Page 464 - Marine Structural Design

440 Part IVSh.uctural Reliability

there are uncertainties in the statistics of response. The use of a simple Rayleigh distribution
can result in a bias toward values that are too high in severe seas.
The operation of the ship may also contribute to the uncertainty of wave-induced bending
moment, including:
Cargo distribution and resulting drafts
Ship headings to the sea
Ship speed
Still-water bending moments
It is relatively easy to calculate still-water bending moments if the distribution of cargo and
other weights is known. However, the still-water bending moments vary between voyages, and
in any cases they are seldom recorded. Hence, very little statistical data are available.
Estimates can be made on the basis of calculations customarily made for every new ship
design
Load Combinations
Correlation exists between the loads discussed in the above. For example, high dynamic loads
may often occur in rough seas when large low-frequency loads also occur, but high thermal
effects may generally coincide with calm, sunny days when wave-induced loads are relatively
mild. It is difficult to combine quasi-static and high-frequency wave-induced loads.
24.5.3 Uncertainties in Ship Structural Capacity
When considering structural failure, separate analyses are necessary for all possible failure
modes such as
Tensile failure
Buckling and collapse
Brittle fracture
Fatigue
Buckling and collapse are an important subject because strength in buckling failure mode is
much lower than the tensile failure mode. Brittle fracture failure has been controlled through
improved material toughness and design of structural details, workmanship and use of crack
stoppers to provide “fail-safe” design. Fatigue failure is an important subject even though
fatigue cracks do not normally in themselves threaten the complete failure of the hull girder.
Ultimate failure is complicated by the fact that buckling may occur progressively in different
segments of the structure and the first occurrence of a buckle does not usually constitute
failure. Loads may successively transfer from buckled areas to those that are still effective.
The objective uncertainties are measurable and include:
Main dimensions of hull
Material properties including yield strength, ultimate strength and Young’s modulus
Variations in material thickness and shape dimensions.
Manufacturing imperfections, including variations in fabrication tolerances, weld quality,
alignment, and residual stresses in welds.