Natural gas will still be important as fuel in the energy mix until 2050 according to IEA.  Unfortunately, a large percentage i.e. 43% of the remaining natural gas reserves contain high levels of H2S and high CO2. C-Mn steels are susceptible to hydrogen damage mechanisms such as sulphide stress cracking (SSC) and hydrogen induced cracking (HIC).

In 2013 C-Mn a natural gas and an oil line in a severe sour pipeline project suffered leaks after only a few days in operation. The root cause analyses showed the pipe internal surface had undetected Local Hard Zones (LHZs) formed during steel plate manufacture involving accelerated cooling as part of the Thermo Mechanical Controlled Processing (TMCP). The leaks led to expensive replacement of the pipelines with CRA clad pipe. Neither NACE MR0175/ISO 15156 or DNV Supplementary Requirements suffix ‘S’ for sour service pipe qualification could have led to detection of Local Hard Zones (LHZs.

Extensive research has since then led to new approaches to monitor steel plate manufacture to prevent LHZ formation and procedures for qualification of the pipe for sour service pipelines.

Testing in simulated operating environment is required to measure the fracture toughness and fatigue crack growth rates of the girth welds to provide essential input to the Engineering Critical Assessment (ECA) performed as per DNV-RP-F108 to develop flaw acceptance criteria for the AUT for pipeline installation.

This paper presents experience of sour service ECAs and associated testing since the first issue of the DNV-RP-F108 in 2006. Results of testing in NACE MR0175/ISO 15156 Region 1, 2 and 3 environments are presented. Further, it is shown how the execution of the common rising displacement tests for sour service fracture toughness testing and fatigue crack growth testing may significantly affect the ECA based flaw acceptance criteria.