Application of Probabilistic Concepts for Determining Tolerance Limits of Initial Imperfection of Ship Plating

Abstract

Generally, ship structure is constructed by welding and so it's member is always accompanied by welding residual stresses and initial imperfections.
These residual stresses and initial imperfections reduce the ultimate strength, therefore it is very important to clarify the acceptable initial imperfection for safety design.
This paper deals with studies on the tolerance limits of initial imperfections of the tanker's deck plates from the view point of strength.
First, in the case of deck plate with initial imperfections and residual stresses under compressive load, stochastic natures of strength and load are studied by means of Extreme Theory.
Then, tolerance limits are determined by applying reliability concepts to estimate the safety.
To this end, two kinds of calculations are carried out. Firstly, the upper limit of initial imperfections is calculated so that the risk of failure of deck plate becomes equal to that of setting values. Secondly, it is selected the standard risk of failure to that of the mild steel deck plate. And the initial imperfections of the High Tensile Steel deck plate (in case of larger ship) is calculated in the same manner.
It is found that the tolerance limits of initial imperfections in JSQS is reasonable around S/t= 40, but the larger the values of S/t, the smaller the safety of deck plate.
These conclusions are obtained from comparison between ships smaller than 100, 000 DWT and larger one, therefore the conclusion is correct only in comparative sense.