Analysis of full scale structural vibrations on S.A. Agulhas II

A ship is exposed to external and internal oscillatory forces. These forces may cause structural vibrations leading to fatigue failure and reduction to the operational life of the vessel. Over the years hull condition monitoring by full scale measurements proved useful in terms of assessing ship loads and responses. This is because it allows for early detection of structural faults and can be used by ship masters in decision making. This thesis presents a preliminary analysis on acceleration data collected during full scale measurements on S.A. Agulhas II. Twelve environmental conditions were processed with the objective to observe the ship’s structural responses in a variety of contemporary or extreme environmental conditions. Frequency analysis was applied to identify engine harmonics and hull girder vibration modes. Results were compared to previous studies of the vessel and deviations were explored. To understand how slamming events affected the structure, an algorithm for detecting slamming events was developed and applied for different environmental conditions, including ice and open water navigation in wave states deemed as calm, medium and severe. Frequency and time domain analysis were carried out to investigate structural responses at different measurement points. In way of the ship bow, whipping response consisted of 2nd and 3-node vibrations. However, in the stern response was dominated by 2nd node vibration. Phase distortion between stern and bow revealed a phase lag of 0.88 sec. Parametric studies of the dynamic response in relation to different ship headings, wave heights and vessel forward speed, have shown that head-on waves give the highest acceleration levels. In other headings mitigating bow slamming, increased stern acceleration was also noticed