Skadeanalys och omkonstruktion av HMS Belos A-ram

Sammanfattning: In order to rescue a distressed submarine crew, the Swedish Navy is able to use a deep submergence rescue vehicle, DSRV. This mini submarine ascends, docks to the downed submarine and picks up the whole crew. The submarine rescue ship HMS Belos has a special crane located in the aft of the ship to lift the surfaced DSRV aboard, which then docks on to the on board pressure chambers located below deck. During inspections the aforementioned crane has shown to have problems with cracks in the structure, most of them located in the bolted joint between the top part of the crane and the two leg parts. A simulation of the forces acting on the structure in heavy sea condition was conducted; these values were then used as input values in a Finite Element Method, FEM, analysis of the joint. Results show that the crane construction is not made to withstand the stresses that occur when the ship is in high sea. The cyclic nature of the stresses in the crane structure, due to the roll motion of the ship, causes material fatigue with crack initiation and crack growth. This can be mended by a relatively easy modification of the bolted joint. The modification, which stiffens the flange, contributes to lower stress concentrations around the previous problem areas, spreading the load more evenly.