Review of Cabling Techniques and Environmental Effects Applicable

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