Leak location

Details Leak location Leak location

1999-10-21

2001-07-31

Larkin, Daniel S. (Department: 2856)

Measuring and testing

With fluid pressure

Leakage

C073S592000, C073S049100, C073S040000

Reexamination Certificate

active

06267000

ABSTRACT:

FIELD OF INVENTION
This invention is concerned with leak location, and relates in particular to the determination of the location of a leak of fluid in an underground pipe. It has regard especially to leak location by means of a surface detector array which detects ground motion.
BACKGROUND OF THE INVENTION
There are many situations in which an underground pipe carrying a fluid, a gas or a liquid, develops a leak, which leak must then be located and repaired. A good example is the case of water distribution pipes buried at a depth down to a few metres below ground. One common location method makes use of the fact that the leaking fluid often creates a noise, typically a hissing sound, which is conducted along the pipe as vibration. The sound is also transmitted through the ground. Although the sound is rapidly attenuated by the ground, it is possible to hear the sound with suitable highly sensitive ground microphones, and therefore ground microphones are often made available to field crews in the water industry as a means of identifying and locating leaks.
It is naturally desirable to locate the leak simply by listening to the sound at the ground surface. Crews do sometimes try to use a ground microphone for final fixing of the position of a leak by moving the microphone around and searching for the point at which the leak noise is loudest. Unfortunately, the sound levels can be grossly distorted by underground heterogenities, and can easily be confused or even swamped by other sound sources above ground. In general, then, ground microphones do not seem to be effective, mainly because background noise obscures the signal, and this has reduced their credibility with field crews. The invention deals with these types of problems encountered by water leak location teams by listening to the leak sound source from a multiplicity of different positions spaced all around the sound, listening either simultaneously or sequentially, and by then suitably combining and processing the received signals to provide the desired indication of the source's location.
SUMMARY OF INVENTION
In one aspect, therefore, the invention provides a method of determining the location of a leak in an underground pipe carrying a fluid, in which method the movement of the ground caused by the fluid as it leaks from the pipe is listened to using sensors at a multiplicity of positions spaced over the ground surface and disposed within an area extending over the suspected location of the leak, and by suitably combining and operating upon the signal information received at each position there is determined the actual location of the leak.
The invention concerns determining the location of a leak in an underground pipe carrying a fluid. The pipe may be of any sort, that is, made of any appropriate material and of any size, and the fluid may likewise be of any sort, gas or liquid. The invention is, however, of particular use in connection with water-bearing pipes.
Underground water-bearing pipes can be of a wide variety of diameters ranging from a few centimetres (for local distribution to domestic property) to more than a metre (for mains service pipes). The depth of burial can also vary from the order of half a metre down to several metres.
The type of ground in which the pipe is buried varies a great deal in terms of its homogeneity, its stiffness, and the top cover. For example, the surrounding ground may be a roadbed, in which case it will be well consolidated, and propagates sound well. It could equally well be a field or grass verge, which is less well consolidated and so sound travelling therethrough propagates less well. Moreover, the actual top surface may be one or more of several materials, typically concrete, tarmacadam or soft earth.
The material of the pipe is likely to depend upon both the application and the age of the pipe. Common materials encountered are cast iron, ductile iron, high or medium density polyethylene, and UPVC (ultraviolet-stabilised polyvinyl chloride). The wall thickness of the pipe is likely to vary (with the material and the application) from a few millimetres upwards over a range factor of the order of five—thus to perhaps twenty-five millimetres and more.
Depending on the pipe's purpose, the pressure within the pipe can range from little more than atmospheric pressure to several Bars. Any such pipe can develop a fault which results in a leak. The fault can be a small diameter hole, a circumferential crack, a longitudinal crack, or a fault at a joint from which the pipe's contents can seep slowly or rapidly depending upon the size of the fault. Very slow seeping leaks which can occur at poorly fitting joints are not likely to generate sufficient leak noise to be heard at the surface of the ground. Other leaks are known to generate noise at the leak which propagates along the pipe wall, and through the fluid in the pipe, through the ground around the pipe, and through any combination of these paths.
A pool of fluid-saturated ground may develop around the leak. There may be a washed out volume around the leak, so that the leak flows directly into the pool rather than directly into the ground.
The pipe material, wall thickness, diameter, ground condition and nature of the leak will all influence the magnitude, the frequency range and frequency spectrum of the leak noise generated, but will not prevent the leak noise occurring. The magnitude of the ground motion will attenuate rapidly with distance from the source. Similarly, the frequency range will fall rapidly with distance from the leak (the rate of attenuation normally increases with frequency, so that higher frequencies are preferentially attenuated as distance increases). The level of ground motion expected will be of the order of 0.01 m/s
2
, being higher or lower by an order of magnitude or more depending on the leak, the depth of burial, the type of ground, and the lateral distance from the leak. Similarly, the frequency range can be expected to be up to a few kilohertz, depending on circumstances, sometimes it will be below one kilohertz. The motion of the ground is liable to be predominantly vertical with some lateral motion.
In the invention the movement of the ground caused by the fluid as it leaks from the pipe is listened to from a multiplicity of positions spaced over the ground surface and disposed within an area extending over the suspected location of the leak. The multiplicity of positions at which the motion of the ground is measured can consist of a number of discrete locations forming an array, a geometrical array, possibly a one-dimensional (or linear) array, but preferably a two-dimensional (or area) array, over the area of ground. Such a system of discrete locations involves an individual measurement sensor sited at each discrete position, and is referred to herein as a fixed array system. If possible, where the array is linear, or has linear components, then the, or one of the, lines of sensor positions should preferably be aligned with the underground pipe being checked.
Alternatively, the ground may be scanned by a remote measurement system, in effect a single sensor is moved, position by position, through and over the chosen area so that it may listen from a large multiplicity of positions one after the other. Indeed, the multiplicity of positions can be so large, and the individual, adjacent positions can be so close to each other, that the ground motion is effectively measured as a continuous distribution with respect to position on the surface. A system using a sequence of positions is referred to herein as a scanning system.
The invention uses the measured motion of the ground surface above the leak in order to determine the location of the leak. The method involves the detection of ground motion over an area of ground extending over the suspected location of the leak. Although in principle this area could be as small or as large as desired, most preferably the area's lateral dimensions do not greatly exceed the depth of the pipe in which the leak has occurred, say, not more tha

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