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Keeping the noise down

Measuring underwater radiated noise in BC Ferries Island Class vessels

Damen Shipyards Group has set itself the goal of becoming the most sustainable maritime solutions provider in the world. There are many ways in which the company is going about trying to achieve this ambition.

Naturally, a lot of effort at Damen is invested in making its vessels as efficient as possible throughout the lifecycle, thereby lowering their carbon output.

Broad definitions of sustainability

But Damen’s definition of sustainability is a broad one, and CO₂ (equivalent) is not the only thing to be emitted by a ship. There are growing concerns about the effects of underwater radiated noise (URN).

This is the sound emitted by the vessel into the water. There are multiple causes of this including the engines and the propeller.

Sound investment

Damen began looking into URN many years ago and since 2014 its Research, Development & Innovation (RD&I) department has coordinated a dedicated Sound and Vibration team. With this, Damen aims to develop, evaluate and install predictive tools, measurement methods, and design and control methods for the project, engineering, and production phases of shipbuilding.

One example of the team’s work was the 2016 in-house development of a process to measure the URN caused by vessels, according to the DNV-GL standard.

With this, Damen lowers a hydrophone system from a small boat, onto the seabed. From this, a cable runs to a support ship that records and measures the sound of the target vessel over a specified distance, at a set speed. In order to do this, Damen developed practical solutions including a patented mounting design for the equipment.

Increased wellbeing onboard

There are numerous benefits associated with reduced levels of noise (and vibration) onboard ships. Amongst these is an improved onboard experience for crew and passengers.

While onboard the situation has improved considerably in recent years, it has been suggested that URN associated with shipping has actually increased in recent decades.

Particularly susceptible to the effects of URN are marine mammals such as whales that rely on sound to perceive the underwater world around them. Due to the limitation of light at certain depths, they use sound for both echolocation and communication. While vessel noise is not directly harmful, many studies show that marine mammals can clearly hear vessels, and that their sounds can be lost amidst the noise (so-called masking), reducing their range of communication and their ability to determine location, making hunting difficult and causing the animals to become stressed

Mitigating Impacts on At-Risk Whales

For Damen’s client, Canada-based BC Ferries, this is a serious concern. The ferry operator has a fleet of 37 vessels, with more being planned, connecting communities along the British Columbian coast. The operator’s routes include services in waters that are home to over 30 species of marine mammals, most notably of which is the Southern Resident Killer Whale (SRKW).

Federally listed as endangered by Canada and the USA, SRKWs are a fish eating species, primarily feeding on Chinook salmon with a seasonal distribution spanning from Alaska in the north to California in the south. Over the past few decades, numbers have been in decline and there are thought to be in the region of just 73 SRKWs alive today.

BC Ferries has been an early, active participant in efforts to understand and mitigate the effects of URN from its fleet.

Safe navigation

The company has taken numerous steps towards this goal. In 2014, for example, BC Ferries became a founding and active member of the Port of Vancouver Enhancing Cetacean Habitat and Observation (ECHO) program in 2014. BC Ferries also developed, in partnership with the Vancouver Fraser Port Authority and Ocean Wise, the Whales in our Waters: Safe Navigation in the Presence of Whales tutorial.

This training tutorial, which all BC Ferries deck and bridge crew must complete, covers a range of topics including tips for identifying and reporting sightings, and best practices for navigating ships in the presence of whales.

Additionally, BC Ferries was the first ferry operator in Canada to publish a Long Term Underwater Noise Management Plan.

As part of its strategy, BC Ferries strives to ensure that each generation of ships is quieter than the one that preceded it. Amongst its tactics, the company engages in research and development partnerships with commercial vendors to support the improvement of URN performance of its vessels.

It was with this aim in mind that BC Ferries worked with Damen during the construction of the first six of its Island Class vessels.

Measuring the Island Class: Black Sea

The double ended Island Class vessels are based on the design of the Damen Road Ferry 8117 E3. The vessels are equipped with batteries and diesel-hybrid technology. BC Ferries intends to operate the vessels on electric power alone once shoreside charging infrastructure becomes available.

To establish levels of URN, measurements were taken from the first six Island Class vessels at two different locations.

The first measurements were taken, according to the DNV-GL Silent Methodology, in the Black Sea during Sea Acceptance Trials.

With this, a hydrophone was positioned on the bed of the Black Sea at a depth of 70 metres. The underwater sound emitted by the vessels was then measured over two ship lengths in different operating conditions. The propulsion is always electric, however, at speeds over 10 knots, the electricity is generated by one or two diesel generator sets.

In both cases, the URN was measured over various frequencies, including those relevant to the biometrics of SRKWs, as follows: 10Hz – 100 kHz for behavioural and psychological impact, 500Hz – 15 kHz for impact on communications, and 15 kHz – 100 kHz to assess the risk of masking echolocation clicks.

The ships are nominally identical, having all been built at the same shipyard, Damen Shipyards Galati in Romania, to the same drawings, using the same components.

The results showed, however a difference between the individual vessels due to a variety of uncertainties – different, nominally identical equipment and different weather conditions, for example.

This was expected, and the 3 dB average variation observed, is lower than the achievable uncertainty as mentioned in the ISO-17208-1:2016 Underwater acoustics standards.

Measuring the Island Class: Boundary Pass

Following delivery, measurements were again taken on the Boundary Pass shipping lane on the approach to the Port of Vancouver in Canada. On this occasion, the vessels’ URN was measured by the static Underwater Listening Station operated by JASCO Applied Sciences (Canada). Here, multiple hydrophones are installed at a depth of 190 metres.

The hydrophones are capable of autonomously recording underwater sound of passing ships, which are then identified and tracked using the vessels’ AIS systems.

At the two different locations, the findings of the measurements were, broadly speaking, in line with one another. In the Boundary Pass measurements, the ferries were seen to be, on average 1 dB higher than in the Black Sea between 10Hz and 100kHz, 1 dB lower in the communication band between 500Hz and 15kHZ, and 8 dB higher for the frequencies between 15kHz – 100 kHz associated with echolocation.

This is most likely due to the specifics of the location and the strength of the current in the Boundary Pass, which, depending on the operator, requires vessel correction to stay on course and, therefore, more steering movements, creating increased propeller cavitation.

Interestingly, no discernible difference is observed between operation in electric, hybrid, or diesel modes. Additionally, the peaks seen in URN in both locations, approximately between 125Hz and 400Hz, suggest that the dominant source of URN from the vessels is not the diesel engine, but the gear transmissions. At higher frequencies, the propeller is found to be the dominant source.

Towards the next generation

This information provided the means to optimise the design for the next generation of vessels, in line with BC Ferries’ ambitions. At the time of writing, Damen is constructing four more Island Class vessels at Damen Shipyards Galati.

These four vessels will be fully electric-ready. They also feature adaptations based on the results of their URN measurements taken both in the Black Sea and at Boundary Pass to deliver quieter performance.

Based on the findings, Damen has applied extensive computational fluid dynamics (CFD) calculations to optimise the hull. Specifically, this modification will reduce pressure resistance and wave making resistance in the water, leading to both a reduction in URN and an increase in efficiency.

Additionally, Damen has worked together with propeller supplier Schottel to optimise the propeller design in an effort to further reduce the URN. As we have seen, however, at peak URN the dominant contribution is from the gears of the thrusters. Here too Damen has collaborated with Schottel to adjust the gear transmissions to reduce URN.

While measurements will still need to be taken, it is expected that these modifications will ensure that vessels are 3 dB quieter than the previous generation of Island Class vessels.

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Thank you to our past Members of the Month

July 2020