Modeling of fatigue in RORO ships

Sammanfattning: The largest modern Pure Car and Truck Carriers (PCTC’s) are typically 230 meters long and have 13 cargo decks. In order to facilitate rapid loading and unloading these ships have been subject to a development of reducing any obstructing structures in the cargo hold, meaning that the transversal shear preventing structures, i.e. the racking bulkheads, has been taken to a minimum. Previous studies have concluded that some points on the racking bulkheads, as a result of the stripped down design, are subject to high stresses resulting from wave induced accelerations of the ship. In this M.Sc. Thesis the fatigue life of a corner of a transverse bulkhead opening in a 230 meter long PCTC with a capacity of 7200 cars is calculated with different methods. •Fatigue life is calculated from recorded ship motion data with the notch stress method in conjunction with rain flow counting and the cumulative damage principal. • Fatigue life is calculated according to (DNV CN. 30.7, 2010), based on a Lloyd’s Register FE model load case. • Actual findings on the ship are compared to the calculated results. Due to the lack of inspection data this comparison is not very extensive and only more briefly discussed. It is concluded that the fatigue life of the examined point, calculated from recorded motion data is 9.6 years and the fatigue life according to DNV is 8.0 years. It is also found that the fatigue damage is cumulated in almost discrete portions and thus the calculated fatigue life can be inaccurate when a short period of time is evaluated as is done in this thesis. A modification to the racking bulkhead with respect to fatigue life is also analyzed and it is concluded that the fatigue life in the examined point could be extended significantly by some simple modifications to the geometry.