In 2018, the International Maritime Organization released an initial strategy on the reduction of greenhouse gas emissions from ships with a clear framework to reduce ship emissions. In an effort to lower carbon emissions and to comply with increasing Energy Efficiency Design Index (EEDI) Phase III levels, various energy saving technologies, such as air lubrication, have been developed to meet the industry need. Air lubrication systems are recognized by IMO as aCategory B-1 “Innovative Energy Efficiency Technology” as described in MEPC.1/Circ.815.
In ship resistance, the three main components are frictional resistance, form resistance and wave resistance. For higher speed displacement vessels, the frictional resistance is approximately 40%, but for low speed displacement vessels, the frictional resistance is the dominant contributor and can reach 85% of the total resistance. For low-speed vessels, a reduction on the frictional resistance would result in an even higher reduction in fuel consumption in addition to what is achieved by the traditional optimization of the ship’s hull form, addressing the form resistance and wave resistance components.
Air lubrication is achieved by pumping air beneath the hull, reducing the area of hull in direct contact with the liquid flow, or in the case of discrete bubbles, by modification of momentum transport and average density in the boundary layer. There are three major categories of air lubrication technology being studied now: Bubble Drag Reduction (BDR), Air Layer Drag Reduction (ALDR), and Partial Cavity Drag Reduction (PCDR).
Bubble Drag Reduction (BDR)
The more efficient Bubble Drag Reduction uses very small or even micro-sized bubbles. The size of the these bubbles is generally less than 0.1 mm. However, micro-bubbles can be difficult to produce on a full-scale ship, and are less effective at low speeds due to buoyancy. As bubbles grow in size they can no longer maintain their spherical shape, making them prone to deform in turbulent flows. There is currently no well-established theory on the mechanisms of skin friction reduction by bubbles. One mechanism which appears to be agreed on by researchers is that the addition of bubbles to a liquid effectively reduces the liquid density, hence the Reynolds stress, resulting in a skin friction reduction. Other two possible mechanisms are:•The turbulence suppression effect, wherein the bubbles suppress the turbulence in the boundary layer which reduces skin friction •Bubbles decrease the effective viscosity of the flow due to the increase of the void fraction
Air lubrication manufacturers technology
A) Mitsubishi Air Lubrication System (MALS) – It was one of the first commercial air lubrication systems developed in the marine industry by the Japanese Shipbuilder Mitsubishi Heavy Industries (MHI), based on research developed since the 1980s in Japan. The MALS is a patented air lubrication system using the BDR method. MHI developed their own turbo blowers specifically used for the MALS, named Mitsubishi Turbo-blower for air lubrication.
B) R&D Engineering – Winged Air Induction Pipe System (WAIP)Winged Air Induction Pipe (WAIP) comprises a series of small air chambers fitted with a foil for ultra-fine micro-bubble generation and was developed in Japan by Yoshiaki Takahashi and Professor Yuichi Murai. The research was conducted by scholars in Japanese universities beginning in 1998. The WAIP system has been the subject of various tests and is marketed via R&D Engineering Inc.
Samsung Heavy Industries – SAVER System (SAVER Air)Samsung Heavy Industries (SHI) developed an air lubrication system, referred as the SAVER system. The SAVER system uses a series of air dispensers installed on the bottom of the ship to spray air bubbles that form an air carpet at the bottom of the ship to reduce frictional drag resistance.
Silverstream System -Its origins lie in the DK Group, which applied large air cavities to reduce frictional drag. The company evolved into Silverstream Technologies in 2014 to further the commercialization of its patented air lubrication technology using smaller air chambers. The Silverstream system employs their patented air release units in the hull to create a layer of micro-bubbles to reduce frictional drag resistance.
Foreship Air Lubrication System -(Foreship ALS)Foreship, a Finnish ship design and engineering company established in 2002 developed an air lubrication system with air dispensers contained in a box to be added to the underside of the hull, which Foreship has optimized hydrodynamically and stated that it would nxot increase skin friction when the air lubrication system is not in use.
Air Layer Drag Reduction (ALDR)
After many years of studying the BDR method, the focus of the air lubrication method extended to the air layer. When sufficient air is injected into the near-wall region of a turbulent boundary layer of water, the injected air will coalesce to form a continuous or nearly continuous layer of air separating the solid surface from the water flow. When the injected air increases, the coalesced air layer is able to maintain a fully continuous air layer covering a larger wetted surface and subsequently results in a greater skin friction drag reduction.
Partial Cavity Drag Reduction (PCDR)
The partial cavity concept utilizes a recess or cavity on the hull bottom where air is injected from inside the hull so that an inflated air cavity is formed and persists to the rest of the hull, separating the hull from with water, therefore reducing frictional resistance. The air layer in the cavitating flow in the cavity is much thicker than the turbulent boundary layer on the ship hull and requires smaller air injection rate to maintain the air layer than BDR or ALDR.
Damen’s Air Chamber Energy Saving (ACES) System
The Air Chamber Energy Saving (ACES) system is an air cavity lubrication system developed by Damen Ship and initiated as a PELS 2 Project. Air cavity chambers are fitted on the flat bottom of a vessel where air is fed into the chambers to separate the water from the bottom hull. APPLICATIONS Up to 2018, 23 vessels were identified to have air lubrication system installed on board.
SYSTEM AND STRUCTURE CONSIDERATIONS
Air lubrication systems generally consist of piping, pneumatic and control systems, and air dispensers.
Structural modification associated with air lubrication systems usually involves installing air dispensers on the hull to dispense air. Local stress concentrations may require reassessment due to the additional openings on the vessel hull. Commonly, equipment, piping and ventilation may be need to be rearranged fo r the installation of air lubrication system.
Stability and lightship weight might need to be re-evaluated due to the additional weight of an air lubrication system. Stability tests and stability calculation changes might be called for if:
•The change in lightship displacement exceed 2% of the lightship displacement from the most recent approved lightship data;
•The change in lightship Longitudinal Center of Gravity (LCG), relative to the most recent approved lightship data exceeds 1.0% of the Length Between Perpendicular (LBP)
Conclusion and Benefits
The application of this technology for cruise installations has now been proven with net efficiency gains of more than 5% in draughts of 8m to 9m with speeds ranging from 10 to 25 knots, the results of which have been verified by reputable 3rd party organisations. Vessels like LNGC’s and Ro-Ro’s which tend to have larger flat bottoms can experience net savings in the range of 8-10%. In addition to energy savings, other benefits from use air lubrication include minimised noise and vibration and reduced fouling as confirmed from previous installations.