Abstract:
An apparatus and method for detecting and locating electric current leakage paths along the length of an electrically conducting elongate member. The apparatus has a variable supply device and a meter device for monitoring the impedance of the member ( 9, 11 ), the supply device being operable to drive current in opposite directions along the member, so that a movable node can be established, the node being where the net change in potential between the member and its surroundings is substantially zero. When the apparatus is in use, the position of the node is varied by varying the current driven along the member and the resultant impedance is monitored by the monitoring meter device, whereby the position of the node at which there is a local minimum in the impedance indicates the position of a current leakage path.

Description:
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 the detection of the regions of the pipe which span hollows in the sea basin. 
     Another object of this invention is to provide an improved method of detecting oscillations of an underwater pipe. 
     According to yet another aspect of this invention there is provided a method of detecting oscillatory motion of an underwater electrically conducting elongate member comprising monitoring the impedance of the elongate member as a function of time, comparing the measured impedance with an expected impedance and using this information to determine whether or not the elongate member is oscillating. 
     The elongate member may be a pipeline or a cable. 
     The expected impedance may be calculated using a theoretical model of the impedance of the pipe, or may be determined from the results of the previous monitoring of the impedance of the pipe as a function of time. 
     The method is preferably utilized as part of a continuous monitoring process, throughout the working lifetime of the pipe. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A method of detecting and locating a current leakage path on a pipe, and method of detecting oscillations of an underwater pipe will now be described by way of example with reference to the accompanying drawings of which: 
     FIG. 1 is a longitudinal section of a length of pipe; 
     FIG. 2 is a representation of the pipe with a single corrosion point; 
     FIG. 3 is an equivalent circuit for one arrangement for carrying out the present invention; 
     FIG. 4 is an equivalent circuit for another arrangement for carrying out the present invention; 
     FIG. 5 is a graph of voltage against length of pipe for the arrangement shown in FIG. 3; 
     FIG. 6 is a graph of voltage against length of pipe for the arrangement shown in FIG. 4; 
     FIG. 7 is a representation of a pipe with i (i=1, 2, 3 . . . ) leakage paths; and 
     FIG. 8 is an underwater pipe which is resting on the sea bed, part of which has been eroded away. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method of detecting and locating current leakage paths in which the present invention is embodied may be applied to an electrically conducting pipe, or a cable, or any other electrically conducting elongate member, which, as is shown in FIG. 1, is provided with an electrical insulator  4  which in its ideal state electrically insulates the pipe, or cable, or elongate member from its surroundings. The surroundings should be able to act as an electrical ground, so pipes  1  underground or under water are suitable for the application of the method. Most such pipes  1  are provided with electrical connection points  3  to allow the pipe  1  to be tested and/or to allow a cathodic protection system to be utilized. 
     Corrosion or other forms of damage, for example piercing of the insulator  4 , along the length of the pipe  1 , may result in electric current leakage paths being formed between the electrically conducting pipe  1  and its surroundings. For the sake of simplicity, firstly consider a pipe  1  with a single point of corrosion  2  which can be represented as a T-junction of electrical impedances, as shown in FIG. 2, in which Z 1  and Z 2  represent the impedance of the pipe  1  in regions adjacent the corrosion  2 , and Z c  represents the corrosion impedance, which is in contact with the surroundings. 
     To detect and locate corrosion, two voltage supplies are connected one at each end of the pipe  1 , with each voltage supply being connected in series with an ammeter. Equivalent circuits for this arrangement are shown in FIGS. 3 and 4 for DC and AC voltage supplies respectively. The voltage applied to the ends of the section of the pipe under examination are selected so that a node is formed at a position along the pipe, the node being where the net difference in potential between the pipe and ground of the circuit is zero volts. For the sake of simplicity, the method of the present invention will be described assuming that the ground is at substantially the same potential as the potential of the surroundings. However, it will be understood that there may be a potential difference between the surroundings and the pipe, which would have to be taken into account in any calculations. 
     Using DC supplies  8  and  10  as shown in FIG. 3, a node can be established by connecting a positive voltage supply  8  to one end of the pipe and a negative voltage supply  10  to the other end with both supplies  8  and  10  having a common ground, at substantially the same potential as the surroundings. On an ideal pipe, the voltage distribution is linear along the length of the pipe  1 , as shown in FIG.  5 . The node  13  is the point at which the voltage is at substantially zero volts. 
     The node  13  can be moved along the length of the pipe  1  by varying the voltage supplies  8  and  10 . In FIG. 5 the lines  14 A and  15 A are the voltage distribution plots for two different sets of voltages applied to the ends of the pipe  1 , each set of voltages resulting in a node  13  being established at a different position along the length of the pipe  1 . When the position of the node  13  coincides with the position of the corrosion  2 , which forms a leakage path to the surroundings, that is the common ground potential, the potential difference across the impedance Z c  is 0 volts and so no current flows in the leakage path. Therefore, the current measured at each end of the pipe  1  by ammeters  9  and  11  should be substantially the same. When the position of the node  13  does not coincide with the position of the corrosion  2 , the potential difference across the impedance Z c  is finite and so current flows in the leakage path through Z c , so that the current measured at each end of the pipe by ammeters  9  and  13  is not the same. Hence, the impedance of the pipe  1  should be when the position of the node  13  coincides with the position of the corrosion  2  than it is when the position of the node does not coincide with the position of the corrosion  2 . Therefore, by modeling the position of the node  13  as a function of the voltage supplied at either end of the pipe  1  and measuring the impedance of the pipe  1  as a function of the position of the node  13 , the position of the corrosion  2  can be identified from the minima in the graph of impedance of the pipe against the position of the node  13 . 
     When there is more than one leakage path resulting in a series of Z ci  impedances to ground (where i=1,2,3 . . . ) the pipe can be represented as a distributed network of T-junctions, as shown in FIG.  7 . The position of each corrosion point  2  can be estimated from the position of local minima on a graph of the impedance measured from the ends of the pipe  1  against position of the node  13 . It would be necessary under these circumstances to use iterative techniques to accurately determine the position of corrosion  2 . 
     If there are no electric current leakage paths to the surroundings then the impedance of the pipe  1  should be substantially constant, as a function of the position of the node  13  along the pipe  1 . 
     If a cathodic protection system is in use along the length of the pipe, then it is necessary to take the voltage applied by this system into account when selecting the DC voltages to be applied at the ends of the pipes. 
     To determine the position of electric current leakage paths using AC voltage supplies, techniques similar to those used for DC voltage supplies are employed. The circuit used is shown in FIG.  4 . 
     Using AC voltage supplies  16  and  17 , a node  13  can be established by connecting a first AC voltage supply  16  to one end of the pipe and a second AC voltage supply  17  to the other end of the pipe and applying voltages selected so that an equal current flows from each end of the pipe but in opposite directions, the voltages supplied are phase locked to the same frequency, with an analytically determined phase and amplitude. This results in an electrical standing wave forming along the length of the pipe. On an ideal pipe  1 , the voltage distribution along the length of the pipe as a result of applying equal AC voltages to opposite ends of the pipe  1  is represented by a standing wave, as shown in FIG.  6 . The node  13  is the point at which the voltage is at substantially zero volts. In this case the frequency of the AC voltage supply is such that only one node  13  is formed. In order to ensure that the supplies are of suitable phase, frequency and amplitude, the two supplies  16  and  17  should be phase locked together and the amplitude of each should be carefully controlled. 
     The node  13  can be moved along the length of the pipe  1  by varying the AC voltage supplies  16  and  17 , for example by varying the phase of the signals supplied. In FIG. 6, lines  14 B and  15 B are voltage distribution plots for two different sets of AC voltages applied to the ends of the pipe  1 , each set of voltages having a different phase, thereby resulting in a node  13  being established at a different position along the length of the pipe  1 . The techniques used to determine the position of electric current leakage paths are similar to those described for the DC supplies. Again, the position of the node  13  is moved along the length of the pipe  1  and the corresponding impedance between the ends of the pipe  1  is measured. 
     When AC supplies are employed, the effects of a cathodic protection system are less significant than when DC supplies are employed. 
     If more than one node  13  is formed along the length of the pipe iterative techniques may be required to determine the position of any electric current leakage paths. Although, if practicably possible, the frequency will be chosen to avoid multiple nodes. 
     When using AC supplies it may be necessary to make corrections to the estimate of the position of the node  13  to take into account any attenuation of the signal along the length of the pipe  1 . This is particularly of significance for underwater pipelines as the attenuation increases very significantly with frequency. 
     It is proposed that the model of the pipe is updated and verified by using it to calculate a theoretical value of the impedance and comparing this with the measured value. When a discrepancy occurs the model is altered to account for this. This is repeated in an iterative manner under software control until there is convergence. 
     In order to accurately determine the location of any leakage paths along the length of a pipe using the method described above, it may be necessary to make a coarse estimate of the located of the leakage paths and then probe the pipe within the region indicated by the initial coarse estimate to make a more accurate second estimate. This may need to be repeated several times. 
     FIG. 8 shows a pipe  23  which is spanning a hollow  25  in the sea bed  27 . The length of the section of the pipe spanning the hollow can be of the order of 100 meters. Hence, any oscillation of such a section due to the motion of the tide can cause severe structural damage to the pipe. The motion of the tide may cause the section of the pipe to oscillate either in the direction  29  of the tide or in a direction  31  which is perpendicular to the direction  29  of the tide. 
     When a metal pipe is immersed in water the surface of the pipe is covered with a fixed layer of positive charge. This layer attracts mobile ions in the water which move towards the pipe&#39;s surface to form a diffuse mobile layer. The diffuse mobile layer is held near the bar by electrostatic attraction. The fixed layer charge and the diffuse mobile layer are together an electrical double layer which can act as a capacitor with a fixed plate and a moveable plate. This capacitor can be charged and discharged like any other capacitor. 
     The impedance of the double layer capacitor can be measured by connecting an AC supply between the pipe and ground. The application of an AC voltage to the pipe causes the mobile layer to move from a minimum radial distance from the pipe to a maximum radial distance from the pipe, as the voltage swings from its maximum to its minimum value. Hence, the mobile layer oscillates towards and away from the pipe. 
     The diffuse mobile layer is also affected by the motion of the tide which can cause it to move towards and away from the fixed layer of the pipe. The capacitance of the double layer is inversely proportional to the distance between the fixed layer of charge and the mobile layer of charge. Hence, the impedance of the double layer capacitor varies depending on the tidal motion. This motion can be characterized by continuously monitoring the impedance of the pipe. If any region of the pipe is oscillating freely, then the impedance should vary as a function of time in a manner which cannot be accounted for solely by taking into account the motion of the tide. Hence, by measuring the impedance of the pipe as a function of time, oscillations of that pipe can be detected. 
     The detection of the oscillation of pipes when oscillating in the direction of tidal flow is highly advantageous as at this stage the damage may not be significant. The variation in the impedance as a result of such oscillations may be very small. Therefore, the impedance should be measured in a very narrow band width. 
     The measurement of the impedance of the pipe as a function of time may be used to calculate the frequency of oscillation of the oscillating section of pipe, the mode of that oscillation and the length of the section of pipe which is so oscillating. 
     Ideally the impedance of the pipe should be measured as part of a continuous monitoring process, throughout the working lifetime of a pipe. Continuous monitoring increases the accuracy with which oscillations can be detected and improves on the reliability of the detection. 
     The detection of oscillations of a pipe, by monitoring its impedance and comparing the measured value with an expected value, enables the elimination of routine visual inspection of the pipe and so greatly reduces the cost of maintaining the pipe. It restricts the need for inspection of the pipe to when oscillation of the pipe is detected. 
     It is envisaged that the impedance data may also be used to locate the position of the sections of pipe which are oscillating, thereby eliminating the need for inspection of the pipe altogether.