ENVIRONMENTAL STATEMENT BARDOLINO DEVELOPMENT

Bardolino Development Environmental Statement
source would make no significant contribution to the total source noise level at the site during
the piling operations (Norton, 1989).
Noise generated during pipeline installation operations
The underwater noise that is produced by vessels arises from two sources – propeller
cavitation and the propulsion machinery inside the vessel. Vessel noise may be considered
to be a continuous, rather than transient noise source, which is a combination of broadband
noise and tonal sounds at specific frequencies (Richardson et al., 1995).
The noise that would be generated by the vessels would be determined by the types and
numbers of vessels that are present at the site at any one time. Because the decibel scale
used to measure sound is a logarithmic scale, the presence of several sources of noise at any
location at any one time leads to only a small increase in the total source level of noise at the
site.
Published data on the sound source levels and frequencies for different types of vessels are
provided in Table 6.6. Sound levels and frequency characteristics are depended on ship size
and speed, with variations among vessels of similar classes (Richardson et al., 1995). Noise
from shipping is roughly related to vessel size; larger ships have larger, slower rotating
propellers, which produce louder, lower frequency sounds. Broadband source levels of ships
between 55 and 85 m are generally around 170-180 dB re 1 µPa @ 1 m (Richardson et al.,
1995), with most energy below 1 kHz. The use of bow thrusters can increase broadband
sound levels by 11 dB and includes higher frequency tonal components up to 1 kHz
(Richardson et al., 1995).
There are currently two pipeline options which would require the operation of a number of
different vessel types in the Bardolino area over a total period of 6 weeks. All would be
typical of the vessels routinely used in the UK North Sea for oil and gas operations and other
activities.
• A dynamically positioned (DP) reel lay vessel would be used to lay the piggy-backed
pipelines simultaneously onto the seabed.
• Trenching would be carried out using a seabed plough towed by an anchor handling
tug vessel (AHTV).
• Rock dumping would be achieved by the careful placement of graded crushed rock
over the pipelines, using a dedicated rock dumping vessel.
• A DP dive support vessel (DSV) would be in the field for several different phases,
such as tie-in spools and hydro-testing.
• Guard vessels would be on site throughout the pipeline operations.
• A DP survey vessel would be used to ensure that the pipes were laid in the correct
location and that trenching activities were performed satisfactorily.
The noise produced by subsea trenching operations depends on the equipment used and the
nature of the seabed sediments. A trenching noise spectrum reported in Richardson et al.
(1995) has peak levels of 178 dB re 1µPa @ 1 m at 160 Hz, with an overall source level 185
dB re 1µPa @ 1 m; this is similar to the data reported by Nedwell et al. (2004). These levels
are for mechanical dredging operations, and may be noisier than the proposed pipeline
installation operations for the Bardolino development which would use a towed plough.
During the pipeline installation operations the vessels that are likely to be present in the area
include DP pipelay vessel (180 db), AHTV for trenching (170 dB), dive support vessel (180
dB), guard vessel (180 dB), survey vessel (180 dB) and rock dump vessel (180 dB) (Table
6.6). The estimated noise level from this combination of vessels operating simultaneously at
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