The use of specialist experimental techniques, such as OMA, can aid problem resolution after a ship has entered service and validate computational techniques. An illustration of this is given with two case studies that the authors dealt with recently. Case Study 1 – Cracking of tank bulkheads as a result of propeller excitation Cracking of water tank bulkheads of an LNG carrier occurred 1 year after delivery of the vessel. Despite continuous repairs and modifications to these bulkheads, cracking persisted for several years in service. The tanks were located directly above the propeller, as shown in Figure 1, and contained distilled water, for use in a steam turbine, and fresh water, for human consumption. Hence contamination could have serious consequences for staff and machinery. Panel vibration was measured at the centre of the affected panels and midway between stiffeners. Typical overall velocity amplitudes of two tank bulkheads are shown in Figure 2. Vibration levels had typical maximum amplitudes of around 200 mm/s and thus exceeded 30 mm/s, the upper limit for safe panel vibration in Reference 3, from relatively low speeds and powers. It was therefore likely that cracking had occurred as a result of vibration and in order to assess how much excitation and response contributed to the issue both were investigated. Water tanks propeller excitation aft peak tank Aft Forward 0 50 100 150 200 250 55 60 65 70 75 80 85 shaft speed [rpm] vibration amplitude [mm/s] 0 5 10 15 20 25 hull pressure [kPa] overall hull pressure amplitude overall vibration amplitude (bulkhead 1 & 2) 30 mm/s limit Fig. 1 Tanker aft body showing the location of water tanks and the propeller Fig. 2 Hull pressure amplitudes and vibration amplitudes of two tank bulkheads As discussed previously, machinery is usually the principle source of vibration excitation on board a vessel. The main engine of this ship was a steam turbine which, in general, does not cause a lot of vibration excitation and, therefore, the propeller was the most likely source of excitation. Propeller radiated hull pressures were measured and considered high, with overall pressure amplitudes exceeding 15 kPa (Figure 2). A typical time series of hull pressure excitation during one blade passage is shown in Figure 3. There is an underlying blade rate pressure fluctuation resulting from the non-cavitating pressure field around the blade. Superimposed on this is the contribution of the pressure caused by cavitation. This pressure consists of a cavitation growth phase as the blade enters the wake peak, possibly causing a gradual reduction in pressure, which is followed by a high pressure peak resulting from the sudden collapse of the main body of sheet cavitation. The three secondary pressure pulses have been observed on other vessels and could be related to tip vortex activity. 282
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