ABSTRACT

The paper presents a numerical investigation of the wave-structure interaction using a hybrid model, qaleFOAM, which combines a two-phase Navier-Stokes model (NS) and the fully nonlinear potential theory (FNPT) using the spatially hierarchical approach. The former governs a limited computational domain (NS domain) around the structures, where the viscous effects may be significant, and is solved by using OpenFOAM/InterDyMFoam with a modified solver for the six degrees-of-freedom (6DoF) motions of rigid bodies. The latter covers the rest of the domain (FNPT domain) and is solved by using the Quasi Lagrangian Eulerian Finite Element Method (QALE-FEM). In the numerical simulation, the incident wave is generated in the FNPT domain using a self-correction wavemaker and propagates into the NS domain through the coupling boundaries (inlet of the NS domain). An improved passive wave absorber is imposed on the outlet of the NS domain for the wave absorption, whereas a new module for solving six-degree-of-freedom structural motions is also applied. The accuracy of the qaleFOAM on modelling wave-structure interaction is preliminarily investigated in terms of both the wave propagation and the responses of the structures. Finally, the qaleFOAM is applied to the CCP-WSI Blind Test and some results are presented for demonstrations.

INTRODUCTION

A reliable prediction of the responses of the offshore structures in a realistic extreme sea plays a fundamental role in the safe and cost-effective design of coastal and offshore structures, and marine renewable devices. Numerous numerical models and software have been developed based on wide ranges of theoretical models.

Aiming to quantify the accuracies, efficiencies and reliabilities of different models in their practical applications, a CCP-WSI blind test workshop has been organized in ISOPE 2018 (Ransley, et al., 2019; Yan et al, 2019). In the workshop, the case with a fixed FPSO subjected to extreme wave conditions was numerically simulated using various numerical models, ranging from the fully nonlinear potential theory (FNPT) to Navier-Stokes (NS) theories, which are solved by different numerical methods, including conventional mesh-based methods, e.g. finite element, finite volume methods. Both single-phase and multiphase models with/without considering turbulence modelling have been attempted. Due to the fact that the experimental data was released after the participators submitted their numerical predictions, minimizing the possibility of numerical calibrations or tuning. Thus, the performances of the participated numerical models can largely reflect their reliabilities in practices. One conclusion is that the accuracies of the FNPT models, such as the Quasi Lagrangian Eulerian Finite Element Method (QALE-FEM, Ma and Yan, 2006, 2009; Yan and Ma, 2007), is at the similar level as the NS models (Ransley, et al., 2019; Yan et al, 2019). Primarily, this is because that, in these cases, the size of the FPSO model is relatively large with reference to the significant wave length (relative size), and therefore, the viscous effect is insignificant.

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