Sour Service grades
The worst enemies of H2S
The design rules for completions have become more stringent since the Macondo platform explosion, with oil companies now insisting on a triple protective barrier against H2S: tubing, liner and production casing. The 3 tubular elements must be capable of resisting H2S-induced Sulfide Stress Cracking (SSC), one of the greatest threats to the integrity of oil and gas wells. Invisible and odorless above 100 ppm, H2S can destroy lives in minutes and equipment in several hours. The only way to counter its force is to use Sour Service resistant grades, such as those developed and produced by Vallourec.
Definitions and standards
H2S gas (hydrogen sulfide) is made up of combined atoms of sulfur and hydrogen. It can occur in bacterial decomposition of organic matter in oxygen-poor environments (methanation). It is also naturally present in oil, natural gas, volcanic gases and hot springs. It is an inflammable, colorless gas and can numb the olfactory nerve above 100 ppm. It reacts especially with metals such as steel, even stainless steel.
ISO 8044 standard defines corrosion
Corrosion is a physicochemical interaction between a metal and its environment that results in changes in the properties of the metal, and which may lead to significant impairment of the function of the metal, the environment, or the technical system, of which these form part.
The National Association of Corrosion Engineers (NACE) formed a committee in the 1950’s to help understand and avoid failures caused by H2S. A first standard (MR0175) was published in 1975. It defined Sour Service conditions as a threshold on the partial pressure of H2S above 0.05 psi.
Taking pH into account…
Today, a revision of NACE MR 0175 provides a new international standard when merged with ISO 15156. It combines the findings of the European Federation of Corrosion (EFC) with those of NACE. This standard defines the metallic material requirements to provide resistance to Sulfide Stress Cracking (SSC). It introduced pH as an additional major factor of influence on the environment, along with specific tests that have become standard in order to qualify material for Sour Service environment.
… and temperature
High pressure often relates to high temperature which is well known to be an inactivating factor of the SSC phenomenon (see table). In fact, high temperatures carry energy to hydrogen atoms which circulate more freely in metal. High temperatures therefore allow the use of steel grades classified as non-Sour Service.
The risk actually occurs at the top of the well where the pressure is still high, but temperature has decreased due to thermal losses, thus activating Sulfide Stress Cracking. This is particularly critical for offshore wells where deep sea temperatures rarely exceed 4 or 5°C. In this case, materials defined in API 5CT (ISO 11960) comply with NACE MR0175, depending on temperature level. Outside these parameters, proprietary grades are required.
Test specimens differ in the way stress is applied and can fit into 2 categories:
Sulfide Stress Cracking
The physical phenomenon associated with Sour Service environment is known as Sulfide Stress Cracking (SSC). H2S in combination with water and low pH will release free hydrogen. Due to their small size, the hydrogen atoms are absorbed by the material and interact with the steel which becomes brittle. Two key activating factors are low temperature and high stress state of the material. Under unfavorable combinations of all these factors, a crack can appear in the material and propagate until catastrophic failure.
It is a brittle fracture, without prior plastic deformation and very sudden, with no warning sign. This is what makes it a particularly dangerous phenomenon.
Choice of material and risk management
It is readily apparent that choosing an unsuitable material can have very serious consequences. There are two possible scenarios: if an operator drills in a known environment (field already explored), he will rely on past experience and either opt for Sour Service grades or not depending on the presence of H2S. If he intervenes in unknown territory, he is better off choosing premium Sour Service products in order to limit the risks. This is the strategy now adopted by most oil companies, the additional costs for Sour Service products being very low compared to the consequences of a potential pipe leak.
The objective is to assess whether a given material is suitable for Sour Service conditions. NACE TM 0177 has defined and normalized 4 tests of which methods A to D are the most used for OCTG. Specimens are generally tested in a critical and normalized Sour Service environment called “environment A”. In fit for purpose conditions, modified environments called “environment C” may be used.
To be considered suitable for Sour Service environments, a material has to pass at least one of these tests.
Test specimens differ in the way stress is applied and can fit into 2 categories:
L80, C90, T95 and C110 are dedicated API grades suitable for Sour Service environments. They comply with the following requirements:
– Chemical composition
– Grain size finer than ASTM 5
– Hardness limitation
– NACE tests (TM 0177), methods A, B or D.
Grades defined in API do not cover all situations faced by oil & gas operators. Nowadays, Sour Service environments pose further challenges with deeper wells, higher pressures and extreme corrosive conditions.
Vallourec material suitable for such conditions provides solutions to push these boundaries in a safe and reliable way.
It has been proven that material suitable for Sour Service environments should be developed and manufactured by carefully designing and controlling the following parameters:
– Cleanliness > To avoid inclusions and segregation
– Grain size > Fine martensitic microstructure
– Homogeneity > Even distribution of characteristics
– Stability > Complete release of quenching and straightening stresses
To achieve such quality, Vallourec relies on its knowledge and extensive experience in steel grade development and manufacturing processes, notably:
– Chromium – Molybdenum steel chemistry
– Low Sulfur and Phosphorus content
– Improved Grain size finer than ASTM 7
– Dedicated quenching process, adapted to heavy products up to 2” in wall thickness (ID-OD quenching)
– 100% Non-Destructive Examination (NDE)
– Extensive database of NACE test results (more than 10 000)
All this leads to a wider range of applications and an enhanced safety margin through SSC resistance, guaranteed and demonstrated by up to 90% of the Specified Minimum Yield Strength vs. 80% with API (NACE method A test).
In line with its strategy as world leader in premium tubular solutions, Vallourec has constantly invested so that its clients can meet the most complex challenges. The number of specialists dedicated to new steel grade development has increased five-fold in the last fifteen years, while testing capabilities have recently doubled in Brazil and France. Steel and tube mills have been further improved in order to produce more sophisticated and reliable materials. Sour Service products are a significant part of this.
Vallourec OCTG offer: a complete range of products
> S series “Sour Service”
– Available in grades 80, 90 and 95 ksi
– Same chemistry and process as SS series
– 85% threshold stress guaranty, higher than 80% in API
– Reduced lead time, as fast as for standard grades, thanks to the mill quality record that proves SSC resistance without additional NACE method A test
> SS series “Severe Sour Service”
– Available in grades 80 to 125 ksi
– Fully compliant with API (or its QA/QC philosophy for VM125 SS grade)
– 85% / 90% threshold stress guaranty, higher than 80% in API
– Delivered tested with NACE method A
> SS-D series “Severe Sour Service NACE-D”
– Available in grades 95 to 125 ksi
– Fully compliant with API (or its QA/QC philosophy for VM125 SS-D)
– 85% / 90% threshold stress guaranty, higher than API
– K1SSC guaranty, higher than API
– Delivered tested with NACE method D
> IRP series “Severe Sour Service NACE A&D”
– Available in grades 80 to 95 ksi
– Fully compliant with API
– 90% threshold stress guaranty, higher than API
– K1SSC guaranty, equal or higher than API
– Delivered tested with NACE methods A & D
> HCSS series “High Collapse & Sour Service”
Vallourec has combined its expertise on both Sour Service applications and High Pressure environments to cope with the most demanding High Pressure – High Temperature (HP-HT) projects. This has led to a High Collapse & Sour Service product line.
> Tailor-made solutions
Beyond API and proprietary Sour Service grades, tailor-made solutions or characteristics may be required to achieve our customers’ highest profile projects.