Balancing risks of injury and disturbance to marine mammals when pile driving at offshore windfarms

1. Offshore windfarms require construction procedures that minimise
impacts on protected marine mammals. Uncertainty over the efficacy of
existing guidelines for mitigating near-field injury when pile-driving
recently resulted in the development of alternative measures, which
integrated the routine deployment of acoustic deterrent devices (ADD) into
engineering installation procedures without prior monitoring by Marine
Mammal Observers. 2. We conducted research around the installation of
jacket foundations at the UK’s first deep-water offshore windfarm to
address data gaps identified by regulators when consenting this new
approach. Specifically, we aimed to a) measure the relationship between
noise levels and hammer energy to inform assessments of near-field injury
zones, b) assess the efficacy of ADDs to disperse harbour porpoises from
these zones. 3. Distance from source had the biggest influence on received
noise levels but, unexpectedly, received levels at any given distance were
highest at low hammer energies. Modelling highlighted that this was
because noise from pin pile installations was dominated by the strong
negative relationship with pile penetration depth with only a weak
positive relationship with hammer energy. 4. Acoustic detections of
porpoises along a gradient of ADD exposure decreased in the 3-hours
following a 15-minute ADD playback, with a 50% probability of response
within 21.7 km. The minimum time to the first porpoise detection after
playbacks was > 2 hours for sites within 1 km of the playback. 5.
Our data suggest that the current regulatory focus on maximum hammer
energies needs review, and future assessments of noise exposure should
also consider foundation type. Despite higher piling noise levels than
predicted, responses to ADD playback suggest mitigation was sufficiently
conservative. Conversely, strong responses of porpoises to ADDs resulted
in far-field disturbance beyond that required to mitigate injury. We
recommend that risks to marine mammals can be further minimised by: 1)
optimising ADD source signals and/or deployment schedules to minimise
broad-scale disturbance; 2) minimising initial hammer energies when
received noise levels were highest; 3) extending the initial phase of soft
start with minimum hammer energies and low blow rates.Minhyuk Seo