Abstract
This publication examines the dominant corrosion and integrity issues affecting the offshore rigid and flexible pipelines and risers that exist in the North Sea, as identified from the experience of the authors and their companies. The aim is not to examine all possible integrity issues, but to identify those that appear to be causing the most problems. For many aging rigid pipelines extension of life to beyond the design criteria is becoming increasingly common, and the increasingly dominant integrity issues are associated with internal and external corrosion. With respect to external corrosion issues, anode depletion is an increasingly common phenomenon below the water line. At and above the water line, coatings often passed the end of their service life in the late 90s and were not renewed due to low oil prices and reduced fabric maintenance spending. For internal corrosion, different corrosion mechanisms dominate in gas, multiphase, oil and water pipelines. Dominant issues for gas and multiphase production are preferential weld corrosion and localized CO2 corrosion mechanisms occurring under scales, deposits or due to increasing H2S levels. In oil transport pipelines, as oil volumes reduce, water drop and MIC are increasingly problematic due to low flow, and pigging operations become vital. In water injection pipelines, aging infrastructure leads to both inefficient de-aeration and an increase in MIC due to contamination over many years. All these issues lead to a need for effective corrosion and integrity management.
The experience of use of flexible pipes is increasing exponentially and by definition it is ageing. Over the coming years an increasing number of flexible pipes will reach the end of their design life, therefore prudent operators should focus on understanding the integrity status of their flexible pipes. Understanding and effectively managing the integrity of flexible pipes is necessary to prevent, predict, and detect the presence of any loss of integrity. This publication presents an update on the guidelines (based on an ongoing Oil and Gas UK JIP) for developing and implementing an integrity management strategy. This approach utilizes the correct mix of inspection and assessment tools, and operating procedures that will allow the operator to assess the opportunity for life extension of flexible pipes. For specific issues, damage or rupture of the outer sheath and subsequent annulus flooding is also examined, as this is the commonest integrity problem that is encountered in practice.
