Interdependencies between variables in fatigue analysis of a weight-optimised naval ship

Modern warships are often constructed from aluminium alloys or high tensile steel, and their increasing range of roles indicates exposure to harsh seaway loads including slamming. These factors can lead to fatigue cracking, which can reduce operational availability. The objective of the present study is to improve understanding of the influence of variables in the fatigue analysis of a weight-optimised warship. The objective is met by analysing hull monitoring data acquired from a 56 m naval aluminium patrol boat, to determine the long-term importance of slamming and the correlation between the hourly number of slams, ship speed, significant stress, and fatigue damage at two structural details. It was found that the effect of the ship’s speed on the fatigue damage is not statistically significant, which differs from the results of a numerical fatigue analysis based on the patrol boat’s long-term speed profile. In addition, a sizable proportion of the fatigue damage accumulated when the patrol boat experienced slamming at low to moderate ship speed. As such, it is suggested that the use of long-term distributions of the significant wave height and wave period as well as ship speed may mask voluntary and/or involuntary speed reduction, which affects the probability of the ship experiencing slamming and in turn the fatigue damage. This finding can lead to improved requirements setting and through-life structural management of weight-optimised warships.