Fracture arrest evaluation of X100 TMCP steel pipe for high pressure gas transportation pipelines

In recent years, gas companies have show na increasing in the possible uso of higher grade steel pipes (Yield Strengh=690 MPa, equivalent to X100 steel grade) for the construction of long distance gas pipelines. As a result, research programs sponsored by both gas companies and steel pipe producers, investigated the suitability of the equivalent of X100 grade steel for high pressure pipeline use, with the main emphasis placed on establishing their fracture behavior. One of the essential points was dealing with ductile fracture arrest capability, in order to consolidate existing know-how regarding the definition of the minimum toughness requirement to control the ductile fracture propagation event. Toughness evaluations were carried out to determine Charpy V-notch shelf energy, DWTT shelf energy, and new promising toughness parameters such as the DWITT specific total/propagation energy values, and a measure of the Crack Tip Opening Angle (CTOA). To achieve this general aim extensive uso of the full scale burst tests was also necessary. The results of full scale tests, in terms of arrest/propagation conditions, discussed together with available results in the literature on API X80 and X100 pipes, show that X100 large diameter pipes are operating at the upper bound of the arrest/propagation ductile fracture propagation conditions; moreover also the applicability of the ‘Battelle Two Step Analysis’ and their straightforward extrapolation from API X80 pipes to X100 grade operating at very high hoop stress values (=500MPa) is highly questionable. So a devoted or specific ‘design’ is mandatory to prevent/control the ductile fracture propagation event in long distance X100 grade large diameter onshore gas pipeline. On specific fracture issues this paper presents also some conclusions useful for the safe design of long distance X100 grade gas pipeline; with regard to this specific issue the CSM proprietary code PICPRO® is presented as a valid tool for enlarging the information gathered from a single full scale burst test, to provide a solution to overcome the problems connected with the lack of a reliable procedure to transfer laboratory toughness results to the real pipe case and finally to achieve a new safe design criteria for crack arrestors. (AU)