CFD-Based Numerical Wave Basin for Global Performance Analysis

Abstract

Several recent benchmark studies have demonstrated that Computational Fluid Dynamics (CFD) is capable of capturing both nonlinear and viscous effects in offshore marine hydrodynamics and predicting well certain wave- and current-induced offshore platform motion. In order to apply CFD for practical global performance analysis of a complete hull-mooring-riser coupled floating system, we develop an advanced numerical wave basin that combines CFD, nonlinear irregular wave modeling, and finite-element mooring modeling. Specifically, CFD is used to simulate the violent free-surface flow with hull motions; nonlinear wave modeling is applied to generate a realistic wavefield and provide initial and far-field conditions to CFD for efficient long-duration simulation; and mooring modeling is two-way coupled with CFD to account for dynamic mooring response and its effects on hull motion. In this study, to demonstrate the capability of such tool, the global performance of a semi-submersible with 4 mooring lines in a 3-hour extreme sea state is simulated for both head and quartering sea. The simulation results are compared to model test data of hull motion, mooring line tension, and relative wave elevation around the hull for validation. It is shown with spectrum and statistics that the simulations predict well the platform’s global performance in all frequency ranges, including low frequency where the mooring lines have the greatest influence on the motion response. Compared to the predictions from a conventional global performance design tool that is based on diffraction analysis and empirical coefficients, the CFD results show significant improvements. The encouraging results from this study indicate that a CFD-based numerical wave basin, although still computationally expensive, is technically ready to be a complementary tool to physical wave basin for offshore platform global performance design.

Development of a Protocol to Couple Wave and Cfd Solvers Towards Reproducible Cfd Modeling Practices for Offshore Applications

“…The one-way coupling in Star-CCM+ has been made by the Euler-Overlay Method [1,15,16]. Source terms are added to the transport equations of the volume fraction and fluid momentum.…”

“…However waves in the ocean are nonlinear and this has implications in the long-term extreme statistics relevant to the industry. Furthermore, the most popular CFD methods are nonlinear so coupling with nonlinear wave model might help to reduce the CFD domain size in avoiding meshing a zone only for the wave nonlinearities to develop [1]. Some potential wave kinematics solvers (PWKS) are able to generate nonlinear irregular waves (see e.g.…”

Linking Experimental and Numerical Wave Modelling

“…CFD methods do not have this limitation, but they are computationally expensive. For instance, the authors in [63,64] show that it is possible to model a floating platform in a three-hour wave realisation in CFD, leading to good results for the motion response functions. Long-duration wave reproduction in CFD is also discussed in the present example, with an emphasis on accurate deterministic numerical reproduction of the wave crests, considering possible unwanted basin and numerical effects.…”

Technical and Economic Readiness Review of CFD-Based Numerical Wave Basin for Offshore Floater Design

“…The coupling of potential-based numerical wave and CFD solution near the offshore structure has been validated in many recent studies (e.g., Bockmann et al, 2014;Wu et al, 2014). The coupling of mooring/riser model and CFD solution have recently been validated in global performance of Semisubmersibles and TLPs (Wu et al, 2016;Teng et al, 2016).…”

“…In addition, the good agreement of the relative wave elevation spectra from model test and CFD shown in Figure 12 also proves that MrNWB can be used for air gap prediction. More detailed information can be obtained in Wu et al (2016).…”