Structural reliability approach to strain based design of offshore pipelines

High longitudinal strains on the pipeline may be activated by sharp bent configurations during the different lifetime phases. A strain based design may be, in these cases, applied given that the pipe is predominantly loaded in global displacement control. The failure is related to the ductile tearing of the defective girth welds: the maximum allowable longitudinal strain is identified by guaranteeing the pipeline integrity with respect to this limit state. The applications of deterministic fracture mechanics assessment procedures to the predictions of fitness for purpose requires the use of data that are often subject to considerable uncertainty. The extreme bounding values for the relevant parameters can lead, in some circumstances, to unacceptably over-conservative predictions of structural integrity. An alternative approach is to use structural reliability methods to allow for the uncertainties in the parameters and to assess the probability of failure of structures containing flaws. A probabilistic fracture mechanics analyses is carried out according to the assessment procedures described by British Standards codes (BS7910, 1999) and using a statistical simulation method (Monte Carlo) for solving the limit state equation. The uncertainties (i.e., specific statistical distributions) of the following parameters are taken into account, expected to most affect the evaluation of the pipe criticality in the high strain conditions: applied longitudinal strains, strength of the base material, strength of the weld metal, CTOD, flaw size. The acceptable probability of failure depends on the consequences of failure for the structure; for the offshore pipeline reference is made to the standard OST101 (DNV 2000). This approach avoids the use of "all purpose" safety factors for ECA evaluations and allows to optimise the final pipeline configuration on the seabed in high strain conditions and to considerably reduce the amounts of the intervention works still keeping the appropriate level of safety.