Mooring-Configurations Induced Decay Motions of a Buoy

Changqing Jiang,Thomas E Schellin,Ould El Moctar

doi:10.3390/jmse9030350

Changqing Jiang, Thomas E Schellin + Show 1 more

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https://doi.org/10.3390/jmse9030350

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Abstract

The hydrodynamic damping of a buoy stationed with three different mooring configurations was estimated using a Navier-Stokes (NS) equations solver coupled with a dynamic mooring model. The mooring configurations comprised a catenary system, a catenary system with sub floaters, and a catenary system with sub floaters and clump weights. Systematic simulations were achieved by adopting the overset grid scheme instead of the conventional morphing grid scheme, which required regenerating the entire mesh when the buoy’s initial position changed, thereby avoiding mesh distortions and numerical instabilities. Motion decay simulations in heave, pitch, and surge were conducted with and without various mooring systems. The analyzed results comprised decaying oscillating motions, natural periods, and associated amounts of damping. The mooring systems influenced not only restoring force characteristics, but also total damping of the moored buoy, which demonstrated the importance of considering mooring-induced damping when investigating moored offshore structures.

Highlights

For different mooring models, using quasi-static mooring models coupled with potential flow solvers or viscous flow solvers [20], we found that the hydrodynamic damping of the moored floating structure was generally underpredicted [21]
Here, we systematically studied the effects of mooring-induced damping of different mooring configurations on decay motions for a buoy type wave energy converters (WECs)
The mooring systems’ restoring forces were similar, their influences on buoy motions and hydrodynamic damping deviated significantly

Summary

Introduction

Over the past several decades, there has been a continuous stream of innovations in ocean-based structures and systems, applied in the oil and gas area, and more recently in the floating offshore wind energy and wave energy sectors [1]. Owing to the scarcity of land, very large floating structures are being designed to cater for an increasing population in coastal areas [2]. All of these structures must maintain station; that is, their mooring systems must withstand the forces of the ocean over long asset lives in the corrosive sea–air interface, while operating under various environmental conditions. The significant nonlinearities inherent in single-point mooring systems, mainly due to hydrodynamic response and control forces as well as mooring system restoring forces, may lead to multifarious dynamic phenomena, such as self-sustained oscillations

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