The detrimental effects as a result of sour service can range from small pinhole leaks to catastrophic failure in pipelines owing to a number of phenomena as a result of the sour gas environment, namely: stress corrosion cracking, sulphide stress corrosion cracking, hydrogen-induced cracking, hydrogen embrittlement and exfoliation, and sulphide-oriented hydrogen-induced cracking.
Mitigation and/or control of cracking in sour gas pipelines can be approached in a number of ways, namely steel manufacturing control, materials and fabrication processes, controlling the environment, and isolating the components from the sour environment.
Mitigation at the design stage
Mitigation must start at ‘square one’ – namely, materials selection, which requires careful review, testing and control such that they will be stipulated as ‘fit-for purpose’ for sour service. The materials selection process should reflect project-specific requirements, intended design life, costings, failure evaluations as well as environmental considerations, etc. As an absolute minimum, the following should be taken into consideration:


Design life and system availability;
  • Pipeline system design – avoidance of deadlegs to mitigate stagnant conditions, correct pipeline sizing to reduce water hold ups and solids deposition;
  • Facilities and process systems design and layout – gas dehydration;
  • Full evaluation of operational and process conditions – H2S, CO2, O2 contents, pressures, temperatures, flow velocities and regimes, entrained solids, biological activity, etc.;
  • Damage mechanism and failure modes with respect to health safety and environmental consequences; and,
  • Materials availability and cost implications.
Notwithstanding the use of carbon and low-alloy steels as sour service linepipe and its susceptibility to various types of cracking, the various materials treatment/processes such as heat treatment, cold working or both must be carefully controlled. Where cold working or rolling of the steel plate may occur, thermal stress relief must take place to mitigate any residual stresses that may remain within the steel. Similarly, where production fluids contains a sulphur or CO2 content which is too high for the corrosion-resistant properties of carbon steel alone, a corrosion-resistant alloy (CRA) is often employed.
It provides a good balance between the mechanical properties of carbon steel and the corrosion resistant properties of a CRA. The use of CRAs such as Inconel 625 or 825 to form solid pipe is neither considered to be the norm nor can be economically justified owing to its prohibitive costs. Therefore, the synergistic combination of carbon steel and CRA together provides a cost-effective and optimum combination of materials.
Such a combination of materials can be manufactured through metallurgical bonding, known as clad pipe (through co-extrusion, hot-rolled bonding, explosive bonding), or by only a mechanical bond between the CRA and steel (through thermohydraulic gripping), known as lined pipe.
Mitigation at the manufacturing stage
Manufacturing of sour linepipe requires optimum steel chemistry and ‘steel cleanliness’. The presence of free sulphur during steel manufacture causes a reduction in overall steel mechanical properties, especially toughness; which dictates the requirements for very low sulphur concentrations; typically 0.005–0.010 per cent.
The use of manganese as an alloying element (and having particular importance as a de-sulphuriser and de-oxidiser element) during steel manufacture provides a mechanism whereby any remaining sulphur can be removed to form manganese sulphide.
The manganese sulphide inclusions formed have a significant influence on the mechanical properties of steel – such as toughness, hardness etc., with their size, composition and numbers influencing steel cleanliness – and must therefore be removed. Such inclusions are usually benign in low-strength steels, and their transverse ductile toughness is sufficient to prevent any ductile fracture. However, for the typical higher strength steels used for linepipe applications, higher transverse toughness values are required.
As part of linepipe manufacture, steel plate is hot rolled, causing the inclusions to transform into an elongated morphology known as ‘stringers’, owing to their ductility. Often with sour service pipelines, the presence of atomic hydrogen can diffuse through the steel structure accumulating at the apex of stringers. Recombination of atomic into molecular hydrogen causes a pressure build-up at these stress concentrator sites, causing crack initiation and subsequent propagation over time. Characteristic cracking or ‘stepwise’ cracking where successive cracks are ‘joined’ are well documented in practice and in the literature.
In addition, the type of rolling is important in order to produce steels which are grain refined, can achieve high strength and toughness in the heat-affected zone (HAZ), provide excellent weldability and formability and, importantly, high-resistance against cold cracking. This is achievable through a process known as thermomechanical rolling process (i.e. deformation without recrystallisation) and thermomechanical controlled process, which combines thermomechanical rolling with accelerated cooling.
To ensure that the final steel product is free from detrimental ‘stringers’, the use of compounds such as calcium or cerium are typically employed, whereby its combination with sulphides is greater than with manganese and promotes a transformation of the elongated forms into spherical particles. In this way, any potential sites for hydrogen diffusion and potential steel cracking cannot now take place.
Prior to fabrication of production linepipe, pre-qualification testing is a normal procedure that includes hardness testing in accordance with NACE specification (NACE MR0175), and is carried out to ensure that during production, welding of linepipe achieves a hardness value on materials testing of no greater than 22 HRC (Rockwell Hardness scale) in order to obviate brittle fracture within the weld itself, parent metal and HAZ. Similarly, the welding procedure must introduce strict controls for welding parameters, welding consumables as well as control and storage of welding rods.
The hardness of parent materials and of welds and their HAZs play important roles in determining the sulphide stress cracking resistance of carbon and low-alloy steels. Hardness control can be an acceptable means of obtaining sulphide stress cracking resistance.
Mitigation at the operational stage
So far examined have been a number of mitigation methods which provide certain controls in terms of safeguarding sour service pipelines. In addition, can also be introduced (additional measures) during the pipeline operational and maintenance stage in the form of a robust pipeline management system.
Pipelines and chemicals management plays a critical role in all pipeline operations and more so with respect to sour service pipelines. Through the implementation of a well-planned risk-based inspection plan, a thorough internal and external examination of pipelines can be carried out and their condition assessed using methodologies such as remotely operated vehicles, divers, and intelligent pigging (in-line inspection). The collective data from the inspection and surveys provide a condition assessment of the internal and external pipeline condition, as well as providing a confidence statement as to the continued pipeline’s ‘fitness for purpose’ or otherwise to the pipeline operator.
In this case therefore, pipeline management must be robust such that both physical and chemicals maintenance is applied to pipelines in sour service. Pro-active pipeline pigging – both intelligent and maintenance – will ensure that the pipeline is kept clean but also that the internal/external condition of the pipeline shall be known. Periodic pigging of pipelines removes liquids, solids, various debris, and other contaminants. In addition, pig ‘trash’ analysis is a useful way to understand what is being transported within the pipeline.
Pipeline pigging is one of the most effective methods of not only cleaning the pipelines but also reducing the potential for any bacterial colonisation – such as sulfate-reducing bacteria (SRBs) – leading to microbiologically influenced corrosion attack, and under deposit corrosion effects, leading potentially to loss of pipeline containment or complete failure. It is important therefore that the selection, type and size of pigs is correctly made in order to ensure complete effectiveness and resulting cleanliness for the pipeline, as this plays an extremely important part in terms of improving the effectiveness of corrosion inhibitor and biocide treatments for sour pipelines in terms of chemical volumes and contact time.
Inhibitor and biocide treatments provide a barrier between the corrosive elements and the pipe surface itself and, dependent on requirements, can be applied either by a batch or continuous programme. All corrosion inhibitors and biocides used for pipelines will be recommended in accordance with manufacturers’ recommendations and specifications, as well as undergoing trials and testing (laboratory and field) to ensure that the correct materials are used. In the case of sour pipelines, determining the correct inhibitors, biocides as well as dosage rates and application methodologies becomes a critical task.
Corrosion monitoring plays an essential part in providing information as to the internal condition of the pipeline. The ongoing monitoring of acid gases such as H2S and CO2 should be periodically assessed, as changes in operating conditions over the lifetime of a pipeline will inevitably occur such as increasing water cuts, dissolved metals and entrained solids such as sand.
Sour service pipelines under the influence of SRBs have been well studied and documented to date. The use of bio-spools in low- and high-pressure systems in conjunction with other corrosion monitoring methods as discussed, can be well placed to monitor such bacteria presence and yield extremely useful data on corrosion activity and efficacy of biocide treatments for corrosion control.
Conclusion
Sour service pipelines carrying fluids or gases in addition to a wet internal environment causes problems to the pipeline leading to corrosion and potential loss of containment or complete breakdown of the pipeline. In addition, the presence of SRBs also play a critical role in generating hydrogen sulphide gas and equally cause potential pipeline corrosion problems.
The article outlines a number of mitigation methods with respect to sour service gas pipelines, such as during manufacturing and fabrication stages and, importantly, during the pipelines operational and management phases. The use of well-planned and structured inspection, maintenance and repair regimes together with a robust campaign of pipelines and chemicals management will be well placed to mitigate the effects of pipeline deterioration and potential failure as a result of sour gas.

source : http://pipelinesinternational.com/news/sour_gas_pipelines_how_do_we_deal_with_them/065073/