Session: 06-03-01 Fluid-Structure, Multi-body and Wave-body Interaction I

Paper Number: 100668

100668 - Nonlinear Dynamic Feature of the Fluid Resonance in a Recessing-Type Moonpool 

Moonpool is referred to as the vertical opening through the drill ships or a few floating production storages and offloading (FPSO) ships. The possible resonant phenomenon in terms of the violent motions of water waves in the moonpool is of great concern because of its huge threat to the safety of field operations and the rich physics behind it. Two main patterns of water motions in the moonpool have been demonstrated, namely, the piston-mode resonance, i.e., up and down motion like a rigid body, and the sloshing-mode resonance, i.e., back and forth motion like in the tank. Recently, the recess floor was equipped in the traditional clean moonpool to facilitate the arrangement of drilling equipment, the asymmetry due to the recess, however, strengthens the higher-order sloshing-mode motions, which cause a rather complex nonlinear coupled water motion inside the moonpool. Hence, the main focus of this work is to highlight the nonlinear dynamics of fluid resonance inside the recessing-type moonpool in the time domain based on the fully nonlinear potential flow solver.

Two-dimensional fully nonlinear numerical analyses in the time domain are systemically performed with the varied incident wave amplitudes and a wide range of the incident wave frequency covering the natural frequencies of the piston and the first-order sloshing modes. Duffing-type features with nonlinear stiffness of the amplitude-frequency responses at different incident wave amplitudes are demonstrated, which suggests that the resonant frequencies are highly dependent on the response amplitudes. In addition, the super-harmonic resonances are found around half and one-third of the first-order sloshing mode natural frequency in the recessing-type moonpool, where the higher-order harmonics rather than first-order dominate the responses, and the frequencies of higher-order harmonics are almost identical to the first-order sloshing mode natural frequency. Such a nonlinear phenomenon, however, disappears in the response of fluid oscillation within a traditional clean moonpool. The harmonic analysis illustrates that the introduction of recess leads to the energy transferring to the sloshing mode, thus exciting the sloshing motions. The joint works of the asymmetry due to recess and the free-surface nonlinearity results in the super-harmonic resonances inside the recessing-type moonpool. The evolutions of fluid oscillations in the recessing-type moonpool, being a complex nonlinear dynamic process, are further examined through the phase dynamics by performing Hilbert Transformation on the time series. Phase trapping and phase slipping states are confirmed in the responses of fluid in the recessing-type moonpool, which reflects the nonlinear evolutions of response developing from partial synchronization to de-synchronization. More results demonstrating the significance of the asymmetry and the free-surface nonlinearity in the fluid resonance within the recessing-type moonpool will be presented at the 42nd International Conference on Ocean, Offshore & Arctic Engineering (OMAE2023).

Presenting Author: Zhiwei Song Dalian University of Technology

Presenting Author Biography: Zhiwei SONG is currently a postdoc at the State Key Laboratory of Coastal and Offshore Engineering. He received his Ph.D. degree in Port, Coastal, and Offshore Engineering from the Dalian University of Technology in 2022. His research interest is the nonlinearity of fluid resonance in the narrow gap/moonpool.

Authors:

Zhiwei Song Dalian University of Technology
Guoqiang Tang Dalian University of Technology
Lin Lu Dalian University of Technology