By Andrew Hobbs
ONE OF Australia’s leading experts in marine corrosion has developed a theory that could change the corrosion threat to parked pipelines, but needs industry support to investigate it further.
University of Newcastle civil engineering professor Robert Melchers told Oil & Gas Australia that some corrosion in parked pipelines could be prevented by removing nitrates from the water inside the pipelines.
Parked pipelines – pre-installed pipelines that are not being used, but which remain in place for potential revitalisation at a later date – are commonly filled with high pressurised, often deaerated water while out of use.
Speaking to Oil & Gas Australia, Professor Melchers said the theory centred on a discovery made while he and others were looking into microbially influenced corrosion (MIC) – where corrosion in pipelines is much increased by bacteria.
“Industry experience shows that in some cases the pipelines have … corrosion at the bottom of the pipeline on the inside – not anywhere else, and in other cases there is nothing,” he said.
Analysing maintenance pigging data supplied by Statoil over a period of two years, Professor Melchers said he and researchers at Swedish research group SWEREA-KIMAB had noticed this phenomenon was often caused when deposits settled to the bottom of the pipeline after the flow was turned off.
This created the perfect conditions for under-deposit corrosion, he said, often exacerbated by microbial activity.
“The important part is that bacteria don’t do anything if they don’t have any energy, and, importantly, if they don’t get the right nutrients,” he said.
“One of the things that the water injection pipelines are used for is to pump nitrates into the oil wells, and nitrates are the critical nutrient for microbial activity in seawater.”
“So if you don’t pump nitrates in, you’re not going to have MIC but you are still going to have underdeposit corrosion.”
“So if your water is dirty and you have got bacteria or you have got nitrates in the water you are going to have all these problems. The only thing you can do is put in very clean water and nothing will happen,” he said.
Professor Melchers said he and research colleagues were trying to launch a joint industry partner research project to validate the theory against a wider range of carefully controlled field observations.
“It would be better to have a broader base of field data and some more industry involvement, and it is probably going to mean rewriting the currently accepted conventional wisdom,” he said.