Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Whipping Loads Due to Aft Body Slamming
Whipping Loads Due to Aft Body Slamming
Pages 333-347
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From page 333... ...
It is concluded that the measured loads can also be used on a finite element model of the ships construction to predict whipping stresses. Whipping stresses are important for fatigue loads and the extreme vertical hull bending moment.
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From page 334... ...
All of these test results were within acceptable limits. The purpose of the stern slamming seakeeping model tests and hull girder structural finite element investigation was to determine if sufficient aft shape was present and if sufficient structural rigidity and strength existed to avoid the vibration and structural problems present in those ships that encountered severe slamming.
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From page 335... ...
This essentially rules out measuring the longitudinal stresses midships by means of a strain gauge system, just as a segmented stern model measuring the total impact force was not considered acceptable, as discussed earlier. However, the whipping component on the vertical bending moment is one of the important consequences of aft body slamming, so a way to measure it must be developed.
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From page 336... ...
Figure 3 Instrumented steel plate to measure vertical bending moment.
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From page 337... ...
This is illustrated in Figure 9 The impact is shown to be a moving pressure front with peak pressures of about 300 to 500 kPa in the initial stage of the impact, reducing to 100 200 kPa in the second stage. The width of the high pressure ridge is about 2.0- 3.0 m.
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From page 338... ...
Therefore, a proper spatial integration of the pressure signals requires a careful selection of the integration method. A simple and robust pressure integration technique, denoted (S)
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The direction and velocity of the moving pressure ridge is determined by the pressure-time signals at the three corner points. The velocity vector at each corner point is known, but normally only one of them actually crosses the subject panel.
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From page 340... ...
The model of the cruise vessel was subjected to an impulsive load when lying at zero speed in calm water. The results of these tests, the frequency of the wet eigen mode of the model and the damping, are presented in Table 2.
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From page 341... ...
Hydro-elastic effects are again lower, so it is concluded that, for practical purposes, they can be ignored in the problem of aft body slamming. P28rl P28fl P28rc P28fg O -100 D tr 2 3 4 Figure 17 Impact of a regular wave on the flexible and the rigid model.
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From page 342... ...
Regular following waves, amplitude 2.6 m, period 8 sec. TUNING OF THE WHIPPING MODEL The mathematical model requires as input the properties of the model and the stiffness CH and damping BH of the connecting spring.
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From page 343... ...
The slamming force shows some spikes when the simple integration method is used, but the model does not respond to such short duration, low impulse spikes. The result of the simulation shows an identical response of the model in the sense of the vertical bending moment RESULTS OF EXPERIMENTS IN WAVES Tests were carried out in following sea conditions at zero speed.
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From page 344... ...
Figure 24 through Figure 26 present the results of the simulations with the whipping model compared to the measured vertical bending moment for three cases. Figure 24 shows how a relatively small impact increases the whipping moment some 20%.
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From page 345... ...
The general good agreement between the calculations using the whipping model with the actual measured vertical bending moment shows that the whipping phenomenon is 'understood', and that the response of the model can be reliably predicted by the whipping model for individual cases. STATISTICS OF WHIPPING LOADS The previous section showed that the amplitude of the whipping moment is very much dependent on the timing of the slam relative to the existing whipping motion.
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From page 346... ...
Vessel at zero speed in following wave conditions. 700 600 500 400 300 200 Figure 28 Extreme values of the whipping part of the Vertical Bending Moment compared to the un-tuned results of the whipping model.
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From page 347... ...
It is therefore concluded that the impact force could be applied to a structural model of the ship to determine the whipping moment and peak stresses for the full scale vessel. Further conclusions are: · A vessel having a modern stern shape lying at zero or low speeds in following wave conditions can experience heavy stern slamming.
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