Scantling Optimization of Ship Structures Considering Fatigue at the Early Design Stage

ABSTRACT: Fatigue aspects are leading criterion for the scantling optimization of ships structures. LBR-5
software is an integrated package which performs, at the early design stage, cost and/or weight optimization of
ships structures (scantling). This software takes into account structural constraints (yielding, buckling, displacement,
ultimate strength). Until now, the fatigue failure issue was not implicitly included in the optimization
loop. At the early design stage, limited information and details are available. Therefore simplified methods
have to be applied. LBR-5 software provides the nominal stress, so chosen procedure uses the nominal
stress and the Miner’s rule. This approach requires a library of stress concentration factors for predefined typical
structural details. The goal of the present work is to develop a tool for fatigue assessment that can be integrated
on LBR5 optimization process, to realize global optimization taking into account the fatigue issues.
Until now, the integration of the fatigue tool is not finished. Only fatigue verification on the optimized structure
is possible. On the present paper, the procedure adopted is presented and a longitudinal scantling of the
mid-ship section of a LNG is optimized with LBR-5. Production cost is considered as objective functions. The
optimized scantling is checked by the fatigue tool. The fatigue criterion is not respected on some panels. Corrections
are performed on these panels to have an optimized scantling without fatigue problems.