Fellow’s Corner

This series of features in Corrosion Management intends to highlight industry wide engineering experiences, practical opinions and guidance to allow improved awareness for the wider public, and focused advice to practicing technologists.  The series is prepared by ICorr Fellows who have made significant contributions to the field of corrosion management with superlative past industry involvements.  The first contribution in this series is “ Metallic Materials Optimisation in Hydrocarbon Production Systems”, by Bijan Kermani, FICorr.

Metallic Materials Optimisation in Hydrocarbon Production Systems

Hydrocarbon producing facilities are potentially subject to both external and internal corrosion threats: in the case of the former, from hostile and geographically remote operating environments, and in the latter from the presence of wet produced fluids and acid gases. Both of these threats continue to impact materials selection, engineering design and through life integrity management.  Selection and optimisation of appropriate materials, which can tolerate given production scenarios together with effective whole life corrosion management, remain key operational challenges and underpin successful hydrocarbon production, economy, safety and security. Correct choice of materials and their optimisation in such systems at the design stage is therefore an essential element of an effective corrosion management programme to achieve high reliability and trouble-free operation. The choice is governed by a number of principal parameters including adequate mechanical properties, corrosion performance, joining integrity, availability and cost. 

This Fellow’s Corner gives an outline of a materials optimisation strategy by combining a number of these key ingredients.  It takes advantage of materials with proven track record while describing attributes essential for such a holistic approach.  It focuses on internal corrosion by produced and injected fluids as the principal criteria in materials selection.  While descriptive, a basic of knowledge of materials and corrosion is nevertheless, highly advisable so that a fit for service solutions is achieved.

Amongst the parameters outlined above, two elements are elaborated further.

i. Corrosion Threats: given the conditions associated with hydrocarbon production and that of gas/water injection, internal corrosion must always be seen as a potential risk.  The risk becomes real once an aqueous phase is present and able to contact the material, providing a ready electrolyte for the corrosion reaction to occur.  The need to reliably handle wet hydrocarbons arises from the increasing number of fields where significant levels of CO2 and H2S are present under more arduous operating conditions.  In addition, the growth in the need for increased production which invariably entails water and/or gas injection to maintain reservoir pressure and/or enhance recovery can introduce O2 and the potential for microbiological activity which presents a different type of corrosion threat. 

While most classical forms of corrosion are encountered in hydrocarbon production, the principal types where the majority of failures occur remains limited.  The most prevalent types of damage encountered include metal-loss corrosion and localised corrosion manifested in the presence of CO2 (sweet corrosion) and H2S (sour corrosion) dissolved in the produced fluids and by the presence of dissolved oxygen in water injection systems.  These three types of corrosion threat should be addressed specifically in the material optimisation process when assessing corrosion risk. In addition, the potential risk of environmental induced cracking needs to be addressed effectively.

ii. Metallic Materials: The oil and gas industry sectors continue to lean heavily on the use of carbon and low alloy steels (CLAS) which are readily available in the volumes required and able to meet many of the mechanical, structural, fabrication and cost requirements.  The technology is well developed and for many applications these materials represent an economical choice.  However, the inherent corrosion resistance of CLASs is relatively low. Consequently, their successful application invariably requires combination with one or more whole-life forms of corrosion mitigation against both internal and external exposure conditions. 

A materials optimisation strategy requires an integration of all the above key parameters to allow the selection of the most suitable, safe and economical material option and corrosion control procedure.  The parameters in such a strategy should take advantage of two key elements of trusted/proven methods reflecting past experience as well innovative solutions. Considering the above elements, the methodology described here has adopted a stepwise process by first exploring the feasibility of using CLAS followed by its use in combination with corrosion inhibition (CI) and/or a corrosion allowance (CA). If this approach is not feasible, then corrosion resistant alloys (CRAs) need to be considered, all based on whole-life costing.

Governed by system corrosivity and the feasibility of corrosion mitigation measures, materials are selected accordingly. It is important to note that while CLASs are chosen primarily according to their general and localised metal loss corrosion resistance, with adequate resistance to different types of H2S induced cracking, CRAs are normally selected primarily based on their resistance to environmental induced cracking.  This latter threat includes sulphide stress cracking and chloride stress corrosion cracking or a combination thereof, as affected by the operating temperatures and conditions.  The exception for CRAs is under extreme conditions – typically a combination of high temperature, low pH, high CO2 and H2S – where general corrosion may also, or exclusively, have to be considered in the overall selection strategy. 

The simple overall approach to the optimisation strategy, shown in the figure, captures these necessary steps in finalising the materials choice. This simplified roadmap includes several key elements taking on board and incorporating (i) corrosion risk evaluation, (ii) operating conditions, (iii) corrosivity assessment, (iv) erosion velocity, (v) window of application of individual alloy, and last but no least (vi) whole life costing of potential materials options and corrosion mitigation methods. The simple methodology is based on utilisation of past successes and lessons learnt in effective use of CLASs and integration of key parameters to allow the selection of the most suitable, safe and economical material option and corrosion control measures. 

The roadmap follows a methodical route to highlighting options and the most appropriate and cost effective materials, and outperforms similar models through the unique integration of key parameters.  The strategy is applicable to optimisation of materials for all applications including downhole completions, surface and transportation facilities. 

A distinction should be made here between materials used for subsurface (wells), where welding and CA may not be applicable, in contrast to materials for above surface facilities (subsea, topside or transportation) where corrosion mitigation in the form of CI deployment, or CA, become feasible. 

Throughout the process, in the absence of reliable data, a methodical approach to performance evaluation needs to be put in place and be implemented.  This provides a flexible structure to allow realistic testing to enable input of complementary data to provide further confidence on their application.

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