Springing/Whipping Response of a Large Ocean-going Vessel-Investigated by an Experimental Method

摘要

Full scale measurements on two bulk carriers by DNV show that wave induced vibrations can contribute as much to fatigue damage as wave frequency loads. Numerical calculations, using four nonlinear codes, predicted vibration levels below the measurements. The inaccurate description of the excitation mechanisms in design analyses may consequently cause expensive repairs for ship owners.rnThis paper describes an experimental method employed to investigate the wave induced vibrations in head seas by towing a large flexible model in ballast and cargo condition. Prior to the model tests, hypotheses about the possible excitation sources were established. None of these excitation sources are, at the time, accurately represented in any numerical codes. However, simplified numerical analyses are carried out to illustrate one source of the vibration, namely stem flare whipping. The possible contribution from various excitation sources is then investigated by model tests.rnHigher order harmonics are examined and transfer functions are derived by tests in regular waves, while the fatigue damage is calculated by Rainflow counting in realistic sea states. The vibration damage is compared to the wave frequency damage. The results confirm the importance of the vibrations. The second order excitation appears to be important, but also third order effects contribute. Experimental investigations of wave induced vibrations are challenging and subjected to many sources of uncertainty. Recommendations about experimental procedures to limit the uncertainties are given.