Titanium for Offshore and Marine Applications

In all fields of engineering, but nowhere more than in marine and offshore service, designers, fabricators and end users are readier than ever before to consider titanium for a continually widening range of applications. Today, with several thousand tons of titanium in service offshore, old - and false - notions about cost, availability, and fabrication are less likely than ever to prejudice engineers who can see clearly for themselves all of the excellent benefits which titanium brings to marine and offshore operations. Titanium is not an 'exotic' metal, it is relatively inexpensive and widely available. A large number of suppliers and fabricators regularly supply components and equipment at prices which emphasise that the metal is easier and less expensive to fabricate and weld than most alloy steels and nickel alloys. The fact is that for sea water applications there is no other material which can approach, economically or technically, the performance offered by titanium.
Titanium is as strong as steel, yet 45% lighter. The high strength, low density and corrosion resistance of titanium contribute positively towards cost reduction. Weight saving is of great importance for offshore platforms. A weight reduction of one tonne topside saves more than £100,000, NOK 1 million, in steel on the subsea jacket. All up weight on semi submersible platforms including tension leg platforms (TLPs) is equally critical, the reduction in hang off weight can be matched by a 3 - 5 times weight reduction in the platform structure, flotation and mooring system. On fast ferries weight reduction is a critical factor contributing to increased payload and speed with reduced fuel consumption.

Titanium requires no corrosion allowance so equipment can be designed to satisfy the minimum requirements for mechanical strength and handling. The outstanding corrosion resistance of titanium even in heavily polluted sea water, offshore produced fluids and all but a few non produced fluids is due to the metal's stable, tenacious and permanent oxide film. In flowing or static sea water at temperatures up to 130°C, titanium surfaces are immune to corrosion and resist erosion in conditions which cause rapid deterioration of other commonly used metals and alloys. Titanium is immune to crevice corrosion up to at least 70°C in sea water, conditions in which some stainless steels are limited to 10°C.
The lessons of past expensive errors made in selection of less serviceable alloys for corrosion resistant duties have been well learnt. Offshore the cost of replacement is several times that of a similar onshore facility. Likewise the penalties are ever increasing on military and commercial vessels for equipment failure and unscheduled outages.
Specification of titanium from the outset, coupled with cost effective design, fabrication, installation and use is a fundamental element in safe, and reliable performance. This is as true for ships and other vessels, where high availability and reduced maintenance costs are an essential requirement, as for offshore installations which are planned for service lives of up to 70 years. Titanium will frequently be competitive on first cost, but will reliably give the lowest cost of ownership and always be winner of the life cycle cost contest.
Corrosion Performance of Titanium Alloys in Natural and Polluted Seawater Relative to Other Alloys

Mode of Corrosion	Copper based alloys	Stainless Steel 316	Stainless Steel 6 Mo and Duplex	Titanium Alloys
General Corrosion	Resistant/Susceptible1	Resistant	Resistant	Resistant
Crevice Corrosion	Susceptible	Susceptible	Susceptible (>25°C)	Resistant (<80°c)2
Pitting Attack	Susceptible	Susceptible	Resistant	Immune
Stress Corrosion	Susceptible1	Susceptible (>60°C)	Resistant	Resistant3
Corrosion Fatigue	Susceptible	Susceptible	Susceptible	Immune
Galvanic attack	Susceptible	Susceptible	Resistant	Immune
Microbiological Corrosion (MIC)	Susceptible	Susceptible	Susceptible	Immune
Weld/HAZ Corrosion	Susceptible	Susceptible	Susceptible	Resistant
Erosion Corrosion	Susceptible	Resistant	Resistant	Highly Resistant

1.       Dependent on Pollution level/sea water chemistry

2.       Grades 7,11,12, 16,17,20,21,24, 28,29 resistant to at least 200°C

3.       Standard Grade 5 has finite susceptibility, Grade 23 (ELI) has improved K_1SCC values

TITANIUM OFFSHORE - CURRENT APPLICATIONS

The number and variety of applications of titanium and titanium alloys offshore continues to increase. From no more than a few hundreds of kilos in chlorination systems and heat exchangers twenty years ago, total consumption now approaches three thousand tons, principally as sea water and process fluid management systems and heat exchangers. These major applications are complemented by a wide range of miscellaneous duties, many critical to platform operation and safety.

Offshore engineers concerned at continuing failures of stainless steel and copper based alloys designated for sea water use have increasingly turned to titanium. Titanium is available at competitive and stable prices and there has been supporting growth of fabrication industry experience and capability to supply a wide range of titanium products, particularly pipes and fittings and systems required by the marine and offshore industries. A mature body of titanium fabricators has long existed in the EC Countries, serving the European chemical, petrochemical and power plant as well as the growing offshore applications market. Since 1990 some fifteen Norwegian fabricators have developed the ability to supply titanium, taking only a relatively short time to become skilled in all aspects of machining, bending, and welding. he development of cold bending of thin wall titanium pipework has provided a breakthrough in the overall competitiveness of titanium systems.

SELECTED OFFSHORE APPLICATIONS OF TITANIUM