The occurrence of Hydrogen Induced Stress Cracking (HISC) is well documented in offshore pipelines constructed from duplex steel. The conditions required for HISC to occur are exposure to water, presence of a negative electrical potential from cathodic protection, and application of a high tensile stress. There is evidence that through wall HISC crack growth can occur in forgings when these conditions are present for just a few hours.
Onshore pipelines may also be constructed from duplex steel to provide corrosion resistance, particularly flowlines carrying well fluids. Where the fluid has a high pressure or low temperature, high tensile stresses may occur. These stresses are further increased at localised stress raisers such as girth welds and attachment fillet welds. The presence of water in the soil and the use of cathodic protection is likely for most buried onshore pipelines. Therefore all the conditions necessary for HISC to occur may be present.
Guidance for assessing the susceptibility of offshore duplex steels to HISC is available in DNV-RP-F112, based on laboratory testing and industry experience following failures caused by HISC. However, historically it has not been commonplace to use duplex material for onshore high pressure pipelines, so no standard guidance is available. A detailed engineering assessment is therefore required. However, many onshore design engineers may not be familiar with the HISC failure mechanism, and may not consider it at all when designing a pipeline using duplex material.
This paper discusses recent experience in applying the design rules in DNV-RP-F112 in an onshore pipeline design project.
Firstly the paper covers the conditions necessary for hydrogen generation to occur. It discusses the differences between onshore and offshore cathodic protection systems, with reference to the electrical potential required for hydrogen generation. It concludes that in some situations, control of the cathodic protection potential will be the sole barrier to hydrogen generation, and thus its reliability can be of critical importance.
Secondly the paper describes the detailed analysis required to determine peak stresses and strains at localised stress raisers, for assessment against the limits provided by DNV-RP-F112. The definition of ‘peak’ stress and strain is discussed, since this is different from the definition more commonly used (for example in fatigue analysis). The HISC phenomenon is driven by dislocation movement due to applied stress, but DNV-RP-F112 provides limits in terms of both stress and strain. A method is presented to determine strains caused exclusively by stress and not by thermal expansion or the Poisson effect.
It is concluded that some interpretation of the DNV-RP-F112 guidance is needed for application in an onshore environment. Further work, including testing, is required to demonstrate whether the DNV-RP-F112 guidance can be applied reliably onshore.
