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The Peak Stress Method (PSM) is a simplified, finite element-based technique to readily estimate the Notch Stress Intensity Factors (NSIFs) at the tip of sharp-V-shaped notches. More precisely, it was shown that the maximum principal stress evaluated at the V-notch tip by means of a rather coarse finite element analysis is proportional to the mode I NSIF, as far as mode II and mode III stresses are negligible. When fatigue strength assessment of fillet welded joints is performed according to the NSIF approach, the weld toe profile is modeled as a sharp V-notch having the toe radius equal to zero (the worst case hypothesis) and the peak stress can be used as design stress combined with a properly calibrated design scatter band. Due to its computational simplicity, the PSM appears rather useful for the everyday design practice of the shipbuilding industry. In the present paper the PSM is briefly described and applied to assess the fatigue strength of joints between HP stiffeners with fillet-welded cover plates adopted by the shipbuilding industry, for which full scale fatigue test data are available. By applying the PSM, good agreement between theoretical and experimental fatigue lives is obtained despite the complex geometry and the different loading conditions of the tests.

In this work, the possible exploitation of fiber-reinforced composites in the context of maritime transportation of compressed natural gas (CNG) is investigated. In addition to a more conventional steel configuration, two different fiber materials, carbon and glass, are considered as construction materials for pressure vessels (PVs) to be stored on board ships, with thickness optimized by FEM analysis. The considered scenario is represented by the transportation of CNG from an offshore well to a terminal on shore. Fleets of ships carrying CNG in pressure vessels manufactured with the investigated materials are generated by means of a ship synthesis model (SSM) software and compared on the basis of technical and economical indicators. The choice of the construction material influences considerably the weight of the PVs, which represent a major item of total ship weight and reflects directly on the general transport performances in terms of resistance, seakeeping and reliability in the service. On the other hand, capital as well as operating expenditures are considerably affected by the choice. When exploring the design space, the ship synthesis model is able, at a preliminary stage of the design, to account for the various technical and economical aspects, their implications and relationships. Results are presented of computations carried out in a specific case, identified by the annual gas production and other characteristics of the well terminal and a cruising route for the ships. The comparison is carried out on the basis of the cost per transported unit of gas and of the percentage of success in the transportation process. The computations show that the choice of the PV material has a key influence on the results in terms of optimal number, dimensions and speed of the ships.