Technical Basis for Revision of API 574 Piping Structural Minimal Thickness Values

Introduction

In March 2020, Marsh JLT Specialty published a study of the 100 largest industry failures (each failure must be > $175 million in loss) from 1972 to 2019 [1]. Over that 47-year stretch, the study shows an average loss of $432 million per failure, or $43.2 billion in total loss. However, this average increased dramatically when reviewing the most recent industry failures from 2018 to 2019. Those eight failures had an average loss of $562 million per failure, or $4.5 billion in total loss. Further, in the full time period, 39% of the 100 largest losses occurred in refineries, 26% in petrochemical facilities, and 24% in upstream facilities. When reviewing the 2018 to 2019 time period, refineries accounted for 50% of the asset losses, with petrochemical facilities at 25% and gas processing at 25%. Meanwhile, upstream facilities accounted for 0% of the major failures noted from 2018 to 2019 (see Table 1). Refining represents the largest portion of the industry failures, and when investigated further the biggest culprit is associated with piping, particularly thinning of piping (the other common causes were start-up/shutdown incidents, natural disasters, or not disclosed).

To manage piping systems for safe and reliable operation, a robust mechanical integrity program will include a focus on piping inspection and provide guidance on acceptable limits for evaluating the piping thickness readings being obtained. Establishing a required minimum thickness (i.e., structural minimum thickness) is a key element for this evaluation. Unlike vessels, the piping structural minimum thickness (T_struct) is not solely governed by hoop stress ( ; where P=internal pressure, r=outside radius, and t=pipe thickness) due to pressure only, but can be governed by longitudinal stress ( ; where F=axial force, A=pipe cross-section area, M=bending moment, and S=pipe section modulus) resulting from pressure and bending moment due to weight and/or thermal loads (see plot of stress orientation in Figure 1). To determine if the required thickness is governed by longitudinal stress, the system would need to be assessed for sustained loads (weight plus pressure), and assessed for thermal loads, if appropriate.