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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.
Fire Systems
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).
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Figure1.
Pipework
for offshore use made from commercially pure titanium.
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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.
Riser Applications
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.
Corrosion Resistance
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.
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Figure
2. Section
of a drilling riser made from Ti-6Al-4V, with booster line made from
Ti-3Al-2.5V.
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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.
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Figure
3. Typical
K1C and KISCC values for Ti-6Al-4V and Ti-6Al-4V ELI
in various conditions.
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Galvanic Corrosion
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.
Conclusions
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.
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