Air Cavity Systems require that the bottom of the vessel is structured in a way that allows air to be trapped below the vessel. Various tests have shown that, in addition to extra cost for the specialised structure, it is very difficult to maintain the quantity of air and that provision of extra air eliminates the savings.
Air Lubrication Systems, abbreviated to ALS and sometimes called ‘bubble’ or ‘bubbler systems’, work on a very different principle by maintaining a constant injection of air bubbles to the flat bottom of a ship. The air reduces the friction/ resistance/drag which means that the power needed for propulsion can be reduced, thereby saving fuel and the subsequent emissions.
Silverstream Technologies’ air lubrication system is specially designed to use very little energy thus the fuel savings are increased. The Wärtsilä Encyclopedia of Marine Technology explains further.
When the air is applied there is an increase of speed due to the reduced resistance. In today’s market the ship would adjust to normal operational speed and benefit from fuel reduction and lower OPEX cost. However, in some instances, it might be advantageous that the owner/operator can obtain higher speeds with the same power by installing this system.
No matter how a ship owner chooses to improve the efficiency of a fleet, whether with changes to the hull, bulbous bow, fins, ducts, waste heat recovery system or similar, air lubrication will be synergistic.
In the case of specialised paints, the constant flow of air discourages marine growth on the flat bottom of the vessel, our system would complement antifouling painting or coating, which, first and foremost addresses fouling on the sides of the vessels.
Air lubrication in general is one of few technologies that can be measured ‘live’ simply by switching the air ON and OFF. We would welcome a standardised method approved by the industry for all companies offering air lubrication or air cavity systems.
Any ship with a reasonably sized flat bottom will benefit from air lubrication. Approximately 20% Flat Bottom Area (FBA) to Wetted Surface Area (WSA) ratio will give measurable savings. The higher the FBA, the higher the expected savings. Obviously, ships with a large fuel consumption will benefit the most as savings are generally a percentage of the total fuel spent. This translates to financial savings and proportionate reduction in total emissions.
The typical dry-dock period for a retrofit installation would usually span 4 and 12 days. The installation is relatively simple and the majority of work does not interfere with normal dry-dock procedures. Good planning is essential with the most important item being integration of the ARUs into the double bottom of the hull. This involves inserting the ARUs in the forward part of the hull and all hot work being completed in order for painting to be finished prior to undocking of the vessel. In an example retrofit installation on a large cruiseship, ARU installation was completed in 5 days with the ship spending a total of 8 days dry-dock. In our first ever installation, all works were completed in dry-dock within 12 days and the Silverstream® System was fully operational when the vessel left the yard.
Our system is about fuel efficiency, about saving fuel, and, by saving fuel, emissions are reduced. We believe ‘green’ is used too casually in the shipping debate and is intrinsically linked to fuel savings; more efficient ships will automatically drive more environmentally friendly ships.
Regarding the IMO EEDI index which addresses the reduction of greenhouse gasses (GHG) or CO2, air lubrication is acknowledged as a technology that will be included in the overall efficiency calculation.
EEOI, SEEMP and ISO 19030 are concerned with measurement and monitoring of performance. This is certainly an area that would benefit from the simplicity of verifying our system, by switching it on and off at short intervals.
Not directly, yet very much so, indirectly. The huge reduction required of sulphur oxides and nitrous oxides – also called SOX and NOX – demands either cleaner fuels or abatement systems. However, the cost of cleaner fuels such as MDO, MGO, ULSFO, VLSFO etc., are significantly higher than normal heavy bunker fuel, HFO or IFO380 and therefore fuel reduction becomes even more relevant.