Reliability evaluation of a structure at sea

Abstract

In general a safety factor or a conventional partial coefficient is used for the design of structures at sea. These methods are classified as Level I (deterministic) methods. However, such approaches do not allow the determination of the reliability (or the failure probability) of the design, and consequently it is neither possible to optimize, nor to avoid over-design of a structure. In order to overcome this problem more advanced probabilistic methods must be applied where the uncertainties (the stochastic properties) of the involved loading and strength variables are considered. Methods where the actual distribution functions for the variables are taken into account are denoted Level III methods. Lever II methods comprise a number of methods in which a transformation of the generally correlated and non-normally distributed variables into uncorrelated and standard normal distributed variables is performed and reliability indices are used as measures of the structural reliability. Both Level II and III methods are discussed in the following. Described is also an advanced partial coefficient system which takes into account the stochastic properties of the variables and makes it possible to design to a specific failure probability level. A concept for the calculation of partial coefficients corresponding to given failure probability within given structure life is presented. Two partial coefficients gamma_H, and gamma_Z are applied to a design formula. Two partial coefficients are calculated from formulae (68) and (69) in which two failure mode specific coefficients, k_alpha and k_beta, are used together with characteristic return period values of H_s, extracted from the site specific long term distribution of H_s. So far the k_alpha, k_beta coefficients have been calculated only for the failure modes which are described by existing uncertainty evaluated formula.e. However, it is easy to expand the system as more failure mode formulae appear. It is important to notice that the reliability of the formulae must be documented, e.g. in terms of a standard deviation, in order to implement them in the partial coefficient system.