Environmental Engineering Reference
In-Depth Information
water column and into the sediments, from which they pass successively back into
the water column. Larger diameter, longer piles require relatively more energy to
drive into the sediments, which results in higher noise levels. For example, the SPL
associated with driving 3.5-m-diameter piles is expected to be roughly 10 dB greater
than for a 1.5-m-diameter pile (Thomsen et al., 2006). Pile-driving sound, while
intense and potentially damaging, would occur only during the installation of some
marine and hydrokinetic energy devices.
Some ocean energy technologies will be secured to the bottom by means of
moorings and anchors drilled into rock. Like pile-driving, hydraulic drilling will
occur during a limited time period, and noise generation will be intermittent. The
Department of the Navy (2003) summarized underwater SPL measurements of three
hydraulic rock drills; frequencies ranged from about 15 Hz to over 39 Hz, and SPLs
ranged from about 120 to 170 dB re 1 µPa. SPLs were relatively consistent across the
entire frequency range.
During operation, vibrations of the device's gearbox, generator, and other moving
components are radiated as sound into the surrounding water. Noise during the oper-
ation of wind farms is of much lower intensity than noise during construction (Betke
et al., 2004; Thomsen et al., 2006), and the same may be true for hydrokinetic and
ocean energy farms; however, this source of noise will be continuous. Measurements
of sound levels associated with the operation of hydrokinetic and ocean energy farms
have not yet been published. One example of a wave energy technology, the WEC
buoy (a version of OPT's PowerBuoy) which has been tested in Hawaii, has many
of the mechanical parts contained within an equipment canister or mounted to a
structure through mounting pads. Thus, the acoustic energy produced by the equip-
ment is not well coupled to the seawater, which is expected to reduce the amount of
radiated noise (Department of the Navy, 2003). Although no measurements had been
made, it was predicted that the acoustic output from the WEC buoy system would
probably be in the range of 75 to 80 dB re 1 µPa. This SPL is equivalent to light to
normal density shipping noise, although the frequency spectrum of the WEC buoy
is expected to be shifted to higher frequencies than typical shipping noise. By com-
parison, Thomsen et al. (2006) reported the ambient noise measured at five different
locations in the North Sea. Depending on frequency, SPL values ranged from 85 to
115 dB, with most energy occurring at frequencies less than 100 Hz.
The Environmental Statement for the proposed installation of the Wave Dragon
wave energy demonstrator off the coast of Pembrokeshire, U.K., predicted noise
levels associated with installation of a concrete caisson (gravity) block and steel
cable mooring arrangement, installation of subsea cable, and support activity (Wave
Dragon Wales, Ltd., 2007). The installation of gravity blocks is not expected to gen-
erate additional noise over and above that of the vessel conducting the operation.
Vessel noise will depend on size and design of the ship but is expected to be up to 180
dB re 1 µPa at 1 m. Other predicted installation noise sources and levels stem from
operation of the ship's echosounder (220 dB re 1 µPa at 1 m peak-to-peak), cable
laying and fixing (159 to 181 dB re 1 µPa at 1 m), and directional drilling (129 dB
re 1 µPa rms at 40 m above the drill). There are no measurements available for the
noise associated with operation of an overtopping device such as the Wave Dragon.
Wave Dragon Wales, Ltd. (2007) predicted that operational noise would result from
