We have found several webpages presenting marketing material disguised as “articles” or “technical papers” that discuss underwater cathodic protection (CP) surveys in a manner that is inaccurate or misleading. This page is intended to provide background discussing some unfounded and misleading statements and is written for personnel with a technical background in CP and CP survey techniques. Items discussed include the “Trailing Wire” survey technique, “Towed Fish” surveys, and close-to-remote survey techniques. For a discussion of requirements and pitfalls for underwater CP inspections, click here.
Trailing Wire Technique
Does the Trailing Wire Technique Work? Absolutely.
The method is the same technique utilized annually in thousands of miles of onshore pipeline close interval surveys (CIS) where a spool of insulated light gauge wire is grounded to a test station, then deployed while pipe to soil potentials are continuously measured and recorded using a portable data logging system. Using the trailing wire technique, potential values recorded are typically within 20mV of direct contact potentials at the same location, assuming the reference electrode is remote from platform/structure CP systems, and that IR drops associated with these systems are accounted for. As with any functional testing technique, properly trained personnel and the correct equipment are essential.
Trailing Wire Technique History
The trailing wire technique was developed by HARCO Corporation in the 1970’s, utilizing the towed fish method (before reliable remotely operated vehicles were available). The first remote operated vehicle (ROV) assisted trailing wire surveys were performed in 1983. The technique has been utilized in the US in both State and Federal Waters (AK, CA, LA, MI, MS, NY, and TX to name a few), and overseas in Southeast Asia, Gulf States, India, Mediterranean, South America, and the North Sea. MPM’s personnel have successfully completed both towed fish and ROV supported trailing wire surveys in AK, CA, LA, the Caribbean, South America, and the North Sea.
Does a Towed-Fish Survey Work? Yes. It is the most cost effective CP survey method for submerged pipelines in shallow water to 500-feet, and is the only cost effective method if the pipelines are buried.
The objective of a towed fish trailing wire survey is to determine the general level of cathodic protection on a submarine pipeline. A towed-fish survey can identify general areas of low CP, electrical interference from foreign CP systems, and other major anomalies such as shorted casings. It is not suitable for identifying small coating defects, individual bracelet anodes or other localized anomalies. As a general rule, the further the CP electrodes (on the fish) are from the pipeline, the more generalized the potential profile. A key component to an accurate survey then, is the ability to keep the fish near to the pipeline, and as such the pipeline’s as-built x-y coordinates and tow fish / pipeline elevation must be utilized in conducting the survey. MPM’s tow fish system incorporates a depth transducer, surface and subsurface navigation, and a vessel-mounted fathometer to accurately locate the fish in relation to the seafloor and a pipeline’s as-built route.
Are there issues with a buried pipeline towed-fish survey? No (with caveats).
The primary effect of burial is to increase the distance from the towed electrodes to the pipeline. As noted above, increasing the electrode to pipe distance simply decreases the sensitivity of the survey to minor anomalies. General areas of low potential can still be identified, then slated for more detailed investigation and/or remediation. As a general rule, if potentials gathered during a towed-fish survey are less negative than -850mV to Ag/AgCl, further investigation is warranted.
How does a Close-to-Remote Survey Work?
A close-to-remote survey operates under the principle that a moving Ag/AgCl cell, located outside (or remote) from both a pipeline’s and survey vessel’s electrical fields, can serve as a stable reference in lieu of physically grounding to the pipeline. The method was developed in the late 1970’s to facilitate performance of ROV assisted close interval potential surveys. What is critical in a close-to-remote survey is that direct contact (stab) potentials be acquired at the beginning and at regular intervals throughout the survey to establish the offset between the remote electrode and the pipeline and to correct for potential drift of the remote electrode as it is moved along the pipeline by the survey vessel.
Are there quality issues associated with Close-to-Remote Surveys? There can be.
We have found that subsea inspection contractors, without oversight, frequently struggle to get this type of survey right. While not particularly difficult, calculations are involved so a close-to-remote survey is not as straight forward as simply reading a meter. An offset generated incorrectly affects all of the subsequent data and undermines the integrity of a survey. The ability to use offsets can also open the door for field personnel to “tweak” the data to mask deficiencies in their equipment or procedures.
SIMS places a high priority on transparency and traceability. Any applied offsets are saved alongside of the raw data stored on each line of recorded data. During an ROV SIMS assisted survey, the offset generation process is overlaid onto the project video in real time for recording onto whatever storage media is being used by the ROV. Because offsets are so important, offset data overlay remains on the ROV screen for a short period to give post-inspection video reviewers (Owner, Operator, Agencies) who may be fast-forwarding through the video, a chance to note the offset and value(s)