Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
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<strong>Review</strong> <strong>of</strong> <strong>Cabling</strong> <strong>Techniques</strong> <strong>and</strong> <strong>Environmental</strong> <strong>Effects</strong> <strong>Applicable</strong> to the Offshore Wind<br />
Farm Industry – Technical Report<br />
thus ensuring the cable is simultaneously buried as the cable ship <strong>and</strong> cable<br />
plough make forward progress. The cable plough is fully instrumented <strong>and</strong> has<br />
hydraulically activated functions to allow for varying depth <strong>of</strong> burial. In addition,<br />
the instrumentation package ensures that all critical aspects <strong>of</strong> the operation can<br />
be controlled <strong>and</strong> monitored back on the host vessel as a real time operation.<br />
Between the late 1980s <strong>and</strong> the early 2000s, technologies improved the types<br />
<strong>and</strong> range <strong>of</strong> plough systems <strong>and</strong> these various types are described in detail<br />
in subsequent sections. During this same period <strong>of</strong> time, a number <strong>of</strong> tracked,<br />
remotely operated vehicles (ROVs) <strong>and</strong> free swimming ROVs were also<br />
introduced into the market. These vehicles were primarily used on projects<br />
where shorter lengths <strong>of</strong> cable burial were specified or where work in shallow<br />
water was required. They were also used where the ground conditions were<br />
beyond the capabilities <strong>of</strong> a conventional subsea plough <strong>and</strong> specialist cutting<br />
tools were required to cut into rock strata.<br />
3.7.6 CABLE FAULT ANALYSIS<br />
A number <strong>of</strong> studies have reviewed cable fault rates based on the depth <strong>of</strong><br />
burial <strong>of</strong> the installed system. This research has shown that, on average, most<br />
‘hits’ on cables result from fishing activity <strong>and</strong> that surface laid cables would be<br />
regularly hit with fault occurrence directly related to the level <strong>of</strong> fishing activity.<br />
Cables buried to 0.6m depth are likely to only experience one or two hits in a<br />
10 to 15 year lifetime (probably in areas <strong>of</strong> shallow burial or where s<strong>and</strong>-waves<br />
are mobile) <strong>and</strong> cables buried to 1.0m are likely to have a high probability <strong>of</strong><br />
remaining fault free. These statistics (which have been extracted from various<br />
papers submitted at SubOptic 1997 <strong>and</strong> SubOptic 2001) only provide guideline<br />
information <strong>and</strong> some systems are likely to remain fault free for their operational<br />
life.<br />
Various research has now resulted in some guidelines for the depth <strong>of</strong> burial<br />
when using ploughs, depending on the threat which the installed cable system<br />
may encounter during its operational life. Table 3.4 presents the target burial<br />
depth for subsea ploughs to place cables below the ‘threat line’ for the different<br />
hazards which are expected to pass over the installed cable <strong>and</strong> for varying<br />
seabed conditions. The figures quoted in the table include a 33% safety factor<br />
which recognises that the target depth <strong>of</strong> burial is not always achieved in the<br />
field for operational reasons.<br />
Figure 3.9 shows an average fibre optic fault rate (normalised per 1000km length),<br />
plotted against the age <strong>of</strong> the cables, for global cable systems installed in the<br />
period 1990 to 1999. Trends are plotted for cables installed in water depths up<br />
to 1000m <strong>and</strong> also for cables installed in water depths greater than 1000m. The<br />
decreasing trend in cable faults with the age <strong>of</strong> the cable is probably attributable<br />
to any exposed sections <strong>of</strong> cable being buried during the life <strong>of</strong> the cable.<br />
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