Page 28 - 2021 - Q2 - Minerva in Focus

MINERVA GAS

An introduction to Air Lubrication Systems (ALS)

Due to the increasing regulations to re- Air lubrication systems are recognized by IMO as a Category B-1 “Innovative Energy
duce greenhouse gas emissions from ships Efficiency Technology” as described in MEPC.1/Circ.815.
and the constant pursuit of reduced op-
erating costs, new energy-saving technol- BACKGROUND
ogies have been emerging. Today’s clear Air lubrication is not a new technology. One of its earliest applications was the Prai-
framework for reducing emissions and the rie/Masker Air System developed post-World War II by the US Navy. The Masker and
need to comply with the Energy Efficiency Prairie systems were designed to decrease the ship’s radiated noise, thus reducing
Index (EEDI) Phase III requirements have the risk of being detected by hostile submarines.
increased the popularity of such technolo- The Masker portion targeted engine silencing, while Prairie would reduce the noise
gies, including air lubrication systems. created by screw cavitation in the propeller. The Masker portion typically consists
of two bands fitted to the outside of the hull adjacent to the vessel’s engine rooms,
forcing air bubbles around the hull. Masker’s working principle is based on the fact
that acoustic waves encountering material with a radically different speed of sound
do not penetrate but are reflected back. Thus, sounds within the ship hull, which
would otherwise propagate for a long distance into the water, are reflected by the
blanket of air bubbles and are eventually dissipated. On the other hand, the Prai-
rie Air System pumps air along the propeller blade leading edge, thus reducing the
hydrodynamic noise originating at the propeller. Hydrodynamic noise is reduced be-
cause the air bubbles fill the vacuum left by the rotating blades as the water “boils,”
allowing cavitation bubbles to contract more slowly.

ALS TECHNOLOGIES
For slow-moving vessels, frictional resistance has been reported to contribute up to
80% of the total resistance, requiring a thorough investigation to reduce it. Frictional
drag reduction (FDR) includes passive methods, such as smooth coatings to the hull,
and active methods, such as air lubrication systems.
Air Lubrication uses air as a lubricant by pumping it beneath the hull. There are
three major categories of air lubrication technology: Bubble Drag Reduction (BDR),
Air Layer Drag Reduction (ALDR), and Partial Cavity Drag Reduction (PCDR).
1. Bubble Drag Reduction (BDR)

The concept of Bubble Drag Reduction (BDR), which is the most widely used air lubri-
cation technique, is summed up as injecting small bubbles into the water near the
ship’s hull through several nozzles.
Operation Principle:
In Bubble Drag Reduction (BDR), gas is injected into the boundary layer through a
slot, porous material, or perforated plate. The gas is separated into bubbles that
reside predominantly in the boundary layer of the hull. The dispersed bubbles act to
reduce the density of the air-water mixture, hence the Reynolds stress, resulting in a
skin friction reduction.
Bubble size is critical because as bubbles grow in size, they can no longer maintain
their spherical shape, making them prone to deform in turbulent flows.
Limitations:
• The biggest challenge here is to keep the bubbles inside the so-called bound-
ary layer because the bubbles tend to migrate from the near-wall region of
the hull, even when the gas is injected beneath a horizontal surface. Hence,
the persistence of pure BDR is said to be poor unless gas injectors are placed
in adequate locations.
An analysis by Dimitra Karamolegkou • The pressure of injected air must be increased as the vessel’s draught increas-
Fleet Support Officer, es; thus, it is more energy-demanding in laden condition.
Minerva Gas • The higher the speed, the higher the friction, thus the higher the flow rate
required.
26 MINERVA IN FOCUS – ISSUE 16 / Q2 2021