Stress Concentration Factors for a Drilling Riser Containing Corrosion Pits

Abstract

The residual fatigue life of a corroded riser joint can be evaluated by means of a fatigue analysis based on S-N data. In this case nominal stresses are determined through a global riser analysis in which the drilling riser is modeled as a tensioned beam subjected to loads throughout its length and with boundary conditions at each end. The effect of the corrosion defects is taken into account multiplying the nominal stresses by stress concentration factors (SCFs) derived by local Finite Element (FE) analyses of the riser joints containing corrosion defects. In this paper stress concentration factors for a drilling riser containing corrosion pits are calculated using solid FE models. These pits are situated on the external surface of the riser joints. Three shapes of corrosion pits are considered: semi spherical, cylindrical wide and cylindrical narrow. Five depths of corrosion pits are considered: 12.6%, 20.1%, 30.2%, 40.3% and 50.3% of the riser wall thickness. The riser outside diameter and the riser wall thickness are 533.4 mm (21 in) and 15.9 mm (0.625 in), respectively.