Effect of correlated input parameters on the failure probability of pipelines with corrosion defects by using FITNET FFS procedure

The paper presents a probabilistic methodology considering the correlations between the input variables for the failure probability evaluation of corroding pipelines based on the corrosion module of the FITNET FFS procedure. A computer program based on this method is developed to calculate the failure probability of pipelines by considering different numbers of defects and different elapsed times. In case of one defect, the correlation between the initial defect depth and the initial defect length has the most significant impact on the failure probability of the pipeline. If the correlations between these two parameters for an individual defect are not considered, the prediction results are nonconservative when the failure probability is below 40% and conservative when it is above 40%. In case of multiple defects, the independent assumption of variables generally leads to a conservative estimate of the failure probability. The conservatism increases if the elapsed time and/or the actual correlation coefficients of the variables increase. The correlation of the operating pressure, the initial defect depth and material ultimate tensile strength at the location of different defects has a larger impact on the failure probability than the correlation of other parameters at different defects. The upper bound failure probability calculated by engineering method corresponds to that calculated using the presented method without considering the correlation between input parameters. This confirms the validity of the presented model.

► Correlation between d0 and L0 has the biggest impact on the probability.
► Independent assumption of variables generally leads to a conservative estimate.
► The conservatism increases with the elapsed time and correlation coefficients.
► Upper bound of engineering method corresponds to that without correlation input.