There have been significant efforts taken so far by the maritime industry to reduce the levels of noise and vibration emissions by various types of ships. The issue was initially addressed so as to prevent structural fatigue damage to the onboard machinery and heavy-duty equipment. For the last three decades, the comfort of passengers as well as the health of the crews has been increasingly considered by all the stakeholders such as ship owners, shipyards and regulatory classification societies. However, with the technological advancements and research, there is no doubt that the increase of underwater noise related to anthropogenic activity at sea induces grave risk on marine life. The adverse effects of the use of powerful sound sources such as low frequency active sonar, air guns which are used by oil industry, pile driving for installation of offshore platform etc. are visible for all to see and have been reported. The hazards created because of underwater noise generated by commercial shipping are presently becoming more acute because of the steady increase of ship traffic and vessel size. Despite the fact that noise levels generated by shipping as compared to other sources such as active sonar, the radiated noise occurs continuously and it has been established that it tends to impacts large maritime areas. The harassment effect on the aquatic life can cause large disturbance on the biologic functions of some marine species, and in the long term, lead to habitat loss and negative consequences on biodiversity. Excessive levels of underwater noise can be especially dangerous for sea creatures like whales and dolphins, hindering their ability to communicate, hunt, migrate and echolocate.
URN and its significance:
The underwater acoustic output generated by commercial ships contributes significantly to ambient noise in the ocean. Underwater noise from commercial ships is generated during normal operation, most notably from propeller cavitation which is known to peak at 50–150 Hz but can extend up to 10,000 Hz.
The major harmful impact for all marine mammals is due to the reduced available dissolved oxygen and thereby creates a plethora of impactful problems.
The Underwater Radiated Noise (URN) Management on-board marine platforms is an interesting research area with varied stakeholder interests. The first is the ship design and manufacturing for efficient operational & maintenance related aspects. The second is the acoustic stealth related naval application for enhanced deployment efficiency to avoid detection by enemy sonars and also acoustic mine avoidance. The third is the growing marine conservation related application pertaining to Acoustic Habitat Degradation. These are multi-dimensional requirements related to safety of the ship, sustainability of the shipping operations and also growth related to the shipping sector.
Effect of URN on Marine Eco-system:
A sound becomes audible when the receiver is able to perceive it over a background noise. The audible range of hearing for marine fauna spans from as low as 5 Hz up to about 200 kHz. Marine mammals use hearing as their primary sense of perception and are highly dependent upon noise / sound for their navigation and communication. Various other fundamental activities such as finding food, reproduction and hazard detection are also based on sound perception and hence are likely to be sensitive to the increase in environmental noise.
Acoustic masking is considered to be a threat to marine fauna, especially those species that communicate on low frequencies, such as baleen whales. Therefore, an excessive high level of ambient noise in the low frequency range can have a negative impact on their population. The predominant noise levels associated with large vessels are in the frequency range of 5–1000 Hz. Noise levels at higher frequency (above 1000 Hz) will normally decrease with increasing frequency. Therefore, the predominant noise in the low-frequency band will affect the ambient noise over a large ocean area. Moreover, this low-frequency band happens to overlap with the frequency band in the audible range used by some marine mammals. Concerns about the potential impact of ocean noise on marine fauna prompted the International Maritime Organisation (IMO) to release a non-mandatory guideline for the reduction of underwater radiated noise (URN) from commercial shipping in 2014.
Frequency relationships between marine animal sounds and sounds from shipping
Sources of URN:
There are two main groups of underwater noise sources. The first is propellers, jets and other underwater propulsion systems.
Propellers constitute a major source of underwater noise because of the rotating blades operating in non-uniform flow. The propeller induced URN can occur in two ways. First is the direct radiation of noise from propeller blades due to their vibration. Second is due to the transfer of forces which create imbalanced moments from these blades to hull, which causes the vibration of the hull and ultimately causes radiation of noise.
The second is the machinery vibration caused by propulsion and auxiliary machinery. Machines which have rotating or reciprocating parts generate noise at the fundamental (natural) frequency and their multiples (harmonics). There are numerous principle and auxiliary machineries located at multiple decks inside the ship. The mechanical vibration from these machineries is radiated from the hull through their mounts and the decks in a very complex configuration. The classes of machinery can be divided into two types based on their functions: propulsion machinery and auxiliary machinery. The first contributor is the main propulsion system. Because diesel engine speed varies according to propulsion demand, the noise is generated at frequencies that depend on ship speed. Propulsion turbines, turbine generators, and reduction gears are the dominant sources of propulsion system noise on steam turbine equipped ship. Noise components from rotating auxiliary machinery and other shipboard equipment contribute to the ship overall noise signature, but usually at lower levels than propulsion systems.
Considering the impact of URN on marine eco-system, it is imperative that solutions must be found to mitigate these harmful effects of the low-frequency noise. To address this issue, the project AQUO “Achieve Quieter Oceans by shipping noise footprint reduction” (www.aquo.eu) started in October 2012. The AQUO project was built in the scope of FP7 European Research Framework. The final goal of AQUO project is to provide to policy makers practical guidelines, acceptable by shipyards and ship owners. Two types of solutions taking into account bioacoustics criteria are provided: solutions regarding ship design (including propeller and cavitation noise) and solutions related to shipping control and regulation. The overall objective of AQUO is to assess and mitigate noise impacts of the maritime transport on the marine underwater environment, mainly for the protection of marine species, to support the requirements of Directive 2008/56/EC (Marine Strategy Framework Directive MSFD) and related Commission Decision on criteria for Good Environmental Status. The which is followed is shown in the figure below.
Methodology for noise mitigation measures assessment in the AQUO Project
Reduction of URN:
The underwater radiated noise analysis typically requires detailed modelling of propeller behaviour as well as the hull and machinery configurations. In recent years, active research and development have improved the accuracy of analysis tools capable of modelling the broadband noise and tonal noise. To establish a consistent metric for the underwater noise emitted from commercial ships, significant efforts have been made to develop standards for underwater noise measurement, including ISO 17208 and ANSI/ASA S12.64. Due consideration is given to the effect of site selection, environmental conditions, requirements of measurement instrumentation, test procedures, and measurement data analysis and interpretation during sea trials. Additional requirements are provided in classification society rules, and the further development of ISO standards specifically for sea trial measurements in shallow water is currently underway.
There are numerous mitigation measures for different noise sources in practice. The fundamental and widely used method is the Propeller design to reduce propeller cavitation and increase cavitation inception speed. Under the stated working conditions, the propeller creates a pressure field on the blade with a region below the vapor pressure of seawater – causing a seawater phase change. As the water vapor enters a favourable pressure gradient, the vapor pocket collapses back into a fluid creating noise. To lower propeller-radiated noise, cavitation needs to be reduced. This can be achieved by enhancing the propeller design or improving the inflow to the propeller by wake optimization. A well-regulated hull wake can enhance propulsive efficiency and reduce propeller cavitation, and propeller-radiated underwater noise. There are a variety of wake improvement devices, such as Schneekluth duct, Mewis duct, Grothues spoiler, and stern flap. It is important to ensure that the selected device is suitable for the hull shape, propeller design, and operating profile of the vessel. In addition, propeller polishing can remove marine fouling, repair erosions, and reduce surface roughness, which helps to reduce cavitation.
Emerging technologies must be implemented to test the efficiency of the said systems. One such invention is the use of acoustic coatings for noise reduction. There are two types of acoustic coatings that are efficient technological solutions: decoupling coatings and anechoic coatings. Normally, both consist of relatively thick viscoelastic layers with some voids and other inclusions in the matrix. The role of decoupling coatings is to reduce the transmission of hull vibrations to the water, and the role of anechoic coatings is to reduce acoustic reflection from the hull by absorbing incoming sound waves. The figure below is a schematic of a possible solution that includes full hull protection, including an anticorrosive primer system, a noise reduction primer and a foul release, environmentally friendly coating. The solution will potentially provide full environmental protection and superior hull protection for ship owners.
Coating solution that is environmentally friendly and offers full hull protection
Machinery-induced underwater noise is mainly generated by structure-borne sound. The machinery vibration can first transmit to the foundations and then propagate to the hull structures, resulting in the radiation of underwater noise. Reducing this vibration and isolating the vibration source from the ship’s hull are effective ways to mitigate machinery-induced underwater noise. Hence, machinery treatment to lower the machinery vibration source level and reduce the vibration energy transferred to the hull structures is one of the most critical measures to diminish the effect of URN. Some of the processes in this type of treatment include using quieter machinery equipment, installation of resilient mounts to reduce the vibrational energy transferred from the equipment into the ship’s structure, using a 2-stage isolation system, acoustic enclosures to absorb engine airborne noise and active vibration cancellation by employing a secondary excitation such as a shaker to cancel the original vibration induced by machinery equipment.
Another one of the famous methods used is hull treatment. Hull treatment solutions can enhance the ship’s hydrodynamic performance and therefore improve the wake flow into the propeller and reduce power requirements. Commonly used methods include hull form optimization, installation of hull and propeller appendages such as flow equalizers, and regular cleaning of the hull. Acoustic decoupling coatings and structural damping tiles can also be applied to reduce the radiation efficiency of the hull vibration.
Clearly, there is an emerging demand to build quieter commercial ships with lower underwater radiated noise emissions – a trend that aligns with the overall goal of developing a more sustainable global maritime industry. In addition to technical viability and cost considerations, an important backdrop amid growing interest in quieter ships is the maritime industry’s quest for decarbonization with challenging goals of achieving the IMO’s greenhouse gas (GHG) emission reduction targets. Underwater radiated noise mitigation measures should, therefore, be implemented in such a way that they do not undermine energy efficiency. Efforts are being made to improve understanding of the impact of various underwater noise mitigation measures on ship energy efficiency and to explore potential co-benefits or trade-offs. The current findings are encouraging in that various measures such as improving propulsion efficiency and reducing ship speed can, in general, reduce fuel consumption and underwater radiated noise at the same time.
B.Tech Final Year Mechanical Engineering student at Vishwakarma Institute of Technology, Pune