Reliability based methodology for the assessment of cumulative life-time hydrodynamic loads and structural capacity

Abstract

A methodology for the assessment of global wave load combinations on a container ship is developed taking into account load modelling uncertainties such as the sea state, the speed with and direction in which the ship travels etc. Data from wave statistics are used to determine the sea state which is required for specifying the sea spectrum associated with a given sea state. The sea spectrum together with the response amplitude operators (RAOs) of the ship are used to determine the stochastic responses of the ship in waves. This load combination methodology employs stochastic analysis in conjunction with cross-spectral analysis to determine a given wave-induced load that is associated with a principal wave-induced load which have been determined a priori via common spectral methods. All the global wave-induced loads are assumed to be random variables that are Gaussian distributions with zero means and variances obtained from their respective zeroth spectral moments. The peaks of the responses are modelled as Rayleigh distributions and the combination of any two responses modelled by a copula function which is a bivariate Rayleigh distribution with parameters such as the zeroth spectral moments of the responses concerned and their interactions in form of a correlation parameter obtained via a cross-spectral analysis of the given responses. Load combinations are obtained with respect to any given design extreme global wave-induced load as the principal load whose effects may be a source for concern at the design stage of the ship. The results were compared to results obtained from existing methods of load combinations. A Spearman’s rank correlation test shows a very good covariation between the compared methods. Nonlinear finite element modelling and analyses on the midship section of a 10,000 TEU container ship are used to assess its ultimate strength under combined loads such as the vertical bending moment, horizontal bending moment and torsional moment. The primary loads that act on an actual ship hull girder are of different modes corresponding to the six degrees of freedom hence it is more realistic to consider the ii E. Alfred Mohammed effects of loads in other modes other than the dominant vertical bending moment in the determination of the longitudinal strength of the hull girder. Torsional moment is particularly important because it can be a serious problem for container ships which have wide open hatches and hence low torsional rigidity. A three-compartment finite element model for the container ship OL185 is developed from the transverse section detail of the ship. A progressive collapse analyses are performed on it using the Abaqus finite element code to determine the ultimate strength of the container ship under combined vertical bending, horizontal bending and torsional moments. These provide a basis for load interaction and combination studies via interaction relationships featuring the relevant load combinations. Finally, the interaction relationships are used to show the hull girder safety margin via a comparison with a predetermined global wave-induced load combination for the midship section of OL185 container ship obtained using the cross-spectral probabilistic methodology.