Identification of the two-degree-of-freedom system for heave motion responses of twin hulls with a moonpool

The heave motion response of the hulls is significantly affected by the fluid resonance in the moonpool, where the two-peak variation with the incident wave frequency is the most important phenomenon. Theoretical analysis based on the potential flow model is developed in the present work to identify the physics of the two-peak phenomenon. A conceptual model is also established to reveal the essential mechanics of the two peaks. It is disclosed that the heave motion response and free surface oscillation are essentially a two-degree-of-freedom system with the main and attached spring oscillators. The two peaks are the first and second resonant frequencies of the system with the in-phase and out-of-phase resonant modes, respectively. A new approach to the free decay test is also proposed to determine the resonant frequencies and resonant modes.