Impact of H2S Souring on PDO Facilities Materials of Construction

Abstract One of the raising challenges in managing Petroleum Development of Oman (PDO) facilities is field souring. Increasing trends and alarms of H2S in station inlet have resulted in changing the facility design criteria from sweet to sour facility. The increasing levels will also influence the downstream facilities and export pipelines. This technical paper presents assessment of the impact on the Materials of Construction (MoC) of the on-plot facility and identify other H2S limits (e.g. crude and gas exports). Given the unpredictable nature of Sulphide Stress Cracking (SSC) and Stress Corrosion cracking (SCC), it is not possible to estimate remnant life or time to failure as this form of degradation is more dependent on process conditions. The approach described in this material assessment follows ISO-151561 design limit. Suitability of materials against current H2S conditions was firstly done through verification of H2S measurements and pressure readings. The facilities were assessed based on corrosion loops as per the Corrosion Management Manual. Piping and equipment material types were confirmed based on PEFS and GA drawings across different loops to define H2S partial pressure limit as per ISO 151561. H2S threshold in ppm was then calculated based on operating, trip, and design pressure as worst case. Based on above assessment, it was found that H2S levels have exceeded acceptable value based on design pressure. However, it was acceptable based on trip set point and operating pressure. For CRA material in wet gas system, the chloride values were confirmed to be low (e.g. less than 5000ppm) which gives more tolerance in H2S partial pressure limits. Other limitation in aging facilities is the unknown material condition for CS whether sour rated or not. Thus, site hardness testing was conducted to verify hardness values were on the acceptable range for sour rated CS material. Mitigations were then identified with the asset based on identified risk. These mitigations include temporarily reducing H2S levels through production deferment, material upgrade, ALARP methodology. H2S levels shows increasing trend which means future values may exceed trip set point values. Therefore, short term mitigation must be implemented to avoid H2S level exceedance including temporarily reducing H2S levels through production deferment. Furthermore, long term mitigations were implemented including material upgrade for certain identified equipment and piping. There is currently a lack of procedures and methodologies to be followed to address this challenge. This paper showcases thoroughly the methodology followed to assess the impact of field souring on the materials of construction.