Underwater Noise Thematic Assessment

Response measures addressing underwater noise

Continuous noise

Mitigation strategies for continuous noise focus on reducing masking and disturbance of sensitive marine animals such as mammals and fish. The main source of continuous noise is shipping. Measures to reduce shipping noise are most effectively aimed at reducing the source levels (‘quiet ships’). Secondly, operational measures such as speed reduction can be considered, and thirdly, measures can be aimed at protecting specific vulnerable areas, for example through rerouting. Such areas could be designated as Particularly Sensitive Sea Areas (PSSAs). In these areas special measures for shipping can be applied. A PSSA is an area that requires special protection through IMO measures because it is significant for recognised environmental, socio-economic or scientific reasons and, at the same time, may be adversely affected in its ecological functions by the impact of international shipping traffic from activities in the sea area.

Technical measures have a high potential to reduce shipping noise in the long term. IMO (2014) has published voluntary guidelines on how the noise of vessels’ main sources, notably propeller noise or machinery vibrations radiated into the water through hull structures, can be reduced. In Europe, the AQUO (Achieve QUieter Oceans by shipping noise footprint reduction) and SONIC (Suppression of UW Noise Induced by Cavitation) projects produced a set of guidelines covering ship design, including for propellers, and traffic management. The World Organisation of Dredging Associations has also published guidance on underwater noise associated with dredging, which mainly offers the maintenance and cleaning of equipment as an effective way of reducing sound (WODA, 2013).

Some of these measures concern ships’ hull design and thus are feasible only on newly built vessels. Others, such as optimised new propellers, would not only reduce noise in existing ships but at the same time save fuel costs, generate income if shipping is included in emission trade systems, and help achieve commitments to reduce carbon dioxide emissions. A cost-benefit analysis of technical options and incentives for reducing shipping noise can be found on the Netherlands government portal on the North Sea (Strietman et al., 2018), and covers incentives such as the inclusion of noise in green shipping indices and voluntary or mandatory measures to promote slow shipping speeds. In 2019 Vard Marine Inc. published a report that describes various means to mitigate underwater radiated noise by ships (Vard Marine Inc., 2019). This study was commissioned by Transport Canada.

To date, it is thought that the 2014 IMO guidelines have not yet had an effect on overall shipping noise, and some modelling suggests that underwater noise has increased in EU waters since 2014 (EMSA/EEA, 2021). In 2021, the IMO Maritime Environment Protection Committee (MEPC) approved a proposal, initiated by Canada, the United States and Australia, for a review of the IMO guidelines. This proposal was supported by the EU and many other OSPAR Contracting Parties.

The costs of some technical measures may be a barrier to rapid noise reduction. Thus, additional operational measures need to be put in place if noise is to be reduced in the short term. These can include reductions in speed or optimisation of operational parameters (e.g. adjustments of rotational speed and screw blade pitch to reduce cavitation) with respect to noise emissions. In addition, rerouting measures and avoidance of areas critical for sensitive species offer a short-term solution; however, such rerouting needs approval by IMO.

Impulsive noise

The mitigation strategies for impulsive noise focus on avoiding injuries to marine animals and the disturbance of sensitive animals such as mammals and fish. A combination of operational and technical measures can be used to mitigate adverse effects, together with assessment during the planning phase of infrastructure developments.

There are technical measures for some impulsive noise-generating activities such as pile driving or blasting. Among the technical measures to reduce piling noise are Big Bubble Curtains, Isolation Casings, and Hydro Sound Dampers (OSPAR, 2016). These three are currently considered state-of-the-art at water depths of up to 40 m (Bellmann et al., 2020). In increasingly large diameter monopiles, a combination of these technologies is advised in order to meet national standards.

A number of emerging technologies have a high potential to reduce noise, but may need some further development. Some alternative low-noise foundation methods are already market-available but are rarely considered by the industry due to their higher costs or limited applicability at specific sites (Koschinski & Lüdemann, 2020). As a means to reduce the shock wave of underwater explosions, which can injure marine vertebrates at ranges of many kilometres (von Benda-Beckmann et al., 2015; Siebert et al., 2022), the Big Bubble Curtain was shown to be effective in reducing the radiation of blast pressure during systematic experiments in coastal waters (12 m depth) conducted between 2008 and 2012 (e.g. Nuetzel, 2008; Schmidtke, 2010; Schmidtke, 2012). These might need to be adapted if they were to be used in deeper waters. Mitigation techniques such as the Big Bubble Curtain, together with other noise reduction techniques such as deflagration or recovery of munitions, should be further tested and evaluated. Such consideration would need to take account of wider possible impacts on the environment as well as safety implications.

For geophysical surveying, noise from seismic surveys can be reduced to a small extent by changes in the design and configuration of airgun arrays. To achieve significant reductions, alternative technologies are needed; marine vibrator technologies are under development.

A number of operational mitigation measures are available to complement the technical mitigation methods. For example, timing and location can be critical for reducing adverse effects on biota or habitats and avoidance of sensitive times and places (e.g. feeding areas, reproductive sites, areas of seasonal migrations). The selection of a less critical time or the relocation of an activity can be simple and effective. Protected species observers and passive acoustic monitoring help in detecting animals within the vicinity of the activity. Safety procedures which allow the ceasing of noisy activity when animals are sighted or acoustically detected can prevent the most serious injury. Pre-activity monitoring of sensitive species give indications whether animals are in the vicinity and in what densities, and thus help determine efficient and cost-effective mitigation methods. Post-activity monitoring at sea or of stranded animals shows whether mitigation was successful and may allow medical treatment of injured animals.

Despite the availability of measures, their application in oil and gas industry activities is likely to be very varied within the OSPAR area, as guidelines to prevent or minimise the impact of noise on marine mammals currently vary and there is international recognition of the need for more consistent, evidence-based guidelines.

OSPAR response

OSPAR has published an inventory of measures to mitigate the emission and environmental impact of underwater noise, last updated in 2020. This includes annexes covering noise from pile driving and from seismic surveys (the latter is due to be updated).

OSPAR Contracting Parties have also engaged with the International Association of Oil and Gas Producers, which has coordinated a Joint Industry Programme on Exploration and Production Sound and Marine Life.

OSPAR’s North-East Atlantic Environmental Strategy 2030 commits OSPAR to agree a regional action plan on underwater noise by 2025, which will set out a series of national and collective actions and, as appropriate, OSPAR measures to reduce noise pollution (NEAES Operational Objective S8.01).