The return period to be assumed in a given case depends on the importance of the structure's mission, available fabrication technology, maintenance schedule, and social impact of the structure. In the present case, these considerations made it necessary to assume a return period of 50 years. The environmental loads were then computed using the conditions associated with a fifty-year design period. These conditions are shown in Table 1.
2.2 Hull construction and arrangement
The prototype of Mighty Whale is a steel floating structure with the appearance of a whale. As shown Figure 2, the hull construction has air chambers for absorbing the wave energy at the front (windward), buoyancy tanks and a stabilizer slope for reducing pitching motion in the waves.
A machine room has been installed on deck in the front part of the hull construction to house turbine generators, an auxiliary diesel generator, batteries and other assorted equipment. In the front center of the hull construction are a measurement room and control room which house measuring systems, communications equipment and the control equipment for the turbine generator system. An air conditioner is located in the measurement room to prevent room air temperature from rising in the summer as well as for dehumidification. To prevent air from being enclosed between the deck and water surface and exerting excessive pressure on the deck, holes are positioned in the deck (slope) connecting the front and rear buoyancy tanks. These holes allow air to pass freely into or out of the space under the deck in accordance with water level variations.
Design of the hull construction accounts for wave external forces like bending and torsional moment and variable underwater pressure that were determined by a 3D source distribution method for wave that become breaking waves with a wave height/wave length ratio of 1/10. The results were then used as external-force conditions in structural analysis to verify the strength and safety of the structure. Various parts of the body were coated with paint that has a service life of five years and the pattern of a whale was drawn on the front and side face of the hull. Anodic protection (with a service life of five years) was used on the underwater portion of the shell plating, in the air chambers and under the "full" mark in the buoyancy tanks. Although the service life of the hull construction is 15 years, paint with a five-year service life has been used because the open sea tests are scheduled for only a two-year period.
2.3 Turbine and generator system
The method of absorbing wave energy, which is important for wave power generation, is on a principle known as the oscillating water column principle. This principle works as follows to extract wave energy: the under water front wall of each air chamber is open to allow entry of the wave. When a wave enters the air chamber, the water surface inside it moves up and down, producing an oscillating airflow, which passes through the opening at the top of the air chamber. This airflow is used to drive the air turbine and generator.
To convert the oscillating airflow produced in the air chambers into electric energy, tandem Wells turbines and generators are mounted on each air chamber as shown in Figure 3. The Wells turbine has a symmetrical blade cross-section and always rotates in the same direction even in an oscillating airflow.
