Abstract
Equipment for drilling risers and subsea oil exploration involve use of many metals with the risk of galvanic corrosion, if not protected or maintained properly. Paint coated steel accounts for the majority for of such structures, but temporary coated steel, stainless steel and hard face alloys are also applied for special components. The time for completing subsea wells typically extended over few weeks in the past, with the possibility of maintenance on the rig between the operations. As drilling technology evolves, the well completion may now last several months, which presents new challenges in respect to corrosion protection. Many of the individual parts of complex drilling equipment are usually electrically isolated by paint coating and thereby not intended for cathodic protection (CP). In some cases, wear of the coatings and the extended exposure periods have caused excessive galvanic corrosion of low alloy steel or hardface coatings coupled to stainless steel. However, the opposite effect has also been observed, i.e. inadequate stainless steel grades are being protected by the contact to low alloy steel. The observed corrosion issues are not consistent, and cannot readily be correlated with equipment design, exposure history, installation depth etc. Consequently major effort have been made to identify aggravating circumstances that occasionally lead to excessive corrosion. This analysis has reviewed many possible factors, such as special environmental conditions, stray currents, paint coating quality, maintenance etc. In some cases, excessive galvanic corrosion was accelerated by the prolonged exposure periods leading to a fully developed marine biofilm on the stainless steel parts. This provides highly oxidizing conditions that are 10-100 times stronger than those observed during short-term exposure. The paper reviews literature on marine corrosion in regions where deep sea drilling is performed. Examples of corrosion issues are presented and mitigating actions are discussed, such as CP, partial coating, maintenance or galvanic insulation. Lastly, the paper addresses developments within coatings and point at future initiatives to further enhance the longevity of the marine drilling riser.
