Petromin marine and offshore Jan-Feb Issue

prime mover is a diesel engine, then
it is called Diesel-Electrical Propulsion.
The most commonly used diesel
electrical propulsion systems are not a
new concept. In the past these systems
were usually diesel engine driven D.C
generators that supplied power to D.C
motors. Their applications were generally
limited to vessels that required a
degree of low speed maneuvering.
Layout of diesel electric propulsion
in ship
Vessels such as ferries, harbor tugs, and
various other applications used diesel
electrical systems for features that were
not available in mechanical systems at
that time like speed control and maneuverability.
To date, electrical propulsion
systems have been used mainly for specialized
vessels rather than for cargo
ships in general. These include dredgers,
tugs, trawlers, lighthouse tenders,
cable ships, ice breakers, research
ships, floating cranes, and vessels for
the offshore industries. Electrical-drive
systems have made substantial progress
in recent years.
Types of Diesel Electric
Propulsion
The two systems dominating the market
today are Frequency controlled A.C
Motors and SCR controlled D.C Motors.
Frequency controlled A.C Motor drive
system were generally more cost effective
below 500 H.P and SCR controlled
D.C motors systems at the higher end.
The offshore drilling industries favor
SCR controlled DC drives.
Modern SCR and frequency controlled
systems have efficiencies approaching
97% in power conversion. The selection
of one over the other is an application
issue. The deep draft cruise
ship industry, due to the high hotel-like
power requirements, is adopting highpower
diesel electrical propulsion systems
in most of its new builds.
Both technologies have a proven
record of efficiencies and reliability.
For a direct current propulsion motor,
the electrical power may be from
one or more DC generators or may be
form an alternator and then delivered
through a rectifier as a DC supply. The
power for direct current motors is limited
to about 8 MW, and so AC machines
are used for high outputs unless
an effort is made to install DC motors
in tandem. The rectification scheme
can incorporate speed control and a
means of reversing.
Electric power generation and
propulsion is a solution optimized
for an environmentally
aware era, and which answers
shipyard and ship-owner’s needs
for fuel economy, higher flexibility
(both for installation and operation),
better comfort on board,
increased availability and an inherent
capacity to be configured
for fault tolerance and graceful
degradation.
Power for AC propulsion motor is supplied
obviously by an alternator; the
prime movers may be a diesel engine,
a gas turbine, or a boiler and steam
turbine installation.
The choice of diesel electrical system as
the power source for a propulsion system
of a vessel has nothing to do with
hydrodynamic efficiency. The propulsion
system of a vessel provides thrust to move
the vessel and is still chosen by the de-
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January to February 2018