Structural reliability assessment of aging ships using the concept of level of certainty

Abstract

The input data in strength calculations (e.g., loads, geometric; and material properties) may have different levels of certainty/uncertainty. In most cases, the level of certainty of the result (e.g., stresses) is unknown. The knowledge of this certainty is important for all parties involved in ships' design, construction, and operation. In some sense, it indicates the likelihood of the calculated result to happen. When the probabilistic distribution of the stresses is available, the level of certainty of any calculated stress can be determined. However, deriving this distribution is not always possible. Therefore, an approximate method for its assessment is proposed that does not require knowledge of the probabilistic distributions of the input parameters or development of new computer programs. It is based on the percentiles of the input parameters calculated from the statistical analysis of the sample of data for each input parameter. This approach is applied for probabilistic assessment of residual plate thickness, hull girder bending stresses, and stresses derived by finite element modeling. It allows more realistic strength analysis especially of old vessels when the statistical data are scarce. Examples are given for 25K dead weight ton (DWT) bulk carrier. Nothing can ever be proved by statistics, one can only show that there is a certain probability that a given hypothesis is not false, which is not the same as showing that it is correct. © Marine Technology.