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.