Casing and tubing are generally designed against burst by requiring a safety margin between the calculated stresses and the yield strength of the material. This same formula is then often used to determine the remaining burst strength of casing that suffered from drillstring wear or wireline wear, in the latter case sometimes including an additional penalty to account for stress concentrations. This approach is very conservative and, moreover, leads to actual safety margins against burst that vary widely over different grades.
Improved and novel approaches towards tubular design and fitness-for-purpose call for models that can accurately predict failure as opposed to conservative assessment procedures. Much progress has been made in developing verified burst strength models for ‘regular’ pipe, accounting for the acceptable wall thickness variability. In this paper attention is paid to pipes with cracks, but the main focus is on the accurate quantification of the degradation of the burst strength due to longitudinal wear.
Burst tests have been carried out on a number of pipe specimens with a wear groove machined at the inside; some of these with axial cracks at the outside. Also finite-element analyses have been performed, simulating the burst of these pipes and also others from different grades and sizes, with defects of different depths and shapes ranging from drillstring wear and wireline wear to sharp cracks.
By combining the results of the tests and the analyses, a simple analytical burst model has been developed that shows an excellent performance for regular pipe and worn pipe. The equations developed here are also applicable to describe the limit behaviour of pipes with crack-like defects, provided ductile tearing is taken into consideration and fracture instability does not occur. More details on the approach to account for flaws in the burst strength of OCTG is presented in a companion paper by Stewart and Klever .
This work is directly applicable in the design and fitness-for-purpose analysis of casing and tubing.