ABSTRACT
Very large floating structure (VLFS) may sometimes be constructed in a location where the sea state is rather harsh such as along the Pacific coastline of Japan. Under such severe wave conditions, the VLFS cannot be moored safely and thus an offshore breakwater is required to reduce the wave forces impacting on the floating structure. The design wave height and period are estimated from forecasted and
observed wave data in rough weather. Several numerical wave-forecasting models are available for this purpose (e.g. The WAMDI Group 1988). For offshore wave observation, a network called NOWPHAS (Nagai and Nukata 2004) is employed for Japanese harbors. The design wave condition is categorized using the return period Tr which indicates the occurrence probability of a target wave height. If the information of the wave period is not sufficient in determining the wave-period probability, the wave period may be derived using the relationship H/L = 0.04, where H is the wave height and L is the wave length. Figure 6.1 shows examples of the arrangement of breakwaters for a VLFS moored in a water area protected by breakwaters. Takada et al. (2002) proposed a table showing the design wave heights
along the Japanese coastlines. In the paper, the design wave height and period are presented for different return periods, design tsunami heights and storm-surge deviations in each harbor area. Figure 6.2 shows an example of the design wave height distribution. Each representative offshore point is plotted in the area with a deep-water depth. Usually a breakwater is constructed in shallow water areas and thus, the offshore wave height Ho has to be replaced by the coastal design wave heightH. The coastal design wave height H is calculated from the following relation:
H = KrKdKsHo (6.1) where Kr is the wave reflection coefficient, Kd the wave diffraction coefficient and Ks the wave shoaling coefficient. Meanwhile, the equivalent
deep-water wave height H′o which includes the influence of wave reflection and diffraction in the shallow water is given by
H′o = KrKdHo (6.2)
In the design of breakwaters,H′o is employed as the offshore wave condition. Takada et al. (2002) have also incorporated the influence of wave refraction and diffraction as the coastal coefficient. The coastal coefficient in Figure 6.2 is calculated as the value at the point with the depth of 10 m as derived from the wave-energy balance method. The return period of the design wave is determined based on the impor-
tance and usage condition of the sheltered area. Usually the industrial harbor is designed for a 50-year return period. Therefore, the 50-year return period is suitable for the design of a VLFS. However, a higher return period is recommended when the value of a VLFS is very high, such as a residential complex constructed on the VLFS. In this chapter, the effectiveness of breakwaters in reducing the wave
height acting on a moored VLFS will be discussed. The different types of breakwater including the slope and vertical type are introduced in Section 6.2. In Section 6.3, the wave-action formula for estimating wave force is presented. In Section 6.4, the experimental formula for estimating the wave-transmission coefficient at the breakwater line is demonstrated where it is decreasing behind the breakwater as the height of breakwater crown increases. Section 6.5 describes an experiment for the motion of the VLFS moored behind a breakwater by considering directional waves. It is shown that the construction of the breakwater is effective in reducing the motion of the VLFS. The influence of the overtopping wave is negligible as the relative crown height hc/Hin becomes greater than 0.6.
