PIPE SLEEVE REPAIR ANALYSIS OF AXIAL SURFACE CRACKS TO IMPROVE REMAINING LIFE

The industry widely uses steel repair sleeves to repair defects in pipelines. This paper focuses on the types of repairs used for crack-like defects where there may be concerns of crack propagation in service from operating pressure cycles. The length of the repair sleeve, the pressure at the time of the repair, and the fillet welds of a Type B sleeve can all effect the future integrity of the repair. Type B sleeves (fillet weld connecting the sleeve to the pipe) are widely used in the pipeline industry to repair crack-like flaws. Current standards require a minimum of two inches of sleeve material past the edge of a defect. A case study is presented to show if this length is adequate under various operating conditions to ensure the defect does not propagate in service to extend beyond the fillet welds. A pipe with a surface crack and the sleeve repair are modeled using finite element analysis to compute the crack front stress intensity. Contact interaction between the pipe's outside surface and the sleeve's inside surface provides the load path from the pipe to the sleeve. The contact interaction is varied from frictionless to fully bonded to vary the amount of crack opening permitted by the sleeve. The stress intensity is compared for each case to quantify the effect of the sleeve repair on reducing the hoop stress and reducing the crack's stress intensity. Operators often take a pressure reduction during an in-service repair both for safety reasons and to assist with obtaining a tight fit-up between the carrier pipe and the steel sleeve. An additional benefit of a pressure reduction can be gained in relation to in-service crack growth. A case study is presented that shows how a pressure reduction can extend the fatigue life of a resident defect. Comparing the crack front stress intensity shows the effect on the crack of the pressure reduction when the sleeve is applied. Fatigue crack propagation analysis of several cases shows how the pressure reduction effects the crack growth rates. The analysis method can be used to examine trade-offs between the amount of pressure reduction on fatigue life versus operational requirements.