Background

Materials selections must be given detailed attention at every stage of the design, construction and operation of systems and equipment for application in offshore oil and gas production. Full attention must be given to general corrosion resistance, selective corrosion resistance (by pitting and crevice attack) and stress corrosion cracking susceptibility in sour hydrogen sulphide environments if failures, loss of production and costly maintenance are to be avoided. Even more important than these considerations is the need to maintain offshore safety. Thus the specification and use of materials which combine corrosion resistance with high mechanical strength is a fundamental requirement.

A greater understanding of the offshore environment and more detailed knowledge of the conditions under which offshore structures and systems have to operate will obviously contribute to the selection of the correct materials.

Corrosion in Sea Water and Offshore Environments

Sea water is highly corrosive and offshore installations are often exposed to temperature extremes. The corrosion resistance of a material is therefore equally as important as mechanical strength. The introduction of chlorine by adding hypochlorite solution to sea water to give biofouling resistance can reduce the corrosion resistance of certain stainless steels, particularly under crevice conditions. Hydrocarbon process systems often have to withstand the potentially corrosive effects of hydrogen sulphide and acid conditions associated with the dissolved carbon dioxide which is often present. Corrosion can weaken elements of an otherwise well designed ,structure or affect individual equipment components to such an extent that they cease to be serviceable. Unfortunately, the fight against corrosion itself can lead to equally damaging side effects such as the release of nascent hydrogen. This can be generated as a result of cathodic protection measures adopted to protect a structure or by dissimilar metal coupling. The presence of such hydrogen can given rise to hydrogen-induced cracking of steels and nickel base alloys.

Alloys for Offshore Applications

Metals manufacturers have spent much time and effort in developing alloys specifically to meet offshore needs. The alloys developed have had to be suitable for shafts and bolting as wellas many other applications. These have included sea water and process pipework, water injection and booster pumps, line shaft pumps, emergency shutdown valves, anchorages and tensioners for riser protection systems, multiphase pumps and remotely operated vehicle components.

The Development of Marinel

One particularly significant corrosion-resistant alloy (CRA) development led to the introduction of an ultra high strength cupronickel alloy (Marinel), approximately five years ago. This alloy was added to the range of alloys available for selection with reference to particular equipment where corrosion and hydrogen embrittlement could occur offshore. Most high strength iron and nickel based alloys and titanium alloys are prone to hydrogen embrittlement, the effect usually becoming more severe as the strength increases. Thus these alloys when operating in a high-stress condition will be more susceptible to hydrogen embrittlement than the same alloys operating under lower stress. Hydrogen embrittlement is of particular concern where high strength (usually B7 carbon steel, 720 N.mm^-2 yield point) bolting is used on subsea structures. The operating stress level usually taken to represent a critical situation with respect to hydrogen embrittlement is that given by the yield stress of B7 carbon steel which has the value of 720 N.mm^-2.

Use of Cathodic Protection

Cathodic protection by sacrificial anodes or impressed current is extensively used to protect subsea structures from corrosion. This technique can generate hydrogen which, if absorbed, may lead to embrittlement of metallic components with the resultant danger of premature failure. The time-dependent nature of the ingress of hydrogen may mean that an apparently unaffected subsea critical component, for example a bolt, fails in an instant after it has performed satisfactorily for several years in service. Failure occurs when the residual ductile core is reduced in area by an encroaching hydrogen embrittlement front to a cross-section which cannot carry the load placed upon it. As an example, the failure of alloy K-500 riser clamp bolts has been reported in the April 1985 issue of Materials Performance (p37). Charging of UNS N 05500 (high strength 70Ni-3OCu alloy) with hydrogen has been shown to result in the hydrogen embrittlement of nonmagnetic drill collars. This has been thought to be due to galvanic coupling of the collars with carbon steel (see the October 1986 issue of Materials Performance, p28). It has also been suggested that a documented example of cracking in high strength steel legs of jack-up rigs was associated with hydrogen-induced stress corrosion cracking, the hydrogen being generated by the cathodic protection system operating in hydrogen sulphide contaminated seawater (February 1989 issue of Veritec Offshore Technology Journal).

Transport of Hydrogen into a Metal

The entry of hydrogen into a metal can be purely diffusion-controlled, or can be assisted by dislocation transport and the latter effect has been experimentally demonstrated by the measurement of hydrogen permeation rates through nickel whilst it is undergoing plastic deformation (see volume 13, 1979 of Scripta Metallurgica, pp 927-932). Dislocation sweep-in of hydrogen from the surface in the case of several different metals has been found to be consistent with the calculated energy of activation of hydrogen-induced cracking (see pp 233-239 of the proceedings of the 1976 TMSAIME international conference on the effects of hydrogen on the behaviour of metals). During hydrogen transport, the hydrogen can be deposited at various ‘trap-sites’ or internal discontinuities such as grain boundaries or precipitates.

Susceptibility to Hydrogen Embrittlement

These can take the form of ‘reversible’ traps which the hydrogen can subsequently leave, or ‘irreversible’ traps, which the hydrogen cannot leave and which tend to encourage local fracture through a lowering of the surface energy of the material. The effectiveness of the traps in promoting hydrogen embrittlement is related to the degree of strengthening present in the material matrix, as it is well established that materials in a higher strength state (i.e. cold worked or age hardened) are more susceptible to hydrogen embrittlement than the same materials in a lower strength condition. Thus, measurement of both the hydrogen entry kinetics of a metal (or alloy) and the ability of the metal to trap hydrogen would give an indication of its hydrogen embrittlement susceptibility. Overall solubility of hydrogen does have an influence on hydrogen embrittlement characteristics, as iron, nickel and titanium have relatively high hydrogen solubilities (>1cc/cc) and these materials are more susceptible to hydrogen embrittlement than aluminium and copper alloys, whose solubilities are generally less than 0.1 cc/cc. The hydrogen diffusion coefficients of steel and titanium are greater than 10^-6 cm².s^-1, whereas the hydrogen diffusion coefficients of nickel, aluminium and copper alloys are approximately 10^-10 cm².s^-1, although this does not take into account dislocation transport or grain boundary diffusion.

Nickel-Copper Alloys and Hydrogen Embrittlement

Two alloys which are interesting to compare are the age hardening nickel-copper alloy K-500 and age hardening cupronickel Marinel, which have similar mechanical properties and hydrogen diffusion characteristics. In comparing the chemical composition of these two alloys, see Table 1, it is apparent that they contain almost the same basic elements, the major difference between them being the Cu:Ni ratio. In the case of Marinel the high Cu:Ni ratio renders the alloy immune to hydrogen embrittlement and this has been found to be largely due to the reduced ability of this alloy to trap the hydrogen irreversibly.

Table 1. Typical composition of bolting.

Material	Ti	Cr	Mn	Nb	Cu	Ni	Fe	Al
K-500	0.6	-	1.0	-	30	Bal.	1.0	2.8
Marinel	-	0.4	5.0	0.7	Bal.	18	1.0	1.8

Marinel in Offshore Applications

In offshore situations many developments have widely employed Marinel bolting for splash zone and subsea. Bolting subsea has been used with 13Cr steel, 22Cr duplex and 25Cr duplex steel manifold, valve and choke flanges. Subsea developments using the alloy include Lyell, Strathspey, Nelson, Heidrun, Johnston and Nelson.

Good galling resistance obviates the need for a lubricant during assembly and nuts can be readily removed after a period of service if required.

For the Conoco Lyell subsea manifold Marinel bolting was chosen for its greater mechanical strength and corrosion resistance compared with grade 660 steel. The bolts were bolt tensioned and assembled without lubricant. Stud bolts have been subjected to a laboratory examination after 18 months service (nearly 12 months with the manifold in operation) and apart from the expected calcareous deposit, appeared completely unaffected by service.

Duplex Stainless Steels in Offshore Applications

A most significant contribution to the fight against corrosion offshore has been made by duplex stainless steels. These have often been adopted on offshore structures in preference to carbon steel or other stainless steels. The value of the duplex stainless steel is that it combines the basic toughness of the more common austenitic stainless steels with the higher strength and improved corrosion resistance of ferritic steels. The optimum chemical composition of these steels provides a high level of corrosion resistance in chloride media together with high mechanical strength and ductility. Other benefits include the ability of some duplex stainless steels to be used at quite low sub-zero temperatures and be able to resist stress corrosion cracking.

A significant feature of duplex stainless steel is that its pitting and crevice corrosion resistance is greatly superior to that of standard austenitic alloys. Pitting resistance equivalent numbers (PREN), a standard industry measure, are often in the high 30s while the latest duplex alloys exceed a PREN of 40. This is an increasingly common specification for certain offshore duties. However, PREN numbers only provide an approximate grading of alloys and do not account for the microstructure of the material. An acceptance corrosion test on material in the supply condition is so much more meaningful.

The Evolution of Duplex Stainless Steels

Ferralium alloy 255 was the world’s first commercial 25% chromium duplex stainless steel when it was introduced over 20 years ago. It pioneered the use of a deliberate nitrogen addition in order to improve ductility and corrosion resistance. Further research has demonstrated the importance of using duplex stainless steels containing both nitrogen and copper.

Super Duplex Stainless Steels for Offshore Applications

For offshore and indeed, onshore applications, the availability of a super duplex (25% chromium) stainless steel alloy in a variety of forms is important. For example, bar, forgings, castings, sheet, plate, pipe/tube, welding consumables, flanges, fittings, dished ends and fasteners are available. In terms of other benefits, the high allowable design stress of this alloy type in comparison with other duplex stainless steels and austenitic stainless steels, including 6% Mo type, is significant. It also offers excellent castability, weldability and machinability. These features are complemented by excellent fatigue resistance and galvanic compatibility with other high alloy stainless steels.

Twenty-two percent chromium stainless steels provide better pitting resistance and resistance to crevice corrosion than type 316 stainless steel by virtue of a more stable passive film and also have greater mechanical strength. However, for optimum corrosion resistance, a 25% chromium high alloy duplex stainless steel is required and these alloys are often referred to as super duplex stainless. Even within this category, it is important to select the correct grade of material to get versatility in handling a wide range of corrosive media and for confidence that the alloy will cope with any excursions or transient operating conditions which make the environment more aggressive.

Materials Selection for Offshore Applications

Offshore structures themselves present different requirements of materials depending upon whether their application is topside, splash zone or subsea. Topside, duplex materials are suitable for a wide range of bolting applications and material such as Ferralium alloy 255 provide up to B7 steel strength, excellent corrosion resistance and a service life equal to the life of the system, thereby contributing to reduced maintenance costs. In the splash zone, the alloy has already demonstrated its suitability for sea water resistance with over 15 years service on North Sea installations and has been widely employed for riser bolting and components on riser protection system on TLPs.

Emergence of New Super Duplex Stainless Steels

Improved materials in the super duplex stainless steel category continue to be developed by manufacturers offering better or differently combined characteristics, features and benefits. These alloys, generally with a PREN > 40, are produced to conform to a number of UNS designations which appear in ASTM product form specifications. Castings and wrought forms are available. Typical of recent developments is Ferralium alloy SD40 (conforming to UNS S 32550) with a PREN > 40.0 and providing a minimum 0.2% proof stress of 550N.mm^-2 and a UTS of 760 N.mm^-2. This 25% chromium super duplex material results from a carefully controlled composition and balanced austenitic/ferritic structure with a substantial content of molybdenum and nitrogen.

Applications for Super Duplex Stainless Steels

Applications which can benefit from the use of these high alloy super duplex steels involve piping systems, pumps (where the good erosion and abrasion resistance is employed), valves, heat exchangers and diverse other equipment.

Recently, the excellent corrosion resistance of the new super duplex Ferralium alloy SD40 has been exploited for subsea electrical connectors on the Saga Snorre and Total South Ellon developments. In one case the super duplex material was chosen to replace standard austenitic stainless steel which had suffered from corrosion attack.

Figure 1. Super duplex stainless steel alloy is available in a variety of forms for both on and offshore applications.

Conclusions

Several types of alloys have been developed in recent years to combat the degradation of existing alloys by corrosion attack and in some cases hydrogen embrittlement in the harsh offshore environment. Super (25 Cr) duplex stainless steels and an ultra high strength cupronickel have provided the solution to many material selection dilemmas.