Structural strength of work boatsand high speed crafts with floatingframes

Sammanfattning: This thesis investigates the usage of floating frames in boats. A floating frame is a transverse frame fittedto the longitudinal stiffener flanges without contact with the shell plating, as opposed to the traditionalfixed frame which is welded to the shell plating with the stiffeners most commonly fitted through cut outsin the frame.To study the floating frame structure in a bigger perspective a finite element analysis is performed on amid ship compartment of an existing 60 m high speed catamaran ferry. The analysis is performed on amodel with scantlings as the original craft but with introduced floating frames. Stresses are analysed withrespect to maximum allowable stress as given in the DNV-rules for HAZ.High stresses are found in the bottom of the frames due to the reduced bending stiffness without effectiveflange from the shell plating. A large deformation in the shell plating relative the transverse frames isfound, creating high stresses in the stiffener webs. This deformation is induced by a large verticaldeformation of the frames.It is concluded that the transverse frames requires an increased stiffness to achieve acceptable stress levels.Possible solutions to increase stiffness are discussed, further studies are required to achieve an acceptablestructure.A design criterion for stiffeners in floating frame constructions is evaluated. The criterion considers theinteraction between a concentrated contact force and a bending moment with the purpose of simplifyingthe design process of stiffeners. The criterion is a combination of design methods from DNV HSLC andEurocode 9.The design criterion is found to give conservative results, although not unreasonably conservative. Thecriterion is suitable for the design of smaller work boats where the scantlings traditionally are not veryoptimized.