Browsing by Author "Bossau, J. C."

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    The detection and quantification of wave slamming from full-scale measurements on a polar vessel
    (Stellenbosch : Stellenbosch University, 2020-12) Bossau, J. C.; Bekker, Annie; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: The SA Agulhas II is a polar supply and research vessel. Her extended transom design predisposes her to stern slamming, which may lead to significant high transient whipping responses. Slamming is a concern for the fatigue life of the vessel and may lead to local damage. Slamming and whipping are investigated through full-scale measurements of ship responses, recorded concurrently with the associated vessel operational parameters and environmental conditions encountered during purposely executed open water manoeuvres. A systematic operational study was conducted accordingly, whereby the relative heading and vessel speed were regulated in consistent wave states. Firstly, slamming events need to be accurately detected from signals measured with an accelerometer array in the vessel hull. The MATLAB findpeaks function, the continuous Morlet wavelet transform and a spectrogram method were implemented. The continuous Morlet wavelet transform and findpeaks method showed the most promising slamming detection capabilities. It was determined that the proposed slamming detection methods require a threshold to identify structurally significant slams and reject slams of smaller magnitude. The peak acceleration amplitude at the time of impact is extracted and the impact site is determined. Typically, more slams are detected at the stern and the peak magnitude of the acceleration measurements is also greatest at the stern. A structural vibration analysis reveals that high velocity levels are potentially a concern for fatigue and may lead to structural damage. Operational modal analysis techniques determined the hull flexure vertical bending response magnitude resulting from wave-induced vibration. The frequency of stern slams is shown to be highest when the vessel is held on station in following and stern-quartering seas and the corresponding acceleration magnitude is largest at the stern under these conditions. It is shown that stern slamming gives rise to higher vibration velocity levels, which indicate that damage is probable for conditions associated with prevalent stern slams of larger magnitude. The frequency and magnitude of slamming incidence detected at the stern can similarly be shown to concur with the associated flexural response, as the first bending mode dominates the response when the ship encounters following and stern-quartering waves during stationary or low-speed operations. The slamming and associated vertical bending mode whipping response is reduced under these conditions as the wave encounter frequency is decreased with increasing vessel speed. Conversely, as the speed increases, more slamming and a greater bending response is observed for head and bow-quartering seas. Consequently, results indicate potential structural damage attributed to stern slamming and whipping, which is exacerbated in following seas when the vessel is stationary. The quantification of wave slamming presented in this thesis therefore warrants a further investigation into the contribution of whipping to fatigue damage in conjunction with a conventional fatigue analysis.