FRICTIONAL RESISTANCE REDUCTION ON SHIPS - HULL AIR LUBRICATION METHOD

The motion of a ship through water requires energy to overcome resistance, i.e. the force working against movement. As the resistance of a full-scale ship cannot be measured directly the knowledge about the resistance of ships comes from model tests. The total resistance on calm water can be divided into three main components: frictional resistance, residual resistance and air resistance.

The frictional resistance depends on the size of the wetted area. It represents often about 70-90% of the ship total resistance for low-speed ships (bulk carriers and tankers), and sometimes less than 40% for high-speed ships (containers and passenger ships). Residual resistance comprises wave resistance that refers to the energy loss caused by waves created by the vessel and viscous pressure resistance. This residual resistance normally represents 10-25% of the total resistance for low-speed ships and up to 40-60% for high-speed ships. Air resistance normally represents about 2% of the total resistance.

What is Air Lubrication Method

Air Lubrication System is a method to reduce the drag (frictional resistance) between the ship's hull and seawater by creating a bubbly flow or gas blanket resulting in energy-saving effects in terms of reduction in ship’s fuel consumption between 5 – 20% depending upon various loading , operation conditions, vessel design and adaptation of type of air lubrication method for vessel.

In order to implement air lubrication method in any ocean-going vessels, the amount of net energy saving rate becomes significant. The Net energy saving rate is obtained by subtracting the energy required to inject the air from the energy saving due to drag reduction.

Working of Air Lubrication

The Air Lubrication method for Ship’s hull works on the principle of trapping layer of air bubbles beneath the ship’s hull. An air blower or a dedicated system is used to generate / inject air bubbles to pass them continuously beneath the ship’s surface. Air bubble outlets are arranged at different locations along the bottom of the hull, symmetrically on both the sides of the ship’s center line.

The air is blown at a constant rate to form a layer of bubbles, which reduces the drag and resistance between the ship and the seawater. However, the efficacy of this system is dependent upon following major factors: -

• Bubble size
• Location of bubble / air ejectors
• Injector configuration
• The air bubble distribution around the hull surface is an important parameter for reducing the resistance working on the hull and must therefore be predicted accurately.

Air Lubrication technique can be classified in three categories

• Micro Bubble Drag Reduction (MDR)
• Air Layer Drag Reduction (LDR)
• Partial Cavity Drag Reduction (PCDR

Micro Bubble Drag Reduction (MDR): Micro bubbles are bubbles smaller than one millimeter in diameter, but larger than one micrometer. In this technique small micro bubbles are injected into the turbulent boundary layer developing on the Wet surface area. However, its application is confined to large ships, such as tankers, since they are very large, flat bottomed and move very slowly. Thus, making them more suited to micro bubbles application.

Air Layer Drag Reduction (ALDR): In ALDR, a continuous air layer is created between sea water and the hull surface, which reduces the frictional drag on the area covered by the air layer

In case of MDR, for lower range of gas injection rates, drag reduction rate is less than 20%. However, above a critical gas injection rate, the ALDR with volumetric air flux being approximately 50 percent greater than MDR gives rise to drag reduction rate over 80 percent. The net energy saving rate for ALDR becomes higher than that for MDR.

Further benefits are accrued at higher speeds and shallower drafts. However, with a sufficiently long persistence length, a net energy savings can be found, even at deeper drafts. Thus, the length of developed air layers is a deciding factor on Net energy savings

Partial Cavity Drag Reduction (PCDR): In PCDR technique, a continuous lubricating gas layer like ALDR method is created. In order to do so, a recess is created on the bottom of the hull that captures a volume of gas and creates a cavity of air between the hull and water flow resulting in more than a 95% decrease in frictional drag for the area covered. The air supply pressure in the cavity is retained by low pressure air fans.

However, PCDR method requires more modifications at the bottom of the hull. In addition, the initial investment cost of PCDR is more than ALDR, while its operating cost is less and results in larger frictional drag reduction with a lesser gas flux.

Pros and Cons

The inclusion of an Air Lubrication method on an ocean-going vessel will result in following advantages: -

• • Reduced carbon emissions
• • Substantial fuel savings
• • Lowering underwater hull noise, radiation and shocks
• • Reduce wave drag.

Though a promising technology, the Air Lubrication System has its own concerns regarding its implementation and performance on ships. Some of them are as follows: -

• • The Air Lubrication method can only be used for certain types of ships having flat bottoms. Ships having V-shaped hulls, such as certain warships or recreational vessels might not be able to reap the benefits of the air lubrication system.
• • Trapping the layer of bubbles beneath the ship’s hull is a challenging task. One solution to this problem could be protruding ridges at the edges of the hull can help in trapping the blanket of bubbles. But this again has its own disadvantages. Further the effect of air bubbles being sucked onto ship’s propeller also needs to be properly studied and analyzed by computational fluid dynamics (CFD) to predict the effect of Air Flow and design the ship’s stern or hull in such a way that it traps the air bubbles beneath the hull. However, this would substantially increase the building cost of the ship.
• • In PDCR method, the air cavities made for trapping the air bubbles would affect the handling and stability of the ship especially in rough seas and the system may not function as it is desired to perform.
• • The air bubbles leaving the hull surface flow into the ship’s propeller. This can influence the efficiency, noise, and vibration of the propeller. Thus, the same needs to be included in study when designing the system.
• • Size of air bubbles and their uniformity is of vital importance. The injectors orientation and distribution need to properly configured to have desired effects. Since any change in air bubble diameter would drastically affect the air bubble distribution beneath the hull.