This paper provides a demonstration on the use of probability-based ship structural safety analysis and enumerates the benefits in comparison with traditional methods. Reliability techniques are developed to determine safety levels of existing vessels, taking into consideration uncertainties in loads, strength and calculation procedures. Ultimate, serviceability and fatigue limit states are developed and applied to an existing tanker. The corresponding load models are examined and, in some instances, further developed. Safety indices associated with yielding, buckling, and fatigue are calculated. Conclusions are drawn regarding the benefits and drawbacks of using probability-based ship safety analysis. This paper provides a demonstration on the use of probability-based ship structural safety analysis and enumerates the benefits in comparison with traditional methods. Reliability techniques are developed to determine safety levels of existing vessels, taking into consideration uncertainties in loads, strength and calculation procedures. Ultimate, serviceability and fatigue limit states are developed and applied to an existing tanker. The corresponding load models are examined and, in some instances, further developed. Safety indices associated with yielding, buckling, and fatigue are calculated. Conclusions are drawn regarding the benefits and drawbacks of using probability-based ship safety analysis.

Ship structures must be designed with adequate safety and reliability, and their designs must be acceptable from an environmental and economical point of view. Target reliabilities have to be met in the design rules in order to ensure that certain safety levels are reached in design according to the rules. There are several steps involved in a process of determining target reliabilities in agreement with a risk based approach as recommended by Formal Safety Assessment, IMO (1997, 2001). The study concentrates on the second step of FSA, i.e. calculation of the reliability level inherent in existing rules representing past practice when state-of-the-art models for environment, loads, response and capacity are adopted. Buckling of a ship deck in the extreme sagging conditions is considered. The suggested procedure is illustrated by three examples. The reliability calculations are carried out for the ‘as build’ stiffened deck plate thickness. Uncertainties involved in the suggested analysis procedure and their consequences on implicit reliability are presented. Implementation of the results in the risk analysis is discussed.

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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.