Page 351 - Practical Design Ships and Floating Structures

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and the fluctuating loads covers wave bending moments, accelerations by ship motion and wave
dynamic pressures. The detailed loading components such as external wave pressure, internal pressure
and hull girder bending moment are defined in the probability level of loading to be lo4.
The accelerations due to ship motion produce the loads by cargo in hold or sea water in ballast tank,
and these loads act on hold or ballast tank as inertia forces.
The dynamic external pressure is considered as the largest of the combined pressure dominated by
pitch motion in headquartering seas, or by roll motion in beandquartering seas.
The dynamic internal pressures from liquid cargo or ballast water are calculated for acceleration
components in vertical, transverse and longitudinal directions and the maximum pressure due to
accelerations of the internal mass may be taken as the internal fatigue load.
4.2 Stress Combinations for Fatigue Andysis

A simplified approach to determine the distributions of long-term stress ranges for closed or semi-
closed hull cross sections is expressed as Weibull distributions. Stress ranges for fatigue analysis are
defined by combining local stress components due to simultaneous internal and external pressure loads
with global stress components induced by hull girder wave bending.
The local dynamic stress components Aal are defined by external and internal dynamic pressures as
following formula with a consideration of occurrence phase.

The total local stress amplitude due to external and internal pressure loads are the sum of individual
local stress components such as local secondary bending stress, local bending stress of longitudinal
and local tertiary plate bending stress.
Global stress range is defined as the combination of vertical bending and horizontal components, Since
two components of bending stress ranges, Aav and Aqg, never occur at the same phase, global stress
range Ang should be combined as

The long-term sailing routes of the ship is considered by reduction factor fe and the effect of mean
stress is considered by reduction factor fm .
Using the global and local stress ranges above, consequently, the stress range AGO for fatigue damage
calculation is taken as [DNV, 19981

Aa, = f,f, Max(A0, + 0.6Au, ,0.6Aa, +Act) (3)

Stress range for fatigue analysis should include the effect of stress concentration due to detail structural
geometry and welding geometry. Therefore, the stress concentration factor K is considered in the
calculation of each stress component.
4.3 Fatigue Damage Assessment

When the distribution of long-term stress range follows Weibull distribution, fatigue damage ratio D
indicating the intensity of cumulative damage is given by [DNV, 19981