Pipeline mapping and interrupter therefor

Details Pipeline mapping and interrupter therefor Pipeline mapping and interrupter therefor

2001-03-23

2003-09-09

Barlow, John (Department: 2863)

Electricity: measuring and testing

Of geophysical surface or subsurface in situ

For small object detection or location

C340S551000

Reexamination Certificate

active

06617855

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the mapping of an underground cable or pipe. It also relate to an interrupter for interrupting the cathodic protection voltage applied to such an underground cable or pipe.
It is well known to apply a cathodic (negative) voltage to an underground cable or pipe (hereinafter “pipe”) to reduce corrosion of that pipe. If the pipe is at a positive voltage relative to ground, electrolytic effects occur which damage the pipe. It should be noted that such cathodic protection voltage may be applied even when the pipe is coated to insulate it from the ground, because it is common for that insulation to develop holes or other faults which could result in localised damage.
However, it is also common for such pipes to experience electrical fields due to other objects such as adjacent pipes, or other electrical conductors such as railway lines, etc. Such localised fields sometimes result in the pipe experiencing a positive voltage relative to ground, so that corrosion occurs. It is therefore desirable to investigate currents in the pipe or cable to detect points where corrosion may occur. If stray currents are detected, these may be due to electric fields from other objects (such currents hereinafter being referred to as stray currents), appropriate action can be taken, such as repairing the fault or taking corrective action elsewhere by suitable voltage control, or even by providing sacrificial anodes at an adjacent fault.
In order to detect the stray currents, it is possible to make use of the magnetic fields generated by those currents, and detect those fields remotely from the pipe itself, such as at the surface. Detection of such magnetic fields is generally known, although special techniques may be needed because of the low magnitude of the currents.
There is also the problem that there may be several pipes in the area being investigated, and the currents from those other pipes may confuse the measurement.
SUMMARY OF THE INVENTION
Therefore, it is preferable that the cathodic protection voltage is modulated to enable the currents generated thereby to be more easily recognised by remote detectors. At its simplest, the modulation could be applied by a relay controlled switch connected between the pipe and the voltage source which supplies the cathodic protection voltage. However, the present invention, in its several aspects, seeks to develop stray current mapping arrangements, and also to provide an interrupter for applying a modulation to the cathodic protection voltage, and hence to the stray currents, and to improve the investigation of those stray currents.
Before discussing these aspects of the invention, however, it needs to be borne in mind that one source of stray currents is due to the cathodic protection voltage applied to other pipes which pass adjacent or across the pipe being investigated. Stray currents on the pipe being investigated may thus have multiple components, particularly when there are a large number of pipes adjacent each other.
Thus, a first aspect of the present invention proposes that the cathodic protection voltage of the pipe being investigated is modulated with a first modulation signal, and the cathodic protection voltage of a second pipe which passes adjacent the first pipe is modulated with a second modulation signal. Then, the stray currents on the pipe being investigated are analysed on the basis of the different modulation signals applied to the two pipes. If the stray currents are analysed at the first modulation, those current components due to the pipe itself, or due to other perturbations such as electric railways are determined. Then, if the stray currents are investigated at the second modulation, ie the modulation applied to another pipe, the effect of the linking between that other pipe and the pipe being investigated can be determined. This process can be repeated for multiple pipes by modulating with a distinct signal for each pipe.
Where the cathodic protection voltage of two pipes is to be modulated, whether it be the same modulation signal or with different modulation signals as in the first aspect, separate interrupters will be fitted to each pipe, and although the interrupters may be driven by oscillators having the same nominal frequency, manufacturing tolerances etc mean that synchronisation cannot be reliably achieved. Therefore, the interrupters need an additional system to provide synchronisation. The synchronisation represents a second aspect of the present invention.
Synchronisation could be achieved by directly connecting the two interrupters, so that the clock signal of one can be used as a reference against which the other is synchronised. However, in many situations, it is not practical for the interrupters connected to different pipes themselves to be connected. The pipes may only pass adjacent each other at a particular point along their length, and if the interrupters cannot be connected at that point, physical interconnection is problematic. Therefore, in a third aspect of the invention, synchronisation makes use of an arbitrary pre-set time for all interrupters. When any individual interrupter starts to operate, after it has stopped operating for some time, it determines the time interval between that pre-set time and the time of start of operation, and the interrupter signal (which is modulated by an appropriate modulation signal), is commenced at the point in the signal which corresponds to that which the signal would be if the signal had commenced at the pre-set time. In this way, the interrupters are synchronised as if any operating interrupter had started at the pre-set time, irrespective of the time which has past since that pre-set time.
In order for this operation to occur, the interrupter needs to determine the time between the pre-set time and the time at which the interrupter starts operation. If an absolute reference clock was available for each interrupter, that reference clock could be used. However, it is not normally economic to provide such accurate time measurement within an interrupter. Therefore, it is preferable that the interrupters make use of an external signal. If an external time signal is available (such as the Rugby signal in the UK), then that could be used. A further alternative is to make use of the Global Positioning System (GPS). Whilst those signals are primarily to provide positional information, they also provide a synchronised clock signal from which the interrupters can determine the time between the pre-set time and the time of starting the interrupter, and so can determine at which point in the interrupter signal the operation is to start.
To put this aspect another way; if all interrupters operated continually, synchronisation would be achieved by starting them all at the same time (the pre-set time). However, since the interrupters are to be turned on and off, a calculation is made whenever they are turned on to determine the point in the cycle they would have reached if they always had been turned on (from the pre-set time referred to above), and the signal is started at the appropriate time in the cycle corresponding to that which would have occurred if the interrupter had been on all the time.
Another aspect, which applies to the modulation of any single pipe, as well as modulations applied to multiple pipe as discussed above, is that the modulation is preferably an irregular modulation, rather than simple regular square-wave modulation. This makes the stray currents being investigated easier to distinguish from other currents in the vicinity of the pipe being investigated.
Since the currents on one pipe may be due not only to the modulation applied to that pipe, but also to the modulation applied to other pipes, it is important that the resulting field generated by the combinations of those modulations must be such that the individual modulations should be separately identifiable. It is possible to do this by modulating at different frequencies, but this has high power requirements. Therefore, in a further aspect

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