Pipline integrity technology

By Dr John Leeds

To date, pipeline integrity has been driven largely by US statutory requirements and utilises predominantly inline inspection (ILI) tool studies, concentrating on determining the current status of pipelines. ILI data identifying metal loss areas is important as it represents a pipeline’s vulnerability to a leak situation but inspection tools are not selective. ILI tools detect a variety of pipeline defects such as corrosion sites, dents, gouges, laminations, cracks, hard spots, weld joints, general background noise and so on.

Most defects in pipeline steel are considered static defects. Their contribution to pipeline vulnerability does not change significantly with time although third party interference can add to their number. They could of course become stress risers for SCC failure. Dynamic defects, typically corrosion metal loss areas, can change significantly with time and are in fact the real parameters that determine a pipeline’s on going vulnerability. Dynamic defects can and often develop at static defect locations.

Repeat ILI inspections are useful in locating changes in the number of static defects and in particular the growth of dynamic defects and how pipeline vulnerability changes with time.

ILI’s biggest contribution is to locate and assess dynamic defects, but ILI studies of dynamic defects are in fact chasing the symptom of the real problem, the breakdown in effective application of corrosion mitigation processes (coating and CP) on pipelines that causes them. Further, a pipeline’s maintenance costs and life are predominantly determined by effective operation and control of corrosion mitigation techniques as applied to dynamic defects. If dynamic defects grow and their number increase, then there is something significantly wrong with the effective operation and control of corrosion mitigation techniques.

There is a current picture painted that ILI tools can serve as the panacea for determining pipeline integrity to the detriment of other techniques that could be applied. However, ILI tools like all techniques have their limitations. They do not detect all static and dynamic defects. Accuracies are approximately 85 percent but these are further reduced by a number of operational difficulties encountered when running such tools. Tool speed variations, waxes and asphaltenes, corrosion product and metal debris etc. build up on tool sensors which can strongly influence detection capabilities. There is the question of tool data interpretation, and whilst it is improving it is still very much a subjective activity. Some companies are well aware of the limitations of ILI Tools and of data errors such as distance or complete sections of pipe being left out of survey reports.

ILI industry also promotes a risk assessment approach incorporating fitness for purpose and remaining life assessment. Corrosion is never a uniform process but starts and stops depending upon forces driving the corrosion process. Unfortunately ILI studies mostly use results from historical pipeline activities (past failures of corrosion mitigation techniques that resulted in metal loss). Most pipelines fail because of external corrosion and it is the pipelines protective coating integrity and continual application of effective cathodic protection that will determine ongoing fitness for purpose and remaining life. This will invariably differ widely from ILI predictions as invariably corrosion control activities are reviewed and upgraded usually as a result of initial ILI surveys thereby invalidating ILI predictions. In quite a number of cases, projections are a dubious waste of money as they are fed by incomplete or inaccurate data gathered by different surveyors using a variety of inconsistent techniques and equipment with limited capability.

For most pipe lines, over 99 percent of all coating faults have little or no metal loss associated with them. CP has a beneficial effect, although it may be poorly controlled. Alternatively, 80 to 90 percent of all external metal loss occurs at coating faults. There can never be total coincidence between metal loss detected by ILI, and coating faults, due to the wide range of defects that metal loss tool detects some of which are not coating-related (e.g. out-of-specification construction grind outs or previous repaired areas).

Future application of ILI should really be viewed as an audit mechanism to establish how effective the corrosion mitigation techniques have been applied and if effective, there should be little difference in data between two ILI inspections taken seven or 10 years apart.

About the author

Dr John Leeds is Managing Director of DC Voltage Gradient Technology and Supply Ltd and Pipeline Integrity Management Ltd. He has worked in the industry since the 1980s on internal corrosion and xmas tree failures. For the last 17 years he has consulted internationally in 43 different countries on pipeline integrity problems.