ssc-367 - Ship Structure Committee

structure is unique and an allowable stressmethod based on typical
structures and a typical environmentwill only provide information
on relative susceptibility of various joints/details to fatigue
failure. In addition,newer vesselsare often constructedfrom high
strength steel, allowingthe use of thinner plates. Ship structure
scantlingsizes are basedon strengthrequirementsand any reduction
in scantling sizes without due consideration for fatigue phenomena
is likely to make the allowable stress method unconservative.
Therefore, allowable stress methods can be used as a “screening
process” and a detailed fatigue analysis is recommended to ensure
integrity of the design.
Ship Structures
The use of a linear ship motion theory is appropr”ate for fatigue
analysis of most vessels. For most vessels structural dynamic
amplifications,wave nonlinearities,and effects such as springing
due to high forward speeds have negligibleeffect on overall fatigue
lives. However, some vesselsoperating in harsh environmentsmaybe
subjected to appreciable fatigue damage due to harsh environment
loading. For such vessels the ability to predict wave
nonlinearities and vessel hogging, sagging and racking effects
accurately may become important. In such instances, a non-linear
ship motion theory may be preferred over a linear ship motion
theory.
Fatigue is a local stress phenomena and it necessitates accurate
definition of stresses for very complex geometries. In addition to
primary hull girder bending in horizontal and vertical axis,
substantial secondary girder bending moments will occur due to
external dynamic loads on vessel bottom and internal inertialloads
due to vessel contents. Thus, a beam theory based nominal stresses
due to primary hul1 bending are inaccurateboth due to complexityof
geometry and the local load effects. A finite element model should
be developed to represent the behavior of the vessel and to
determine the local stress distributions accurately.
5-14
HL