ABSTRACT
It is common practice to conduct a model experiment in a water basin in order to investigate the behavior of a vessel or a marine structure in waves. Although the accuracy of estimating the dynamic characteristics of a vessel or a marine structure by the theoretical analysis and numerical simulation has been improved based on the recent development of electronic computers, the model experiments of vessels and marine structures have been conducted over 100 years and new phenomena are often discovered by such model experiments even in these modern times. In particular, for a very large floating structure (VLFS) that has never been built, even if the reliability of the theoretical analysis is very high, it is crucial to validate the theoretical analysis or investigate the behavior by a model experiment of VLFS (see Figure 7.1). Basically, there are three types of objectives in conducting model exper-
iments of floating structures (SNAJ 1997). First to validate the result of the theoretical analysis; that is to assess how correct the analysis is. The oscillation-response experiment of VLFS in regular waves of small wave height falls in this objective category. Second, to investigate matters that
are difficult to estimate theoretically or analyze. The investigation of fluid forces due to the fluid viscosity or the study of the nonlinear effects of wave heights to the behavior of a VLFS in large wave heights is in this category. Third to investigate the overall performance characteristics of a VLFS in a realistic situation as far as possible. By assuming that VLFS is moored to several mooring devices, the experiment to show how the VLFS will behave in multidirectional irregular waves falls into this category. The construction of a VLFS requires that the floating unit modules be joined together on the ocean. A model experiment of a VLFS on the joining operation of multifloating modules falls into this category. Though model experiments of a VLFS are very important, there are pecu-
liar problems in these model experiments. The first problem is the fact that an actual VLFS is, by definition, “very large.” In order to conduct a model experiment of a large floating structure in an available water basin, it is necessary to reduce the scale of the geometrically similar model of a large floating structure to an extraordinarily small scale. This may cause a problem in the generation capability of a water basin (i.e., how to accurately generate short waves) and the accuracy of measuring the behavior of the model. The second problem is that the bending rigidity in the vertical direction is
relatively small and that the response and deformation of a VLFS as an elastic body are quite large in the vertical direction. This is because the dimensions in the vertical direction of VLFS are extremely small compared to the ones in the horizontal directions as mentioned before. For an ordinary vessel, its deformation as an elastic body does not pose many problems. However, for VLFS, this “hydroelastic problem” is quite important and it should be taken into consideration for the estimation of a VLFS’s behavior. In order to examine these phenomena in the model experiment, it is necessary to make the rigidity of the model similar to the rigidity of the actual VLFS. However, it is very difficult to make the rigidity of a small-scale model similar in order to meet the law of similarity. This chapter describes how to conduct an accurate model experiment by
overcoming these problems and then introduce some examples of model experiments of VLFS.
