Investigation of interaction between extreme waves and a moored FPSO using FNPT and CFD solvers

Abstract

To assess the survivability of marine structures, numerical tools that can predict the interaction between extreme waves and structures are needed. Considering the significant nonlinearity associated with the problem, fully nonlinear models, including the fully nonlinear potential theory (FNPT) and general viscous flow theory based on the Navier-Stokes equation (NS) and Continuity equation, are necessary for a reliable prediction. Both methods have relatively higher computational cost compared to the linear or second order wave theories, which are popular in routine design practices. Although the FNPT model generally requires less computational efforts compared to the NS model, its theoretical assumption, i.e. the flow is incompressible, irrotational and inviscid, invalidates its applications to those problems with significant viscous effects and/or breaking waves. It is, therefore, necessary to conduct a comparative study on the accuracy of the FNPT in various problems to quantify its range of application. In this paper, both the Quasi Arbitrary Lagrangian Eulerian Finite Element (QALE-FEM) method based on the FNPT model and the open source Reynolds Average Navier-Stoke (RANS) based code, OpenFOAM, are used to predict the interaction between extreme waves and a moored Floating Production Storage and Offloading (FPSO) model. The extreme waves are generated using the NewWave theory and different wave steepnesses are used. The results, including the wave runup, pressure and force on the FPSO, are compared with the corresponding experimental data obtained from the ocean basin at the COAST Laboratory, University of Plymouth. Satisfactory agreement between the numerical predictions and the experimental measurements are observed. It is also concluded that the differences between the QALE-FEM results and the OpenFOAM results are mainly caused by the effectiveness of the wave generation in the corresponding simulations; the viscous effects may be considerable in the rotational motion of the FPSO when subjected to extreme waves.

Non-hydrostatic modelling of the wave-induced response of moored floating structures

“…Perturbation-type BEM models are inherently limited to weakly nonlinear sea-states as the assumptions of the perturbation approach may be violated in extreme sea states. Fully nonlinear potential flow models (e.g., Ma and Yan, 2009;Yan et al, 2019;Hu et al, 2020), e.g. using the Finite Element Method (FEM), can also intrinsically account for nonlinear wave-structure interactions but are typically not applied in extensive coastal regions.…”

On the Efficacy of Turbulence Modelling for Sloshing

“…Finitevolume discretisation combined with the Volume of Fluid (VoF) method is available in many commercial and open-source CFD solvers and presents a common methodological approach to solving sloshing-type problems. Higuera et al [31], Ransley [32] and Hu et al [18] used VoF to study waves and their interactions with fixed and floating structures, with a good agreement with experimental data, and the successful use of VoF for sloshing problems was demonstrated in [33][34][35][36][37][38][39][40].…”

“…Hu et al [16], Yan et al [17] studied the impact load of breaking waves on coastal structures. The survivability of a moored floating production storage and offloading model was assessed by Hu et al [18]. The coupled rolling motion of a ship with a tank sloshing was studied by Lee et al [19] and Huang et al [8], showing a rolling damping effect when the tank's dimension and filling level are carefully chosen.…”

Modelling of focused wave interaction with wave energy converter models using qaleFOAM

“…Grilli et al, 2001;Ma et al, 2001Ma et al, , 2006Ma et al, , 2015Ning et al, 2008Ning et al, , 2009Stansby, 2013;Engsig-Karup et al, 2016;Wang et al, 2018). For simulating structural responses, the FNPT model can also deliver a promising accuracy if the structure is relatively big compared with the wave length (Celebi et al, 1998;Kashiwagi, 2000;Tanizawa and Minami,2001;Wu and Hu, 2004;Bai and Eatock Taylor, 2006;Yan and Ma, 2007;Ma and Yan, 2009;Hu et al 2020), due to insignificant viscous effects involved in such problems. This was further confirmed by the final report of the first CCP-WSI (Collaborative Computational Project in Wave-Structure Interaction) blind test held in ISOPE 2018 (Ransley et al 2019), in which cases with a fixed FPSO subjected to extreme wave conditions were numerically simulated using various numerical models and compared with the experimental data.…”

“…The viscous effects may also be significant when the motion of the structure is significant (e.g. Yan & Ma, 2007;Hu et al, 2020) and/or the fluid is sloshing in a confined zone (e.g. Yan and Ma et al 2019).…”