Since external corrosion is only responsible for a very few of the documented pipeline failures, we could truthfully say that, in general, the combination of CP and coatings is doing a good job.
However, we must not be led into a false sense of security. The only reason the external leaks have not started in earnest is that the old systems were unknowingly over-designed. Thus, a 25-year design life has effectively turned into 30, 35 or even 40 years.
There is a practical limit on how long sacrificial anodes will last, and it is based on the auto-corrosion rate of the anode material. If we were to assume that pipeline systems are all good for at least 30 years, then there should be several thousand miles of pipeline with depleted CP systems (Figure 1). The question, then, is why are we not seeing more external failures?
In truth, the answer to that question is that we probably are seeing a higher external corrosion leak rate than we have at any time in the past. But when will it peak? The pitting rate of steel in seawater on a well-coated pipeline in the absence of cathodic protection anodes could vary between 0.01-0.05 inches per year. Thus, it could take anywhere from 5 to 25 years to pit through an inch of steel. This amount of loss could be sufficient to cause a pipeline failure. Higher corrosion rates can be generally expected when the pipe coating has a combination of large damaged areas and adjacent pinhole defects, and when the pipe is exposed to seawater rather than mud. There is also a particular risk of microbiologically influenced corrosion (MIC) on buried lines with bitumastic-type coatings and depleted cathodic protection.
What Is The Risk?
On pipelines in excess of 30 years old, the risks are quite high. If the cathodic protection systems have depleted, then corrosion will begin at numerous sites all over the pipeline. Unless detected and retrofitted, the first leak could be the end of the pipeline, as the next several hundred won't be far behind. There are only so many clamps that an operator can afford to install before economic concerns dictate pipeline replacement or abandonment. Given the cost of laying pipelines offshore today, many of the lines will never be replaced, and this could result in early deaths of the oil and gas fields they service. Other old lines are the critical links between the new deep water fields and the shore-based markets. Loss of these lines will present an interesting and unenviable dilemma for operators.
What Is The Answer?
There are three basic strategies that a pipeline owner can adopt:
1. Survey the pipeline cathodic protection system.
2. Retrofit the cathodic protection anodes on pipelines of a certain vintage.
3. Do nothing (and hope that the laws of electrochemistry will ignore your pipeline), essentially ignoring the problem.
Cathodic Protection Surveys
Close-interval cathodic protection surveys are the most logical strategy, but strangely enough, operators in the Gulf of Mexico survey very little. When a survey is actually run, it is usually of little value because the method used (trailing wire) inherently produces erroneous data.
There are accurate survey systems available; these either involve physically contacting the line at intervals or utilizing remotely operated vehicles (ROV's) (Figure 6) to track the pipeline and carry reference electrode arrays above the pipeline at known locations (a typical plot from such a survey is shown Figure 7). This type of survey will let the operator see the condition of the line and make informed decisions regarding retrofitting.