Fatigue damage assessments and the influence of wave directionality

A fatigue assessment is a complex and time-consuming process with an uncertain outcome. The result is sensitive to comparatively small differences in the structure (globally or locally) or the loading environment. The predominant cause of fatigue damage for marine structures is usually wave action. It is then often asked what the beneficial effect of wave directionality is. There are two sources of wave directionality, i.e. firstly, the variable direction of uni-directional seas, and secondly, wave spreading around a given mean direction. These two effects need to be distinguished and considered separately. The influence of wave directionality is not easily predictable and its quantification normally requires extensive and detailed analysis by numerical calculations. However, under two general restrictions the influence can be rather easily determined, i.e. for a rotationally symmetric structure (a vertical circular cylinder) and for wave conditions that are of the same severity in all directions. The paper investigates this special problem and presents quantitative results for various cases.