Abstract
Injury to marine mammals due to sound from impact piling of the foundations for offshore wind farms has become a significant concern in the UK and worldwide. With increasing pile sizes and hammer energies being proposed for future developments, this concern remains a significant consenting risk.
The use of soft-start (where the initial hammer energy is reduced for a period at the start of piling) and the use of slow-start (where the hammer strike rate is reduced) are common mitigation strategies for reducing the potential for injury to marine mammals in the UK. More recently, the use of Acoustic Deterrent Devices (ADDs) has become commonplace as an additional mitigation strategy for deterring marine mammals from the area where piling will take place.
The range at which injury can occur depends upon numerous different factors, including the source level of sound radiated by the pile into the water column, hammer energy, strike rate, swim-away speed, timings for each phase, and propagation of sound away from the pile. For the mitigation strategy to be most effective, any near-by animal needs to swim some distance from the pile before full-power piling occurs. However, at present there is no definitive methodology for determining the most effective combination of ADD, soft-start, and slow-start in order to reduce the risk of injury to as low as practicable.
This chapter examines the various factors that affect the efficacy of a combined ADD, slow-start, and soft-start piling mitigation protocols, and makes recommendations as to how to design the most effective mitigation strategy.