Historically, offshore pipeline cathodic protection monitoring has relied on the use of portable survey techniques. This has typically relied on ROV assisted or surface deployed survey methods. These methods have been shown to have technical as well as economic shortcomings; this is particularly true of buried offshore pipelines where accuracy is always questionable. As more focus is being placed on offshore pipeline integrity, it was time for a new method to emerge. The technology discussed involves the retro-placement of permanent clamp-on monitors onto the pipeline which can measure pipeline to seawater potential as well as current density. The sensors can be interrogated locally using light powered subsea voltage readouts. Application of the technology can be either during pipeline construction, during installation of life extension CP systems, or during routine subsea pipeline interventions. The new method eliminates the need for long cables or expensive acoustic or modulated data transfer and provides all the information required to fully verify CP system performance, thus eliminating the need for expensive close-interval surveys. Some deployment case histories will be presented along with feasibility of application on deepwater pipelines and comparative economics.
Keywords: Offshore Pipeline, Cathodic Protection, CP Monitoring, Fixed Reference Electrodes, Pipeline Integrity, Pipeline Surveys
There have been many survey methods developed in an attempt to accurately assess CP on existing offshore pipelines. Many of these have produced questionable results. A survey that produces questionable results has zero value. The writer’s company has been at the forefront of technology improvement in this area for over twenty years . After having surveyed thousands of miles of offshore pipeline with divers, ROV’s and using towed survey arrays we have learned the following important lessons:
- The accuracy of any offshore pipeline survey is only as good as the number of contact potentials measurements which are used to correct the potential profiles.
- Towed surveys (without frequent re-calibration) produce data that is of little or no value, and can be misleading. It is for this reason that the writer’s company has discontinued it’s version of this method.
- The accuracy of survey on buried pipelines, even with a ROV is questionable. This has to do with difficulties in obtaining the required number of calibration contacts, and the inability of the current technology to measure the very small field gradients that are present at or above the mudline.
- The cost of offshore vessels and ROV support is a volatile variable; this can result in excessive cost to run a survey that yields questionable results.
It is for the aforementioned reasons that CIS surveys on offshore pipelines are virtually non-existent on the OCS of the Gulf of Mexico where virtually all the pipelines are buried in water depths less than 200 feet. Operators are reluctant to spend the money when they cannot take the results to the bank.
While we have all this skepticism about offshore pipeline survey, the industry is coming under increasing regulatory scrutiny, and asset integrity is becoming a higher priority with pipeline operators. There is however light at the end of the tunnel. Two new technologies could provide the answer to meaningful, cost effective monitoring of offshore pipeline CP.