Full Scale Measurements of Fatigue and Extreme Loading Including Whipping on an 8600TEU Post Panamax Container Vessel in the Asia to Europe Trade

Abstract

Large container vessels are known to vibrate heavily in head sea storms due to its flexible hull girder, pronounced bow flare and high vessel speeds. Strong vibrations are mainly excited by bow impacts causing transient vibrations referred to as whipping. This whipping response increases both fatigue and extreme loading. Further resonance vibration by springing response is also well known to contribute to increased fatigue loading. On these large container vessels it may however not be only the vertical vibration mode that is excited but also horizontal and torsional vibrations (coupled). The current paper describes the measurement system installed on a vessel operating between East Asia and Europe. Sensors for global loading, local loading and transverse hatch opening distortions are supplemented by navigational and environmental data. The system is an extended hull monitoring system analyzing data onboard and providing decision support onboard as well as providing statistical and time series of data to shore for further assessment. The measurements confirm that the fatigue loading of critical details are dominated by the vibrations, and that the fatigue loading level in deck in a storm is higher than ever measured before, also leading to high extreme loads above IACS rule values. The full scale measurements do to some degree confirm previous model tests of the same vessel, but the real vessel has been operated at reduced speeds. So far the fatigue loading on this route is at a comfortable level, partly due to reduced speeds, but also the encountered sea states may be less severe than the route specific scatter diagram. The loading may however increase if the vessel speed is increased, and the loading may become uncomfortable high if the vessel is put on a more harsh trade like North Pacific (or North Atlantic). Also torsional and horizontal vibrations are observed, but the transverse hatch opening distortions are moderate. Stern slamming is not measured to any significant degree, but bow flare slamming is measured in the storms. Side shell fatigue loading is at a comfortable level due to the CSA-2 notation involving direct hydrodynamic load calculations in the design introducing a sufficient safety margin against fatigue cracks. Vibration is however contributing significantly also in the side shell. Only a few storms have been encountered so far. The fatigue damage is concentrated amidships and the affect of warping in front of the superstructure does not increase the fatigue loading to a level of concern.