Feature 1 | DENMARK
and South America without measuring significant or consistent performance gains. We can’t get the same results twice as different conditions, such as vessel speed and draſt, wind, waves or currents, change its effectiveness,” said Dr de Kat. Maersk is confident the issues with
the system can be solved and that ALS will offer fuel savings in the future also for vessels with a fine hull form like Olivia Maersk, but for the time being the company intends to develop a better understanding of the system and potential improvements in drag reduction. Te feeling is that the system needs more air at a higher pressure to operate successfully on a larger vessel. “When we started we didn’t
know enough about the creation of microbubbles under pressure,” admitted Dr de Kat and the company is looking for partners to help develop the system further. Initial tests on the WAIP system
were carried out in MARIN’s High Speed Basin wi th underwater photographs showing the creation of the microbubbles necessary to make air lubrication work at speeds of between 12-20knots. However, the WAIP system differs from the ALS on Yamatai which rides on an aircushion that is pumped out directly beneath the vessel. The attraction of
the WAIP system over
other ALS’s for Maersk was that WAIP uses comparatively little power to create its air lubrication. Maersk said: “One of the main
challenges for this type of ship [Olivia Maersk] is the hull shape with relatively small portion of flat bottom surface (as opposed to Yamatai which has a flat, barge like bottom). Injection of air with ‘conventional’ large size bubbles in the bottom of Olivia Maersk would result in a marginal reduction in resistance. The WAIP system produces microbubbles, i.e. bubbles having a size in the order of microns, which are more apt to stay within the boundary layer for some time and distance along the hull.” The original concept envisages using
a relatively small amount of energy to compress air, and “to generate microbubbles through instability of the air-water interface at each opening in
34
Micro Bubbles at the WAIP tank test at Marin.
the hull. Friction exerted by the fluid should then be reduced, provided the bubbles stay within the boundary layer close to the hull,” said Maersk. A complex piping system delivers
pressurised air to 124 ducts in the bow of the ship, below the waterline. Microbubbles act like smoke in water and should, in theory, remain close to the hull providing an aircushion the length of
the hull as the movement
of the ship takes it over the bubbles. Maersk said that the system requires half a bar pressure for every 5m draught and so for Olivia Maersk, which has a 12m draught, 1bar pressure should be sufficient to reduce the resistance of the hull in the water. One theory as to why WAIP may
not perform as it was expected is the absorption of bubbles into the surrounding fluid over a long distance along the hull, which could exacerbated by disturbances in the boundary layer caused by weld seams, but there may also be other, as yet undetected problems. “The system must have more air supplied and we must increase the flow rates, we need more air flow than we originally anticipated,” said Dr de Kat, but he added that
the company
needs to find a method of observing the full scale operation of Olivia Maersk and the air flow beneath the water surface to fully understand the problems and their remedies. “Finding a way to make those flow observations is not so easy,” he explained. Investment in the system has already run into millions of dollars, said Dr de
Kat, but the benefits of a working system would soon repay this outlay. Installation of WAIP on Olivia Maersk
“was preceded by detailed engineering which included besides the model basin tests, design of the air supply and piping system, global strength analysis and local strength analysis, including the ability to withstand peak loads associated with slamming, and damage stability assessment. Te construction of the system was a massive undertaking, with complex discussions with the yard that saw more than 2000m of galvanised piping being procured purely for the retrofit of the WAIP system. All these pipes were pressure tested and the installation of the system and the vessel modifications were approved by Lloyd’s Register.” Costs associated with this system
would be expensive to retrofit onto Maersk ships. However, the pay back from the
its operational standard could also be considerable. AP Møller-Maersk estimates that its
entire fleet “consumed approximately 10million tonnes of fuel annually when this project was conceived. A 10% [fuel] saving would enable a CO2
reduction of
approximately 3million tonnes annually. Tis prompted the group to take on a sizeable technological risk in pursuing this fuel saving initiative.” In reducing fuel consumption by
1million tonnes and assuming that fuel will cost around US$600/tonne, as is confidently predicted by many in the industry, an effective ALS could bring
The Naval Architect April 2011 system when it achieves
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80