1.
"Downhole injection of raw sea water", Dr Roger Francis, Weir
Materials
Injection
of raw sea water is being considered for a number of offshore projects where
space/weight is paramount e.g. floating production platforms.
The
corrosion problems were outlined both for raw water injection and re-injection
of produced water, and the materials options available. The most difficult area is mixing produced water with raw sea
water, and this was shown to cause some very corrosive mixtures, particularly
when H2S was present. The
materials options available were outlined, as were the areas where there is a
lack of data. Cooling of the
produced water prior to mixing with sea water is an option for reducing
aggresivity prior to injection. This
has been adopted by the Statoil Nome project, which is using superduplex
stainless steel. An alternative is
to have separate injection lines for raw water and produced water.
2
"The RCP method for prevention of local corrosion on stainless
steels in saline water piping systems", Mr Craig Donald, CorrOcean Ltd
Some
local corrosion failures have been experiences with highly alloyed stainless
steels in chlorinated seawater piping systems used in oil and gas production. The
present paper presents the RCP method - Resistor controlled Cathodic Protection
- as a means to avoid such corrosion problems in a simple and economic way. The
paper outlines the principles of the method and a more detailed discussion for
the case of 6Mo stainless steels in chlorinated seawater. The
discussion comprises the protection potential, the current density requirements,
the resistor control and the anode design. Example
calculations of the CP design including anode size, spacing and lifetime are
presented. Other applications are
also briefly discussed.
3
"Some corrosion mechanisms of tubes and pipes", Mr
Ritchie Brown, BAeSEMA
When
inspecting heat exchanger tubes it is important to know the potential mechanisms
of failure, their morphology and the methods available for inspection. The
aim of this is to ensure defects can be rectified during planned maintenance
periods thus avoiding expensive unplanned shutdowns. Often a successful
maintenance schedule is dependant on the customer knowing the fluids in the heat
exchanger and the effect these have on the tube material. Armed
with this information the NDE specialist can advise the most suitable inspection
technique.
The
presentation reviewed thirteen potential corrosion and cracking mechanisms with
photographs showing examples of the morphology of the defects produced and the
way these can vary depending on material selection and the environment. From
a range of corrosion mechanisms, under flowing and static conditions, the
resulting defects can be on internal and external surface and can range from
general wall thinning to isolated pitting. The
introduction of stresses can lead to axial and circumferential linear defects as
a result of stress corrosion cracking and corrosion fatigue. Vibrations,
particularly between heat exchanger tubes and tube support plates, can also
result in fretting damage.
4
"Welding techniques for high alloy austenitic stainless steels",
Dr Trevor G Gooch, TWI
Factors
controlling corrosion resistance of weldments in high alloy austenitic stainless
steel were described, with emphasis on microsegregation, intermetallic phase
precipitation and nitrogen loss from the molten pool. The
application was considered of a range of welding processes, both fusion and
solid state.
Autogenous
fusion weldments have corrosion resistance below that of the parent, but low arc
energy, high travel speed and use of N2-bearing shielding gas were recommended
for best properties. Conventional
fusion welding practice is to use an overalloyed nickel-base filler metal to
avoid preferential weld metal corrosion, and attention was given to the effects
of consumable composition and level of weldpool dilution by base steel. With
non-matching consumables, overall joint corrosion resistance may be limited by
the presence of a fusion boundary unmixed zone: better performance may be
obtained using solid state friction welding, given appropriate component
geometry.
Overall, the effects of welding on superaustenitic steels were covered, and the materials have given excellent service in welded fabrications. The presentation summarised recommendations on preferred welding procedure.
1
"Seawater and firewater systems", Dr Mike Swidzinski,
Phillips Petroleum Co.
The
principal uses of seawater for offshore installations was considered with
specific reference to the selection of materials for seawater handling, pumping
and distribution.
Key
considerations influencing material selection for seawater duty were addressed -
issues such as galvanic compatibility, marine growth fouling, chemical
treatments, seawater velocity, crevice and pitting corrosion were discussed in
the context of identifying the most effective materials for seawater and
firewater duty.
Materials
that are commonly considered for seawater distribution and firewater systems
were discussed. These included:
Carbon
Steel, Galvanised Carbon Steel, Austenitic Stainless Steels, Duplex Stainless
Steels, Copper Alloys, including Copper-Nickel, Titanium, Nickel Based Alloys
and Fibre-Reinforced Materials
The
performance and general offshore industry experience encountered with the use of
these materials was debated in open forum.
No
clear conclusions regarding material selection could be drawn. Each
of the materials considered was shown to be fully effective for given aspects of
seawater service - similarly each material considered was found to have inherent
installation or performance limitations which would not favour its application
for certain duties. Consequently,
no one material appeared as clear favourite for all offshore seawater and
firewater applications.
The
issue of material selection for seawater applications still requires close
scrutiny by the Materials and Corrosion Engineer. The optimum balance between CAPEX and OPEX still appears to be
offered by a hybridised materials selection strategy – relying on simpler and
cheaper materials for less arduous duties - through to the more exotic and
corrosion resistant materials for the manufacture of more sensitive and
strategically important items where whole-life reliability is paramount.
2
"Comparison of the corrosion properties of cast aluminium bronze
with stainless steels", Mr Vin Calicut, Copper Development Association
Aluminium
bronzes have a long and successful history of use in seawater, it is the
standard material specified for ships' propellers and is preferred for seawater
systems in submarines. Many workers
have studied the corrosion resistance of aluminium bronzes, others have studied
ferrous materials. Few workers have
studied both simultaneously.
This
project has collated data comparing the properties of cast nickel aluminium
bronze (NAB), in particular NES 747 Part 2, with cast standard and
superaustenitic and duplex stainless steels and Ni-Resist for use in offshore
sea water applications. Properties
compared are physical, mechanical and fabrication but the bulk of the work has
related to the corrosion aspects of the materials.
Corrosion
properties compared are general corrosion, pitting, resistance to polluted sea
water, crevice corrosion, erosion corrosion, cavitation, stress corrosion
cracking, corrosion fatigue and galvanic corrosion. An
overall composite ranking for the corrosion resistance has been established
showing the superduplex stainless steels to be the best followed by the
superaustenitic alloys with NAB close behind in third place. The
standard duplex, Ni-Resist and standard austenitic stainless steels make up the
six.
Within
this composite ranking NAB stands up well against superduplex alloys in terms of
general corrosion, pitting, cavitation and corrosion fatigue. A
pitting resistance temperature indicator (CPT or PREN) is not relevant to
aluminium bronze. Its erosion corrosion, although not as good is reasonable; it
is immune to chloride stress corrosion cracking. With regard to other properties it has comparable strength to
the austenitic materials and can, by heat treatment have properties comparable
to the duplex alloys but this may be accompanied by a loss in corrosion
resistance.
The
alloy has excellent heat transfer and electrical conductivity properties as well
as good wear resistance and anti fouling properties. Its
weldability is considered comparable with superduplex stainless steels. NAB
should not be used in polluted sea water containing sulphides.
Overall,
providing the price of cast nickel-aluminium bronze is significantly lower than
cast superduplex or superaustenitic stainless steels, there are many
applications where it can compete successfully, based on cost linked with
corrosion properties. Typical
examples of these applications are rising mains for fire hydrants, pumps and
valves, desalination equipment and sea water valves for submarine applications.
From
a technical point of view these marine applications would use the major
strengths of NAB compared particularly with superduplex stainless steel. These
are:
Good general corrosion resistance
Good pitting corrosion resistance
Reasonable
erosion corrosion resistance (for a Cu base alloy)
Comparable
resistance to cavitation and fatigue
Immune
to chloride SCC
Comparable
strength
Excellent
heat and electrical transfer properties
Excellent
wear resistance
Much superior anti-fouling properties.
3.
"Materials selection for corrosion control in desalination
plant", Dr Trevor Hodgkiess, Glasgow University
Dr
Hodgkiess provided an overview of corrosion problems and their control in
desalination plant with emphasis on the role of materials selection together
with an indication of current issues in this subject area. The talk was
illustrated throughout by corrosion and materials selection case studies from
desalination plant. The two main
seawater desalination processes are distillation and reverse osmosis and the
emphasis in the presentation was on the former process in which corrosion
problems are more widespread.
The
review commenced with a detailed description of corrosion in seawater/brine and
the role of oxygen control was identified before turning to the historical and
current strategies relating to materials selection. This involves the widespread use of Cu/Ni alloys or stainless
steel either in solid form or as a cladding on carbon-steel. The
behaviour of stainless steels in the high temperature/deoxygenated environments
within a thermal desalination plant was discussed. The
range of materials available for heat transfer tubing was also reviewed. The speaker then moved on to the topic of corrosion in the
vapour systems. This is currently a
cause of major concern in this respect are vapour chemistry, venting design and
materials selection. Vapour space corrosion can involve oxygen and carbon
dioxide from the normal seawater feed and can be further exacerbated by the
presence of chlorine, bromine, ammonia or hydrogen sulphide.
In relation to reverse osmosis units, the main corrosion issue is crevice corrosion of the high-pressure pipework components which are generally constructed from stainless steel. The role of stainless steel grade was discussed. Finally, corrosion and material selection issues associated with ancillary components such as pumps and pipes was summarised.
1
"Development
in materials technology for marine pumps",
Mr Len Phillips, Weir Pumps.
Centrifugal pumps are widely used in marine environments
for handling seawater. Pumps vary
in size and complexity, dependent on the desired application. These
can a range from a relatively small single stage pump at low head for general
service to a high pressure multi-stage pump for seawater injection duties.
A wide range of materials are employed. For pump casings,
cast iron and cast steel are viable for seawater only where corrosion protection
is applied. Austenitic cast iron
affords increased corrosion and erosion resistance and provides galvanic
protection to more noble pump internals. Copper based alloys such as gunmetals
are also employed for pump casings, whilst nickel aluminium bronze provides a
stronger, more corrosion resistant alloy where required.
Stainless steels have seen major developments.
Although
very popular for many years in seawater applications, 316 austenitic stainless
steel has relatively poor crevice corrosion resistance and only modest strength.
The development of duplex stainless steels has provided alloys with excellent
resistance to crevice attack and stress corrosion cracking at high temperatures.
Their increased strength allows the possibility of reduced cross sections and
hence weight saving.
For impellers, impingement and cavitation erosion
resistance are most important. A
similar range of materials as that used for casings can be used for impellers.
Maximum performance will be obtained from the duplex stainless steels.
The wear rings must possess good corrosion/erosion
resistance and be mutually compatible. Where
suspended solids are present, hard particle erosion resistance is also required.
Austenitic cast iron, bronzes and
stainless steels are all employed. For
more aggressive environments, one or both rings may be coated. Examples include
stellite weld overlay, plasma spray CrO and HVOF tungsten carbide. Solid materials such as sintered carbides and ceramics have
also been used to combat excessive sand erosion.
Possible future developments may include more corrosion
resistant stainless steels and nickel based alloys and the increased use of
titanium. Where solids are present,
the use of hard coatings and solid ceramics/hard metals may be more widespread.
2
"Performance of marine materials under erosive flow
conditions", Dr Robert Wood, Southampton University.
This talk highlights the importance of erosion as a wear
process and its affect on marine equipment, should it be ignored. The
main causes of erosion are introduced and discussed; namely where flows induce
solid particle impingement, liquid droplet impingement and cavitation. The
parameters which influence slurry erosion arc introduced and key parameters
which must be controlled in any experimental approach identified. Solid
particle impingement results are presented for a wide range of marine materials
and advanced materials/coatings obtained using a slurry jet erosion facility.
The potential of the ultra hard coatings for erosion resistant
applications is noted. The
difference in erosion mechanisms for ductile and brittle materials are also
discussed. The influence of jet
impingement angle and kinetic energy (Ek) on erosion rates are explored. Erosion maps for ductile and brittle materials based on
kinetic energy terms, Ekv0.5 and Ek are
presented as a useful marine materials selection tool. Hydrodynamic cavitation erosion mechanisms are discussed and
the resistance of marine materials to such damage mechanisms are reviewed. The
ranking of such materials under cavitation erosion conditions are compared with
those obtained under slurry erosion conditions and the differences discussed. The
importance of studying the performance of materials under realistic field
conditions are highlighted by exploring the synergistic effects between erosion
and corrosion. Current research projects in the area of surface engineering for
marine applications are explained as an indication of possible future
developments and use of advanced coatings.
3
"Structural
Adhesives",
Mr Alan Hunter, Altra Consultants Ltd.
This presentation addresses the rationale for the use of
structural adhesives in marine environments. A summary of Altra experience to date is given including both
offshore and onshore usage. Detailed
discussion of outstanding issues is covered.