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Windward side Leeward side Meters measuring absolute wave amplitudes Meters measuring relative amplitudes Meter measuring incident wave amplitude Fig. 4: Placing of meters for wave measurements. The absolute wave amplitudes were recorded at fixed distances from the ship at all times. This was achieved by mounting rods to the carriage and attaching wave meters to these rods. Six meters were placed on the windward side and two on the leeward side. The dis-tances from the ships hull to the meters on the windward side were 5, 20, 25, 30, 35 and 40 meters in full-scale and 5 and 10 meters on the leeward side. In addition one meter was placed far upstream and aft of the ship to measure the incident waves, see Fig. 4. The placement of this meter was chosen so it should not be affected by reflected waves. The test setup for the wave measure-ments can also be seen in Fig. 5. Fig. 5: Test setup for wave measurements. Instrumentation The instrumentation used for the ship motions, wave measurements and evacuating equipment are described below: for ship motions: • • • • • • − optical tracking system for the mother ship mo-tions; − video recording equipment. for wave measurements: − 13 resistive wave probes; − video recording equipment. for lifeboats launching: − optical tracking system for the lifeboat mo-tions; − 3 accelerometers, which were placed in the centre of gravity of the lifeboats. The sampling rate was 1 kHz. − video recording equipment. for the chute system: − optical tracking system for measurements of the chute length; − video recording equipment. Test Program Test Program when Studying the Wave Climate The test program designed to study the wave climate close to a ship in beam seas consisted of three different scenarios. The scenarios were: • the ship was soft-moored; • the ship was free to drift transversally; • the ship was forced to drift transversally with an additional drift force. Since the tests were performed in a wave tank, the in-fluence of environmental factors such as wind was ex-cluded. Therefore, in the third scenario, an extra drift force was added to resemble the extra velocity a full-scale ship experiences due to wind. The following parameters were varied in all three sce-narios: incident wave amplitude; incident wave length. The model was exposed to beam seas consisting of regular incident waves with different angular velocities. The angular velocities of the incident waves varied from 0.3 to 1.4 rad/s in full-scale, which corresponds to wave periods between 20 and 4.5 seconds.
These waves were chosen since the zero crossing periods for these waves (except the longest wave) are representative for the waves most commonly found in the Atlantic Ocean and Baltic Sea (Hogben, Dacunha and Olliver, 1986). The incident wave amplitudes varied between 0.5 and 2 meters. The relation in percent between the incident wave amplitude and the incident wave length was lower than 2 percent for these tests. This indicates that the waves have properties according to linear potential theory, which implies that waves can be linearly super-imposed. Therefore the results from the tests could be made non-dimensional by piding the measurements with the incident wave amplitude to simplify compari-son between tests. Test program for Lifeboats Launching In these tests the ship model was free to drift transver-sally in regular beam seas. About 40 tests of the conventional lifeboat were carried out. The following parameters were varied (all parame-ters are presented in full-scale): • • • • • • • • • • • • • • wave height (1 - 3 meters) wave period (5, 9 and 12 seconds) launching side (windward- and leeward side) launching speed (0.7 and 1.3 m/sec) release time About 50 tests of the “fall” lifeboat were carried out. The following parameters were varied (all parameters are presented in full-scale): wave height (1 - 3 meters) wave period (5, 9 and 12 seconds) launching side (windward- and leewardside) launching acceleration (0.3, 0.5 and 0.7 g) release time Tests program for Chute System In these tests the ship model was free to drift transver-sally in regular beam seas. About 40 tests of the chute system were carried out. The following parameters were varied: wave height (1 - 3 meters) wave period (5, 9 and 12 seconds) launching side (windward- and leeward side) load condition – number of occupants on the plat-form / in the life raft (0/0, 25/25 and 50/50). Evaluation of Risk When examining the tests with evacuating equipment the most important result is the level of risk that the passengers are exposed to when using these equipment. Therefore this section describes how the evaluation of risk is computed for the lifeboat, the “fall” system and the chute system. Risk Evaluation for the Lifeboat The following risks connected with evacuation by the lifeboat system were detected: • impacts against the ”mother” ship during lowering due to lifeboat swing motion; • impacts against the ”mother” ship after water entry due to severe wave climate close to the hull; • impacts against the ”mother” ship due to late re-lease; • potential hazard that the lifeboat will be caught on the some part of the “mother” ship (e.g. fender). The risk estimation is based on measurements, video recordings of the behaviour of the equipment and as-sumptions about human tolerance. By analogy with previous tests evaluation (Tsychkova and Rutgersson, 2001a) the estimated risk, connected with evacuation, has been classified in three different categories: low, moderate and high risk (Table 2). The classification was based on acceleration limits for free-fall boats and some assumptions about human tolerance. The impact tests of the model lifeboat were carried out to compare accelera-tions at collision against the mother ship with full-scale impact tests. Research into human injuries is constantly in progress; when new knowledge is available the esti-mation of the risk can be re-evaluated. Table 2: Limits for risk estimation of tests of the life-boat/davit system corresponding to the full-scale lifeboat Risk level Events connected with risk Low Moderate High Acceleration limits. Co-ordinate axis Y and Z [g] ≤6 6-9 ≥9 Max roll angle [deg] ≤ 30 30-70 ≥ 70 It is assumed that the probability of human injures is about 50% for the cases with high risk, 5% for the mod-erate risk and 0,5% for the low risk cases. Because the lifeboat launch starts at random time rela-tive to the ship motions there is some uncertainty in the results of lifeboat impacts and measured accelerations during launching. Due to the limited number of tests, the stochastic influence is hidden in the results. Risk Evaluation for the “Fall” System The evaluation of risk of the “fall” system is based on measurements and video recordings. In a similar way as for the lifeboat the estimated risk for the “fall” system has been classified in three different categories: low, moderate and high risk. In Table 3 the acceptable roll angle and acceleration limits used for estimation of evacuation risk with the “fall” system are presented. For the roll angle of the lifeboat only two different risk levels are suggested. Moderate risk level means that 5% of passengers will be injured when the maximum roll angle of the lifeboat is about 50 degrees or more.