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Corrosion Resistant Alloy for OCTG

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Corrosion Resistant Alloy for OCTG

At a glance the OCTG Corrosion Resistant Alloy (CRA) market is a very small part: about 0.1% of the world wide stainless steel production. But this part is fundamental for the exploration and production of gas mainly in the more and more severe conditions.

Definition and history

There are several ways for managing downhole corrosion, from doing nothing to implement immune materials like CRA. For the end user, the main purpose of CRA tubing is not to resist corrosion but to find out the overall less expensive solution. In the oil and gas exploration, in manyareas effluents of the wells are corrosive and the trend is on the increase.
The stainless steels appeared around 1870, and at the beginning they were named “rustless”.
The first target was to avoid corrosion in the guns for weapons. Those rustless alloys then demonstrated their advantages for many other usages like in chemical industry. The true development of stainless steels large mass production started in the sixties with the steel melting and refining technologies such as AOD and VOD. In the seventies the development of gas research and production in corrosive wells leaded to develop specific OCTG grades.

(© Tubacex)


Metallurgists consider steel as stainless if it contains a minimum 10.5% Chromium that creates a protective passive layer at the surface of the product against corrosion. Some elements like Molybdenum, Copper… can reinforce the corrosion resistance.
Stainless steels are sorted by ISO 13680 and API 5CRA in 4 classes related to their metallurgical structure: martensitic, ferritic, austenitic-ferritic (or duplex) and austenitic. Martensitic (mainly 13%Cr and super 13%Cr) are hardened by appropriate heat treatment, but their use is limited to environment with low H2S partial pressures. Contrary to duplex and austenitic, ferritic alloys are not used in OCTG: their mechanical characteristics are very difficult to strengthen by cold hardening.
Cold hardening (or also known as cold working or strain hardening) is the strengthening of a metal by plastic deformation. Shaping the product at ambient temperature it allows reaching desired yield strength and final dimensions.  It is the main process used for CRA.


Worldwide volume

Worldwide volume of stainless steel production is about 30,000,000 tons/year (flat and long
products, all industries) and main alloys used for production are Chromium (Cr), Nickel (Ni), Molybdenium (Mo) and Tugnsten (W). Price of those alloys (ex : 3 USD/Kg for CR, 20 USD/Kg for Ni, …) and their proportion in the final product (31% of Ni in a 28% Cr for ex) as well as their quotations on stock markets explain their high costs and their daily fluctuations.

Production process

Stainless steel production also requires specific mills. The high alloy content and the required thermo-chemical equilibrium lead to refine steel using AOD (Argon Oxygen Decarburisation) or VOD (Vacuum Oxygen Decarburisation) operations. Steel is then casted into ingots, then transformed into bars that will be used to produce seamless pipes after piercing, extrusion (extruded product are so called hollow) and cold working. This specific process participates also to their price level.


Choice of alloy depends upon the severity of the environment and the strength requirements and because presence of H2S and chlorides complicates the selection of alloy for a specific service ISO 15156 / NACE MR0175 provides guidance on the material selection process.
OCTG CRA shall be used as soon as partial pressure of CO2 (pCO2) is above 2psi (12kPa). Then depending on H2S partial pressure (pH2S) NACE MR0175 recommends:

- For pH2S up to 0.1 bar (1.5 psi): Martensitic (13% Cr and Super 13% Cr)
- For pH2S from 1.5 to 3 psi: Duplex (25% Cr and Super 25% Cr)
- If pH2S higher than 3 psi: Austenitic (28% Cr and above)

Well temperature and demanded mechanical resistance in situ are also 2 other discriminating factors.
Duplex and austenitic have weak tensile performances: their Specified Minimum Yield Strength (SMYS) is between 60 and 80ksi. To get stronger tensile performances, cold working is needed (either drawing or pilgering). The yield strength range is related to the material ability to get the adequate reduction ratio (Reduction Of Area or ROA) between hollow and cold worked product. Cold hardening can achieve on CRA SMYS 110 to 140ksi: interest is to keep the same level of resistance for lower weight i.e. lower cost.


Market evolution

Worldwide OCTG yearly demand is estimated at about 7,000,000 tons among which :
- Martensitic alloys represent about 200,000 tons/year
- Duplex and austenitic demand is between 25,000 to 30,000 tons/year.
CRA used in OCTG (from 225,000 to 230,000 tons/year) is then a niche among the worldwide stainless steel market (30,000,000 tons/year).
Meanwhile, despite its small market volume and their costs:
- Market trend for Duplex and austenitic used in OCTG is at growth: it is estimated to be at about +25% on the 2012-2015 period. Additional needs will mainly be brought by the Brazilian field of Tupi which long term estimated volumes (around 10,000 tons/year by 2015) should compensate the decrease that will happen in other market areas mainly because of depletion of oil and gas fields.
- CRA contribution to sales is not negligible and has to be disconnected of the volumes sold. Although in 2010, CRA represented a negligible 0.2 % of the total volume of pipes delivered
by Vallourec it represented 1.1 % of its sales. Those 2 proportions are expected to significantly increase in the coming years.
Major market areas are located in North Sea (ex: Elgin Franklin), Middle East (ex: Shah, South Pars…), Brazil (ex: Tupi) and GOM. Austenitic represent 70% of the OCTG CRA market and large diameters (6-5/8” and 7” mainly) represent 65% of the worldwide need. On a long term, market trend is to the request of larger diameters (9-5/8” and above).


Vallourec expertise

Vallourec is an integrated manufacturer of seamless pipes OCTG from carbon steel up to 13% CR and Super 13%Cr steels. For CRA (22% Cr and above), Vallourec has established its strategic partnership with the Spanish Supplier Tubacex, an integrated world leader in the manufacturing of seamless stainless tubes, in order to optimize the production of seamless stainless tubular for OCTG. Partnership was signed and announced on February 2009.
In that alliance Tubacex is in charge of supplying high quality CRA tubular when Vallourec has responsibility of technical and communication relationship towards end-users, threading and commercialization of final.
This organization is aimed at offering to the market an optimized synergy of the respective areas of excellence of both companies:
- Vallourec expertise in threading and testing of leader proprietary connections of the VAM family
- VAM field service support on VAM connection
- VAM licensee / repair network on VAM connection
- Tubacex expertise in stainless seamless tube production for Oil & Gas and non Oil & Gas products
- Tubacex integrated production including steel mill and tubular production
- Cooperative R&D and technical expertise on stainless grades production and testing, with full support from Vallourec corrosion and metallurgy research center (Vallourec Research Aulnoye – VRA)
The partnership also beneficiates from Vallourec past experience in CRA supply for OCTG. Tubacex and Vallourec have notably conducted thorough industrial qualification of OCTG production according to Vallourec proprietary specifications in order to ensure consistency in quality of supply.
All CRA grades are produced according to proprietary Vallourec specifications and can be threaded with major connections in the VAM family. VAM connections have an extensive qualification history on CRA and are threaded on about 80 % of all OCTG CRA supplied worldwide.
Present on the OCTG CRA market since the eighties, Vallourec has already delivered numerous customers on that different market areas:
- Total, British Gas, BP, … in North Sea
- ADMA, ADCO, KOC, … in Middle East
- Petrobras in Brazil
- But also: Gazprom, Exxon, Shell, Sinopec, Talisman, Tullow … in other regions
Like for all OCTG, CRA Customers follow a qualification process that includes check of Quality Assurance / Control, ability to produce volumes and grades and quality of the product through corrosion tests. Two Customer policies co-exist on this last item:
- One is based on specification without reference to field conditions, which are unknown by supplier
- Other is based according to a medium that approximate field specific conditions
In these 2 ways of qualification, Vallourec can give some advices, but final choice of grade remains in all cases under Customer responsibility.
The main type of corrosion tests demanded is with an applied stress. There are also tests without stress that quantify the resistance of localised or generalised corrosion. They are performed at different temperatures. Examination of coupons after test quantifies the level of corrosion. Measurement of attack allows Customer to rank material from different suppliers and reinforce its selection choice.
Tests with stress are split between test within elastic domain and above elastic. They are screening tests with different partial pressures of CO2, H2S and temperatures and all other parameters that could be given by customers and depending of well conditions.
Tests within elastic domain are trials to understand the material behaviour as in the well. Demanded tests are NACE A (Tensile) or C (C-ring) as described in NACE TM0177.
Test above elastic domain is SSRT (Slow Strain Rate Test) conducted up to rupture. “Slow” because 1 test lasts about 1 day.

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