Predictive maintenance (PdM) is the leading edge type of maintenance. Its principles are currently broadly used to maintain industrial assets [16]. Yet PdM is as yet not embraced by the pipeline industry. The paper describes a comprehensive practical risk based methodology of predictive maintenance of pipelines for different criteria of failure. For pipeline systems the main criterion is integrity. One of the main causes of loss of containment is pipe wall defects which grow in time. Any type of analysis of pipeline state (residual life time, probability of failure (POF), etc.,) is based on the sizes of discovered defects, which are assessed during the ILI or DA. In the developed methodology pipeline strength is assessed using one of the five internationally recognized design codes (the B31G, B31mod, DNV, Battelle, Shell 92). The pipeline POF is calculated by the comprehensive Gram-Charlier-Edgeworth method [14]. Having in mind that the repair actions are executed on particular cross-sections of the pipeline, the POF are calculated for each defect present in the pipeline. When calculating POFs, the defect sizes (depth, length and width), wall thickness and pipe diameter, SMYS of the pipe material, the radial and longitudinal corrosion rates, and operating pressure (OP) are considered random variables each distributed according to its PDF. In the proposed method of PdM of pipelines the remaining life time can be assessed using following criteria: POF = Qth; dd = 80%wt; SMOP = MAOP; ERF = MAOP/SMOP, if ERF ≥ 1, the pipeline needs immediate repair; dd = 100%wt. Here Qth is the ultimate permissible POF, dd is the depth of the most dangerous defect, wt is pipe wall thickness, SMOP is the maximal safe operating pressure SMOP = DF·Pf, MAOP is the Maximum Allowable Operating Pressure, Pf is the failure pressure, DF is the design factor (for B31Gmod DF = 1.39), ERF is the Estimated Repair Factor. The above criteria are arranged in descending order according to the growing level of their severity in time. The prediction of future sizes of growing defects and the pipeline remaining life time are obtained by using consistent assessments of their corrosion rates CRs. In the PdM methodology these CRs may be considered as deterministic, semi-probabilistic or fully stochastic values. Formulas are given for assessing the CRs using results of one ILI, two consecutive ILI, with or without verification measurements, and for the case when several independent types of measurements are used to assess the defect sizes. The paper describes results of implementation of the developed methodology on a real life pipeline. The time to reach each of the limit states given above was calculated, using results of two consecutive ILI divided by a three year interval. Knowledge of these arrival times permits minimizing the maintenance expenditures without creating any threats to its integrity and safety.
Residual Life Prediction of Gas-Engine Turbine Blades Based on Damage Surrogate-Assisted Modeling