NUMERICAL ASSESSMENT OF SUBSEA PIPELINE PRESSURE CAPACITY DUE TO EXTERNAL CIRCUMFERENTIAL SEMI-ELLIPTICAL CRACK

Abstract

Crack defects in subsea pipelines may lead to pipe wall failure due to leak or rupture. Hence, crack analysis is essential to ensure pipeline integrity by assessing whether it is still fit for service. Cracks are evaluated using stress intensity factor (SIF) by ensuring that this factor does not exceed the pipe material fracture toughness, at which point the crack initiates and propagates, leading to pipeline failure. A parametric study is performed to compute SIF values of circumferential semi-elliptical cracks on the external surface of a pipeline with a combination of crack depths of 4, 5, and 6 mm, and crack lengths of 20, 40, and 60 mm. An API 5L X65 grade pipeline with 254 mm outside diameter and 12.7 mm wall thickness is assessed by utilizing the extended finite element method. The models have been validated using theoretical SIF values calculated using API 579-1, with deviations between 0.37% and 3.65%. Maximum pressure capacities are then assessed to study the effects of initial crack depth and length. It is concluded that for these cases, each millimeter increase in initial crack depth may reduce the pressure capacity up to 12 times compared to the initial crack length.