Chapter 23 DEVELOPMENTS IN STRAIN-BASED FRACTURE ASSESSMENTS - A PERSPECTIVE

When assessing geohazards for pipelines, scenarios where the pipeline may be subjected to strains well into the plastic regime is likely to be encountered. It is of importance to assess the integrity of girth welds in this respect. Most existing fracture assessment procedures were primarily developed for assessing elastic conditions, and there is a need to develop assessment models to provide more accurate estimates of the performance of girth weld with flaws subjected to large strains.

This paper presents a perspective on the development of strain-based fracture assessment procedures. There has been a significant activity in the field resulting in valuable contributions from several parties in the last 10-15 years. The perspective presented in this paper is, however, mainly based on knowledge and results developed in the Fracture Control Offshore Pipelines JIPs.

Assessment of cracks under globally applied large strains is characterized by plasticity governing most of the opening of crack. This poses some challenges related to the applicability of general assumptions in fracture mechanics. Special features related to crack driving force, crack growth calculations and critical conditions are discussed in the paper. Further, the roles of stress-strain curve, mismatch, misalignment, and bi-axial loading in strain-based assessments, and possible ways to account for them, are highlighted.

The paper also contains discussions associated with the use of numerical modelling, strain capacity prediction models, and probabilistic assessments. The paper is concluded with a general discussion on the state of fracture assessments for scenarios with global loading in the plastic regime. It is concluded that considerable progress has been made, however, a more active coupling of recent models or numerical modelling schemes with probabilistic approaches will probably increase the benefit of the developments.