Obiettivo
The goal of this project is to develop a set of design rules which are derived from the application of reliability to ship structural design. The specific objectives of this project are:
-The development of reliability based tools to provide a more rational basis for the structural design through explicit quantification and trade-off between safety and economy, leading to consistent decisions on the allocation of safety margins in all components of the structure;
-the development of reliability based tools to make for an assessment of the integrity of existing ship structures, allowing decisions on the possibility of extending their lifetime
-the development of reliability based guidelines in a semiprobabilistic format for ship structural design which will be a proposal, on a pre-normative basis, for unified European regulations on ship structural design.
This project has identified the significant uncertainties involved in the procedure to calculate the long-term distribution of wave induced load effects due to the different theories of predicting wave induced load effects.
It was also shown that there are significant uncertainties associated with the reliability of the existing wave data which are an input to the calculations leading to design values of wave induced load effects.
Important achievements were accomplished as concerns practical methods of predicting non-linear wave induced loads. This is specially important for ships of fine forms.
A method was developed to calculate the stresses developed in extreme situations associated with ship slamming. However, the approach still needs to be further extended because it does not account for the effect of water on deck which often occurs in the extreme situations.
The knowledge of the behaviour of stiffened panels was improved although there are still some aspects that need more work, namely the collapse under combined loading which had not been considered in the present project.
A major achievement with significant practical implications is the development of a simplified procedure to calculate the hull girder collapse accounting for the post collapse strength of each panel. This is a procedure that is bound to be quickly introduced in design practice and to be considered in the rule requirements of the Classification Societies.
A very complicated procedure was used to determine the fatigue reliability of important joints. While this procedure will give reference values, more simplified approaches are required for current design procedures. However, these approaches can be calibrated with the results of a more demanding analysis like the one proposed here.
A standard reliability approach has been proposed as a basis for comparative studies between different ships and for calibration of safety factors. The procedure was applied to some example cases, providing an indication of the nominal levels of safety in present ships. Partial safety factors have been determined based on that level of reference.
Although the methodology for reliability assessment and for derivation of safety factors was demonstrated, a larger series of analysis and additional refinements resulting from it are required before definite values can be proposed for design.
This project will explore a new way of formulating the rules for ship structural design. The project will develop the methodology for rule development plus the methods to quantify the uncertainty in the variables, to perform a reliability analysis of a ship structure and to derive partial safety factors for the design of the primary structure. The methods will be applied to typical cases of containerships and tankers, indicating how a coherent set of rules can be developed.
The research project will lead to a set of simplified procedures and formulae to be used in design for the prediction of design loads and for the assessment of the strength of structural components. Software packages will also be developed to quantify loading, response and strength for specific ships and components. A methodology for code development and guidance notes for ship structural design will be produced.
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