High Temperature Hydrogen Attack in API RP 581 RBI Methodology: A Commentary

Introduction and Scope

Risk based inspection (RBI) is implemented by many oil and gas operators around the world. It is utilized to obtain an understanding of the risk profile of static assets, establish inspection intervals, and develop inspection plans. RBI methodologies were developed by industry trade organizations such as API, EEMUA, and ASME, as well as by owner/operator companies and consulting and technology providers. Despite the similarities in approach, there are differences in the details of the methodologies. For this reason, it is important to understand the different methodologies and their respective strengths, weaknesses, and relevance, before applying one. In this article, the focus is on API Recommended Practice (RP) 581 Risk-Based Inspection Methodology, 3rd Edition.^[1]

Risk is a combination of the probability of failure (POF) and Consequence of Failure (COF). The damage factor (DF) is one of few factors for the calculation of POF with a focus on statistical quantification of damage in consideration of time and inspection activities. In order to calculate relative risks for equipment in the oil and gas and petrochemical industries which are subject to a large number of damage mechanisms, API RP 581 includes six damage modes: thinning, stress corrosion cracking, external damage, high temperature hydrogen attack (HTHA), piping mechanical fatigue, and brittle fracture. This article explores API RP 581’s approach to HTHA DF calculations to obtain the POF that subsequently affects risk assessment and management. Sample calculations are provided to affirm an understanding of the concept along with reflections on the approach.

A Recap on HTHA

HTHA is a damage mechanism that has been thoroughly covered in Inspectioneering Journal, so this article will simply provide a brief recap. In short, HTHA affects hydroprocessing, catalytic reformers, and hydrogen cleanup units where carbon steel and chromium-molybdenum alloys are exposed to hydrogen at elevated temperature and pressures as described by API RP 571 3rd edition. API RP 571 also describes the stages of formation and development of HTHA from bubbles/cavities formations, cracking, fissuring to blistering. Among listed affected units are hydrotreaters (desulfurizers), hydrocrackers, catalytic reformers and pressure swing absorption units.^[2]