7 Jun 2020

Underwater sound reduction

In focus: ✔ Damen R&D ✔ Fishery Research Vessel 7417 'Baía Farta' ✔ Sigma Frigate & Corvette ✔ Damen Shipyards Galati ✔ KNVTS ✔ Ecomare ✔ International Fund for Animal Welfare (IFAW)


If you cause your ship to stop and place the head of a long tube in the water and place the outside extremity to your ear, you will hear ships at a great distance from you.

The statement itself does not come as a surprise – the connected nature of water molecules relative to, say, air molecules, simply put, ensures sound can travel a lot further underwater. What is perhaps surprising about this is that, when Leonardo da Vinci said it, it was 1490 and ships did not yet have engines.

Significant shipping

Leaving aside for now complexities relating to different frequencies and hearing ranges and keeping things simple, it’s easy to imagine that underwater sound today is both more prevalent and loud. In fact, it’s been estimated that underwater sound has doubled every ten years for the past six decades.

There are numerous factors – military activity, offshore energy exploration and operations and, of course, shipping. Shipping is the most significant. It’s certainly not the loudest, but it is the most widespread and, as vessels ply their trade across oceans and seas, day and night, throughout the seasons, the sounds of shipping are unceasing.

The effects this has on marine life are poorly understood, though it’s clear that there are effects. Perhaps the most obvious impact is that on marine mammals. These creatures have evolved using sound to communicate with each other and to locate their pray. Interruptions in sound can literally compromise marine mammals’ survival.

Moving down the chain

There are indications that the impact of underwater noise may go further down the food chain too. That, for example, fish larvae may be killed by the sounds of pile driving as offshore construction takes place, that certain creatures may suffer not only hearing damage, but also undergo behavioural changes as a result – even very straightforward changes like loss of sleep.

Damen is well positioned to assist with the development of quieter ships – it’s something the shipbuilder has been working on for many years already, as Damen’s Manager R&D valorisation Edwin van Buren explains:

"We have been conducting research into the underwater sound profile of our ships for many years. To begin with, this had little to with ecology, but with ensuring the quiet operation of, for example, naval ships and research vessels."

Taking a different path

Many shipyards outsource the profiling of underwater sound to classification societies or specialist companies, but Damen has always chosen not to follow this path.

Doing it in-house is cost-effective for our clients. It also gives confidence to some clients keen to maintain confidentiality about their vessels’ performance – for example, navies. Besides, it makes sense to measure underwater noise on our own vessels, in the same way we look at characteristics such as speed or bollard pull, for example. Adding underwater sound measurement to our scope gives us control of a wider part of the shipbuilding process.

Continuing in the loop

To these ends, Damen R&D has developed a continual ship design feedback loop based on four stages: predict levels, adjust design accordingly, measure levels, update model.

Tjakko Keizer, principal research engineer at Damen, has been working on underwater sound levels on Damen vessels for almost a decade. “If I look back, it started in 2001 with the Irish research vessel Celtic Explorer. We made a good start; she’s still the benchmark in quiet underwater sound today.

“Then it was a while before we were called upon to design something with quiet performance in mind. In 2011 we started again with a new research ship for Belgium. After that it went fast – there was one after another. The latest has been the Baía Farta fishery research vessel for Angola.

“Then there were the SIGMA vessels (modularly built corvettes and frigates) – this is what we developed the measuring system for initially.” The system he refers to is the patented Damen Underwater Sound Measurement System. The system is a mobile, modular one that features a hydrophone, tetrahedral cage and long measurement cables on a reel, all of which are lowered overboard to the seabed, and a battery powered date recorder, a laptop and a ship-tracking GPS system, operated on board a support vessel.

Location, location, location

It’s a relatively simple piece of equipment, though it usage is not without challenges.

“It’s difficult to find a suitable place for carrying out the measurements. Location is key. The sea floor must be firm. The water must be deep – between 50 and 100 metres. And there must be no background noise.” Given that sound travels so much farther in water, this requires a remote area.

Norwegian good

“The Norwegian Fjords are good, as NATO navies have long known. Though given the right environment, we are able to use the system on location. We’ve used it in Indonesia, for example on the PKR (frigates for the Indonesian Navy) and also in the Black Sea to demonstrate the compliance of the Baía Farta with the contractually agreed underwater sound levels.

“We are planning to use it again soon on the RSV Nuyina, Australia’s Antarctic Supply and Research Vessel that we are currently constructing at Damen Shipyards Galati.”

The findings of Damen’s work on underwater sound, though not originally aiming specifically at improved ecological performance, did, however, turn out to improve the sustainability of its vessels’ performance.

Cutting cavitation

“Quite significantly, in fact,” rejoins Edwin. “The biggest part of the issue comes from cavitation caused by sub-optimally performing propellers. By identifying this and optimising the propeller and other related parts – nozzles, rudder, even the hull form – we drastically reduce the level of underwater sound. At the same time, the optimal performance of the propeller has a direct and significant impact on reducing fuel consumption and thus, emissions. This brings together two benefits for ship owners; attention to underwater sound also generates energy efficiency.”

Damen’s underwater sound research is developing towards greater ecological usage. Damen was recently approached by KNVTS (Royal Dutch Association of Marine Technicians) to give a presentation on a chosen topic. Having realised the potential of what they could do in the field of underwater sound, Damen selected underwater sound as its subject.

“We approached ecological experts Ecomare – specialised in seals in the North Sea to cooperate with us on this. We placed our focus on two things – what is the effect of underwater sound, so far as we know, to marine life and, based on this, what technology have we designed to improve the situation inside and outside of our vessels.”

Regulation ready

Following this, Damen was invited to join a consortium organised by the International Fund for Animal Welfare (IFAW), Rijkswaterstaat and partners including marine biologists, government bodies and research institutes as well as industry representatives.

“Participation in the consortium enables us to anticipate changes in the market. Combined with the experience we already have, we are able to respond quickly. The technology is clear, the measurements are clear. Our job now is to look at how to combine the two and develop solutions. The need for this going forwards couldn’t be clearer. Advice for best practice as regards underwater sound is there already, it’s highly likely such recommendations will pass into regulation. And when they do, we’re ready.”