Development of a Probabilistic Fracture Mechanics Framework for the Life Extension Assessment of Ageing Petroleum Pressure Vessels in Nigerian Refineries

The continued operation of ageing pressure vessels beyond their original design life in Nigeria's petroleum refinery infrastructure requires a rigorous, probabilistic fracture mechanics-based life extension assessment methodology that quantitatively accounts for uncertainties in flaw sizing from inspection, material property scatter, operating stress variability, and environmental degradation, none of which are adequately addressed in the deterministic fitness-for-service standards currently applied by Nigerian refinery operators. This research develops a probabilistic fracture mechanics framework for the remaining life extension assessment of ageing refinery pressure vessels, integrating probabilistic defect characterisation from inspection data, Monte Carlo simulation of crack growth trajectories, and Bayesian updating of life estimates as successive inspection results become available. The framework is anchored on the API 579-1 fitness-for-service Level 2 and Level 3 assessment procedures, extended by first-order reliability method and subset simulation methods to compute failure probabilities for complex multi-mode failure scenarios. Material fracture toughness and fatigue crack growth rate distributions are characterised experimentally for representative pressure vessel steel grades from Nigerian refinery stock using standard fracture mechanics test methods. The framework is applied to three case study vessels at NNPC refineries with documented service histories and inspection records, generating remaining life probability distributions with quantified confidence bounds under continued operation versus different maintenance intervention strategies. Sensitivity analyses identify flaw detection probability and material fracture toughness uncertainty as the dominant contributors to residual life prediction uncertainty. Keywords: probabilistic fracture mechanics, pressure vessel, fitness-for-service, life extension, Nigerian refinery.