Assessment of the Reliability Level Embedded in Pipeline Design Codes

Recently a long time discussion among specialists about the meaning of the probabilities of failure (POF) produced by different reliability analysis methods surfaced in pipeline journals. This paper, which was a long time in the making, is a follow-up on the discussion and analyses the actual reliability level which was empirically embedded in codes for pipeline design [B31G, B31Gmod, Shell92 and Battelle (PCORRC)] and Building Standard (BR) Main Pipelines #2.05.06-85, using a real pipeline as an example. 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) on the pipeline. This approach allows direct connection of the deterministic safety coefficients used in the BR with the level of reliability of the pipelines associated with these coefficients. 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 = Pf − Pop, where Pf is the failure pressure of an ideal pipe, which is estimated by any design code; Pop is the operating pressure. The failure and operating pressure 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 permitted revealing the reliability levels which are actually present in the analyzed international pipeline design codes. In a nutshell, the paper proves that the international codes under consideration are very reliable, as they produce very safe designs of pipelines with very low POF, and, hence, provide large safety coefficients, and that the algorithm developed in the paper permits connecting the current level of pipeline degradation (in terms of POF), with its current safety coefficient, which, in this case, is a function of time. All calculating in the paper where performed using MathCAD. Illustrations of these calculations are also presented in the paper.