Review of Cabling Techniques and Environmental Effects Applicable

Cable types and installation techniques
become exposed as a consequence of moving seabed sediments. Experience
gained from the post installation surveys of subsea telecommunications cables
has shown that sections of cable which may have been left surface laid during
the initial installation have subsequently been re-buried by the movement of
sandwaves, whilst at the same time previously buried sections of cable have
been exposed.
In order to mitigate the effects of mobile sediments, deeper cable burial is often
specified. This solution may work where the amplitude of the sandwaves are
relatively low (typical 1m to 1.5m). However in areas where the amplitudes are
much higher, deeper burial will not be possible on a practical basis. Furthermore,
if deeper burial is attempted in these areas there would be a potential for a
significant impact on the local environment.
To successfully mitigate the effects of mobile sediment seabeds, the only
effective method is to undertake accurate and detailed surveys and to carefully
plan the cable route to avoid any potential problems. The survey data should
provide detailed bathymetric, geophysical and geotechnical information and the
results of this survey should then be reviewed against historical chart data to
ascertain any evidence of the mobility of the seabed in the area where the cables
are planned to be buried.
With all survey and historical data to hand, a detailed cable route risk assessment
can be completed. This will identify the optimum route for the cables and may
well involve a complete route diversion to avoid mobile sediments such as
sandbanks etc. The subsea cables can also be routed to follow the troughs of any
major sandwaves or sandbanks such that any movement of sands would result
in mobile sediments being deposited over the cables as opposed to exposing
them. However, future accessibility of the cables for any potential repair or
maintenance activities needs to be considered if such a policy is adopted.
3.8.8 CROSSING NAVIGATION CHANNELS
When the planned cable routing for the export cables requires a crossing of
either an existing navigable channel or a proposed route for a navigable channel,
it is necessary to install the cables to a suitable reference elevation to protect
the cables from any future planned dredging operations along the line of the
channel. There are two options which are commonly employed.
Option 1 – the pre-excavation method
The pre-excavation method requires an initial pre-excavation across the
navigable channel before the export cables are installed. Export cables would
then be buried using a subsea cable plough (or possibly a trenching vehicle)
which will pass over the recently pre-excavated channel, during the cable lay
procedure. It is anticipated that the cable crossing corridor across the channel
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