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Research Papers: Offshore Technology

# Benchmarking of a Computational Fluid Dynamics-Based Numerical Wave Tank for Studying Wave Load Effects on Fixed and Floating Offshore Structures

[+] Author and Article Information
Ali Nematbakhsh

Mem. ASME
Centre for Ships and Ocean Structures,
Department of Marine Technology,
Norwegian University of Science and Technology,
Trondheim NO-7491, Norway
e-mail: ali.nematbakhsh@ntnu.no

Zhen Gao

Professor
Centre for Ships and Ocean Structures;
Centre for Autonomous Marine
Operations and Systems,
Department of Marine Technology,
Norwegian University of Science and Technology
Trondheim NO-7491, Norway
e-mail: zhen.gao@ntnu.no

Torgeir Moan

Professor
Centre for Ships and Ocean Structures;
Centre for Autonomous Marine Operations
and Systems,
Department of Marine Technology,
Norwegian University of Science and Technology,
Trondheim NO-7491, Norway
e-mail: torgeir.moan@ntnu.no

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received March 14, 2016; final manuscript received November 25, 2016; published online April 5, 2017. Assoc. Editor: Yi-Hsiang Yu.

J. Offshore Mech. Arct. Eng 139(3), 031301 (Apr 05, 2017) (11 pages) Paper No: OMAE-16-1027; doi: 10.1115/1.4035475 History: Received March 14, 2016; Revised November 25, 2016

## Abstract

A computational fluid dynamics (CFD) based numerical wave tank (NWT) is developed and verified to study wave load effects on fixed and free floating offshore structures. The model is based on solving Navier–Stokes equations on a structured grid, level set method for tracking the free surface, and an immersed boundary method for studying wave–structure interaction. This paper deals with establishing and verifying a CFD-based NWT. Various concerns that arise during this establishment are discussed, namely effects of wave reflection which might affect the structure response, damping of waves in downstream, and three-dimensional (3D) effects of the waves. A method is described and verified to predict the time when incoming waves from wave generator are affected by reflecting waves from the structure which can help in better designing the dimensions of NWT. The model is then used to study sway, heave, and roll responses of a floating barge which is nonuniform in density and limited in sway direction by a spring and damper. Also, it is used to study wave loads on a fixed, large diameter, surface piercing circular cylinder. The numerical results are compared with the experimental and other numerical results, and in general very good agreement is observed in all range of studied wave frequencies. It is shown that for the studied fixed cylinder, the Morison equation leads to promising results for wavelength to diameter ratio larger than $2π$ (kD < 1), while for shorter wavelengths results in considerable over prediction of wave loads, due to simplification of wave diffraction effects.

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## Figures

Fig. 1

Sketch of the numerical wave tank used to study the barge response. The barge is hinged at the center of gravity, allowing to have roll motion in addition to sway and heave. λ is the wavelength.

Fig. 2

Wave amplitude measured at the wave generator for kB/2=0.61. Distorted waves after 53 s due to the re-reflected waves can be seen.

Fig. 3

Interaction of the barge with relatively short length waves (kB/2=0.61). The wave is traveling from left to the right side of the barge: (a) t=55.25 s and (b) t=55.75 s.

Fig. 4

Sway motion of the barge for kB/2=0.61. The sway motion is normalized by the barge breadth (B).

Fig. 5

Heave motion of the barge for kB/2=0.61. The heave response is normalized by the barge breadth (B).

Fig. 6

Roll motion of the barge for kB/2=0.61. A mean negative roll angle is observed.

Fig. 7

Sway (a), heave (b), and roll (c) RAOs of a 2D model of floating barge. Experimental data are derived from Ref. [39] and linear frequency domain results are calculated in Ref. [40]. θ is measured in radians.

Fig. 8

Sway, heave, and roll responses, and zoomed in view plot of the responses at the barge's roll natural frequency (kB/2=0.5)

Fig. 9

Sketch of the numerical wave tank used to study wave loads on a surface-piercing fixed cylinder. In all the simulations, water depth is 2.5 m, and the cylinder diameter is 1 m, corresponding to model scale test [47].

Fig. 10

Wave amplitude measured two wavelengths upstream of the cylinder at two locations as shown in Fig. 9

Fig. 11

One frame of numerical simulation for ka = 0.5 at t = 39.5 s. The zoomed view includes cross section of numerical grid points. 216×100×150 number of grid points in x, y, and z directions are used for numerical simulation.

Fig. 12

Time domain horizontal wave forces and bending moments on fixed circular cylinder. The moment is calculated with respect to the bottom of the cylinder.

Fig. 13

Comparing the CFD results for the wave-induced forces and bending moments at the bottom of fixed surface piercing cylinder, with the experimental data and potential flow theory-based results. “WAMIT,” “CFD model,” and “Morison Eq.” are calculated in the current research. “Exp” and “Potential Flow Theory” results were obtained by Ref. [47].

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