The constraint-based assessment of CTOD toughness requirement is discussed for high-strain line pipes. The biaxial loading caused by a high internal pressure and the strength mismatch between the base and weld metals exert influences on the CTOD and stress fields of a crack in the pipe component. Nevertheless, the pipe component suffers from a significant constraint loss compared with the fracture toughness specimen with a deep crack. It is confirmed that the equivalent CTOD ratio _, standardized in ISO 27306, can be applied to correct the CTOD toughness requirement for constraint loss in the line pipe.
Natural gas industries show a clear-cut trend toward the use of highstrength large-diameter pipes for long-distance high-pressure pipelines. This is to reduce the cost of gas transmission from remote sources to market end-users with an efficient mass gas-transportation. The construction cost of pipelines can also be reduced by the use of highstrength steels because of the reduction in pipe wall-thickness leading to less work time for pipe joining. However, this scenario works with the development of an innovative design for fracture avoidance. Longdistance pipelines are likely to be subjected to a large ground movement if laid in a seismic area or in a permafrost area. Considering a high cracking susceptibility of high-strength pipes, attention should be paid to the increase in the crack driving force in hostile environments. The crack driving force of pipelines will also be increased by factors addressed below. Long-distance pipelines employ a high gas-pressure, e.g. design factor of 0.72SMYS or higher (SMYS: specified minimum yield strength), which enhances a biaxial stress state in the pipe wall. It was demonstrated (Gordon et al., 2007; Ostby, 2007; Liu et al., 2007) that the biaxial loading promotes crack-tip opening of a surface crack on the pipe wall, compared with uniaxial loading.
