Strength and fatigue assessment activities consist in evaluating if the ship structure is adequate to support Class Societies prescribed loads.
Our extensive knowledge and solid experience about the calculation procedure and the approval process required by Class Societies, combined with internally developed tools and software packaging, ensure the high quality of the calculation output and a rapid response to the client. Indeed, during the years, our company has successfully completed several calculations and approval processes for many kinds of ships and different Class Societies.
In order to introduce strength and fatigue assessments from a technical standpoint, first of all, a FEM (finite elements method) model has to be built relying on the scantling and class drawings received from yards or naval designers. The size of the elements is defined on the base of our experience and the type of analysis that needs to be conducted. In addition, for each kind of analysis it is necessary to verify that the ship deflection and stress levels do not exceed the prescribed criteria of Class Societies and otherwise, structural alterations are proposed.
More specifically, strength and fatigue assessments analyses include the following activities:
LONGITUDINAL STRENGTH ASSESSMENT
In this analysis, the ship is loaded with still water and rule waves hull girder loads in terms of bending moment and shear forces. The bending moment and shear force distribution curves are applied to the model in order to reach the theoretical target. Thanks to this analysis, it is possible to evaluate the deck efficiency and the contribution of the superstructure against the hull girder behaviour. Once the stresses are calculated, it is required to proceed with the buckling analysis. The size of the elements used in this specific case is based on the coarse mesh typology. Internal tools have been developed for the application of loads, balancing of the model and post-processing activities (as buckling analysis, for example), allowing to complete the whole longitudinal strength analysis more quickly. Another important step consists in applying global loads on local details of the FEM model and, in order to properly evaluate the openings SCF (stress concentration factor), the local model has to be created by the use of fine mesh criteria.
BASEMENT-FOUNDATION STATIC ANALYSIS
The verification of foundations and basements of different equipments (as cranes and main machinery propulsions) is executed considering SWL (safe working loads), received from providers.
Inertial and environmental loads required by Class Societies rules and DAF (dynamic amplification factor) are taken in consideration later on in the analysis.
The type of mesh has to be chosen from time to time on the base of the analysis that has to be conducted.
DETAILS FATIGUE ASSESSMENT
The details are discretised in order to proper evaluate the SCF and the notch dynamic stress range.
The fatigue assessment of the details is based on the use of S-N curves, from which the allowable value of dynamic stress range is obtained, known the number of the cycles to whom the detail is subjected.
The fatigue checks are exercised in line with the procedures required by Class Societies and, thanks to the various internal tools that have been developed (each related to a determined Class Society), the verification can be performed faster.
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