Page 181 - Introduction to Naval Architecture

STRENGTH 167

connected at that node. This condition and the boundary conditions
enable the nodal forces to be calculated. The strains involved in the
displacements lead to a pattern of stress distribution in the beam. The
finer the mesh the more accurately the stress pattern will be
represented. In a more complex structure such as that shown in Figure
7.21, elements of different shape and size can be used. Smaller
elements would be used where it was suspected that the stresses would
be highest and more variable.
The starting point in a comprehensive structural design approach
would be a finite element analysis of the complete hull using a relatively
coarse mesh. The data from this global analysis would then be used to
define the boundary conditions for more limited areas which would be
studied using a finer mesh.

Corrosion protection

The surface of all metalwork, inside and outside the ship, needs to
be protected against the corrosive effects of the sea environment and
of some of the cargoes carried. Most failures of marine structures are
due to a combination of corrosion and fatigue. Both can be described
as cumulative damage mechanisms. High tensile steels are as liable to
corrosion as mild steel. Hence when they are used to produce a
lighter weight structure, corrosion can assume even greater
significance.

Types of corrosion
These can be classified as:

(1) General corrosion. This occurs relatively uniformly over the
surface and takes place at a predictable rate.
(2) Pitting. Localized corrosion can occur under surface deposits
and in crevices. Pits can act as stress raisers and initiate fatigue
cracks, but the main concern with modern shipbuilding steels is
penetration and subsequent pollution.
(3) Differential aeration. Debris and fouling on a surface can lead to
different concentrations of oxygen which trigger local
corrosion.
(4) Galvanic action. Sea water acts as an electrolyte so that electro-
chemical corrosion can occur. This may be between different
steels or even between the same steel when subject to different
amounts of working or when a partial oxide film is present. In
the 'cell' that is created it is the anodic area that is eaten away. A
few average values of electrical potential for different metals in
sea water of 3.5 per cent salinity and 25°C are listed in Table 7.5.