Impressed current cathodic protection

Abstract

An impressed current cathodic protection arrangement includes an elongate metallic structure to be protected and cathodic protection apparatus which comprises a DC power supply and an anode. One terminal of the power supply is connected to the structure at a connection point and another terminal of the power supply is connected to the anode. The arrangement includes monitoring apparatus for monitoring effectiveness of cathodic protection provided by the cathodic protection apparatus by determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point.

1 . An impressed current cathodic protection arrangement comprising:
 an elongate metallic structure to be protected,   cathodic protection apparatus which comprises a DC power supply and an anode, one terminal of the power supply being connected to the structure at a connection point and another terminal of the power supply being connected to the anode, and   monitoring apparatus for monitoring effectiveness of cathodic protection provided by the cathodic protection apparatus by determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point.   
     
     
         2 . An impressed current cathodic protection arrangement according to  claim 1  in which, the monitoring apparatus comprises equipment for determining the electrical potential of the structure relative to surroundings at a reference point on the structure to be protected which reference point is spaced from said one location. 
     
     
         3 . An impressed current cathodic protection arrangement according to  claim 2  in which the reference point is the connection point. 
     
     
         4 . An impressed current cathodic protection arrangement according to  claim 2  in which the equipment for determining the electrical potential of the structure relative to surroundings at the reference point comprises a second structure which is spaced from the structure to be protected and the anode, and equipment for measuring the potential difference between the second structure and the structure to be protected to allow determination of the electrical potential of the structure relative to surroundings at the reference point. 
     
     
         5 . An impressed current cathodic protection arrangement according to  claim 2  in which the equipment for determining the electrical potential of the structure relative to surroundings at the reference point comprises a memory in which is stored a parameter which is representative of the impedance of the elongate structure as seen from the reference point, equipment for measuring the current supplied to the structure at the reference point and equipment for calculating the electrical potential of the structure relative to surroundings at the reference point using said parameter and the measured current. 
     
     
         6 . An impressed current cathodic protection arrangement according to  claim 1  in which the monitoring apparatus comprises a central station. 
     
     
         7 . An impressed current cathodic protection arrangement according to  claim 6  in which the central station comprises part or all of the equipment for determining the electrical potential of the structure relative to surroundings at the reference point. 
     
     
         8 . An impressed current cathodic protection arrangement according to  claim 6  in which the central station is arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point in dependence on one or more inputs. 
     
     
         9 . An impressed current cathodic protection arrangement according to  claim 1  where the elongate structure to be protected comprises tubing and the monitoring apparatus comprises a tool which is arranged to be located within the tubing. 
     
     
         10 . An impressed current cathodic protection arrangement according to  claim 9  in which the tool comprises a spaced pair of contacts for contacting with an internal surface of the tubing at axially spaced locations, and a control unit comprising a sensor for measuring the potential difference between the contacts. 
     
     
         11 . An impressed current cathodic protection arrangement according to  claim 10  in which the control unit comprises a transmitter for transmitting, to the central station, data indicating a measured potential difference between the contacts. 
     
     
         12 . An impressed current cathodic protection arrangement according to  claim 11  in which the tool is arranged to transmit said data along the elongate metallic structure. 
     
     
         13 . An impressed current cathodic protection arrangement according to  claim 10  in which the monitoring apparatus is arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point in dependence on the potential difference measured between the spaced contacts of the tool as measured by the sensor of the tool. 
     
     
         14 . An impressed current cathodic protection arrangement according to  claim 9  in which the tool is arranged for movement within the tubing. 
     
     
         15 . An impressed current cathodic protection arrangement according to  claim 14  in which the monitoring apparatus is arranged to use the tool to take potential difference measurements one of continuously, and at intervals as it is moved along the tubing. 
     
     
         16 . An impressed current cathodic protection arrangement according to  claim 1  in which the monitoring apparatus comprises a memory in which is stored a model based on parameters specific to the elongate structure and its surroundings which model predicts how the absolute value of a potential applied to the structure at the reference point will decay along the length of the structure away from the reference point. 
     
     
         17 . An impressed current cathodic protection arrangement according to  claim 16  in which the monitoring apparatus is arranged to use the model based on parameters specific to the elongate structure and its surroundings in determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point. 
     
     
         18 . An impressed current cathodic protection arrangement according to  claim 2  in which the equipment for determining the electrical potential of the structure relative to surroundings at the reference point comprises a memory in which is stored a model based on parameters specific to the elongate structure and its surroundings which model predicts how the absolute value of a potential applied to the structure at the reference point will decay along the length of the structure away from the reference point. 
     
     
         19 . An impressed current cathodic protection arrangement according to  claim 10  in which the monitoring apparatus is arranged to determine the electrical potential of the structure relative to surroundings at at least one location which is spaced from said connection point in dependence on:
 i) the electrical potential of the structure relative to surroundings at said reference point as determined by the equipment for determining the electrical potential of the structure relative to surroundings at the reference point; 
 and in dependence on at least one of: 
 ii) the potential difference measured between the spaced contacts of the tool as measured by the sensor of the tool; and 
 iii) the model based on parameters specific to the elongate structure and its surroundings. 
 
     
     
         20 . An impressed current cathodic protection arrangement according to  claim 1  in which the monitoring apparatus is arranged to indicate when the magnitude of the potential of the structure relative to the surroundings falls below a threshold level at at least one point along the length of the structure. 
     
     
         21 . An impressed current cathodic protection arrangement according to  claim 1  in which the monitoring apparatus is arranged to indicate at what distance from the reference point the potential of the structure relative to the surroundings falls below a threshold level. 
     
     
         22 . An impressed current cathodic protection arrangement according to  claim 1  in which the monitoring apparatus is arranged to indicate the potential of the structure relative to the surroundings as a function of distance away from the reference point. 
     
     
         23 . A well installation comprising an impressed current cathodic protection arrangement according to  claim 1 . 
     
     
         24 . A method of monitoring an impressed current cathodic protection arrangement, the arrangement comprising an elongate metallic structure to be protected, and cathodic protection apparatus which comprises a DC power supply and an anode, one terminal of the power supply being connected to the structure at a connection point and another terminal of the power supply being connected to the anode, the method comprising the step of:
 monitoring effectiveness of the cathodic protection provided by the cathodic protection apparatus by determining the electrical potential of the structure relative to surroundings at at least one location which is spaced from the connection point.