Interpretation of the South Stream Ring Collapse Test Program Results

A comprehensive study was conducted on the collapse mechanism of rings cut off from line pipe specimens. It was theoretically established that a ring without end restraints should have a lower collapse pressure value than an equivalent full-scale joint. However, similar collapse pressure test results were obtained for both rings and joints. Ring tests differ from joint tests in end restraint, but both capture collapse resistance integrally, while coupon tests (production tests) only capture the influence of material properties. The ‘length effect’ and the ‘end cap effect’ increase the collapse resistance of a joint, while radial end restraints increase the collapse resistance of a ring. These effects are found to be of same order of magnitude for the investigated combination of parameters (thermally aged SAWL 450 and D/t of approximately 20). Further, influence of the out-of-roundness shape and material properties on the collapse behaviour of rings was assessed.