Importance of Qualitative Risk-Based Chloride Mapping in Material Selection in HC Services
Authors: Al Wahedi Faisal , Khalil Hisham , Gupta Kapil , Al Habshi Shaikh , Al Shehhi Shayma 
Published: October 2022

Material selection of sour hydrocarbon processing plant is vital exercise. The well-known industry guideline for selection of Corrosion Resistance Alloys (CRAs) is NACE MR0175/ISO 15156-3 which is always complemented by either documented field experience or Laboratory qualification testing records. The chloride content is one of key controlling parameters, not only for material selection of new equipment or unit, but also for establishing baseline Integrity Operating Windows (IOWs) in sour service. Nevertheless, the significance of chloride ions in wet gas system was highly recognized by end-users after several failures in sour gas processing units in the last decade which was mainly attributed to Stress Corrosion Cracking (SCC) failure in presence of chloride ions which used to be estimated as nil or less than 50ppmw during equipment design phase. Hence, the objective of this work is to establish an appropriate understanding of chloride distribution in sour gas processing units at early phase of asset development such as early Conceptual Studies and Front-End Engineering Design (FEED) for appropriate risk-based and life cycle cost effective material selection.

A qualitative risk-based methodology has been adopted for estimating potential chloride ions carry over into downstream process streams and equipment. The assessment methodology covers, not only normal operating scenarios, but also other mode of operations such as system startup, gas recycling, gas compressor's settle out conditions, etc. Preliminary chloride levels anticipated in various streams were qualitatively estimated by Process Engineers considering potential scenarios such as formation water carry over, condensation, evaporation, etc. However, a peer-review session was also held in participation of Materials & Corrosion engineers, Mechanical engineers and Operation engineers for validating assumptions and fine-tuning estimated chloride levels.

The risk-based estimated chloride levels have been utilized in material selection of systems such as Slug Catcher gas outlet line, LP/MP Gas Compression Unit as well as Fuel Gas Sweetening and Conditioning Units. Equipment material selection and estimated chloride levels have been additionally evaluated through Corrosion Risk Assessment Study (CRAS) to demonstrate that all potential threats are mitigated as low as reasonably possible (ALARP) with life cycle cost effective barriers.

Chloride mapping in gas system is a newly engineering practice that has been recently performed in recent onshore / offshore project. Although there is no available industry-recognized engineering software that can be utilized for such purpose, it is highly recommended for Engineering Consultants to develop a qualitative or semi-quantitative model for risk-based chloride level estimation in gas system. It should be noted that this work should be carried out at early stage of project such as Pre-FEED and FEED phase and should be stipulated in Consultant's scope of work. Meanwhile, the engagement of end-user's process, corrosion, materials, and integrity engineers is highly required. Lesson Learnt from past project and operating experience of the plant shall also be taken into account while deciding the parameters for the studies