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 />
Noise associated with cable laying will therefore impact upon hearing specialists<br />
to a greater extent. The effect <strong>of</strong> underwater noise on fish can be categorised as<br />
(Nedwell et al., 2003):<br />
● Primary effects: These include immediate or delayed fatal injury <strong>of</strong> animals<br />
near to powerful sources, such as the blast from underwater explosives;<br />
● Secondary effects: These include injuries <strong>and</strong> deafness which may have long<br />
term implications for survival; <strong>and</strong><br />
●<br />
116<br />
Tertiary effects: These are most likely to be associated with cable laying <strong>and</strong><br />
include avoidance <strong>of</strong> the area which could have significant effects in the<br />
vicinity <strong>of</strong> breeding grounds, migratory routes or schooling areas.<br />
Nedwell et al. (2003, 2005) proposed a measure <strong>of</strong> sound that takes into account<br />
the differences between species in terms <strong>of</strong> hearing ability. This measurement<br />
is referred to as dB ht (Species). The derivation <strong>of</strong> dB ht (species) involves the<br />
measurement <strong>of</strong> sound passing through a filter that mimics the hearing ability<br />
<strong>of</strong> individual species <strong>and</strong> provides a species specific measurement <strong>of</strong> the<br />
likely level <strong>of</strong> perception <strong>of</strong> sound by the species. For a full description <strong>of</strong> the<br />
dB ht (Species) measurement <strong>and</strong> the uncertainties involved in its application,<br />
the reader is directed to Nedwell et al. (2003, 2005). While the dB ht (Species)<br />
approach is still being validated <strong>and</strong> reviewed, available information suggests<br />
that species <strong>of</strong> fish <strong>and</strong> marine mammals (see Section 5.6.1) will show a strong<br />
avoidance reaction to sound levels <strong>of</strong> 90 dB ht <strong>and</strong> above. Strong avoidance by<br />
most individuals is likely to occur at 100 dB ht with a mild avoidance reaction<br />
occurring in a minority <strong>of</strong> individuals at levels above 75 dB ht . The effect <strong>of</strong> such<br />
impacts will be dependant upon a combination <strong>of</strong> factors including the type <strong>and</strong><br />
magnitude <strong>of</strong> the noise along with the proximity <strong>of</strong> an organism to the source<br />
<strong>of</strong> the noise.<br />
In the context <strong>of</strong> <strong>of</strong>fshore wind farm construction, much <strong>of</strong> the work relating to<br />
the impact <strong>of</strong> noise upon species has focussed on the effect <strong>of</strong> pile driving, as<br />
this is by far the ‘worst case scenario’ in terms <strong>of</strong> the production <strong>of</strong> high intensity<br />
impulsive sound. Sound levels associated with cable installation have received<br />
considerably less attention <strong>and</strong> very little monitoring data is available.<br />
Nedwell et al. (2003) have reported that cable trenching in s<strong>and</strong>y gravel at North<br />
Hoyle <strong>of</strong>fshore wind farm produced noise at a source level <strong>of</strong> 178 dB re 1μPa @<br />
1m. However, when illustrating the dB ht levels <strong>of</strong> the noise as a function <strong>of</strong> range,<br />
the following figure was produced.<br />
Although based on some uncertainty, due to the high levels <strong>of</strong> variability in the<br />
noise produced, Figure 5.2 clearly shows that, for each species the dB ht level is<br />
below 90. In this situation, significant avoidance reactions amongst fish would<br />
not be expected to occur.