Mitigating offshore noise for marine mammals

In order to maintain marine biodiversity, the EU Marine Strategy Framework Directive (MSFD) requires member states to achieve Good Environmental Status of their seas by 2020. The MSFD includes a criterion specifying that the introduction of energy, including underwater noise, generated for instance during offshore wind farm installation, should be at levels that do not adversely affect the marine environment.

Servicing vessels used during construction and operation can generate continuous noise at frequencies that overlap with many marine mammals’ communication signals. While some impacts of underwater noise on marine mammals and fish have been quantified and specific noise thresholds set accordingly, the impacts that lead to behaviour and wider population impacts are still widely unknown.

Baker Consultants recently worked with developers of the Borkum Riffgrund 1 offshore windfarm, based in German waters. The German government’s response to the MSFD has been precautionary, with the establishment of strict noise thresholds that must not be exceeded during certain activities, including the installation of offshore windfarms. Therefore, in order for the German government to approve monopile foundations for such windfarms, evidence that underwater noise has remained below the threshold is needed periodically during installation before future installation work is approved.

At Borkum Riffgrund 1, the company installed autonomous Passive Acoustic Monitoring (PAM) devices at set distances from each monopile and rotated these on a regular basis to measure noise and for recording harbour porpoise activity in the area.

Baker Consultants also proposed and implemented an innovative method for monitoring porpoise activity using full-spectrum recorders instead of click detectors. The company’s specialist bioacousticians designed their own automatic classifiers to detect the porpoise clicks more efficiently and the resulting detections were manually inspected and verified by experienced observers. This allowed investigation of the data waveforms to minimise the level of uncertainty in the results.

To reduce noise away from the pilings, IHC Noise Mitigation System, a double walled cylinder filled with a bubble layer, was used. This ensured the right trade-off between piling energy and blow count was attained. To mitigate noise to marine mammals, the piling energy and rate were gradually increased and acoustic deterrent devices (ADDs) deployed to warn animals about the upcoming anthropogenic activity and encourage them to leave the area.

Baker Consultants is now working on Gode Wind 1 and 2 offshore windfarms, also based in German waters, to again provide underwater noise and marine mammal monitoring and mitigation for GeoSea.

By Rachael Doyle