Underwater Noise Pollution: Reducing Its Impact And Presence

Underwater noise pollution is a serious issue that poses a threat to marine life and ecosystems. Caused primarily by human activities such as commercial shipping, seismic surveys, and oil exploration, this pollution can have detrimental effects on the communication, navigation, and survival of marine animals. Recognizing the urgency of the problem, various organizations and individuals are exploring ways to reduce or eliminate underwater noise pollution.

This paragraph introduces the topic of underwater noise pollution, its sources, its impacts, and the ongoing efforts to address it. By highlighting the harm caused to marine life, it emphasizes the importance of finding solutions. The following discussion will delve into specific measures that can be taken to mitigate or eliminate underwater noise pollution, with a focus on the most effective strategies and their potential challenges.

Reduce cavitation by improving propeller design and the wake field

The wake equalizing duct (WED) is one of the most common auxiliary propulsor devices used to improve the propulsion performance of a ship. It is also used to reduce the propeller-excited vibrations and viscous resistance forces.

Propellers are a major source of underwater noise pollution, which has harmful effects on marine life. Modifying propeller design and the wake field can help to reduce cavitation and, therefore, noise levels.

Propeller cavitation is strongly influenced by the non-uniform inflow to the propeller. The ship wake is dominated by viscous effects, which are subject to major scale effects. Propeller designers often base their design on the nominal model scale wake distribution, which can lead to an underprediction of cavitation extent, volume, and pressure pulses. This can result in a conservative design that may not perform as efficiently as expected.

To improve propeller cavitation performance, designers can consider the following:

• Access to effective wake data: Propeller designers should have access to accurate and effective wake data, including the full-scale wake distribution, to make informed decisions. This data can be obtained through self-propulsion model tests or advanced computational methods such as the RANS-BEM coupling approach.
• Circumferentially averaged wake field: Using a circumferentially averaged and scaled nominal wake field at the model scale as input for the propeller design can help optimize the radial circulation distribution and improve cavitation performance.
• Consideration of hull-propeller interaction: The presence of the propeller behind the ship modifies the inflow field. Accounting for hull-propeller interaction in the design process can lead to more accurate predictions of cavitation behaviour.
• Optimization of propeller geometry: Modifying the propeller geometry, such as the number of blades, blade area ratio, and skew, can influence cavitation performance.
• Alternative propulsor devices: In some cases, auxiliary propulsor devices like the WED can be used to improve propulsion performance and reduce cavitation. However, the effectiveness of WEDs in reducing cavitation may vary, and optimization of their dimensions and form may be necessary.

Slow down ships

Slowing down ships is one of the most effective ways to reduce underwater noise pollution. The faster and larger ships are, the louder they get. Reducing the speed of the global shipping fleet by 10-20% can cause a reduction of noise emissions by 40-67% respectively.

A general rule of thumb is that for each 1-knot reduction in speed, a ship's noise is reduced by 1 decibel. For example, slowing the global fleet by a modest 10% could reduce the total sound energy from shipping by around 40%. In a real-world example, the Vancouver-Fraser Port Authority in British Columbia asks ships to slow down to either 11 or 14.5 knots (based on ship type) as they approach the port to reduce noise disturbance to the critically endangered Southern Resident orca population. With over 80% of large vessels participating in the slowdown in 2022, underwater sound intensity in the area decreased by as much as 55%.

Reducing ship speed also has the added benefit of decreasing fuel use and harmful emissions. This is especially important in sensitive regions like the Arctic, where underwater noise from ships has an even greater impact.

However, implementing ship speed restrictions is not as simple as it seems. While some organizations like OceanCare have been campaigning for slower ships since 2002, there is a lack of political will to make this a reality. Transnational solutions are necessary, and the onus is on organizations like the International Maritime Organization (IMO) to develop mandatory measures that require ships to slow down.

In the meantime, there are other measures that can be taken to reduce underwater noise from ships. These include improving ship design, such as using quieter propellers and modifying hulls, as well as regular ship maintenance and vibrationally isolating machinery.

Use marine vibrator systems instead of air guns for seismic surveys

Marine vibrator systems are an emerging alternative to conventional air guns in ocean-bottom seismic surveys. They are considered less intrusive than air guns as they emit their energy spread out over time, rather than in a single, high-intensity pulse. Marine vibrators also have the ability to generate low-frequency waves and have a limited adverse impact on marine wildlife.

Marine vibrators have several benefits over air guns. They can be used to create new and flexible source geometries in the 5-100 Hz frequency range, as no air compressors are required. They also have ultra-low-frequency potential in the 2-6 Hz frequency range, which is beneficial for FWI, elastic impedance inversion, and deep signal penetration. The acoustic signature of marine vibrators is continuous or transient in nature, with a rise time of several seconds, and the energy is emitted in a reasonably uniform manner over 5 to 30 seconds or more. This longer exposure time means that marine vibrators can be up to 98% quieter than air guns, reducing the risk of disruption to marine ecosystems.

The use of marine vibrators also presents some challenges. For example, handling the phenomena associated with the Doppler effect and time-dependent source-receiver offsets due to the source motion. Additionally, the time-varying nature of the sea surface during the multi-second acquisition time introduces further challenges for processing and inversion. However, these challenges can be addressed through the development of advanced imaging and inversion techniques.

Overall, the use of marine vibrator systems instead of air guns for seismic surveys has the potential to reduce underwater noise pollution and minimize the impact on marine life.

Implement bubble curtains to dampen noise from wind farm construction

Bubble curtains are an effective method to dampen noise from wind farm construction, which can be highly detrimental to marine life. The noise from wind farm construction, particularly the pile-driving phase, can cause hearing loss and even permanent deafness in porpoises, leaving them disoriented and unable to survive.

Bubble curtains work by slowing down and breaking up sound waves. A perforated hose is laid on the seabed around the construction site, and air is pumped through to create a veil of bubbles rising to the surface. As sound travels faster through water than air, the sound waves are interrupted by the bubbles, resulting in a loss of energy and a quieter sound on the other side of the curtain.

Bubble curtains were pioneered in Germany to protect the endangered harbour porpoise, the only cetacean species in the North and Baltic Seas. The noise from wind farm construction is lowered to a threshold deemed safe for the species, based on scientific research. The curtain's muffling effect may also benefit other marine mammals vulnerable to noise, such as seals.

The use of bubble curtains has now spread beyond Europe, with interest from countries like the US and Taiwan. This technology is a crucial tool to reduce underwater noise pollution and protect marine life during wind farm construction.

Establish acoustic refuges and buffer zones around marine protected areas

Establishing acoustic refuges and buffer zones around marine protected areas is a crucial strategy in mitigating the harmful effects of underwater noise pollution on marine life. Marine protected areas are designated regions that aim to conserve biologically diverse and ecologically significant marine habitats. By establishing acoustic refuges and buffer zones around these areas, we can create safe havens for marine life and mitigate the impact of noise pollution.

Acoustic refuges are areas that are relatively free from anthropogenic noise, providing a refuge for marine species that rely on sound for communication, navigation, and prey detection. These areas can be identified through risk assessments that pinpoint quiet habitats, often referred to as "opportunity sites". By prioritising the protection of these quiet habitats, conservation efforts can be more effective and cost-efficient.

Buffer zones, on the other hand, act as a protective barrier around marine protected areas. These zones can be established by implementing noise reduction strategies, such as slowing down ships, using quieter technologies, or adopting alternative construction methods. For example, the use of marine vibrator systems instead of traditional air guns in seismic surveys can create a "longer, quieter signal" that is less harmful to marine life. Additionally, implementing regulations and restrictions on noise-generating activities within these zones can further minimise noise pollution.

The establishment of acoustic refuges and buffer zones offers a two-pronged approach to combating underwater noise pollution. Acoustic refuges provide a haven for marine life to escape noise pollution, while buffer zones aim to reduce the amount of noise-generating activities and minimise noise levels around protected areas.

By implementing these strategies, we can reduce the harmful effects of noise pollution on marine life, including stress, hearing loss, difficulty feeding and reproducing, displacement, and even death. It is important to note that while these measures are effective, they should be complemented by broader efforts to address noise pollution at its source and enforce regulations that prioritise the protection of marine ecosystems.

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