Page 326 - Introduction to Naval Architecture

SHIP DESIGN 311

experienced, analysed, the corrective action decided upon, agreed and
implemented. Also advancing technology and changing trade require-
ments can lead to ships with new features, and operating patterns,
which have not been fully proven. Testing of hydrodynamic or
structural models and of materials in representative conditions can
help but the final proof of the soundness of a design is its performance
at sea.
A ship can be seriously damaged by, or lost because of:

(1) Water entering as a result of damage or human error in not
having watertight boundaries sealed. This can lead to capsize or
foundering.
(2) Fire or explosion.
(3) Structural failure due to overloading, fatigue or fracture,
possibly brittle in nature. Failure may be of the overall hull
girder or local, say in way of a hatch cover, so permitting the
ingress of water.
(4) Loss of propulsive power or steering, leading to collision or
grounding.

A designer should produce a safety case, identifying how a ship might,
suffer damage, the probability of occurrence and the potential
consequences. With such data an informed judgement can be made
on the level of risk it is reasonable to accept for the design in
question, and what safety systems should be fitted on board. Such
analysis might show a need for external support in some situations.
9
For instance, escort tugs might be deemed desirable in confined
waters or areas of special ecological significance. Many of the factors
involved can be quantified, but not all, making good judgement an
essential element in all such analyses. The important thing is that a
process of logical thought is applied, exposed to debate and deci-
sions monitored as the design develops. Some of the decisions will
depend upon the master and crew taking certain actions and that
information should be declared to the command so that the design
intent is understood.
Safety is no academic exercise. Many ships are lost as is shown by
the annual analysis of ship losses published by Lloyd's Register. In
the early 1990s some 200 ships were lost each year from all causes,
the gross registered tonnage being about a million per year. In some
cases the reason for loss is unknown, suggesting some rapid over-
whelming of the ship. One such case, that aroused a lot of debate,
was the loss of the MV Derbyshire, a British OBO carrier of 192 000
tonne displacement. Too many trawlers have been lost from
unknown causes.