Page 231 - Practical Design Ships and Floating Structures

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structures at sea by the Technical Research Association of Mega-Float in Japan. The prototype
structure is a pontoon type structure with the dimension of 300m length, 60m breadth, 2m depth and
0.5m draft, and installed at real sea having the water depth of 8m. The scale ratio of the model was
1/30, with 9.75m length. A series of tank test were conducted and vertical motions and bending
moments were measured using potentiometers and strain gages, respectively in regular waves. Similar
experiments were carried out by Shin et al. (1999) using a model having the scale ratio of U42.857
with 7m length for the same prototype structure and the experimental results were compared with the
analytical ones.
In this paper, the experimental results using the model with the scale ratio of 1/20 for the same
prototype structure were summarized and, in particular, they were compared with the other existing
experimental data to investigate the response-dependence on the water depths chosen for the
experiment.

2 DESIGN AND FABRICATION OF A TEST MODEL

An 1/20 scaled model of the prototype was designed and fabricated considering the size of the wave
tank and capability of the wave maker of the Korea Research Institute of Ship and Ocean Engineering
that has the dimension of 56m length, 30m breadth and 5m depth. The model was built up by
mechanically bolting 15 units of aluminum honeycomb sandwich plates, each unit of which has the
dimension of lm(L) x 3m(B), using H-beam of the size of 3m length and cross section of 55x3 +39x3
mm. The total thickness of the model was 39mm, in which the thickness of face and bottom plates
were lmm respectively, and that of honeycomb core was 37mm. In order to get the desired draft of
25mm, 593 counter-weights, each having the weight of 1 kg, were uniformly distributed over the
surface of the model. For simulating the dolphin fender mooring system of the prototype, the mooring
device composed of horizontal bar and universal joints was also designed. The complete test model
and the mooring device are shown in the figure 1 and 2, respectively.

Figure 1 : The complete test model

For model tests, it is nearly impossible to fabricate a model to satisfy the similarity laws in both
geometry and strength. In the hydroelastic model tests, it is common to build the model to satisfy the
frequency similarity law between the exciting wave and the structure. The wave frequencies of real sea
wave and the test wave in wave tanks have the following relationship by the similarity law and wave
dispersion relations:
(f,,), = J;x(fLJ, (1)
where r is the scale ratio and the subscripts m and p denote the model and the prototype, respectively.
Therefore, the model should be built to satisfy the following relationship in the natural frequencies of