Page 372 - Marine Structural Design

348 Part III Fatigue and Fracture

18.2 Fatigue Loading for Ocean-Going Ships

For ocean-going ships, two basic sea states are to be considered in the determination of global
bending loads and local pressure: the head sea condition and oblique sea condition. The
cumulative fatigue damages should be calculated for full laden condition and ballast condition
respectively. The probability for each of these conditions is defined by classification rules
according to the type of vessels as below:

Table 18.1 Percentages of Fatigue Loading Conditions (IACS, 1999)

50 %
Bulk carriers 60 %

Two basic sea states are to be considered in the determination of global bending loads and
local pressure: the head sea condition and oblique sea condition. These basic sea states
combine the various dynamic effect of environment on the hull structure. The load
components for these sea states depend on the ship classification rules applied. For instance,
BV (1 998) further defines the hull girder loads and local loads (pressure & internal loads) for
four cases as Table 18.2.

Table 18.2 Load Cases for Ocean Going Ships (BV, 1991
Head-Sea Condition, a! Oblique-Sea Condition, 0
Static sea pressure associated Case 11, Case 21,
to maximum and minimum
inertia cargo or blast loads Amax=-O.45, Amax=-0.30,

Amin=0.45, Amin=0.30,
B=O B=0.45
Maximum (ship on crest of Case 12, Case 22,
wave) and minimum (ship on Amax=0.625, A~~x=-O.~OS~II(Z-N),
tough of wave) wave-induced
sea pressure associated to Amin=-0.625, Amin=O.3Osgn(z-N),
static internal cargo or ballast B4.45 B=0.625
loads

The global loads include still water bending moment M,, for the load condition considered,
and vertical wave bending moment. The vertical bending stresso, is further defined:

0 In sagging condition for maximum internal cargo or ballast loads: