Stress investigation on an elliptical crack in offshore structures

Abstract

Offshore pipelines often experiences large plastic strains during laying or in-services conditions. Moreover, defects such as fatigue cracks can commonly be found within the welded region of the pipelines. With defects presence within the materials and large plastic strains, structural failure due to fractures is highly possible.

There are non-destructive testing (NDT) methods and crack assessments protocols available to check the serviceability of the pipelines. However the NDT methods are usually costly and not accurate while the crack assessments protocols are only applicable to small fracture behaviours and too conservative to be applied to analysis fracture behaviours within offshore pipes.

The evolution of the crack tip opening displacement (CTOD) of a crack has been identify as a possible parameter that can be used to assess fracture capability of a practical pipeline that undergoes large plastic deformations. The main aim of this project is to find a suitable fracture criterion for the fracture behaviour analysis of practical pipelines. By considering the effects of depth of cracks, semi-major axis to semi-minor axis of elliptical cracks and internal pipe pressure on the evolutions of CTOD, it is shown that CTOD is a suitable fracture criterion.