Saving Fuel on Carpet of Bubbles

The continuing quest by owners, operators, shipbuilders and designers to find recession-beating, fuelefficient and more CO² friendly sources of power has been one of the marine industry’s overriding campaigns over the past 10 years.

Sleeker hulls and trials and tests with alternative fuels have been two of the options favoured by leading companies to improve the performances of their fleets. Now designers are exploring novel energy saving and energy-generating technologies. One of these is known as an air hull lubrication system.

Quite simply, the air hull lubrication system enables a reduction in a ship’s through-water frictional resistance by generating a carpet of bubbles underneath the hull. One particular version of this technology is called the Air Cavity System (ACS), which works using compressors to fill with air a series of cavities (chambers) fitted to the leading edge of the flat bottom of a vessel. Then, as water from the surrounding sea flows past the still air in the cavities, it draws micro-air bubbles out of the chamber, which then form on the boundary layer of the hull.

Thus the vessel floats on a thin layer of air bubbles on its bottom, which helps to eliminate any contact or resistance between the hull and the water and enables it to travel more freely and economically on any given route.

By reducing the friction between hull and water, ACS is able to reduce a ship’s need for power and thus fuel consumption by up to an estimated 10%, according to the latest tests.

“We have been working very closely with Lloyd’s Register and we see their brand as vital to this process, lending credibility and independent assurance to the new technology on its route to market,” said Noah Silberschmidt, Managing Director of DK Group, the pioneers of the system.

ACS technology is best suited to large, flat-bottomed vessels, such as tankers, bulk carriers and broad-beamed container ships, with the number of chambers or cavities varying from eight on a small vessel to 30 on a larger one. As well as newbuilds, ACS can be retrofitted to existing vessels in dry dock in an estimated timespan of two weeks. Silberschmidt commented: “I am very excited about the retrofit variant. DKGroup believes that the retrofit solution will be considered by shipowners as an alternative to investing in new tonnage. A potential 10% improvement of the efficiency on existing tonnage will narrow the gap considerably to new and more efficient vessels.’’

An earlier version of the system was tested on a full-scale demonstrator ship in the Norwegian fjords. After that, full scale tests were successfully carried out in a cavitation tank at the HSVA Hamburg Ship Model Basin in Germany. Then DK Group approached Lloyd’s Register’s Technical Investigation Department (TID) to conduct a technology review of the system. Dejan Radosavljevic, TID’s Fluid Dynamics Section Manager, said: “It is clear that at full operation, ACS provides significant reduction in frictional resistance compared to the case without a cavity.”

The final stage of the process is due in December 2013 or January 2014, when DK Group plans to trial the technology on a 45,000 dwt Handymax tanker.

One organisation taking an interest in this hydrodynamic technology is the Sustainable Shipping Initiative (SSI), a 25-year initiative to make the maritime industry more sustainable and environmental throughout the whole supply chain, from producers and manufacturers to end-users that include the world’s poorest countries.

Lloyd’s Register is a founder-member of the SSI, together with leading shipowners, charterers, builders, engineers and financial institutions. LR’s Lead Environmental Specialist Paul McStay, a member of the SSI’s technology workstream committee and LR’s Coordinator of ACS, said: “The SSI is keen to work with technologies that have the potential to deliver step changes in energy efficiency for the industry and the ACS is one of these. LR fully supports this project and after it has been fully tested and the performance validated on a full-size vessel, there is every reason to believe it could be rolled out to owners and operators in the global fleet.

“When we were approached by DK Group, we could see that here was a system that was built on earlier generation ideas to create a much more viable product. As with many new technologies, getting some specific details right is the key to success, and we have been very pleased to participate, not least with DK Group themselves, in order to really understand the technology, but also with our clients to help them explore how to exploit it to meet their needs.

“Lloyd’s Register is always seeking better ways to improve vessel performance and operational efficiency and to prove that projects do what they say ‘on the tin’, as well as satisfying the rules, regulations and, in this case, the SOLAS and MARPOL conventions. ACS has the potential to give owners and operators the environmental benefits of fewer emissions due to reduced fuel consumption,” concluded McStay.

Lloyd’s Register, September 19, 2013