LONG-TERM CORROSION OF PARKED AND ABANDONED OFFSHORE STEEL PIPELINES

Internal corrosion of steel pipelines before use, or temporarily taken out of use, and 'parked' on the seafloor for some time (months), suspended from operation or 'abandoned' for extensive period of time continues to be an issue for offshore operators and regulators. This is irrespective of whether the pipes are gas or oil production or water injection pipelines. Severe pitting corrosion, sometimes observed at the 6 o'clock position, conventionally often is assumed the result of microbiologically influenced corrosion (MIC), particularly if coupled with high bacterial concentrations in the rusts. Dosing with biocides is not always effective and increasingly is considered environmentally undesirable. This paper provides a brief overview of the fundamental requirements for long-term corrosion under low oxygen and essentially stagnant environmental conditions. It shows that any severe internal corrosion at the 6 o'clock position has similarities to that sometimes observed for water injection pipelines and likely the result of under-deposit corrosion resulting from the deposition of rusts and other debris. MIC may contribute providing there is availability of necessary nutrients. Generally similar issues apply for the external and interior corrosion of abandoned pipelines. Avenues for further research and development are outlined, including for pipelines proposed to be abandoned.