Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
1998-02-09
2001-07-17
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S527000
Reexamination Certificate
active
06262578
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for detecting and locating current leakage paths along the length of an electrically conducting elongate member, in particular pipes or cables. This invention also relates to a method for detecting oscillatory motion of such an elongate member under water.
2. Description of the Related Art
The type of pipe or cable to which the method of the present invention can be applied is one which is electrically conducting along its length and in its ideal state is electrically insulated from its surroundings. However, in the event of corrosion along the length of such pipes, electric current leakage paths are formed between the pipe and its surroundings. In order to detect corrosion, or other forms of damage which result in the formation of leakage paths along the length of a pipe, a typical method involves connecting a section of the pipe to a Wheatstone bridge type arrangement thereby enabling its resistance to be measured and using this resistance to determine whether there are any leakage paths formed between the pipe and its surroundings. However, this has the disadvantage that whilst an electric current leakage path can be detected, the location of that leakage path can be difficult to determine. In addition, this method requires that the pipe is probed extensively along its length and this can be time consuming and when the pipe is underground or under water, very expensive. Another method of detecting corrosion is to merely visually inspect the pipes which again is very expensive.
DE-A-3727224 discloses a measuring system for locating leakage faults on cables in which a pair of parallel conductors sheathed in an insulating material are connected into a bridge circuit and the location of a fault in the insulating material that leads to a leakage is determined by balancing the bridge circuit.
SUMMARY OF THE INVENTION
An object of the present invention is to enable detection and location of electric current leakage paths along the length of a single electrically conducting elongate member without having to probe the member extensively, thereby facilitating the location of such leakage paths and reducing the overall cost of the operation.
According to one aspect of this invention there is provided a method for detecting and locating electric current leakage paths along the length of an electrically conducting elongate member which in its ideal state is electrically insulated from its surroundings, said leakage paths and said surroundings being such that current can leak from the elongate member to its surroundings, wherein said method comprises the steps of establishing at least one electric node along the length of the elongate member by applying a respective variable electric signal to each end of the elongate member so that electric current is driven along the elongate member, said node being where the net difference in potential between the elongate member and its surroundings is substantially zero, varying the position of the said at least one node along the elongate member by varying the applied signals, monitoring the impedance of the elongate member as the position of the node is varied, identifying the position of the node at which the impedance has a local minimum and using that identified position as an indication of the position at which there is a current leakage path.
The node may be established by applying a respective electric signal to each end of the elongate member so that current is driven along the elongate member. The position of the node may be varied by varying the signals applied to the ends of the elongate member.
The preferred method of identifying the position of the node involves modeling the potential distribution along the length of the elongate member and using this to estimate the position of the node. The model may be improved using iterative techniques by estimating the impedance of the pipe using the model, comparing the estimated value with the measured impedance, and, if necessary, updating the model, this being repeated until the value of the impedance estimated using the model is substantially in agreement with the measured value. The model can then be used to estimate the position of the node.
According to another aspect of the invention there is provided apparatus for detecting and locating electric current leakage paths along the length of an electrically conducting elongate member which in its ideal condition is substantially insulated from its surroundings, the leakage paths and the surroundings being such that current can leak from the said elongate member to its surroundings, in which the apparatus comprises variable electrical signal supply means for connection to the elongate member, wherein said apparatus also comprises means for monitoring the impedance of the elongate member, said variable supply means is operable to drive current along the elongate member so that at least one node is established at a position along the length of the elongate member, the node being where the net change in potential between the elongate member and its surroundings is substantially zero, and wherein the position of the node is varied by varying the current driven by the supply means, the arrangement being such that in use the position of the node is varied and the resulting impedance of the elongate member is monitored, the position of the node at which the impedance has a local minimum indicating the position at which there is a current leakage path.
The supply means may be connected to the elongate member through connection points provided for a cathodic protection system. The position of the node may be varied by varying the potential applied to the ends of the elongate member. The supply means may be DC or AC voltage supplies. With an AC supply, there may be more than one node formed along the length of the elongate member, depending on the frequency of the voltage source.
For DC measurements, the means for monitoring current may be an ammeter. For AC measurements the means for monitoring current may be an ammeter and a shunt resistor with an oscilloscope, or other means for data acquisition, to determine the phase of the measured current with respect to the applied potential.
The location of the current leakage paths may be determined by making a coarse estimate of the position of the leakage path and then repeating the steps of the method on a portion of the elongate member in which the coarse estimate indicates there is a leakage path, to thereby provide a more accurate estimate. This process may be repeated several times.
Where there is more than one leakage path, there will be a series of local minima in the graph of impedance of the elongate member against the position of the node, each one corresponding to a leakage path.
The electrically conducting elongate member may be a pipe or a cable.
In the case of underwater pipes, there is a particular problem when the sea bed underneath a pipe is eroded so that sections of the pipe span hollows in the sea basin. When this happens, the motion of the tide may cause oscillations of the order of a few tenths of the diameter of the pipe, initially in the direction of the tidal flow. However, if the oscillations are allowed to progress then the span may increase. If concrete weight coating is fitted to the pipe this may be shed thereby resulting in the pipe oscillating in a direction perpendicular to the tidal flow with an amplitude of one or more diameters. This can cause severe structural damage and has significant safety implications.
Underwater pipes are checked typically by routinely scanning the pipe with a camera and when necessary identifying the position of regions of the pipe which are spanning hollows in the sea basin. When a region of the pipe is identified as spanning a hollow, remedial work is required to stabilize the pipe. Usually this is achieved by filling the hollow with rock or using so-called concrete mattresses. This is an extremely costly process, particularly in relation to t
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Flight Refuelling Limited
Hollington Jermele M.
Metjahic Safet
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