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|
|Crevice Corrosion||Susceptible||Susceptible||Susceptible (>25¬∞C)||Resistant (<80¬∞c)2|
|Stress Corrosion||Susceptible1||Susceptible (>60¬∞C)||Resistant||Resistant3|
|Microbiological Corrosion (MIC)||Susceptible||Susceptible||Susceptible||Immune|
|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 K1SCC 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
|Application||Company||Project||Titanium alloy grade|
|Taper Stress Joints||Placid Oil||Green Canyon||23 (Ti-6Al-4VELI)|
|Taper Stress Joints||Ensearch||Garden Banks||23|
|Taper Stress Joints||Oryx Energy||Neptune||23|
|Fire Water Systems
– ” –
|Norsk Hydro||Troll B (Oil)
|2 (Commercially Pure)|
|Fire Water Systems||Elf Petroleum||Froy TCP||2|
|Fire Water Systems||Statoil||Sleipnir West, Siri||2|
|Fire Water Systems||Statoil||Norne||2|
|Sea Water Lift Pipes
– ” –
– ” –
|Ballast Water Systems||Mobil||Statfjord A/B
|Ballast Water Systems||Hibernia||2|
|Penetration Sleeves||Statoil||Sleipnir West||2|
|Penetration Sleeves||Norsk Hydro||Oseberg||2|
|Fresh Water Pipework||Elf||Frigg||2|
|Sea Water Pipework||Esso||Jotun||2|
|Sea Water Pipework||Norsk Hydro||Njord, Visund||2 (110 tons)|
|Seawater systems, fire, ballast and
produced water Pipework
|Statoil||Asgard B||2 (300 tons)|
|Gravity Based System||Statoil||Troll A (Gas)||2(500 tons)|
|Drilling Riser||Statoil (Conoco)||Heidrun||23|
|Booster Lines||Statoil (Conoco)||Heidrun||9 (Ti-3Al-2.5V)|
|Anchor System Pipework||Statoil (Conoco)||Heidrun||2|
|Penetrations and Manholes||Statoil (Conoco)||Heidrun||2|
Because of high strength, high toughness and exceptional erosion/corrosion resistance, titanium is also currently being used for:
• Submarine ball valves
• Fire pumps
• Heat exchangers
• Hull materials for deep sea submersibles
• Water jet propulsion systems
• Propeller shafts and propellers
• Exhaust stack liners
• Naval armour
• Underwater manipulators
• High strength fasteners
• Yacht fittings
• Shipboard of cooling and piping systems
• Many other components in ship designs.
The first all-titanium fishing boat was launched in Japan in 1998. Weighing 4.6 tonnes the 12.5m (41ft.) long vessel can travel at 30 knots with improved fuel efficiency. Operational cost savings include no necessity for hull painting and easier removal of biofouling. The progressive degradation of glass fibre boat hulls by repeated fouling and cleaning is an ongoing penalty for the Japanese inshore fishing fleet.
HIGH PRESSURE SYSTEMS
Riser piping, taper stress joints, production tubulars and liners, flowlines, and similar high pressure systems provide applications where, as service life requirements are extended and operational and safety demands become more exacting, titanium can outperform steel and non bonded flexible pipe The concept of using titanium in such applications is not new.
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 loading 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 was installed offshore for Ensearch (Garden Banks) in July 1995. A total of fifteen Ti-6Al-4V taper stress joints have been delivered for the Oryx Neptune field.
Titanium taper stress joints (TSJ) are typically one half to one third the length, one quarter the weight, and less expensive than their functional counterparts in steel.
Titanium is favoured over heavier more costly non bonded flexible pipe where:
• Weight loading on the vessel or platform is critical
• Production side pressure and temperature <125¬∞C
• Larger pipe diameters
• Collapse limitations in deep water
• Performance reliability and long service are critical.
Titanium is favoured over heavier stiffer and less corrosion resistant steel piping where:
• Vessel motions are great and sea state severe
• Fatigue and bending stresses are a potential problem
• Vortex induced vibration fatigue is a potential problem
• Weight loading on the vessel or platform is critical
• Shallow waters place high bend loads on steel risers
• Corrosive and hot brines and sour fluids are produced.
The continuing discoveries of deep water offshore oil and gas fields places increased emphasis on the development of a long term technical and cost effective solution for riser pipe materials. Interest is concentrated on titanium for its favourable strength weight ratio, flexibility, and low density. Titanium will not influence, interact with or contaminate the natural marine environment.
Minimising weight is an increasingly critical issue for floating platforms as water depth increases. The development of composite risers using a titanium liner of reduced thickness in conjunction with fibre reinforcement marks a move towards further reduction of weight compared to all metal systems
The world’s first all titanium alloy (Ti-6Al-4VELI – ASTM Grade 23) riser is the 400 metre long drilling riser on the Heidrun tension leg platform. Titanium provides for a substantial reduction of topside weight, reduced tensioning requirements and the elimination of buoyancy elements and the expensive and cumbersome flex joints, both traditionally used with steel drilling risers. Grade 23 is also the nominated alloy for the 711 mm (28 inch diameter x 670m (2,200 ft) long free hanging caternary combined gas export and surface pig launching riser for the Asgard platform. Increasing temperature and pressure, (the Heidrun drilling riser is rated to 31 MPa (4500 psi)), and the expectation that the gas and oil extracted will become ‘sour’ in the longer term make titanium a very attractive option. Existing non bonded flexible pipe cannot tolerate the predicted higher product temperatures or thermal cycling. The range of titanium alloys includes the capability to handle product temperatures in excess of 200¬∞C, and thermal cycling is not a problem
The ability to recycle used riser pipes with a relatively high recovery value at the end of their planned life is a further bonus in the favourable life cycle cost equation for titanium.