The outstanding performance of titanium and its alloys in sea water, brackish and polluted waters, and petroleum refinery environments has been fully exploited in recent years by the offshore oil and gas industry. Today, principally in the Norwegian sector of the North Sea, the number and variety of applications of titanium and titanium alloys offshore is increasing at an exponential rate.
From no more than a few hundreds of kilos used in chlorination systems and heat exchangers twenty years ago, total consumption of titanium now exceeds several thousand tonnes, with each new project likely to see greater use of titanium than its predecessor, table 1. Persistent corrosion problems with steels, particularly crevice corrosion, have been eliminated by use of titanium for low pressure ballast, fire and service water pipework. Typical consumption for topside pipework now ranges from 50 to 150 tonnes per platform.
Why is Titanium used in Offshore Applications?
Three principal factors have caused this dramatic switch in materials selection by offshore engineers:
• Firstly, as highly disruptive failures of stainless steel and copper based alloys have increased, concerns have grown for plant safety and protection of the environment at the lowest practicable life cycle cost.
• Secondly, titanium continues to be available at competitive and relatively stable prices, and with this has come supporting growth of fabrication experience and capability to supply a wide range of titanium products, particularly pipes, fittings and systems required by the offshore industry. Since 1990, fifteen Norwegian fabricators have developed the capability 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.
• The third factor in the increased specification and use of titanium has been the improved availability of information to design engineers and offshore operators of the useful combination of properties which titanium uniquely possesses, together with the practical aspects of specifying and using titanium cost effectively. The Titanium Information Group, collaborating with the Norwegian Titanium Technology Forum, has contributed significantly to this achievement. Offshore the cost of replacement is 27 times higher than for similar onshore work. The specification of titanium at the outset, coupled with cost effective design, fabrication, installation and use is seen as wholly appropriate for off shore installations which are now being designed with life cycles of 30 to 50 years. Titanium will frequently be competitive on first cost, and will always be the winner in the life cycle cost contest.
A pilot project in 1994 by Elf Petroleum Norge for the Frigg platform produced results showing that the installed cost of titanium on a 200m by 15cm 2MNm-2 sea water line was 20% below that of carbon steel. The use of cold bending, eliminated more than 80% of the welding work. Fewer bends and fittings were needed and there was less welding. Flanged joints were made by cold flaring of the pipe ends.
The low weight of the titanium pipe considerably eased installation - one man can handle a 6m length of 15cm diameter schedule 10s pipe without assistance. Post installation surface treatments, shot blasting and painting of the titanium were not required.
Uninsulated thin wall welded titanium pipes have passed the NPD H-class hydrocarbon fire test. The unique shock resistance and damage tolerance of titanium provide the maximum possibility for survival in the event of explosion, fire or other disaster (figure 1).
Figure1. Pipework for offshore use made from commercially pure titanium.
UK fire system manufacturers Grinnell offer all titanium sprinkler and deluge systems detectors, nozzles, valves and pipework, installed at minimum cost using cold bending. Titanium fire water systems are now installed on Froy/TCP (Elf Petroleum), Sleipner West (Statoil), Troll B and Brage (Norsk Hydro).
High Pressure Heat Exchangers
Titanium tube and shell high pressure gas coolers, with gas put through thick wall tubes and cooling water on the shell side, are typically large and heavy. A substantial saving of space and weight for such units is now possible through the use of compact heat exchangers, developed by Rolls Laval using titanium alloy Ti-6Al-4V, and superplastic forming and diffusion bonding. The first units occupying one tenth the volume and one seventh the weight of their tubular counterparts are now in service.
The need for extraction from deep subsea locations using floating production storage and off loading (FPSO) platforms has provided a challenging potential market for titanium tubulars for drilling and flexible production risers. Offshore fields for future development include numerous locations with water depths exceeding 300 metres (1000ft), table 2.
Titanium is seen by many engineers as the only material suitable for flexible risers to operate in these water depths, with gas or oil temperatures exceeding 125°C. Existing flexible pipe cannot tolerate the pressure, the higher product temperatures or thermal cycling. The application of titanium alloys for deep water production riser designs will require upwards of 500 tons of alloy per riser system. The qualification of titanium alloys in this application will read across to higher pressure process plant, removing the current restrictions to pressure classes 150 and 300 of commercially pure titanium and the lower strength titanium alloys (300-600MNm-2 tensile strength).
The concept of using titanium in riser applications is not new. In the late 1970s Cameron (now Wyman Gordon) Houston had a one third scale model titanium alloy stress joint tested successfully under conditions simulating the 100 year North Sea wave. A full scale taper stress joint was supplied to the Gulf of Mexico for Placid Oil in the Green Canyon field in 1987. The joint was retrieved in 1989.
Despite the brevity of this period of service, the installation lacked nothing of the most severe test conditions, being exposed to 100 year wave loadings through the occurrence of the Gulf loop currents which persisted for over two weeks during 1988. The titanium alloy joint survived undamaged in any way, and following a period of storage was refurbished and installed offshore for Enserch in July 1995. A further substantial order has recently been placed for Ti-6Al-4V taper stress joints for the Oryx Neptune field.
Titanium resists all produced fluids encountered offshore and all but a few non-produced fluids. Titanium alloys suitable for use down hole are compatible with completion fluids in all oxygen free conditions. Titanium alloys suitable for sour service are immune to corrosion, including pitting and stress corrosion cracking (SCC) in aerated and deaerated chloride-containing waters (e.g. sea water and brines).
Acidising fluids used in conjunction with titanium require more care in their selection. All titanium alloys are compatible with organic acids without inhibition. Subject to the alloy selected, special considerations are necessary for hydrochloric acid. Some of the most resistant palladium-containing alloys may not require acid inhibition.
All titanium alloys are rapidly attacked by hydrofluoric acid, even in very dilute concentrations, and also in fluoride-containing solutions below pH7. Titanium cannot be used if regular HF acidising is anticipated. (The use of titanium risers will bar the use of hydrofluoric acid and provide further opportunities for down hole and topsides application of titanium.)
Methanol is one of the few specialised environments and media which may cause SCC in titanium alloys. Failures of titanium have occurred in - dry methanol, and in methanol/halide and methanol/acid mixtures. Historically, a minimum water content of 2% has been recommended to provide immunity for commercially pure titanium for all but possibly the most severe conditions (for which commercially pure would not likely be used because of service temperature or operating pressure). More recently a revised recommendation of 5% water has been issued to cover all alloys being used offshore and for all anticipated conditions.
Fatigue And Toughness
The fatigue strength of smooth titanium alloy test specimens is typically 50%-60% of the tensile strength values. Notched specimen tests give lower values. Care is always required in design and manufacture to avoid stress concentrating factors, poor surface finish, sharp sectional transitions, unblended radii and corners etc.
Questions continue to be raised over the surface quality required on titanium alloys for riser applications. Currently it is deemed essential to provide a very high standard of finish, with all tears, splits, cracks, laps and other defects likely to arise in production removed, figure 2.
Figure 2. Section of a drilling riser made from Ti-6Al-4V, with booster line made from Ti-3Al-2.5V.
Corrosion fatigue is generally not a problem for titanium and its alloys. Fatigue crack propagation rates in seawater for commercially pure titanium are similar to those in air, but rates for alloys Ti-6Al-4V and Ti-6Al-4V ELI are marginally higher in seawater and other corrosive environments as compared to those in air. The absolute crack propagation rate will vary with specific alloy composition, microstructure, crack orientation and loading, but may be increased by the presence of hydrogen, generated galvanically or from impressed cathodic potentials. Several alloys including Ti-6Al-4V ELI possess fracture toughness (KIC) levels in excess of 80 MNm-3/2 in air, but reduced levels of toughness in seawater (KISCC) and other aggressive environments, figure 3.
Figure 3. Typical K1C and KISCC values for Ti-6Al-4V and Ti-6Al-4V ELI in various conditions.
Where titanium is incorporated into mixed metal plant or equipment, it will usually be the cathode if a galvanic couple exists, or is created. Design strategies used offshore to prevent or limit galvanic corrosion and protect adjoining less noble parts of the system include electrical isolation of titanium through the use of non conducting gaskets and sleeved bolts, chemical dosing, installation of short easily replaced heavy wall sections of less noble metal, or coupling to composite materials or to galvanically near compatible alloys such as the molybdenum bearing austenitic and duplex steels, (254SM0, Zeron 100), and high nickel alloys, (Inconel 625, Hastelloy C).
Several operators have coated exposed titanium surfaces to reduce the cathode/anode ratio. Impressed potential cathodic protection of the base metal should give no more than -0.8V SCE. Similarly, sacrificial anodes if not subject to resistor control, must be selected to produce negative potentials of less than -0.8V SCE. Review of the cathodic protection system is essential when a significant area of titanium replaces steel subsea. Galvanic corrosion of less resistant metals may be harmful to titanium as the cathode if conditions lead to the uptake of hydrogen. Hydrogen absorption may be caused or aggravated by:
• coupling of titanium to a less corrosion resistant metal
• cathodic protection systems producing potentials > -0.8V SCE
• tensile load or residual stress if absorption is occurring
• pH less than 3 or more than 12 increases the risk of uptake
• higher temperatures which cause an increase of corrosion at the anode and higher hydrogen activity at the cathode
• hydrogen sulphide, which will accelerate hydrogen uptake in the presence of a cathodic potential.
From the foregoing it is clear that titanium has rightfully won an established place for oil and gas production equipment in the offshore industry - there are few if any satisfactory cost effective alternatives for both low and high pressure water and product pipework, heat exchangers, vessels and ancillary equipment. The development of deep water fields will require the use of titanium tubulars as flexible risers and work continues fully to identify the parameters of the environment and to characterise alloys from the potential range of candidates which will be able to perform reliably in the application. Speedy and positive progress will be greatly assisted by a substantial and intensive programme of investment, and it is the oil companies themselves, as end users of the tubulars and beneficiaries from the use of titanium who at this time have both the resources and the motivation to make that level of investment.