Experimental Study on Coupled Motions of a Spar-Buoy Under Mathieu Instability

A series of experiments were carried out to investigate the occurrence of the Mathieu-type instability. The main objective of this study is utilization of an auto-parametrically excited oscillation for wave energy converters. In this paper, the subject is the auto-parametrically excited oscillation of a spar-buoy type point absorber with two degrees of freedom. A small spar buoy model with a ballast controlling system was made and the model experiments were conducted to realize the large oscillating motion based on the Mathieu-type instability. The ballast controlling system is installed in the buoy model and the vertical movement of the ballast produces a certain change of the pitching natural period. Using the controlling system, the pitching motion in regular waves under the heave resonant period was measured. In some experiments, it was observed that the large pitching motion occurred suddenly, and the time histories showed different excitation pattern from the theoretical Mathieu-type instability. Based on the model experiments and considerations of the theory of Mathieu-type instability, the occurrence of the large pitching motion is discussed.

Experimental Study on Dynamic Control of Oscillation Characteristics of a Spar-Buoy

Experimental studies were carried out to investigate the possibility of utilization of auto-parametrically excited oscillation based on the Mathieu instability. For the simplicity, the subject is limited to the heaving motion of a spar-buoy type point absorber. The device consists of an inner cylinder and 12 outer cylinders. Each outer cylinder is equipped a movable floating column controlled by a ball screw mechanism and the buoyancy of the outer cylinders can be dynamically changed to induce the Mathieu instability. Based on results of the numerical simulation and the model tests, the possibility of utilization of auto-parametrically excited oscillation for WEC was shown.

Optimization Method for Oscillation Characteristics of a Spar-Buoy

Numerical studies are carried out to investigate the effectiveness of an optimizing method of oscillation characteristics. The main objective of this study is utilization of the auto-parametrically excited oscillation of single degree of freedom. The subject is limited to the motion of a spar-buoy type point absorber. The device consists of an inner cylinder and 12 outer cylinders. Buoyancy of the outer cylinders is controlled by axial-sliding valves in order to change the heaving period. Model tests were also conducted to investigate the basic characteristics. Based on the numerical simulations in time domain, the possibility of utilization of auto-parametrically excited oscillation for WEC is shown.