ABSTRACT:

Model test experiments were conducted in towing tank to measure and analyze the resonance ranges due to flow induced oscillations for circular shaped moonpool. Data analysis was conducted using ‘Empirical Mode Decomposition Method’. The natural frequency of water governs the water oscillations inside the moonpool which remains the same (0.7 Hz) from Froude number, 0.26~1.3 in calm water with forward model speed condition, while it changes in flow and waves condition together at 2 s and 3 s time period. Piston phenomenon was observed between Froude number, 0.26~0.39. Hydrodynamic noise is distinguished from structural noise and relationship between St and Fr number is presented. Noise generated is highest (154.14 dB) at beginning of resonance in circular moonpool, while it is lowest between Froude number, 0.52~0.63 Analysis reveal that sloshing motion in the moonpool couples with the piston mode in waves and flow condition together which are very important factors for safe operation and design.

INTRODUCTION

Water motion mostly occur at the natural modes of the moonpool namely; the sloshing modes, back and forth in-between the vertical walls (like in a tank), and the piston (or pumping or Helmholtz) mode, where the water inside the moonpool heaves up and down more or less like a rigid body. Water motions in the moonpool can be excited through different mechanisms, in waves or in calm water with forward speed of the vessel. In this paper ‘submerged cavity or opening with free surface’ normally termed as moonpool; is hereafter also called a cavity. It is seen that the natural frequency of water inside the cavity does not change with a change in the velocity rather it is dependent upon the physical properties of the fluid medium, shape of cavity, size of cavity and the draught of water inside the cavity.

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