Stress and Fatigue Analysis of a Bow-Door Arm

Most of the bow-door systems constructed today have their bow-doors attached to the hull via arms and open horizontally outwards from the ship’s sides. The arms carry the doors during opening and closing and when they are in open position. Bow door systems are designed according to the class rules, of which the IACS UR S8 rules serve as a base. Even though bow-door arms are subjected to some dynamic loads, they are dimensioned according to static loads. One ferry designed in this way obtained a large crack in the port-side arm. If the arm had cracked off completely, the bow-door would probably have been lost at sea, as the arm was a part of the locking arrangement. In order to investigate if arms, dimensioned according to a static load, can be a part of the supporting and locking arrangement of bow-doors, an analysis of a bow-door arm has been carried out. The analysis was carried out with field measurements, in the form of strain- and acceleration measurements, on an arm during the different load cases it is subjected to. The measured results were applied on a FE-Model of the arm in order to find the stress and stress ranges everywhere in the arm during the different load cases. With this information, areas subjected to large stress amplitudes were identified for which fatigue calculations were carried out. None of the load cases proved to give rise to fatigue of concern, except for the bow propellers, which, in certain situations, excite the arms into resonance cycling. The resonance gives rise to a high number of stress cycles with amplitudes large enough to cause fatigue failure