Method and system for measuring a condition of a structure

A method for measuring a condition of a structure including: moving a measurement probe through a tunnel and measuring the condition of the structure with the measurement probe wherein the tunnel is in electrical potential measurement proximity to the structure.

BACKGROUND

This disclosure relates to testing and evaluating cathodic protection effectiveness on buried or submerged metallic structures, and specifically to the evaluation of corrosion protection levels on pipelines, tanks, piles, and piping systems.

Buried or submerged metallic structures, such as pipelines, tanks, and distribution piping systems are usually coated with non-conductive material to prevent corrosion. If any corrosion occurs in any uncoated areas of the structure adverse effects may occur which will reduce the effective life of the structure. To prevent such adverse effects, most pipelines are provided with corrosion protection comprising cathodic protection, in addition to the non-conductive coating. Cathodic protection provides corrosion protection to any bare metal areas exposed to soil due to coating defects, by causing direct current to flow from the soil into the structure, thereby polarizing the structure as a cathode. The required direct current output of the cathodic protection system is reduced to manageable levels by the coating, which substantially reduces the bare metal area of the structure exposed to soil.

The objective of the cathodic protection is to shift the potential of the structure to a more negative potential. The potential shift must be large enough to mitigate structure corrosion. Potential criteria have been developed by the National Association of Corrosion Engineers (RP0169-92) to provide guidance for determination of safe cathodic protection levels to mitigate corrosion. One of the criteria is based on a single value of potential, measured with a regular high-impedance voltmeter with the cathodic protection system operating. The potential measured with the cathodic protection system operating are identified as “on” potential readings. This measurement is very easy to take, however, it requires a consideration or elimination of voltage drops in resistive materials between the reference electrode and the structure. Another criterion is based on achieving the same value of structure potential immediately after interrupting the operation of the cathodic protection system, and is referred to as an “off” potential reading. A further criterion is based on a single value of the structure potential decay, which is measured from the “off” potential, leaving the cathodic protection system disconnected for several hours or days.

There is no easy and practical method to determine the voltage drop when the “on” potential reading is taken. Therefore, the “off” potential readings, which eliminate the soil voltage drop measured immediately after interrupting the cathodic protection system from the structure, are often used for monitoring corrosion protection levels. However, the “off” potential readings are much more difficult to take than the “on” readings. The interpretation of the “off” potential readings is also much more complex. The “off” potential readings often require use of synchronized current interrupters, fast reacting recorders, oscilloscopes, or wave analyzers. The “off” potential readings after cathodic protection is interrupted can be adversely affected by long-cell currents in the structure caused by currents flowing between more polarized sections of the structure being protected, which occur in the proximity of the rectifiers, and less polarized sections of the structure being protected, which typically occur at sections of the structure that are generally equidistant between sequential rectifiers. Also, the “off” potential readings are often adversely affected by inductive or capacitive voltage spikes, caused by cathodic protection interruption. If the “off” potential reading is taken some time after the spike, some of the polarization is lost and the reading could be therefore invalid.

Meeting the “off” potential criterion often requires that more cathodic protection current be applied than is required to meet the “on” potential criterion, resulting in possible overprotecting of the structure, faster deterioration of the coating, and a higher probability of hydrogen evolution and steel embrittlement within the structure. The “off” potential measurements are not valid in areas where substantial uninterruptable direct currents are flowing through the soil into or from the structure, polarizing the structure. Such conditions exist, for instance in stray current areas, where the structure is affected by stray currents from electric railroads, from cathodic protection systems on foreign structures, and in areas with telluric (earth) currents naturally induced by fluctuations in the earth's magnetic field. Also, the “off” potential readings cannot be used on structures with distributed galvanic anodes directly connected to the structure.

To eliminate some of the disadvantages of the “off” potential readings on the structure, different cathodic protection test probes and coupon/access tube assemblies have been proposed. The probes consist of a short steel pipe section as a coupon, a plastic tube filled with conductive backfill functioning as an electrolytic “salt bridge,” and a porous ceramic plug glued to the end of the plastic tube, representing a potential sensing area. A coupon is a metal electrode, which simulates an area in which the non-conductive coating is not present on the structure and provides a reference from which the cathodic protection system can be measured. Coupons are made from the same or similar metal as the structure, and are electrically connected to the structure to receive cathodic protection. Cathodic protection probes with cylindrical coupons, now commercially available, have been described in Material Performance, published by National Association of Corrosion Engineers, Houston, Tex., June 1996, pp. 21–24.

Cathodic protection probes with cylindrical coupons are difficult to use when the structure to be tested is located beneath a surface obstruction that prevents access to the soil from the ground surface without interactions with the surface obstruction. Conducting periodic inspections of structures located beneath surface obstructions, such as paved surfaces, using currently available methods is costly and destructive.

SUMMARY

Embodiments of the invention include a method for measuring a condition of a structure including: moving a measurement probe through a tunnel and measuring the condition of the structure with the measurement probe wherein the tunnel is in electrical potential measurement proximity to the structure.

Embodiments of the invention also include a system for measuring a condition of a structure including: a tunnel in electrical potential measurement proximity to the structure; a measurement probe receivable inside the tunnel; and a movement system operable for moving the measurement probe through the tunnel wherein said measurement probe measures one or more conditions of the structure.

Further embodiments of the invention include a system for measuring a condition of a structure including: means for moving a measurement probe through a tunnel, wherein the tunnel is in electrical potential measurement proximity to the structure; and means for measuring the condition of the structure using known electrical techniques.

DETAILED DESCRIPTION

Illustrated inFIG. 1is an exemplary embodiment of a system for measuring a condition of a structure10. In one embodiment, which is illustrated inFIG. 1, the system for measuring the condition of the structure10includes a support device, which in this embodiment is illustrated as a conduit12, in electrical potential measurement proximity to a structure, which in this embodiment is illustrated as a pipeline14, and a measurement probe16designed to fit inside the conduit12. The system for measuring a condition of a structure10also includes a tunnel18that is created, and may be excavated, using commonly known methods including directional drilling techniques. The tunnel18is separate from the pipeline14as shown inFIG. 1, and may be any opening suitable to allow use of the probe described herein. After creation of the tunnel18, the conduit12is positioned inside of the tunnel18. The measurement probe16may be inserted into the conduit12at any time. After insertion into the conduit12, the measurement probe16is moved through the conduit12. As the measurement probe16traverses the conduit12, measurements of the conditions of the structure, such as cathodic protection potentials, are taken at specified intervals. In an exemplary embodiment, the measurement probe16may be a corrosion measurement half-cell, which generally is any device that contains an electrode and a surrounding electrolyte, and the conduit12maybe a plastic conduit or any other conduit that is constructed of a suitable electrically permeable membrane.

The conduit12may be pulled or pushed through the tunnel18such that either or both ends of the conduit12are exposed above the ground surface20or at least located to allow for easy access to the conduit12. In an exemplary embodiment, the exposed ends of the conduit12are fitted with removable end caps to facilitate periodic inspections. Optionally, the conduit12can be filled with water and the measurement probe16will be pulled or pushed through the conduit12. The conduit12ensures that tunnel18will remain intact to allow for periodic inspections without destruction or interference with the surface obstructions30.

In another exemplary embodiment, the system for measuring a condition of a structure10includes the measurement probe16and the tunnel18in electrical potential measurement proximity to a structure, such as the pipeline14. After creation of the tunnel18, the measurement probe16may be inserted into and moved through the tunnel18. While moving through the tunnel18the measurement probe16takes measurements of the conditions of the structure. Optionally, a support device may be positioned in the tunnel18to maintain the structural integrity of the tunnel18after it is created, thereby facilitating periodic inspections.

In an alternative exemplary embodiment, the system for measuring a condition of a structure10includes the measurement probe16that is coupled to an excavation tool. The excavation tool is used to create the tunnel18in electrical potential measurement proximity to a structure, such as the pipeline14. After creation of tunnel18the measurement probe16and coupled excavation tool are retracted through tunnel18. The measurement probe16may take measurements of the condition of the structure either during creation of the tunnel18or during retraction of the measurement probe16.

FIG. 2depicts an exemplary embodiment of a system for measuring a condition of a structure10where the measurement probe16is shown inside the conduit12. The measurement probe16may be connected to a measurement device located above the ground surface20by a measurement wire22. The measurement wire22communicates the properties measured by the measurement probe16to the measurement device. A tether cable24will also be connected to the measurement probe16which will facilitate retracting the measurement probe16through the conduit12. Additionally, the tether cable24allows an operator to retrieve the measurement probe16in the event that the measurement probe16becomes stuck in the conduit12.

After being inserted into the conduit12the measurement probe16will be moved through the conduit12using any suitable movement means including, but not limited to, gas or liquid propulsion. In an exemplary embodiment, the measurement probe16is moved through the conduit12by a movement system that employs any suitable form of propulsion. The measurement probe16is then retracted through the conduit12at a controlled rate using the tether cable24. The measurement probe16measures the condition of the pipeline14at specific intervals while it is being retracted. In another exemplary embodiment, the measurement probe16is moved through the conduit12by a movement system that employs any suitable form of propulsion and the measurement probe16measures the condition of the pipeline14while it is being moved through the conduit12. In an alternative exemplary embodiment, the measurement probe16is moved through the conduit12at a controlled rate and the measurement probe16measures the condition of the pipeline14while it is being moved through the conduit12. In yet another exemplary embodiment, the measurement probe16is self-motivated and moves through the conduit12while measuring the condition of the pipeline14.

Turning now toFIG. 3, which depicts another exemplary embodiment of the system for measuring the condition of the structure10where the structure shown is the pipeline14, which is located at typically three feet below the surface obstruction30. As shown inFIG. 3, the tunnel18should be created such that the conduit12, once inserted in the tunnel18, is located approximately within ten feet radially from the pipeline14. Additionally, the tunnel18should be created such that the conduit12, once inserted in the tunnel18, is approximately parallel to the pipeline14. The proper positioning of the tunnel18with respect to the pipeline14ensures that the measurement probe16is able to take accurate measurements of the condition of the pipeline14.

In an exemplary embodiment, the condition of a pipeline14, specifically the presence of external corrosion, is indirectly measured with a measurement probe16using known electrical techniques. Before measuring the condition of the pipeline14, a tunnel18is created in electrical potential measurement proximity to the pipeline14to be inspected, in one embodiment, within 10 feet radially. After creating the tunnel18, a conduit12constructed of any suitable electrically permeable material is inserted into the tunnel18. The conduit12may be inserted such that one or both ends of the conduit12remain above the ground surface20, or at least located to allow for easy access to the conduit12, to facilitate periodic inspections. Optionally, the exposed ends of the conduit12may be fitted with removable end caps. After the conduit12has been inserted into the tunnel18, a measurement probe16is inserted in one end of the conduit12and is moved through the conduit12using any suitable movement means including, but not limited to, liquid or gas propulsion. In an alternative exemplary embodiment, the measurement probe16is self-motivated.

The measurement probe16may be connected to a measurement device located above the ground surface20by a measurement wire22. The measurement wire22is used to communicate measured values from the measurement probe16to the measurement device. Additionally, the measurement probe16may be attached to a tether cable24that facilitates retracting and removing the measurement probe16from the conduit12. The measurement probe16may take measurements of the condition of the pipeline14continuously or at specific intervals either while being moved through the conduit12or while being retracted through the conduit12. In an alternative exemplary embodiment, the measurement probe16may include a data storage device to record the values that the measurement probe16reads from pipeline14.

The system for measuring a condition of a structure10greatly increases the efficiency of periodically testing the conditions of a structure, such as the pipeline14. Once the system for measuring a condition of a structure10has been used to measure the condition of a specific pipeline14, the conduit12may be sealed with one or more removable end caps. Subsequent inspections of the pipeline14can be preformed by removing the end caps from the conduit12, placing the measurement probe16into the conduit12, moving the measurement probe16through the conduit12, measuring the condition of the pipeline14, and retracting the measurement probe16through the conduit12. This process will reduce the time required for subsequent inspections and will eliminate disturbance of surface structures.