Structural strength of work boats and high speed crafts with pre-fabricated, floating panels in aluminum
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- Institutt for marin teknikk 
This thesis evaluates some relevant aspects related to structural integrity for work boats and high speed vessels with floating frames. This structural design is expected to significantly reduce the building cost, but it is also expected to reduce the structural strength of the vessel. A model of a modified version of the JumboCat 60 (JC60) with floating frames is used as a recurring example in the calculations. As of date, the use of floating frames in not acknowledge by the classification societies. The traditional version of the JC60 is however classified according to the DNV HSLC rules, and it is shown that the scantlings for the floating frame version exceeds the minimum requirements in the rules. Finite element models of the traditional and the floating frame version of the JC60 has been developed. These has been analysed for three different load conditions as defined in the DNV Classification Notes 30.8, namely the symmetric bottom slamming, the transverse split force, and the torsional/pitch-connecting moment load condition. These load conditions are assumed to be the most critical. From the finite element analyses, it is seen that the structural response for both models is quite similar. Only the transverse split force load condition analyses showed stresses exceeding the allowable stress levels given in the DNV HSLC rules. However, the results indicated that the hull beam of the traditional model was slightly stiffer, so it is suggested that the plate thickness for the shell plating in the floating frame version is increased at some critical regions. An increase of the longitudinal stiffener shear area is also seen necessary to reduce high shear stresses at critical areas for the floating frame version. A fatigue assessment of the longitudinal stiffener-transverse floating frame connection has been performed. Three different locations were identified as potentially critical in terms of excessive fatigue damage. Those were located at the middle of the stiffener flange at the weld toe, at the edge of the stiffener flange at the weld toe, and at the edge of the frame bottom flange. The long term distribution of stresses was approximated by a twoparameter Weibull distribution. The shape factor was set to 0.81 and the number of load cycles for 20 years of service was set to 100 million. The maximum stress range in the load history was assumed to occur for symmetric bottom slamming. Four finite element models of the structural detail were developed and analysed for determination of the maximum stress range. It was found by Miner summation that none of the locations considered would experience a critical fatigue failure for 20 years of service.