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The waves are effectively reflected against the ship side and cause a severe wave climate on windward side, which could be seen from the wave measurements presented above. This leads to large motions of the lifeboat after it enters the water and impact against the mother ship. Due to small mother ship motions the risk for impacts against the mother ship side during lowering is small. Therefore the influence of the davit parameters and launching speed is also small. The moderate and high risk in the tests on leeward side depends on drift motion of the mother ship. In the waves with a period of 5 sec the wave drift speed of the mother ship model was about 0.8 m/sec. The drift of the mother ship increase the risk for collisions against the mother ship after water entry. In longer waves (a period of 12 sec, an angular velocity of 0.5 rad/s) the ship motions are large and the wave reflection against the shipside is small and therefore the difference in risk on windward/leeward side is small. The launching point highest and close to the hull was found to be dangerous on both leeward and windward sides. An increase in davit arm length reduces the risk of impacts against the ship and large lateral accelera-tions. A decrease in launching height reduces the risk of impact with the ship. An increase of the lowering speed reduces the swing motions of the lifeboat and thereby the risk for impacts against the ship. Wave period [sec] 1 2 35 9 12 Wave height [m] - incoming wave Risk levels: -low -moderate -high Launching parameters: launching height [m]/davit arm [m]/launching speed [m/s] 16/2.5/0.7 16/2.5/1.3 16/6/0.7 16/6/1.310/2.5/0.7 10/2.5/1.3 10/6/0.7 10/6/1.316/6/0.7 16/6/1.316/2.5/0.716/2.5/1.310/6/0.7 10/6/1.310/2.5/0.710/2.5/1.3 Fig. 9: Test results for evacuation by the lifeboat A short disconnection time is important to reduce the risk of collision with the ship or risk of capsizing during disconnection. The high risk in three of the tests in 3m waves with period of 12 sec (Fig. 9) depend on that the lifeboat wasn’t released directly after water entry. This kept the lifeboat hanging from the wires and powerful impacts against the mother ship occurred. Results of the “Fall” System Tests The purpose of testing the “fall” system was to show that the suggested concept (Tsychkova and Rutgersson, 2001c) makes it possible to reduce the risks connected with launching of lifeboats. Since the influence of davit parameters and launching speed is small for the risk evaluation for short waves the “fall” system was tested mostly in the waves with a period of 12 seconds. The influence of the launching acceleration and start time of the launching on the risk for the lifeboat to impact against the mother ship was investigated. The drift of the mother ship was small in these waves and didn’t influence the risk of evacuation. In Fig. 10 the motions of the launching point are illus-trated to simplify understanding of further presented diagrams. 1234path of thelaunchingpoint Fig. 10:
Motions of the launching points During the tests of the “fall” system in waves with a height of 3 meters and a period of 12 seconds the launching acceleration was varied between 30 to 70% of the acceleration of gravity. Results of these tests are presented in Fig. 11. Each circle in the diagram repre-sents the risk level for one test. The circle is placed on the motion diagram of the launching point from Fig. 10 where the launching was started. The launching accel-eration is presented next to the circles. a) LH=16m, DA=6 m b) LH=16m, DA=2.5 mc) LH=10m, DA=6 m d) LH=10m, DA=2.5 mRisk levels: -low -moderate -high 0.3g 0.5g 0.7g 0.5g 0.5g0.5g 0.5g 0.5g 0.7g 0.7g 0.7g 0.7g 0.7g 0.5g0.5g0.5g 0.5g0.5g0.3g 0.7g 0.3g0.3g 0.3g 0.3g0.5g 0.5g 0.5g0.5g 0.5g Fig. 11: Results of the “fall” system tests in 3m waves with a period of 12 sec. LH – launching height, DA - distance between launching point and the mother ship side. Lateral and angular velocities of the lifeboat have maxi-mum values when the “mother” ship is in position 1 and 3 (Fig. 10). If the launching acceleration is too high the lifeboat continues to move in the lateral direction. If launching starts when the lifeboat is on the way towards the “mother” ship an impact against the “mother” ship can occur. If launching starts when the lifeboat is on the way from the “mother” ship the lifeboat can enter the water with a non-zero heeling angle. However, when the launching acceleration is lower the lifeboat path is adjusted by the action of wires and an impact against the ship side or lifeboat water entry with a non-zero heeling angle can be avoided. In the tests close to the mother ship side the distance between the lifeboat and the ship is small and it is diffi-cult to avoid impacts if launching starts when the life-boat is on the way towards the ship (Fig. 10, position 3; Fig. 11, b). The influence of the launching acceleration in these tests was very small (see Fig. 11,b). Decrease of the launching height and increase of the distance between the lifeboat and the mother reduce the risk for impact against the mother ship. Results of the chute tests At the tests in 1m waves the risk of evacuation by the chute system corresponds to the risk in still weather. With increasing wave height the risk increases (Fig. 12). Each circle in the diagram represents the risk level for one test. Wave period [sec] 1 源[自-751*`论/文'网·www.751com.cn/ 3Wave height [m] 5 9 12 Load condition (number of occupants on the platform/in the life raft): 0/0 25/25 50/50 50/50 25/25 0/0 - incoming wave Risk levels: -low -moderate -high Fig. 12: Test results for evacuation by the chute The wave period influences the risk in different ways on the windward and the leeward side of a drifting ship. In the longer waves (a period of 12 sec) when the motions of the “mother” ship are large and wave reflections on the ship side are small the influence of wind-ward/leeward side is small and the risk connected with evacuation is similar for the both sides (Fig. 12, waves with a period of 12 sec). The short waves (a period of 5 and 9 sec, an angular velocity of 1.3 and 0.7 rad/s respectively) cause small motions of the “mother” ship, they are reflected against the ship side and cause a difference in wave climate on windward and leeward side of the ship (see also Fig. 6). The influence of windward/leeward side is large (Fig. 12, 3m waves with a period of 5 and 9 sec). The total weight of passengers on the platform and in the life rafts has only a slight effect on the system mo-tion and thereby on the risk connected with embarka-tion. As shown in Fig. 12 the risk levels are the same for the chute system without occupants and with 100 occupants except the tests on windward side in 2m waves with a period of 5 sec. When the wave climate is hard the relative motion of the mother ship and the platform are very large. This results in that the chute was jammed between the mother ship and the platform in a lot of the tests in the waves with periods of 5 and 9 sec. Conclusions The wave climate close to a ship with no forward speed exposed to regular beam seas has been investigated. Three scenarios were examined, the first scenario had the ship soft-moored. The second scenario had the ship free to drift transversally while an additional drift force was used in the third scenario. The resulting absolute and relative wave amplitudes as well as the smoothness of the waves show no differences between the scenarios. It can therefore be concluded that it is sufficient to use a soft-mooring arrangement for these kinds of tests. Linear regular incident waves were used in all three scenarios and the resulting wave systems all remained within linear potential theory.