Page 135 - Introduction to Naval Architecture

7 Strength

Anyone who has been at sea in rough weather will be only too aware
that a ship is heavily loaded and strained. It moves about quite violently
and the structure groans as the parts move relative to each other.
Looking at the waves causing the motion the impression is one of utter
confusion. The individual will have become aware of two fundamental
difficulties facing a naval architect, those of identifying the loading to
which the structure is subjected and of calculating its response to that
loading. The task of assessing the adequacy of a ship's structure is
perhaps the most complex structural engineering problem there is.
The stresses generated in the material of the ship and the resulting
deformations must both be kept within acceptable limits by careful
design and each element of the structure must play its part. There is
generally no opportunity to build a prototype and the consequencies of
getting things wrong can be catastrophic.
Many local strength problems in a ship can be solved by methods
employed in general mechanical or civil engineering. This chapter
concentrates on the peculiarly naval architectural problem of the
strength of a hull in still water and in waves. From a consideration of
the overall strength and loading of the hull it is possible to consider the
adequacy of the strength of its constituent parts, the plating and
grillages. The global calculations indicate stresses or strains acting in
local areas to be taken into account in designing local details.
The complete structural problem is a dynamic one but, as with many
other aspects of naval architecture, the situation in calm water is
considered first. Even in this state the ship is subject to the forces of
hydrostatic pressure and the weight of the ship and all it carries,
Indeed, care is necessary when loading ships in port to ensure that the
structure is not overloaded. Ships have been lost in harbour. In 1994
the OBO carrier Trade Daring, a ship of 145 000 dwt, broke in half while
loading iron and manganese ore. Although this was a relatively old ship
the lesson is there to be learnt.
A ship's ability to withstand very high occasional loading is ensured
by designing to stress levels which are likely to be met perhaps only
once in the life of the ship. Failures in ship structures are much more
121