Environmental statement - Flyndre and Cawdor - Maersk Oil

Flyndre and Cawdor Environmental Statement
Section 5 Assessment of Potential Impacts and Control Measures
The use of vessels, especially Dynamically Positioned (DP) vessels, will cause elevated noise levels and
it is expected that peak vessel activity will occur during subsea installation. During the subsea
installation phase a number of vessels will be present and may be manoeuvring at relatively high
power. It is also possible that several of them will be using DP which generates a relatively high noise
output, these include anchor handling tugs and supply vessels, reel lay vessels, survey vessels,
trenching and trenching support vessels and DSVs.
In terms of direct physical injuries to hearing structures in marine mammals and potentially fish, it
appears from the available data that quite loud and/or sustained exposures are required to cause
even temporary changes in hearing sensitivity. Consequently, the likelihood that a single exposure of
shipping noise would be sufficient to permanently damage the hearing of marine animals appears to
be remote. However, short term behavioural effects may be observed amongst cetaceans and
pinnipeds, but the overall impact of these is thought to be negligible.
Proposed Control Measures
Minimise number of vessels required
Minimise length of time vessels are on site
PILE DRIVING NOISE
The support frame for the two manifolds will be secured by piles driven into the seabed and during
environmental screening, the noise was assessed as posing a moderate risk to the environment as pile
driving is considered to be the only activity that would have the potential to generate underwater
sounds levels capable of causing injury to marine life.
The manifolds will be secured to the seabed by up to four piles. The piles will have an outside
diameter of 0.60 m and a penetration length of 15 m. An underwater hydraulic pile‐driver suspended
from a support vessel will be used. The duration of the installation period is expected to be 2 – 2.5
hours for each pile and a total of 8‐10 hours for each manifold (based on driving time required for
piles at Affleck field where piles were 0.61 m in diameter with a penetration length of 16.5 m).
In order to provide an objective and quantitative assessment of the likely degree of any
environmental effect upon marine receptors it is necessary to estimate the sound level as a function
of distance. Very few empirical data sets exist for pile driving measurements, the source
characteristic and frequency spectrum that was used in this model was undertaken during piling of a
1.5 m diameter pile at the FINO‐1 Platform installation. The pile driving generated a source level of
228 dB (peak) re 1µPa @ 1m (ITAP, 2005). As can be seen from Figure 5‐1 the dominant piling
frequencies are those less than 1000 Hz. It is expected that piling of the manifolds will generate a
similar spectra of frequencies to that shown below, with the majority of the sound pressure being
generated at frequencies below 2 kHz.
D/4114/2011 5 ‐ 11