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

Abstract

Modern warships are often constructed from aluminium alloys or high tensile steel, and their increasing range of operational 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, 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. In addition, a sizable proportion of the fatigue damage accumulated at low to moderate ship speed, when the patrol boat experienced slamming, rather than at higher speeds. This may be due to voluntary and/or involuntary speed reduction, which is not typically taken into account in numerical fatigue analysis. That is, the use of long-term distributions of the wave environment and ship speed may mask the effects of voluntary and/or involuntary speed reduction on slamming occurrence and the fatigue damage. This finding can lead to improved requirements setting and through-life structural management of weight-optimised warships.