Project Deliverable D50 Report on Best Practices ... - ESONET NoE

when the mechanical requirements are not stringent. Alloys in alpha-beta phase such as 6%
Aluminium and 4% Vanadium (or equivalent Russian grades) are a reliable solution. One
drawback of titanium alloys is their electrochemical potential which may corrode other
metals. It is suggested to protect it by painting for instance to limit its active surface.
Bronze. Among copper alloys, some have a good behaviour for long time exposure to
seawater. They may have the advantage of intrinsic biofouling protection by release of copper
ions.
Aluminium alloys of several kinds are a solution for underwater components. The serie 5000
(according to Aluminium Association standard nomenclature) is not prone to heavy corrosion
and may be used unprotected. The powder produced by corrosion may be a disturbance for
some very precise measurements of particles in the abyss. The 6000 serie and to some extend
7000 serie (with better mechanical performances) are used with hard anodizing specified for
marine application. A cathodic protection with Aluminium-Indium alloys anodes is ensuring
long-term endurance.
Use of thermoplastic materials Thermoplastic materials have the great advantage to suffer no
electrochemical corrosion. Their limitation of use is due to the water ingress and creep.
Thermoplastics with brittle behaviour can only be used in special configurations. They are
used in cable sheathing and overmoulding, for light mechanical pieces, electrical insulation,
o-rings view ports for cameras and water-tightness components. Due to the creep
characteristics, the load must not be permanent for equipments immersed a long time such as
subsea observatories. PEEK or PCTFE have exceptional behaviours but are quite expensive,
they are only manufactured into small pieces in sensors and instrumentation. Polyurethane is
commonly used, but its formula must be especially suited for long-term seawater exposure.
The polyether type of molecule has acceptable performances. The components of
polyurethane and of most thermoplastic materials are changing quite often due to environment
regulations and medical regulations for the workers. This may lead to perform again
acceptance tests or tests on mechanical characteristics. In general, characteristics for underwater
ageing is dependent on the crystalline to amorphous ratio.
However, the improvement of these materials is very promising and may lead to light weight
equipments with long immersion potentials.
Use of composite materials The high mechanical characteristics of composite materials and
the lack of corrosion are excellent arguments for their use at sea. In long-term sea floor
deployments, these performances have been demonstrated. In the telecom cable industry,
repeaters in glass epoxy have been produced and used for the last twenty years by Alcatel for
instance. Components of sensor strings implemented in underwater wells, by industrial
companies such as Schlumberger or academic institutes like IFREMER, have shown their
cost effectiveness. In these applications, thick glass epoxy is machined and used as any
material. Resin The plastic matrix to be reinforced by fibres must be well tested. The criteria
are, as such, a R&D issue in: water ingress, creep, shock, ageing of matrix-fibre interface. The
choice of epoxy and vinyl-esther is acceptable. Other matrices such as polyester are not
recommended. The production methods (responsible of the void ratio) and chemical
components are changing according to the manufacturer. The qualification is specific,
unfortunately existing standards are not sufficient. A good example of methodology was
given by the EC project Composite Housing. Glass fibres. The reinforcement by glass fibre is
providing good performances for the long term. The high glass/matrix ratios are giving better
hydrostatic pressure and compressive strength (70 - 80 % in mass). The use of S or R glass for
the fibre and the choice of manufacturing method such as filament winding, fabric prepreg,
injection have been qualified in several design of underwater equipment. The lander MAP 2
using a glass-epoxy hull and several glass-epoxy components such as amplificating flexural
<strong>Deliverable</strong> #50 - update October 2010 38