ILI tool enhances CP monitoring

Identifying problems with and monitoring the effectiveness of a pipeline’s cathodic protection (CP) system can be difficult, expensive, and time consuming, especially when the pipe is located in difficult-to-access areas. If the CP is monitored via an in-line inspection tool (smart pig), then the protection status can be confirmed or problems identified regardless of the pipeline location, accessibility, or condition of the right-of-way. 

An in-line inspection tool capable of reading and recording the magnitude and polarity of current supplied by cathodic protection has been developed and tested in both crude oil and refined product pipelines. The results show that CP currents can be quickly, accurately and efficiently gathered without access to the outside surface of the pipe. For difficult-to-access areas, CPCM Cathodic Protection In-Line Inspection Services provide for a reliable, cost-effective, time-saving way to monitor, validate, or trouble shoot a pipeline’s cathodic protection system.

From the beginning of cathodic protection, it was understood that the flow of direct electrical current was the key to protecting a pipeline from corrosion.  As A. W. Peabody wrote in his classic book, “Pipeline Corrosion Control,” the basic theory of cathodic protection involves the use of
current to protect against corrosion. He wrote that “When the amount of current
is adjusted properly, there will be a net
current flow onto the pipe surface (at all points). The entire surface then will be cathodic and the protection complete.”

Measuring CP current

Corrosion professionals have long valued the concept of a pig that could measure CP current in the pipe, and several attempts were made to develop a device similar to downhole wire-line tools that measure current in well casings. However, practical
and technical issues proved too difficult
to solve with existing technology, and
those attempts were abandoned. 

Development of CP practice centered around the measurement of the pipe-to-soil potential produced by the CP current because they were measurable, at least in theory, over the entire pipe surface using available technology. This state of affairs lasted until recently when a joint project between Shell Global Solutions, Baker Hughes Pipeline Management Group and the U.S. Department of Transportation (DOT) overcame these challenges with
the development of the CPCM In-Line Inspection Tool. With this tool, the practical collection of accurate CP current information is available over an entire pipeline segment for the first time.

The new tool provides two advantages to the pipeline operator:

•  Measures CP current direction and magnitude in the pipeline to supplement and extend the pipe-to-soil potential data already available.

•  Allows the pipeline operator to easily gather CP information regardless of right-of-way obstacles or obstructions, IR drop issues, or stray current interference.

While interpretation of CPCM data continues to be refined, work to date has shown that the tool generates much useful and accurate information – information that is unobtainable from a potential survey. The tools can quickly and accurately determine current density applied continuously along the pipeline as well as the location and magnitude of current leaving (rectifiers, bonds, anodes) or entering (bonds, shorts) by metallic connections. 

During cathodic protection, steel is polarized by the effect of a flow of direct current, reducing the anodic area of the surface where corrosion occurs. If the current flow and polarization is great enough, the current will enter the steel at all points, eliminating all local anodes and preventing corrosion. The relationship between current, potential, and resistance is represented by the well-known equation E = IR. From this relationship, it is easy to illustrate that polarization and current density provide essentially equal information. Knowing the value of one and the specific coating resistance at any point, it is possible to calculate the other. 

Based on this information, the current survey provided by the new tool provides the same amount and type of information as a close interval survey (CIS). The only advantage to the potential survey is that well-established potential criteria are available to determine protection, while current density criteria are not. 

Development of suitable current density criteria to prove protection would certainly be beneficial, and this project is currently planned. Used together, current and potential measurements would allow for a complete characterization of the CP system and a complete understanding of the performance of the coating system.

Benefits of CP current monitoring

While more work is needed to fully exploit all the capabilities of the inspection tool and the resulting measurements, monitoring CP current has a number of immediate practical advantages over external potential measurements:

•  Right-of-way access issues are eliminated: Because the tool measures current from inside the pipe, access to the outside of the pipe is unnecessary.  The pipeline can be surveyed regardless of right-of-way condition or location. There is no need to deal with landowners, hire crews or boats, submit permits or even know the exact pipeline location.

•  Stray current interference is easily detected: Unlike potential data which requires interpretation to find the presence of stray current interference and can only guess at its magnitude or direction, current data clearly shows the exact location, magnitude, and direction of unwanted current for external sources such as utilities, power lines, DC rail, or other CP systems. 

•  IR drop is unimportant: IR voltage in the ground surrounding the pipe causes unwanted shifts in external potential measurements, but has no impact on the current measurement.  CPCM surveys do not require interruption of CP systems. In fact, rectifiers should be in normal operational mode for best results.

•  Crowded right-of-ways are not problematic: The presence of other pipelines in close proximity in a single right-of-way has no impact on current measurements. Only the current on the pipeline being surveyed is measured, regardless of other pipelines nearby. 

•  Current data provides DCVG information in the same survey: Current density information duplicates the information produced by a DCVG survey, giving the effect of two surveys in a single inspection. (CIS and DCVG).

Field trial No. 1

A 24-in. (610-mm), 34-mi (54.7-km) crude line was inspected with the CPCM tool. The run identified three CP current sources providing 29.5 amps of current to the line. Two “shorts” to other structures were discovered totaling 8.2 amps leaving 21.3 amps to protect the pipeline. The largest short, which was taking 7.9 amps was leaving approx 5,400 ft (1,646 m) of the pipeline void of any appreciable CP current (Figure 1). The inspection data pinpointed the location of the short. It was excavated and found to be a pipeline crossing (Figure 2). The pipelines were separated and the short was cleared. Removal of this short allowed the CP current to effectively cover the area previously void of current. This was proven by observing the potential shifts on the other side of the area previously void of current.

Field trial No. 2

A 12-in. (324-mm) crude oil pipeline experiencing sudden low potentials offered an opportunity for additional validation of the new tool. The tool information gained from this run identified the location of a shorted pipeline causing low pipe-to-soil potentials. The short was subsequently removed and the pipe-to-soil potentials returned to their historical values (Figure 3). 

The data revealed the total current being applied to the pipeline and the pattern of each current source.  From this information one mislabeled rectifier negative was identified, an insulator at an underwater tie-in was verified and the amount of current from the delivering and receiving stations was discerned. 

The tests conducted on this crude pipeline revealed the importance of having a “clean” pipeline. There were areas of paraffin build-up that prevented the collection of usable data. The erratic voltage data collected in this area indicated a lack of contact integrity. This poor contact was confirmed by the contact signal recorded during the tool run. It was determined that paraffin buildup on the pipe wall caused the contact wheels to experience lift off and large voltage swings were recorded. It can be noted that even though some of the data was compromised due to intermittent contact loss the data was of value due to the cumulative nature of reading current in the return path.