Different Types Of Roll Stabilization Systems Used For
ShipsImagine yourself to be one of the first passengers of the
Royal Caribbean International Quantum of the Seas. As soon as you
enter the ship you hear the Master requesting all the passengers
& crew members to fasten their seat belts and remain in their
respective rooms until the voyage is complete! Definitely, this is
something no passenger would like to hear who has spent a fortune
to enjoy the excellent facilities, food and exceptional beauty of
such a marvelous ship.However, this could become a reality if the
ship had not been equipped with machinery or designed in a way to
overcome all the various types of motions felt at the seas. Why
take only cruise ships and ocean liners, think about the cargo
carrying ships such as Bulk carriers, Ro-Ro ships or even Tankers
& LNG carriers. Just imagine how difficult would it be to carry
the cargo (liquid or bulk) safely if we did not have any technology
to control the various motions during the voyage.
In this article we shall be focusing on how we can control the
Rolling motion of the ship and the various principles behind the
different types of Roll Stabilization systems.
Rolling is indeed the biggest problems among all the other
motion in seas. Technological advancements in the design of ships
have already given us naval architects the advantage to study the
wave motions around the hull of the ship and give the best possible
design to minimize such effects & ensure a comfortable and safe
voyage for the passengers and the crew.
Image Credits: George / WikimediaImage Credits: George /
WikimediaRoll motion stabilization can be achieved in conventional
ships by changing their hull forms, however, reduction in roll
amplitudes are possible by other means as well. Stabilization
systems can be broadly classified into
Passive Systems: In which no separate source of power is
required and no special control system like the Bilge keel, anti
rolling tanks (passive), fixed fins & passive moving weight
system.Active Systems: In which the moment opposing roll is
produced by moving masses or control surfaces by means of power
like the active fins, Anti rolling tanks (active), active moving
weight & the gyroscope.Bilge Keels: They are the most popular
and are fitted to the great majority of ships. They are plates
projecting from the turn of bilge and extending over the middle
half to two-thirds of the ships length. To avoid damage they do not
normally protrude beyond the ships side or keel lines, but they
need to penetrate the boundary layer around the hull. They cause a
body of water to move with the ship and create turbulence thus
dampening the motion and causing an increase in period and
reduction in amplitude. Although relatively small in dimension,
they have large levers about the rolling axis and the forces on
them produce a large moment opposing the rolling.Their effect is
generally enhanced by ahead speed. They are aligned with the flow
of water past the hull in still water to reduce their drag in that
state. When the ship is rolling the drag will increase and slow the
ship a little.
Structural Component of Bilge Keel Structural Component of Bilge
Keel (Source: INA Eric Tupper)passive tank system
Passive Tank system (Source: Basic Ship Theory, BST)Anti Rolling
Tanks (Active): They are similar to the principle of passive tank
system but the movement of water is controlled by pumps or by the
air pressure above the water surface. The tanks either side of the
ship may be connected by a lower limb or two separate tanks can be
used. The air duct contains valves operated by a roll sensing
device. This concept uses an axial flow pump to force the water in
the tank from one side of the ship to the other, rather than to
have it slosh under the natural roll, sway and yaw forces, as
happens in a passive tank. In a simplified version of an active
system an accelerometer senses the rolling motions, and signals are
sent from this roll sensing device to a variable pitch pump, which
controls the liquid flow between the tanks. The device can be
either a simple accelerometer or a complicated gyroscopic sensing
system that detects even a small angle of roll by the gyroscopic
precession. Thus, the device can be used control ship motion due to
every single wave. Depending on the sophistication of the system
active tank stabilizers have been found to leave an efficiency of
80% or more in motion stabilization.
Fig. 3: Active Tank system (Source: www.hoppe-marine.com) Active
Tank system (Source: www.hoppe-marine.com)Active Fins: With active
fins a sensitive gyro system senses the rolling motion of the ship
and sends signal to the actuating system which, in turn, causes the
fins to move in a direction such as to cause forces opposing the
roll. The actuating gear is usually electrohydraulic. The fins,
which may be capable of retraction into the hull, are placed about
the turn of bilge in order to secure maximum leverage for the
forces acting upon them. A flap from the trailing edge may be used
to enhance the lift force generated. The capacity of a fin system
is usually expressed in terms of the steady angle of heel it can
cause with the ship moving ahead in still water at a given speed.
The force on a fin varies in proportion to the square of the ship
speed, whereas the GZ curve for the ship is independent of speed.
However, a fin system is not likely to be very effective at speeds
below about 10 knots.
Arrangement of Fins in activated fin systemArrangement of Fins
in activated fin systemfin stabilizer system Fin Stabilizer system
(source: alibaba.com)The following table highlights some of the
major aspects among the Roll Stabilizers as discussed above:
comparision tableTable 1: Comparison between various roll
stabilizers (Source: BST)
From the above observation it can be concluded that each and
every stabilization system has got its own advantages &
disadvantages. Therefore, more amount of effort has to be given to
the hydrostatics and the hydrodynamics of the ship hull motion
through the waves, so as to avoid the need for kind of roll
stabilization system or to use it to its minimum. Also, in
principle methods used to stabilize against roll can be used to
stabilize against pitch, but the powers involved are too great to
justify their use.
Over to you..