Assessment of the reliability level embedded in pipeline design codes

摘要

Recently a prolonged discussion among specialists about the meaning of the probabilities of failure (PoF) produced by different reliability analysis methods appeared in pipeline journals. This paper, which was a long time in the making, is a follow-up on the discussion and analyses of the actual reliability level which was empirically embedded in codes for pipeline design (B31G, B31 Gmod, Shell92, and Battelle (PCORRC)) and the Russian Building Regulations(BR) for Main Pipelines , using a real pipeline as an example. The assessment of the actual reliability level empirically embedded in BR is based on assessing the order of the quintiles of strength parameters (design values of tensile strength and yield strength of the pipe material) and load (internal pressure) for the pipeline. This approach establishes a direct connection between the deterministic safety factors used in the BR and the level of reliability of the pipelines associated with these safety factors. The actual reliability level, empirically embedded in international codes, is calculated as the probability that the limit state function (LSF) of ideal pipeline (without defects) is positive: LSF = P_f - P_(op), where P_f is the failure pressure of an ideal pipe, which is estimated by the design code of interest, and P_(op) is the operating pressure. The failure and operating pressures are considered as random variables.The expression for this probability was obtained analytically and in closed form. Recommendations are also presented for choosing probability distributions and statistical parameters for random variables (RVs). Extensive calculations were undertaken to determine the reliability levels which are actually embedded in the analysed international pipeline design codes.This paper concludes that the considered international codes are very reliable, as they produce very safe designs of pipelines with very low PoF, providing a large safety margin. The algorithm presented in this paper allows for determining a relationship between the current level of pipeline degradation (in terms of PoF), with its current safety margin as a function of time. All the calculations in this paper were performed using MathCAD, and illustrations of these calculations are also presented.