Probabilistic modelling of the hull girder target safety level of tankers

The paper aims at assessing the probabilistic characteristics of the hull girder target safety level of a Suezmax tanker derived from a cost-benefit analysis. The target safety level is obtained considering as risk control option the change in the cross section scantlings of the tanker and its effect on risk reduction expressed by the total expected cost of the hull girder failure. The approach involves the evaluation of the effect of the risk control option on the hull girder failure probability assessed by means of a structural reliability analysis, and the assessment of the expected cost of hull girder failure in terms of property damage of the ship, pollution due to spillage of oil and loss of life of the ship's crew, as proposed by the International Maritime Organization (IMO). The uncertainties in the costs related to the loss of human life and environmental damages are characterised using available statistics and included into the probabilistic framework. Moreover, a probabilistic model is proposed that explicitly accounts for labour and material costs when evaluating the cost of the risk control option. The hull girder failure probability is calculated using the First-Order Reliability Method, considering the ultimate collapse of the midship cross section under vertical bending moment as the failure event. Uncertainty propagation and sensitivity analyses are conducted for different operational scenarios to assess the contribution of the model parameters on the uncertainty of the target safety level of the tanker with the objective of identifying the range of the variation and the characteristic target safety level of the hull girder that would be adequate for risk-based design and reliability-based code calibration studies.