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It is likely that oil and gas extraction will continue as a major source of global energy despite the growth of renewable energies. The proportion of oil and gas production from offshore areas has increased significantly over the last 30 years, and the growth of deep water production is expected to continue as easier fields have been exhausted. Oil and gas production remains a challenging area for material selection as conditions become increasingly severe, whilst cost pressures also affect the viability of such locations against alternatives. Corrosion resistant alloys are well-suited to the unique conditions experienced in oil and gas extraction, and their use will continue to grow.
A basic design requirement of any component is its mechanical strength – its ability to withstand a certain load without deformation. Although a number of mechanical properties are typically shown in product datasheets, the 0.2% proof stress indicates the stress needed to deform the metal plastically i.e. permanently, which is the most fundamental criteria.
Compared with austenitic grades, there is also a significant uplift in general corrosion resistance which can support substitution. Against nickel alloys there is a significant material cost saving available if other elements of the specification are appropriate. Duplex and super duplex stainless steels are of interest as their strengths are typically double those of the commonly available austenitic stainless steels (Alloy 316L, Alloy 254) and Nickel alloys (Alloy 625, Alloy 825). This allows component manufacturers to optimise designs and utilise smaller sections – aside from the obvious material cost saving, the suspended weight of components in subsea applications can be more beneficial.
The corrosion resistance of any given material is heavily dependent upon the specific environmental conditions. As a first review, the Pitting Resistance Equivalent number (PREN) is a theoretical way of comparing the pitting corrosion resistance of various types of metals based on their chemical compositions. Although the root cause of corrosion can originate from a number of different factors, it most commonly presents itself as pitting corrosion.
Typical Pitting Resistance Equivalent numbers for a range of corrosion resistant alloys
Note: PREN = %Cr + 3.3x %Mo + 16x %N
Super duplex stainless steel grades achieve excellent resistance to pitting corrosion through their higher chromium content combined with molybdenum and nitrogen. However, unlike many of the austenitic stainless steels and nickel alloys, their nickel content is relatively low. Given the high cost (and volatility) of nickel prices, this can be an important cost advantage for their specification.
Another simple way to review the suitability of metals is the Critical Pitting temperature (CPT) which is a standardised laboratory test. Samples are exposed to an aggressive corrosive solution, with the CPT representing the temperature at which pitting corrosion is initiated. Due to the somewhat artificial conditions of the test compared with real-world production conditions it ranks metals susceptibility to pitting corrosion but not their actual performance. Sometimes clients will use the basic rule-of-thumb that metals for oil and gas applications should have a CPT > 40oC – something that is met by most metals that Langley Alloys presents to the market.
Typical Critical Pitting Temperature values for a range of corrosion resistant alloys
The above tables represent simple points of comparison, but far more detailed examination is typically required for demanding applications in the oil and gas industry. With respect to mechanical properties, tensile properties at both lower temperatures (-200oC) and elevated temperatures (250oC) may need to be considered, together with impact toughness. Depending upon the application, hardness and galling resistance could also be very relevant.
With respect to corrosion resistance, there are a number of additional environmental factors that exist in oil and gas applications that complicate material selection:
Corrosion in the presence of oxygen is not just a problem on topside installations, but can also be found downhole as it is introduced by water flooding, maintaining pipeline pressure and the use of various fluids during extraction and completion. Oxygen scavengers can be used to remove this gas in an attempt to minimise pitting corrosion.
Where there is any substantial amount of oxygen present in the environment, combined with chloride ions, austenitic stainless steels will readily pit and crevice corrode at a high rate, even at temperatures as low as 10 °C. Therefore they are usually considered unsuitable for applications where there is oxygen in the environment with chlorides, and particularly where there is also H2S or other sulphide species or acids.
Duplex stainless steels are not highly pitting resistant and would not generally be selected for conditions containing oxygen if chloride ions were present at temperatures above 10 – 20 °C. Super duplex stainless steels are used up to 30 °C in aerated seawater and so would provide pitting resistance in some combinations of chloride ions and oxygen.
Nickel alloys will be challenged if exposed to both oxygen and chlorides. In fully aerated warm brines, Alloy 625 is resistant to about 60 °C. Above 90 °C it is generally necessary to consider pure titanium or its alloys for handling hot aerated brines.
Also described as carbon dioxide (CO2) corrosion, this form of corrosion is initiated by CO2 dissolved in water giving rise to carbonic acid (H2CO3), which in turn creates to corrosive H+ ions. The corrosivity increases with the concentration of CO2 and temperature, with corrosion initially being slow and localised in the form of pitting. Corrosion inhibitors can be introduced within the tooling and pipeline to inhibit CO2 corrosion in oil wells.
Austenitic stainless steels, such as Alloy 316L, are resistant to corrosion in sweet environments. For alloys containing added levels of molybdenum, such as Alloy 254, they produce a more stable passive layer and therefore being more suitable for environments with chloride ions present.
Duplex stainless steels (Alloy 2205) are also resistant to sweet corrosion, with little risk of pitting or stress corrosion cracking up to 200 °C, even with levels of salinity significantly above that typically experience in seawater. As expected super duplex stainless steels such as Ferralium has an even greater resistance to pitting, typically requiring temperatures up to 300C higher than for duplex grades to experience the onset of pitting. For very extreme conditions, only found in certain process section of an operation, there is some susceptibility to stress corrosion cracking in concentrated brine slurries at raised temperatures.
Corrosion associated with the presence of hydrogen sulphide (H2S) is more commonly referred to as sour corrosion. However, a number of different potential failure modes may ultimately result from it.
H2S forms an acid which promotes corrosion, the products of which can also accelerate the electrochemical reaction that occurs. Iron sulphide corrosion products form, which in some cases can subsequently protect the surface of the metal from further corrosion. However, it is often porous in nature and leads to a build-up of acidic FeCl2 that prevents FeS precipitating and results in localised pitting corrosion.
The creation of hydrogen (H2) that is absorbed by the metal can result in premature failure when the part is subject to stress – sulphide stress cracking (SSC). Materials can be selected that resist SSC, the process for which is comprehensively covered in the NACE MR 0175/ISO15156-3 standard (see below).
Aside from material selection, careful design of components to limit stress concentrators within the form can also mitigate against premature failure.
If a large volume of H2 diffuses into the metal, blistering or cracking may occur even without the presence of any additional stress, resulting in hydrogen induced cracking (HIC). This phenomenon usually only effects high-strength, hard steels (>90ksi) and relates to the metallurgical structure created during manufacture.
There are a number of material standards utilised throughout the oil and gas industry to inform material selection that are either referenced in our customer’s enquiries or Langley Alloy product specifications.
The American Petroleum Institute collates standards for petroleum and petrochemical equipment and operating procedures. The above standards relate to ‘Corrosion Resistant Alloy seamless tubes for use as casing, tubing and coupling stock’ and ‘Age-hardened Nickel Alloys for oil and gas drilling and production equipment respectively’. Both standards layout specific requirements for mechanical properties, chemical composition, microstructure, but place no specific demands on material selection for sour environments.
The NORSOK standards were developed by various Norwegian authorities in an attempt to reduce costs and improve competitiveness of oil and gas exploration undertaken on the Norwegian continental shelf. As such, the M-001 ‘Material Selection’ standard provides significant guidance on the applicability of materials for both specific environments and applications.
Previous revisions included a table like the one below, although it is no longer included in NORSOK M-001 Revision 5. Therefore, these uncontrolled numbers should not be considered definitive, but they do clearly show a hierarchy of material performance in a range of environments. Generally speaking, as you move down the table the materials are able to match the demands of increasingly aggressive conditions. Obviously, guidelines for H2S limits are only one criteria, so absolute strength and general corrosion resistance will also figure. For instance, duplex and super duplex stainless steels will be utilised in a wider range of environments as customers are able to exploit their inherently higher strengths and/or relative corrosion resistance at a given price point.
Materials Selection: Guidelines for H2S limits for generic CRA classes
NORSOK M-001 also provides an indication of materials typically specified for the following application areas: produced water systems; water injection systems; seawater systems; waste heat recovery units; utility use; subsea production and injection and fastener systems (regular and pressure containing purposes).
For instance, 25% Cr super duplex stainless steels such as Ferralium are suggested for pressure vessels, piping and pumps, drains, instrumentation, piping and fasteners in a number of the application categories.
The National Association of Corrosion Engineers started life as a small group of corrosion engineers working on a regional study, but has subsequently developed into an international association focused on the challenges on corrosion in industrial processes.
NACE MR 0175 (‘Metals for Sulphide Stress Cracking and Stress Corrosion Cracking Resistance in Sour Oilfield Environments’), now also issued as ISO 15156-3, specifies the types of corrosion resistant materials that can be used in specific oilfield environments. One particular feature of this standard is the use of limits for material hardness, as it is the only practical material measurement that can be conducted in the field as a validation of material specification. Despite this restriction, hardness has a reasonable correlation with overall mechanical properties and is applied to both the parent metal and any weld features.
The data presented in the previous table, from an older version of NORSOK M-001, is now maintained and developed at much greater detail within NACE MR 0175 / ISO 15156-3. The standard covers generic product families as well as making specific product references. An example of this is austenitic stainless steels, where various limits are defined for the use of Alloy 316L (with respect to temperature, H2S, chloride concentration, pH, sulphur presence), whereas the superior performance of Fermonic means that its use is specifically defined and can be used in a far broader range of chloride concentrations and pH albeit subject to limits on the other elements.
Langley Alloys supplies a large number of oil and gas Majors, OEMs, fabricators and machine shops with corrosion resistant alloys for application in down-hole tooling, lower completions, pumps, valves, instrumentation and services. Given the differing requirements by application, geography and customer specification it is not possible to be prescriptive, but a few applications are highlighted below.
Our unique high-strength super duplex stainless steels has found application in Xmas tree components, drilling ‘pup’ joints, barrier valves, well screen and sand screen elements and drill riser tensioner (DRT) cylinders as specified by a number of Majors.
Our high-strength austenitic stainless steel is routinely used as drill tooling within the drill string on account of its strength, corrosion resistance and non-magnetic properties. It is also widely used in pumps, valves, fasteners.
The anti-galling, non-magnetic and corrosion resistance properties of this high-strength copper alloy has found application in blow-out preventers (BOP), drilling tooling in logging while drilling (LWD), measuring while drilling (MWD) and directional drilling components, and also parts for drill riser tensioners.
This particular high-performance nickel alloy has previously found application in oil-well drill collars and instrumentation, although it is now more commonly utilised within valve trims, fasteners and pump shafts.
We supply a range of dimensions into well screen components and inflow control devices (ICD). These can be supplied as finished rings, utilising our in-house machining capability to provide cost- and time-effective supply chain solutions.
We also supply this high-strength nickel alloy for application in drilling ‘pup’ joints and barrier valves. It is also used for fasteners, hangers and packers in down-hole applications.
The extremely high-strength, corrosion resistance and working temperature range, as well as resistance to stress corrosion cracking mean that this alloy has found widespread application in the oil and gas sector. We carry an extensive range of dimensions that are finding application as drill tooling, barrier valves, pressure pumps components for both intelligent pump solution (IPS) and electric submersible pump (ESP) applications, as well as gate valves, stems, hangers and other well head components.