Page 330 - Practical Ship Design

288 Chapter 10

IO. 1.2 Redundancy
Whilst a designer aiming for economy will usually try to minimise structural
redundancy, recent bulk carrier casualties suggest that a measure of redundancy is
desirable so that the loss, or a severe reduction in the strength, of some structural
members can be absorbed without catastrophic failure. Particular attention should
be paid to this in the design of parts of the ship which seem likely to be susceptible
to corrosion and/or fatigue.
In some parts of the structure the design should be governed by local strength and/
or vibration considerations; in other areas it may be important to limit deflection.
In a 1992 I.E.S.I.S. paper entitled “Safety of bulk carriers” J.M. Ferguson of
Lloyds Register gave a useful reminder of the many factors which may influence
the safety of a ship and this is reproduced as Fig. 10.1. The paper lists the main
types of defects and their locations as:
(i) cracking at hatch corners;
(ii) plate panel buckling of cross deck strips and stiffening structure;
(iii) cracking of hatch coamings;
(iv) cracking at the intersection of the inner bottom plating and the hopper
plating;
(v) grab and bulldozer damage to the side frames lower brackets;
(vi) grab damage to the inner bottom plating, hopper and lower stool plating;
(vii) cracking at main frame bracket toes;
(viii)both generalised and local corrosion of main frames and brackets;
(ix) cracking at fore and aft extremities of topside tank structures;
(x) corrosion within topside tanks.
(xii) general corrosion of transverse bulkheads.
Although this list refers directly to bulk carriers the importance of good detail
design and good operational practice is equally applicable to all ships, and is worth
emphasising at the start of this chapter on structure.

10.1.3 The variety of structural calculations

Although longitudinal strength is the most important strength consideration in
almost all ships with both the vertical bending moment and the vertical shear
forces requiring investigation, a number of other strength considerations must be
considered. Prominent amongst these are transverse, torsional and horizontal bend-
ing strength, with torsional strength requiring particular attention on “open” ships
with large hatches arranged close together.
In later more detailed scantling calculations, watertight bulkheads must be
designed to meet the hydrostatic loads that they will receive if one of the adjoining
compartments becomes open to the sea, whilst bulkheads of large tanks which may