While deep-water acoustic ranges are preferred for standardized vessel noise measurement, they are less accessible than shallow-water ranges.
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Ships operate in many different depths and oceanographic conditions, but most noise measurement to date has been conducted at deep-water acoustic ranges. The International Standards Organization (ISO), among others, has developed standards to measure the underwater radiated noise from marine vessels at acoustic ranges. These ranges can be in water that is 30 metres (~100 feet) or deeper. Standards ensure the methods to measure vessel noise are consistent and correctly interpreted. Less is known about ship-source underwater noise in shallow waters, but it is different than in deep water. Noise generated in shallow waters interacts more with the seabed and sea surface. These interactions create conditions whereby the marine vessel’s true source levels are not directly reflected in the measurements. Deepwater acoustic ranges suffer fewer interactions with the seabed and are better for acoustic ranging. However, such ranges are few and often less accessible to vessel operators as they tend to be far from shipping lanes. Shallow water ranges are usually located closer to ports, harbours, and other near-shore centres of maritime activity for convenient access and ease of maintenance.
Modelling, Calibration and Trials of Underwater Acoustic Ranges Towards Quieter Vessels
The QVI project Modelling, Calibration and Trials of Underwater Acoustic Ranges Towards Quieter Vessels undertaken by Dr. Mae Seto’s Intelligent Systems Laboratory at Dalhousie University studies and measures how underwater noise is attenuated in shallow water acoustic ranges. Quantifying this underwater acoustic “transmission loss” means measurements collected by the hydrophones from acoustic noise sources (like ships) could be “calibrated” and transformed into a deepwater range equivalent measurement.
Focusing their efforts on two representative shallow-water locations, the team quantified transmission losses with underwater measurements of calibrated acoustic source emissions and noted dependencies like water depth and time of year. The next step was to analyze and interpret these measurements then develop algorithms and models to transform shallow water measurements into their deepwater equivalent.
The measured transmission losses apply to the two bodies of water from which the measurements were made. Ocean conditions such as depth, temperature, bathymetry – the shape and features of the seabed – and the seabed composition, vary from one location to another and impact the local underwater acoustic transmission loss. The models and measurement methodologies developed by Dr. Seto’s laboratory make it possible to quantify and qualify new candidate acoustic ranges, enabling the establishment of more acoustic testing facilities.
Towards a Standard for Vessel URN Measurement in Shallow Water
Another QVI-funded initiative is involved in the development of a new ISO standard for measuring vessel underwater radiated noise at shallow water acoustic ranges. The project was undertaken by JASCO Applied Sciences and involved measuring and comparing noise from several vessels using specialized hydrophone arrays deployed at sites with different water depths.
The work confirmed that it is possible to obtain repeatable and reliable vessel noise measurements in shallow water environments that are comparable to those made in deep water. The investigations compared hundreds of measurements of the same vessels in three different water depths. The shallowest water tests placed hydrophones in a line on the seabed (horizontal line arrays) in waters approximately 30 metres (100 feet) deep and tested two methods for analyzing the vessel noise measurements. The horizontal distances were at least 50 metres (165 feet) for vessels up to 50 metres in length – the distance from the front to the back of the vessel. The distance was increased to at least one vessel length separation for vessels longer than 50 metres (165 feet).
The project compared the shallow measurements with those taken of the same vessels but at deeper depths of 70 metres and 180 metres, which met the depth requirements of the existing deep water measurement standards (ISO 17208-1 and 17208-2). The results demonstrated that highly accurate measurements can be made in shallow water, but the methods require knowledge of the local seabed properties to account for sound reflected from the seabed.
HyDrone: A portable approach
Although the number of acoustic ranges available for measuring vessel noise is increasing, they remain few. It is not always possible for vessels to divert from their usual route or take the extra time to make multiple passes over a fixed acoustic range. One solution is to create portable acoustic ranges. This is the approach BPE Technologies Inc. took with the HyDrone: A Novel Underwater Radiated Noise Measurement Method Using a Waterproof Aerial Drone with Hydrophone project.
The HyDrone is designed to offer portable and easy-to-operate underwater radiated noise measurements of propeller cavitation – one of the most significant sources of underwater vessel noise. It consists of a remote-controlled waterproof quad-rotor drone, a GPS, a hydrophone attached to a rope for measuring noise, and two cameras to allow HyDrone operators to look at the vessel’s propellers while in the water. Operators can launch the HyDrone from their vessel, place it in the water up to five kilometres away, shut down its motors, and take measurements of vessel noise.
The HyDrone can be dispatched in about five minutes, allowing operators to take measurements at multiple distances and different times of day to account for variable conditions and locations as the vessel travels. More conventional mobile setups, which involve an array of hydrophones attached to a surface buoy and support vessel, can take up to four to five hours to deploy.
As with the more conventional mobile setup approach, factors such as wind and wave conditions need to be considered when deploying the HyDrone. The reasons are twofold: 1) the operational limits of the drone itself and 2) limiting noise interference that can reduce the quality of data collected. Overall, the accuracy of the HyDrone’s measurements was comparable to those taken by the support boat.
The development of the HyDrone is progressing with the aim of creating a cost-effective and easy-to-operate marine environmental monitoring tool.
This article is part of a five articles series on technology for detecting and analysing underwater vessel noise.
Continue learning about the new discoveries and challenges in making vessels quieter with the other topics in this series here
The Quiet Vessel Initiative is a federally funded program through Transport Canada. Industry partners and researchers interested in potential research and development collaborations to advance innovative solutions in marine technology are invited to contact the Quiet Vessel Initiative team at Marine-RDD-maritime@tc.gc.ca.
Featured image credit: Todd Cravens via Unsplash
Oceanographic conditions: physical and chemical features of the ocean that vary in space and time. They include factors such as temperature, salinity, currents, waves, tides, ice concentration and thickness, and surface winds.
Underwater noise: sound generated below water by human activity in the ocean environment. Various industries contribute to underwater noise—offshore energy, construction, military operations, and of course vessel traffic. The noise generated by vessels is referred to as underwater radiated noise.
Hydrophone: an underwater microphone that can be deployed individually or in groups. Groups of hydrophones can be arranged either horizontally on the seafloor or vertically at different depths in the water column. Hydrophones detect pressure changes in the water caused by sound waves. These sensors convert the underwater pressure fluctuations into electrical signals, which can then be analyzed to determine the properties of sound, such as volume and frequency.
Algorithm: a process or set of rules to be followed in calculations or other problem-solving operations, especially by a computer. Algorithms act as an exact list of instructions that conduct specified actions step by step in either hardware- or software-based routines.
Cavitation: is a change in phase from liquid to vapour, like boiling, but caused by a change in pressure rather than a change in temperature. When areas of sufficiently low pressure are generated in water, vapour bubbles form. As the vapour bubbles leave the area of low pressure, they collapse (implode). Because the pressure differences are usually large, the collapse of cavitation bubbles is very powerful and loud. Other vessel factors, such as the hull design, also influence cavitation. Measurements have shown that propeller cavitation is more common at higher speeds due to greater loads on propeller blades. Cavitation can also occur when propeller blades are misaligned or damaged. In addition to creating a great deal of sound, cavitation bubbles can damage or degrade metallic surfaces like propellers, reducing their performance and efficiency.
