Titanium for offshore and marine applications

Citation

(2002), "Titanium for offshore and marine applications", Anti-Corrosion Methods and Materials, Vol. 49 No. 4. https://doi.org/10.1108/acmm.2002.12849daf.001

Titanium for offshore and marine applications

Titanium for offshore and marine applications

The unique and outstanding performance of titanium and its alloys in sea-water, chemical brines, brackish and polluted waters has been established over many years of service in a wide range of operating conditions. Similarly well-known is the ability of the metal to survive in aggressive oil refinery environments. In recent years the resistance of titanium to conditions encountered in oil and gas extraction and processing offshore has been more fully explored with wholly satisfactory results.

Today titanium is frequently specified as first choice for marine and offshore piping systems, heat exchangers, and a wide range of ancillary equipment for both water and product management in both critical and general applications. The first titanium drilling riser has been in satisfactory operation on the Heidrun platform for three years. Wider consideration is currently being given to titanium for composite drilling risers, flexible production risers and flowlines. Qualification of the stronger titanium alloys for risers has further extended the application of titanium offshore into higher pressure pipelines, heat exchangers and process plant.

The technology and experience of design, application, installation and use of titanium are as relevant to first-time users as to committed customers. In both offshore and onshore environments, if sea-water- or chloride-contaminated media is involved, or if hydrocarbons are in the process stream, then titanium must be considered. Design and fabrication parameters for titanium are well understood and widely practised, and service performance is proven with an extensive and impressive track record of achievements. Titanium systems which inherently save cost through reliable performance are usually no more expensive, and in many cases cost less in total to install than those in less durable materials.

Why use titanium?

Light, strong, corrosion-resistant titanium should be considered for marine and offshore applications, wherever weight or space are factors or corrosion/erosion is a problem.

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 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 per cent 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. 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 three to five times weight reduction in the platform structure, flotation and mooring system. The cost penalty for weight on a TLP or semi-submersible platform was recently quoted at between £5-13 (NOK65-170) per kilo. 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 of up to 130úC, 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 80úC in sea-water, conditions in which some stainless steels are limited to 10úC.

The lessons of past expensive errors made in the selection of less serviceable alloys for corrosion-resistant duties have been well learned. 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 a winner of the life cycle cost contest.

Titanium offshore – current applications

The number and variety of applications of titanium and titanium alloys used offshore continue to increase. From no more than a few hundreds of kilos in chlorination systems and heat exchangers 20 years ago, total consumption now approaches 3,000 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 markets as well as the growing offshore demand. Since 1990 some 15 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. The development of cold bending of thin wall titanium pipework has provided a breakthrough in the overall competitiveness of titanium systems.

Other applications

In addition to the examples shown above, titanium is used in a wide range of marine and offshore equipment and components on the basis of corrosion resistance, reliability and freedom from maintenance. Applications range from deep-diving submersibles such as the French Nautile, Japan's Shinkai, diving-bell hatch doors, tensioner piston rods for the Snorre TLP, hydraulic cylinders for steering hydrofoil planes, jet foil inducers, process vessels, pumps and valves, encapsulations for deep sea cameras, electronic packages and sonar devices, hydraulic sampling cylinders, instrumentation, cable and pipeline clamping systems, fasteners, springs, impressed current system anodes, and desalination units. These technically successful and cost-effective uses, many of which date from the late 1970s, unquestionably encouraged the wider acceptance of titanium in what are now major continuing applications.

Titanium – the cost perspective

What will it cost? The price per kilo of titanium is no guide to the cost of properly designed equipment. Most designs are based on the use of metal by surface area rather than by weight and, compared with other high performance alloys, the high strength, low density, outstanding corrosion resistance and relative ease of fabrication and welding of titanium are all factors which combine to keep costs down.

Table I compares apparently more expensive titanium with a lower cost by weight (but more dense) steel or nickel alloy in sheet application.

The strength and corrosion resistance of titanium may enable the selection of a thinner gauge of sheet. A reduction of thickness by a third reduces the relative cost of the titanium to 300 – two-thirds that of the apparently cheaper competitive material!

Pipe and tube are normally quoted and sold per metre. Weight for weight a titanium pipe of the same diameter and schedule will be almost twice as long as a pipe made from any one of its more dense competitors. Reduction of the schedule of wall thickness can cut pipe by weight by up to 70 per cent with a substantial reduction in the system purchase price. Further weight and cost reduction may be possible by elimination of flanged joints.

Studied approaches to using titanium usually show lower than anticipated first cost, substantial life cost savings and downstream technical benefits. At the end of service life titanium components will have a high positive recycle value. Costs for future disposal of non-metallic products may be significant by comparison.

Guidelines for cost control

Titanium alloys suitable for marine and offshore applications can be supplied in all the semi-finished and finished forms common to the industrial metals. The selection of the correct alloy, form and manufacturing route to produce cost-effective components and systems is, however, much more important in titanium than when designing with materials of lower basic cost. The simple substitution, detail for detail, of steel or other alloys with titanium will prove to be an expensive exercise and may result in titanium being deselected for an application. Time spent in redesign, using the principles outlined in this guide, will always be well repaid:

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  Do not simply substitute titanium into existing designs.

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  Do not budget for titanium project costs by weight, especially not by the design weight of steel or copper alloy systems or components.

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  Check for standard products and specifications to obtain best availability and lowest cost.

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  Use design strategies based on using minimum material thickness.

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  Exploit the corrosion resistance of titanium to the full.

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  For pipes use cold bending or induction bending to limit the need for fabricated bends, flanges and welding.

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  Check the flange guide for the lowest cost system appropriate to the design code.

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  Consider the use of linings and cladding in preference to solid design, where heavy sections are unavoidable.

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  For cast parts such as valves and pumps, if sufficient numbers justify, produce new patterns to optimise the strength and corrosion resistance of titanium.

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  Always consult suppliers and fabricators at the earliest stage of design.

Further details available from: The Titanium Information Group. Tel: +44 (0)1562 60276; Fax: +44 (0)1562 824851.