Stepwise Cracking of Line Pipe Steels in Simulated Oilfield Environments

Abstract

Abstract Pipelines used in the gathering and transport of natural gas, natural gas 1iquids and other petroleum products may contain significant amounts of hydrogen suffice (H2S) and carbon dioxide (C02). When associated with brine these environments can cause internal corrosion and stepwise cracking of pipe line steels. Such phenomena are of increasing importance in the design and protection of offshore gathering lines as levels of H2S increase in these areas. Stepwise cracking is observed in lower strength (less than or equal to 80 ksi yield strength) line pipe steels which are used in oilfield service. Cracking occurs internally through the accumulation of molecular hydrogen along internal surfaces such as nonmetallic inclusions or low temperature transformation structures. As hydrogen becomes trapped at internal surfaces, b1isters fond, usually along parallel planes, which begin to link up via small cracks. These cracks lower the effective wall thickness of the pipe and may eventually become through wall failures. Research over the years has shown how various metallographic variables can be controlled to increase resistance to stepwise cracking. This study examines the effects of inhibitors on the stepwise cracking characteristics of API line pipe steel. The use of inhibitors was found to be available method of controlling the onset and propagation of stepwise cracking. Introduction Pipelines used in the gathering and transport of natural gas, natural gas 1iquids and other petroleum products may contain significant amounts of hydrogensulfide (H2S) and carbon dioxide (C02). In conjunction with aqueous water, these constituents cause internal corrosion of steel pipelines. Also, pipe line material s exposed to aqueous H2S containing environments may be susceptible to failure by either sulfide stress cracking (SSC) or stepwise (blister) cracking. A study was undertaken to provide an assessment of the state-of-the-art of preventing or minimizing the deleterious effects of these phenomena in the transmission of sour (H2S containing) gas. This investigation was undertaken to provide information regarding:the manufacturing of pipe line materials,operating procedures andfield experience. Background Mechanisms of Pipeline Corrosion Damage When acid gases are present, three distinct forms of corrosion damage in pipeline may occur:Weight-loss CorrosionSulfide Stress Cracking (SSC)Stepwise (Blister) Cracking Weight-Loss Corrosion. Produced oil and gas frequently contain H2S and/or CO2 When dissolved in aqueous systems (water or brine), these gases fond acids of fairly low pH which attack and corrode materials commonly used for transmission lines. This attack may be localized, in the fond of pitting or crevice corrosion, or it may be general attack dependent upon the environmental conditions that exist.