Review of Cabling Techniques and Environmental Effects Applicable

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