Sour service refers to the exploration and production environments in the oil and gas industry that have enough hydrogen sulphide (H₂S) to cause the potential cracking of metallic materials. Operating in environments where H₂S is above this threshold is recognised as one of the toughest challenges in hydrocarbon production.
What are sour service pipes?
These conditions have necessitated the development of sour service pipe product lines to ensure the integrity of assets and operations under the corrosive conditions in crude oil and natural gas extraction pipelines. Sour service pipe, or sour resistant line pipe, uses a type of steel referred to as fully-killed (de-oxidised) steel and is subject to different manufacturing specifications and testing requirements than non-sour line pipe.
The key to manufacturing sour service pipes is to eliminate elongated non-metallic inclusions. Removal of one of the pillars (non-metallic impurities) can be achieved by reducing the sulphur content and keeping the manganese levels as low as possible. Higher carbon, phosphorus, and higher manganese cause a hard bainite microstructure and high hardness in the central segregation zone.
Usage and applications
Pipe corrosion is a significant threat to personal safety and environmental pollution. Sour service pipe is mainly used in H₂S environments. There are two kinds of cracking that present the most serious corrosion problems for pipes:
Hydrogen Induced Cracking (HIC)
HIC is planar cracking that occurs in carbon and low alloy steels when atomic hydrogen diffuses into the steel and then combines to form molecular hydrogen at trap sites. Testing against HIC is conducted on all sour service pipes.
Sulphide Stress Cracking (SSC)
SSC refers to the cracking of metal involving corrosion and tensile stress such as residual and/or applied stress in the presence of water and hydrogen sulphide (H2S). Testing against SSC is done against sour service pipes if agreed to.
The first incident resulting from these corrosion issues occurred on a subsea pipeline in the Persian gulf in 1972. Since that failure, the mechanisms and countermeasures have been investigated and studied and pipe steel resistant to HIC and SSC has been developed. Now there is a range of sour service pipes available that are purpose-built to operate under these corrosive conditions.
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