Structural Safety Analysis of Bow-Doors

Licentiate thesis, 2004

Bow-doors are one of the most vulnerable parts on a RoRo-vessel. At the same time, they are necessary for the profitability of today’s RoRo-vessels. The minimum design rules for bow-door systems, stated in the International Association of Classification Societies (IACS) S8, constitute the basis for the class rules. In this work, the rules of IACS S8 have been assessed regarding the structural integrity for bow-doors of the “clam door” type. The work is based on previous damages, compared to the design rules, so as to find weaknesses in the rules. Damages were mapped and weak points in the rules as well as in the design and construction stages were identified. For the found problem areas part analyses were performed. Fractures and cracks dominate as the cause of damage for bow-door systems. In most of these cases cracks in welds are involved. Fatigue proved to be a major cause for the development of fractures and cracks. Assuming the bow-door as a rigid-body and distributing the forces equally on each support, as prescribed by IACS S8, proved to be an approach that leads to inadequate results. The deformation of the door causes the loads on the supports to differ significantly and the real pressure distribution subjects the lower supports to much higher loads then the upper ones. It is common with gaps between the supports in the doors and the corresponding ones in the hull. These gaps will considerably affect the load distribution on the supports. In certain situations, bow propellers may excite bow-door arms into resonance cycling. This would give rise to a large number of high stress cycles and might even cause fatigue failure.