Connect, September/October 1998
European Marine Contractors is an offshore pipeline contractor whose operations include pipelay using the S-lay method. Girth welds made on the laybarges are subjected to NDT, any flaws which do not meet the specified acceptance criteria are removed.
Defect removal can either be by in-situ repair or by reversing the barge so that the repair or cut-out is undertaken at a location where the pipe is no longer under load. In-situ repair is desirable to improve productivity and reduce cost. However, since local repairs are conducted under loads, which may be as high as 90% of the pipe yield strength, an analysis has to be undertaken to ensure that the in-situ repair is safe.
An analysis procedure has been developed to predict safe repair groove sizes. The procedure is derived from small scale testing on typical offshore welds, with BSI PD 6493:1991 methods used to perform a ductile tearing instability analysis. The procedure has been validated by a series of three large-scale fracture mechanics tests on 36 inch diameter API X65 linepipe (31mm wall thickness) which simulated the removal of defects during weld repair.
More details on procedures or fracture mechanics testing from TWI's Structural Integrity Department. E-mail: firstname.lastname@example.org