Assessing the Integrity of Non-Piggable Pipelines
To the average householder, pipes are simply a convenient means to deliver water or natural gas to the home and are perceived to be of relatively small diameter. These narrow tributaries, however, are just the final stage in a vast and complex network in which some of the larger pipes may have a diameter of as much as a metre. While water-reticulation systems must obviously be extensive, those used to carry oil and gas seldom attract attention, despite being even vaster. Furthermore, they often include non-piggable pipelines.
The term refers to a specially designed item of equipment which is known in the profession as a “pig”. The device is inserted into a pipe where it forms a tight fit and, when propelled along its length, it can be used to inspect or to clean its interior surfaces. The method is particularly useful for inspecting underground installations that would need to be exposed to perform an external inspection. Its success, however, depends on the internal geometry of the pipe. Where this is not sufficiently uniform, the progress of the pig may be obstructed, and the pipeline will be non-piggable.
The usual explanation for this is that the pipe contains bends that are too sharp for the pig to negotiate while, in some lines, the internal diameter may vary by too great a margin in places to permit the device’s continuous passage. While these devices are designed to adjust to small changes in diameter and may even be flexible to a degree, there are limits beyond which their use is no longer possible. Inevitably, such pipelines must be classified as non-piggable but how, then, are they to be inspected and maintained?
Typical of this type of challenge are the tank-farm installations and the transfer lines that lead to and from a jetty. Often, they contain bends of short radius and mitre bends, while having no pig traps installed, thus ruling out inline inspection by means of traditional pigs. In many cases, however, the solution could simply be to employ a new type of pig. Pigging technology has undergone a number of advances since its inception with the result that many previously non-piggable pipelines can now be safely inspected using a much smarter and more adaptable type of pig.
Where the main concern in the past was the possibility of a pig becoming stuck and difficult to retrieve, this risk has since been minimised by the development of a type of calliper pig. The tool is designed to negotiate all typical pipeline restrictions and can be used to perform a preliminary inspection of a line to detect and to report back to the contractor regarding any obstructions that could possibly result in the inspection pig becoming stuck.
Effective for the inspection of many pipelines that were previously considered to be non-piggable, the combination of this innovative tool with an intelligent pig is providing a welcome solution for the gas and oil industry by enabling the continued use of lines that might otherwise have required replacement.
Assessment of the integrity of these so-called non-piggable pipelines is just one of the many technical services offered to this strategic industry in South Africa by Oleum Process & Pipeline Services.