Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Cable types <strong>and</strong> installation techniques<br />
become exposed as a consequence <strong>of</strong> moving seabed sediments. Experience<br />
gained from the post installation surveys <strong>of</strong> subsea telecommunications cables<br />
has shown that sections <strong>of</strong> cable which may have been left surface laid during<br />
the initial installation have subsequently been re-buried by the movement <strong>of</strong><br />
s<strong>and</strong>waves, whilst at the same time previously buried sections <strong>of</strong> cable have<br />
been exposed.<br />
In order to mitigate the effects <strong>of</strong> mobile sediments, deeper cable burial is <strong>of</strong>ten<br />
specified. This solution may work where the amplitude <strong>of</strong> the s<strong>and</strong>waves are<br />
relatively low (typical 1m to 1.5m). However in areas where the amplitudes are<br />
much higher, deeper burial will not be possible on a practical basis. Furthermore,<br />
if deeper burial is attempted in these areas there would be a potential for a<br />
significant impact on the local environment.<br />
To successfully mitigate the effects <strong>of</strong> mobile sediment seabeds, the only<br />
effective method is to undertake accurate <strong>and</strong> detailed surveys <strong>and</strong> to carefully<br />
plan the cable route to avoid any potential problems. The survey data should<br />
provide detailed bathymetric, geophysical <strong>and</strong> geotechnical information <strong>and</strong> the<br />
results <strong>of</strong> this survey should then be reviewed against historical chart data to<br />
ascertain any evidence <strong>of</strong> the mobility <strong>of</strong> the seabed in the area where the cables<br />
are planned to be buried.<br />
With all survey <strong>and</strong> historical data to h<strong>and</strong>, a detailed cable route risk assessment<br />
can be completed. This will identify the optimum route for the cables <strong>and</strong> may<br />
well involve a complete route diversion to avoid mobile sediments such as<br />
s<strong>and</strong>banks etc. The subsea cables can also be routed to follow the troughs <strong>of</strong> any<br />
major s<strong>and</strong>waves or s<strong>and</strong>banks such that any movement <strong>of</strong> s<strong>and</strong>s would result<br />
in mobile sediments being deposited over the cables as opposed to exposing<br />
them. However, future accessibility <strong>of</strong> the cables for any potential repair or<br />
maintenance activities needs to be considered if such a policy is adopted.<br />
3.8.8 CROSSING NAVIGATION CHANNELS<br />
When the planned cable routing for the export cables requires a crossing <strong>of</strong><br />
either an existing navigable channel or a proposed route for a navigable channel,<br />
it is necessary to install the cables to a suitable reference elevation to protect<br />
the cables from any future planned dredging operations along the line <strong>of</strong> the<br />
channel. There are two options which are commonly employed.<br />
Option 1 – the pre-excavation method<br />
The pre-excavation method requires an initial pre-excavation across the<br />
navigable channel before the export cables are installed. Export cables would<br />
then be buried using a subsea cable plough (or possibly a trenching vehicle)<br />
which will pass over the recently pre-excavated channel, during the cable lay<br />
procedure. It is anticipated that the cable crossing corridor across the channel<br />
67