Marine mammal monitoring and underwater noise mitigation is an integral part of the development of offshore wind farms, which, along with many other developments, generate underwater noise that can negatively impact marine mammals. For instance, servicing vessels used during construction and operation can generate continuous noise at low frequencies, which overlap with the communication signals of many marine mammals, such as baleen whales. In fact, each operation in the development of offshore wind farms has its own acoustic signature that must be identified and quantified in order to assess its impacts on species present in the area.
The introduction of noise into the marine environment is a major concern, given that numerous marine species including dolphins and porpoises rely on sound as their principal means of communication and navigation. Noise can be particularly disruptive in the marine environment because sound travels great distances through water. Further, although some of the impacts of underwater noise on marine mammals and fish have been quantified into specific thresholds that cause hearing loss, either permanently or temporarily, and noise thresholds set accordingly, the thresholds that lead to changes in behaviour and wider population impacts are still largely unknown.
The EU Marine Strategy Framework Directive (MSFD) requires member states to achieve Good Environmental Status of their seas by 2020 and part of it includes a criterion specifying that underwater noise such as what is generated during offshore wind farm installation, should be at levels that do not adversely affect the marine environment.
The pectoral fin of a Humpback whale (Megaptera novaeangliae). Credit: Shutterstock.
Two recent offshore wind farms – Borkum Riffgrund 1, which just begun exporting power, and Gode Wind 1 and 2, which is currently under construction and has marine mammal monitoring and underwater noise mitigation in place – are based in German waters in the North Sea. The German government’s BSH (Bundesamt FÜr Seeschifffahrt und hydrographie) maritime agency has established strict noise thresholds for Sound Exposure Level (SEL), which must not be exceeded during piling activities. Consequently, an SEL limit of 160 dB re 1 μPa2 s outside a 750-meter radius for pile-driving operations appears in the licence conditions for offshore wind farms.
Noise Mitigation at the Borkum Riffgrund 1 Offshore Wind Farm. Credit: DONG Energy, KIRBI A/S and Wommian Demant Invest A/S.
In order for the government to approve monopile foundations for offshore wind farms, evidence that underwater noise has remained below this threshold must be given at set intervals during installation before approval is given for any future installations. In the case of Borkum Riffgrund 1, the licence initially only allowed the installation of the first 12 monopiles, with consent for additional monopiles subject to the outcome of noise measurements.
How It Works
These two large offshore wind farm projects in the North Sea, with 77 and 97 turbines respectively, needed evidence that noise thresholds were met and required monitoring of marine mammal activity (harbour porpoises in both instances) via passive acoustic recorders during wind turbine foundation installation. It was also necessary to assess the efficiency of the noise mitigation strategy which in both cases, used the IHC Noise Mitigation System.
For Borkum Riffgrund 1, a methods statement for the monitoring campaign was drafted and approved by the German government in the early phases of the project. After that the installation company (GeoSea), the piling company (IHC Hydrohammer) and the consent managers at DONG Energy were monitored to ensure mitigation protocols were followed and given advice on optimizing the piling strategy to minimize noise.
Construction of the Borkum Riffgrund 1 Offshore Wind Farm. Credit: DONG Energy, KIRBI A/S and Wommian Demant Invest A/S.
The weather played a role in the project. Servicing of acoustic recorders in the North Sea is challenging when a specific schedule must be adhered to, particularly in winter, as the instruments used are very sensitive. To ensure safe working conditions and avoid accidents, servicing must usually be carried out in sea state 2 or lower, which meant that most servicing had to be completed before weather conditions changed, whilst simultaneously fitting around the piling schedule to ensure no data was lost.
Pingers like these are used for marine mammal deterrence. Credit: Baker Consultants.
Acoustic Monitoring and Mitigation Strategy
At Borkum Riffgrund 1, Passive Acoustic Monitoring (PAM) devices were installed at set distances from each monopile and rotated on a regular basis, following the BSH guidance. PAM devices measure noise and record porpoise activity in the area.
A part of the acoustic monitoring included an innovative method for monitoring porpoise activity using full-spectrum recorders instead of click detectors. This allowed investigation of the data waveforms to minimize uncertainty in the results. Specialist bioacousticians designed automatic classifiers to detect the porpoise clicks more efficiently and the results were manually inspected and verified by experienced observers.
The aim of the mitigation strategy was to reduce the underwater noise and keep harbor porpoises outside of the piling zone.
Overarching guidance on noise mitigation traditionally follows three separate lines of approach: material noise control measures along the propagation path, at the receiver and at the noise source location; and modification of operational procedures.
Baker Consultants at work performing passive acoustic monitoring for noise. Credit: Baker Consultants.
In the cases of Borkum Riffgrund 1 and Gode Wind (and other similar projects), the vast area of the underwater environment affected by localized, noise-producing activities negates the option of using the first noise mitigation strategy, as this would not meet best practicable means (BPM). Therefore, material control measures were adopted at the source of noise and involved the use of the IHC Noise Mitigation System, which is a double walled cylinder filled with a bubble layer, to ensure that the right trade-off between piling energy and blow count was attained. Operation procedures, too, were modified through the adoption of a “ramp-up” procedure. This procedure was used as a mitigation measure (along with the acoustic deterrent devices) to warn marine mammals about the upcoming anthropogenic activity and encourage them to leave.
Monitoring underwater noise can be extremely challenging and the costs of these survey techniques have to be balanced with the added value of gathering this information. However, in recent years, the technology available for monitoring underwater noise and marine mammal presence has improved dramatically, with several affordable and good quality instruments now readily available. This means there are now multiple ways in which surveys can be conducted, depending on individual needs. Most importantly, instruments are now available that allow the monitoring of sound underwater both short and long term, autonomously or from a boat, and allow real-time data transmission to a remote location.
As Passive Acoustic Monitoring requires minimal human intervention and can be used when weather conditions are highly adverse, it therefore reduces time and costs over more traditional methods.
Dr. Federica Pace is Marine Technical Director at Baker Consultants Marine and her skills combine undergraduate training in marine biology with advanced studies in underwater acoustics and signal processing applied to marine mammal sounds. Her most recent work includes providing expert advice on the impact of anthropogenic noise sources on marine mammals.