Reliability and Code Calibration

Abstract

The structural reliability of a ship is basically ensured by compliance with a design code. A systematic process for the development of such a code is described. All potential hazards that a ship may encounter during its lifetime need consideration. Relevant hazards with respect to strength requirements can be defined by a set of load scenarios that may occur when the ship is operating. The load scenarios may then be split into a manageable number of different load cases. Code strength criteria are typically specified as design equations or inequalities which require the capacity to be equal or greater than the load. A design equation includes characteristic load effects, representative of the load combination under consideration, and a characteristic capacity, relevant for the failure mode under consideration. Furthermore, one or more safety factors or utilization factors are included, with magnitudes that should ensure an appropriate reliability level. The required reliability level depends on the consequences of failure, and is based on a risk evaluation, which also takes account of service experience. Structural reliability analysis (SRA) is a suitable tool for the development of structural codes. Uncertainties in input and methodology are accounted for by random variables, and the failure probability is calculated. The SRA of the hull‐girder ultimate strength criterion is described in detail for tankers. This is the most critical failure mode for a ship. It is shown how the definitions of characteristic values and the magnitudes of partial safety factors can be adjusted to optimize the code for a target‐reliability level. This calibration was carried out during the development of the International Association of Classification Societies' (IACS) common structural rules for tankers.