Reliability analysis of pipelines during laying, considering ultimate strength under combined loads

Structural reliability methods are applied to establish a measure of safety for pipelines during laying and especially to calibrate semi-probabilistic ultimate limit state criteria based on measures of uncertainty, method of reliability, and a given target level. Ultimate collapse of thick tubes und er combined external pressure, tension, aid bending loads are studied apply ing the finite element method. Nonlinear effects of large deformations, eff ects of initial ovality, residual stresses, strain-hardening, yield anisotr opy, and loading paths were accounted for in the analysis. A set of interac tion equations is proposed Load effects in the pipelines during installatio n by the S-lay method are studied. The effects of uncertainties in yield st ress, mass, stiffness of the stinger, response amplitude operator and peak period for the wave spectrum were accounted for in the analysis. The major factors affecting strain concentration due to concrete coating are taken in to account. A combination of design point calculation and importance sampli ng procedure is used to calculate the probability of failure. The study inc ludes calibration of partial safety factors for the design format selected. The most important random variable is the model uncertainty for bending ca pacity, while the uncertainty of the load effect has minor importance for t he probability of failure. The system effect is taken into account consider ing the correlation along the pipeline. The probability of failure is refer red both to the total laying period as welt as a 3-h period demonstrating t hat the target level needs to be defined in view of the reference time pei- ind. [S0892-7219(00)01501-6].