Risk assessment based on analytical evaluation of structural integrity and life of drilling rig pipe

Abstract

Risk assessment procedure based on structural integrity evaluation has been applied to oil drilling rig pipe. The new concept based on application of structural integrity evaluation to assess risk level according to probability and consequence of failure was used. Analytical expressions for surface cracks are used to calculate stress intensity factors for different crack geometries. Oil drilling rig welded pipe integrity has been evaluated analytically by simple application of the Failure Assessment Diagramme (FAD), i.e. by using the ratio of linear elastic fracture mechanics parameter, KI, and its critical value, KIc, in relation with the ratio of applied stress and its critical value, to define the point in FAD. Probability of failure is then taken according to the position of the point in the FAD, corresponding to oil drilling rig welded pipe data. The same logic is employed in the case of cyclic loading, i.e. fatigue crack growth. In that case, structural life is estimated by using Paris law to calculate crack length vs. number of cycles, to be used in risk assessment procedure and by managers to make decision about further use of damaged pipes, based on data provided by engineers.