control. At each step, winching is paused to allow the test to be effected. Leakage can occur at hot spots or joints for example. Where leakage is at a joint, the presence of detectors  30 - 32  (of FIG. 1) will identify the source of leakage. 
     If a major leak is detected along the pipe at any location, this can cause an alarm or other indication in the P.C. as detected by being unable to balance pressures in the test and seal volumes. 
     Typically leakage measurement is from 0.0028 SCMH (0.1 SCFH) to 1.0 SCMH (35 SCFH) in a low pressure main. Leakage measurement results can be within 10% accuracy or better. 
     Inner Seal Integity Test 
     In the event that the pig stops with an inner seal resting over an intrusion or debris, leakage past the inner seal may be such that test and seal volume pressures remain equalised when the equalisation valve is closed. This would mask any leakage from the test volume, if further testing does not occur. However, if the inner seals are functioning correctly, a forced increase in test volume pressure (e.g. by changing the preset regulation pressure) with the equalisation valve closed would give an increase in differential pressure across the inner seals. By monitoring this effect, seal integrity can be checked at each test step along the pipe. Alternatively, venting of the test volume via another valve to the actual pipeline pressure to achieve a pressure drop will also serve as a mechanism for checking seal integrity 
     Service Location 
     If services are taken from the pipeline, it will be necessary to discriminate between joint leaks and pressure drops due to consumption at service pipe locations. Service pipe junctions could be detected by a magnetic source present in the service pipe, for example. 
     Leakage from services can be measured, if the service is blanked off in the property. In this case, an additional deliberate leak at the property end of the service will be used for service volume quantification. If service leakage measurements are not required, the flow through facility on the pig ensures continuity of gas supply. In this case, leakage measurements while the pig is parked over the service would be masked by the demand from the property and would be discarded. 
     The computer provides a user interface for entry of site details, a running graphical display of leakage versus distance along the main, and software to drive the test sequence and winch control systems. 
     Post inspection data analysis will allow on site graphical or report style presentation of inspection results, showing the position and magnitude of leaks above a user set threshold, together with the positions of joints and services. 
     Inspection time will typically be 20-30 minutes per 100 meters of main. Hence the pig is designed for use in live gas mains typically without interruption to downstream gas supply or services. It incorporates joint and service position detection and will determine the position and magnitude of leakage from mains and services. 
     The leakage pig is unique as it will both locate and accurately measure gas leakage from distribution pipes, dead or live. The source of leakage could be a faulty joint or a pipe defect. 
     The pig is therefore able to: 
     1. accurately quantify leakage both for inspection purposes and for collection of valuable leakage data, 
     2. test integrity of any in pipe repair; and 
     3. locate leaks where barholing and external repair is precluded. 
     The device has been described in terms of carrying out checks whilst it is temporarily stationary over any particular pipe position and utlising the equalisation valve, before the device moves forward again to the next incremented step position. 
     However in a further arrangement the valve could be replaced by an equalisation aperture and the device could move continuously through the pipeline to carry out its tests. 
     This freeflow detection would be particularly suitable for testing small leaks (e.g. of the order of 100 scmh) in a transmission pipeline system, by employing both local pressure drop measurements and pipe bore mapping.