Handbook of best practices

such as filament winding, fabric pre-preg, injection have been qualified in several design of
underwater equipment.
Carbon fibres
Lighter structures may be designed using carbon fibres. Under tensile or flexural strength
design criteria, the additional cost finds good arguments. It is more limited for structures
dimensioned by the compressive strength. The feasibility of carbon epoxy pressure hulls has
been demonstrated by EC projects. The electrical conductivity of carbon must be taken into
account: it is not an isolator.
Syntactic foam
A composite material made up with very small hollow glass spheres inside a plastic matrix is
able to provide buoyant material. It has been qualified for full water depth floats as well as
pipe insulation material. The floats must be unitary tested when they are a safety
component. They must be preferred to glass spheres.
Ø Note that functional properties of non-metallic materials must be checked with
respect to ageing (for instance, impedance of acoustic transducers may vary).
Brittle materials such as glass or ceramics have exceptional compressive strength. But any
tensile or shear stress may lead to rupture. They are used for electric insulation in
connectors with a very stringent manufacturing process.
Glass spheres, although they were the basis of deep ocean exploration in the 20 th century,
are not recommended for ESONET type of long term deployments. Further studies on
probability of rupture have been performed by KM3Net project. The distance between
spheres to avoid the transmission of implosion was investigated by KM3Net Preparatory
Phase (20 m distance for 2000 dbar pressure on 6000 dbar rated spheres is a first
recommendation) [19].
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