Air lubrication has been increasing operational flexibility and reducing fuel consumption and emissions for major players in the maritime industry for over a decade, the most common myths surrounding air lubrication, and how the market-leading Silverstream System is helping to decarbonise the industry, says a press release on Wartsila.
How air lubrication works
Air lubrication fundamentally changes the interaction between water and a vessel’s hull, shearing air from air release units to create a carpet of microbubbles that coat the flat bottom of the vessel.
This reduces fuel consumption and associated emissions by decreasing the frictional resistance – in a nutshell, it takes some of the effort out of pushing the vessel through the water.
The biggest benefit of this is operational flexibility – owners can choose whether to maintain normal operational speed and benefit from a reduction in fuel consumption and emissions or opt for higher speeds with the same power requirement.
Clean technology recognised
Another benefit of air lubrication is regulatory compliance – it is a clean technology recognised under the IMO’s Energy Efficiency Design Index (EEDI) for newbuilds.
It is also one of the key means for existing vessels to improve their efficiency rating on the IMO’s Energy Efficiency Existing Ship Index (EEXI) – a commercial requirement as decarbonisation targets tighten.
The technology also helps vessels to reduce their carbon intensity in order to meet the requirements of the IMO’s operational carbon intensity indicator (CII), which demands that ships of 5,000 gross tonnage and above know their operational carbon intensity to ensure a continuous reduction of emissions within a specific level.
“Owners can choose whether to maintain normal operational speed and benefit from a reduction in fuel consumption and emissions or opt for higher speeds with the same power requirement.”
Myth 1: Air lubrication causes cavitation
When the tip of a propeller rotates with a velocity well above 100 km/h, a low-pressure region is created, which causes the cold sea water to turn into vapour.
This is called cavitation. Once the vapour is out of the low-pressure region, it collapses violently, creating shockwaves and potential erosion.
The bubbles coming from air lubrication are filled with air, not water vapour, and therefore do not collapse at all.
Moreover, in a mixture of water and air bubbles, the fluid properties change quite significantly, which can even suppress the development of cavitation to a certain extent.
The air bubbles also act as a cushion during the collapse phase of the water vapour.
Myth 2: Air lubrication causes underwater radiated noise (URN)
As air lubrication does not cause cavitation, it also avoids the associated sonic boom.
In fact, the Silverstream® System has a dampening effect on URN, a benefit that is currently being researched by Wärtsilä and Silverstream.
It is already known that a small amount of underwater air has the potential to significantly dampen the transmission of URN – for example, bubble screens have been used to mask noise from offshore drilling operations for several years.
Structure-borne noise from a two-stroke engine will be dampened by the air layer, as part of the transfer path from the hull plating to the sea water. And, as mentioned above, the noise from cavitation phenomena will be dampened as well.
Myth 3: Air lubrication isn’t tested in the real world
Unlike other solutions on the market, the Silverstream® System has been verified over a long period of time by independent third parties, including Lloyd’s Register, HSVA and the University of Southampton, to result in 5–10% net fuel and emissions savings.
In fact, air lubrication is one of the few maritime clean technologies that can be measured ‘live’ simply by switching the air on and off. When the system is turned on, there is an instant drop in shaft power and an increase in speed, as the frictional resistance is reduced.
“The Silverstream® System has been verified over a long period of time by independent third parties to result in 5–10% net fuel and emissions savings.”
Myth 4: Air lubrication has a negative impact on hull coatings
Dry dock vessel inspections after multiple years of operation with the Silverstream® System have shown little to no biofouling of system components.
Leading coatings manufacturers have also confirmed that they do not expect air lubrication to cause any detrimental impact to hull coatings.
In fact, there is growing evidence that the microbubbles discourage hull fouling.
Experimental work with Akzo Nobel has quantified this as a 50% reduction in biofouling, which is backed up by underwater surveys and customer business cases.
Reducing hull fouling can easily bring a 10% reduction in resistance, reducing fuel consumption and maintenance costs.
Myth 5: Air lubrication conflicts with other fuel-saving measures
Air lubrication works well with other energy-saving technologies, like optimised hydrodynamic designs, and EnergoProFin, EnergoFlow and GATE RUDDER™ by Wärtsilä. As its benefits can be directly measured by turning the system on and off, you can always be sure that the system is providing additional efficiency gains.
The EU CHEK project is studying the symbiosis of energy-saving devices by developing a wind-optimised bulk carrier that maximises efficiency with a range of innovative technologies that work together, including the Silverstream® System and GATE RUDDER™.
Increasing efficiency with proven technology
Air lubrication is an ideal way to reduce emissions and increase operational flexibility for both retrofits and newbuilds, as some of the leading names in the sector have discovered – companies such as MSC, Shell, Grimaldi, Maersk, Hyundai Heavy Industries, Carnival and Vale have all invested in the technology.
In 2021, MSC signed the largest ever order for a maritime propulsion clean technology to fit Silverstream’s air lubrication system on more than 30 of its cargo vessels, saving the company a forecasted 1.6 million tonnes of emitted CO2 and over €250 million in estimated fuel costs.
The container shipping giant has subsequently ordered even more Silverstream® Systems.
“In 2021, MSC signed the largest ever order for a maritime propulsion clean technology to fit Silverstream’s air lubrication system on more than 30 of its cargo vessels, saving the company a forecasted 1.6 million tonnes of emitted CO2 and over €250 million in estimated fuel costs.”
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