Page 325 - Introduction to Naval Architecture

310 SHIP DESIGN

a costly item. It has been estimated that a frigate costs about £100000
for each day of seatime. It is a matter of striking the right balance
between conflicting factors. To help in making these decisions the
technique of availability modelling can be used.
In availability modelling the various ship capabilities are set down.
Then the equipments involved in conferring that ability on the ship are
noted in a diagram showing how they contribute. Some components of
the diagram will be in series and others in parallel. One capability
might be the ability to move. This requires the main engines, gearing,
propulsor, shafting, bearings and so on. There will also be a number of
supporting functions such as lub-oil pumps, machinery seatings and
bearings as well the need for electrical supplies and fuel. Large items
such as the main machinery can be broken down into their constituent
components. For each item the MTBF can be assessed together with the
probability of a failure in a given time span. These individual figures
can be combined to give the overall reliability of a system using an
approach similar to the way the total resistance of an electric circuit is
calculated from the individual resistances of items in series or parallel.
High reliability of components is needed when many are used in a
system. Ten components, each with a reliability of 99 per cent, when
placed in series lead to an overall reliability of (0.99) = 0.905. Ten
10
units in parallel would have a reliability of l-(O.l) , effectively 100
per cent.
Such analyses can highlight weak links which the designer can
improve by fitting more reliable components or duplication. They also
provide guidance on which spares should be stocked and in what
quantities, that is the range and scale of spares.
In some cases, and the ability to move is one, the capability may exist
at more than one level. For instance loss of one shaft of a twin shaft ship
will reduce the available power to 50 per cent. If more than one diesel
drives one shaft, the loss of one is not as critical as losing all the power
on the shaft.

SAFETY

A great deal of attention is paid to the safety of ships by national and
international regulations and during design and operation. However, a
ship can be designed to meet all existing regulations and yet not be as
safe as it could, and should, be. This is partly because regulations have
to be agreed by many authorities and are often a compromise between
what is regarded by many as the best practice and what others feel to be
unduly restrictive or are prepared to accept for economic reasons.
Partly the difference is due to the time lag between failures being