The Crack Resolution Required to Ensure Fatigue Life of a Rescue Submarine’s Transfer Skirt

Transfer skirt (TS) is a structure with spherical asymmetric sectors, which is connected to a rescue submarine and used to transfer crew between two submarines in the depth of the sea. When this structure is connected to the disabled submarine in deep, it is under violent stresses that when repeated, cause fatigue damage at sensitive regions such as welded joints. In this article, based on the Paris crack growth equation and fracture mechanics criteria in BS 7910 standard, a code is developed, by which the fatigue life of the transfer skirt structure containing various initial crack sizes is evaluated. Stress analysis is performed in order to acquire the stress distribution and to determine the stress concentration spots at the welded joints. At last, regarding the relationship between crack length and prospected life under various conditions, the minimum resolution required for examination instruments to identify defects in the welds is resolved. It seems that a permissible size of an initial crack in the welds of a transfer skirt enduring 50000 fatigue cycles is the utmost 0.65mm.