Strength assessment of damaged steel ship structures

Abstract

In 2012 106 vessels over 100 gross tonnes were lost. During the damage incidents many of these vessels required assistance from shore based emergency response services with regards to their damaged strength to stabilise the situation, preserve life, prevent environmental disaster and limit financial costs to owners and insurers. The research work presented in this Engineering Doctorate Thesis surrounds the strength assessment of damaged steel ship structures, the influence of damage on the strength of steel-plated structures and methods for assessing the residual strength of a vessel in an emergency. The focus of the work is to improve the modelling of damaged steel ship structures within an emergency situation, in order to improve guidance provided to a stricken vessel during a damage incident or salvage process. Literature study has shown that structural idealisation through the use of interframe progressive collapse analysis, to be the current state of the art method for the rapid assessment of intact and damaged ship structures. However, a number of weaknesses have been identified in the method when applied to damage assessment. The literature study has also shown a lack of understanding of the effect of damage on steel-plated structures as specific analysis has not been performed previously. Significant research has been undertaken into the influence of damage, in the form of a hole, on the ultimate collapse strength of steel-plated structures. Three levels of structural modelling have been used, stiffened-plate, stiffened panel and grillage. Comparison of the predicted ultimate collapse strength by finite element analysis (FEA) with predictions using the interframe progressive collapse idealisation, has shown the calculated results to be conservative for the assessment of damaged structure when the failure mode remains in the interframe collapse form. However, changes in the failure mode lead to the interframe progressive collapse method over predicting the ultimate collapse strength. The analysis shows that even small damage events can lead to significant changes to the failure mode and resulting ultimate collapse strength of the structure. Such influences must be accounted for in any simplified method. A new method for the assessment of damaged ship structures is proposed that is capable of modelling a damage scenario more accurately. Demonstration of the method has shown the results to be less conservative than the current state of the art, when compared to FEA, for local analysis of damaged steel-plated structure. The ability of the method to account for the influence of damage, and the resulting failure modes, that may significantly influence the ultimate bending strength of the structure has also been demonstrated. The method implements a compartment level progressive collapse analysis with structural data captured through the use of the response surface method ‘kriging’, using data points provided from FEA. This method allows the critical damage variables to be captured and strength data accessible quickly for use in the analysis. The time to provide a solution to the damage scenario is equivalent to the existing interframe progressive collapse method. Therefore, the method is suitable for application within an emergency response or salvage service.

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