The big question was “high resolution or low resolution?”, relating to the capacity of the equipment to detect pipewall features. Putting this another way, did the operator want a ‘quick-and-dirty’ (and therefore cheap) inspection, or was the ‘Full Monty’ required? The issue revolved around the capacity of the intelligent inspection tools to inspect, and the cleanliness of the pipeline that the tool was to inspect was sometimes considered of less significance.

Now, however, intelligent inspection tools are of an unimaginably greater sophistication, and the general question being asked is no longer to do with their capacity to accurately and precisely detect features, but to do with how clean the pipeline is. A pipeline’s internal cleanliness has, quite properly, become a question of great significance. Nevertheless, there are no published standards of cleanliness and although there are many ways in which deposits can be removed from a pipe wall, ensuring a pipeline is clean enough for an inspection to be carried out remains a subjective process.

It has often been said that the best cleaning tool is a magnetic-flux leakage intelligent pig, and this remains true. While it is clear that each pipeline and its operating regime are different, it seems surprising that it has not been found possible to establish some basic guidelines for achieving cleanliness. Under normal operating conditions, minimisation of pipewall deposits will improve flow conditions as well as a pipeline’s overall efficiency and cost-effectiveness, to say nothing of the effect on reducing the potential for corrosion. When it’s time for an inspection, deposits and other debris must be removed, both to ensure that the tool’s sensors can have unimpeded access to the pipewall, and to remove the possibility of debris clogging-up the tool, and even causing it to become stuck.

The question of ‘how clean is clean?’ is not unfamiliar and, in fairness, is being asked more frequently. One of the most detrimental cleaning problems for gas pipelines is the formation and accumulation of so-called ‘black powder’. This material– which is as fine as flour, although far more dangerous because it is both abrasive and pyrophoric – is one of the least understood but most prominent contamination problems in gas pipelines. Black powder is the name given to the mixture of iron oxides, carbonates, and sulphides found in gas lines. Its sources include millscale, corrosion products, salts and scales from gas wells and wet gas gathering systems, and atmospheric corrosion. The variability of its composition is illustrated by reports of the powder ranging from being completely iron sulphide to completely iron oxide.

Black powder can cause product quality problems and excessive wear and erosion on internal pipewalls and many other pipeline components, including compressors, turbines, and valves. The accumulated solids can plug small orifices and consequently affect measurement equipment and, as the particles settle out of the gas stream, they can fill-in surface pits and other internal pipewall anomalies, preventing accurate inspection. In sag bends, these build-ups can harbour corrosive bacteria.

Although difficult to deal with, the problems caused by accumulations of this material can be overcome – as can most pipeline problems – by careful planning and attention to detail. However, removal of the black powder from the pipeline is not the end of the affair. As the material is hazardous, necessary arrangements for its disposal must be made, and obviously these should be in place before any pigging operations begin.

A special session on this problem is being organised at the Pipeline Pigging and Integrity Management (PPIM) Conference being held in Houston on 17–18 February (see page 46), and other papers at the event will also address the issue.

The more the subject of ‘how clean is clean?’ can be discussed, the more likely it is that shared experiences can lead to a shared solution; at the very least, ‘clean’ needs to be kept in the spotlight of pipeline integrity management and operations.

John Tiratsoo

Editor-in-Chief