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Research Papers: Offshore Geotechnics

Numerical Study of Seabed Boundary Layer Flow Around Monopile and Gravity-Based Wind Turbine Foundations

[+] Author and Article Information
Muk Chen Ong

Department of Mechanical and Structural
Engineering and Materials Science,
University of Stavanger,
Stavanger 4036, Norway
e-mail: muk.c.ong@uis.no

Eirik Trygsland

Department of Marine Technology,
Norwegian University of Science and
Technology,
Trondheim 7491, Norway
e-mail: eirik_trygsland@hotmail.com

Dag Myrhaug

Department of Marine Technology,
Norwegian University of Science
and Technology,
Trondheim 7491, Norway
e-mail: dag.myrhaug@ntnu.no

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 17, 2016; final manuscript received March 2, 2017; published online May 5, 2017. Assoc. Editor: David R. Fuhrman.

J. Offshore Mech. Arct. Eng 139(4), 042001 (May 05, 2017) (7 pages) Paper No: OMAE-16-1029; doi: 10.1115/1.4036208 History: Received March 17, 2016; Revised March 02, 2017

Computational fluid dynamics (CFD) has been used to study the seabed boundary layer flow around monopile and gravity-based offshore wind turbine foundations. The gravity-based foundation has a hexagonal bottom slab (bottom part). The objective of the present study is to investigate the formation of horseshoe vortex and flow structures around two different bottom-fixed offshore wind turbine foundations in order to provide an assessment of potential scour for engineering design. Three-dimensional CFD simulations have been performed using Spalart–Allmaras delayed detached eddy simulation (SADDES) at a Reynolds number 4 × 106 based on the freestream velocity and the diameter of the monopile foundation, D. A seabed boundary layer flow with a boundary layer thickness D is assumed for all the simulations. Vortical structures, time-averaged results of velocity distributions and bed shear stresses are computed. The numerical results are discussed by studying the difference in flows around the monopile and the gravity-based foundations. A distinct horseshoe vortex is found in front of the monopile foundation. Two small horseshoe vortices are found in front of the hexagonal gravity-based foundation, i.e., one is on the top of the bottom slab and one is near the seabed in front of the bottom slab. The horseshoe vortex size for the hexagonal gravity-based foundation is found to be smaller than that for the monopile foundation. The effects of different foundation geometries on destroying the formation of horseshoe vortices (which is the main cause of scour problems) are discussed.

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Figures

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Fig. 1

Geometry of the monopile foundation

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Fig. 2

Geometry of the hexagonal gravity-based foundation

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Fig. 3

The size of the computational domain and the boundary conditions

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Fig. 4

Three-dimensional views of the mesh on the surface of the monopile and hexagonal gravity-based foundations

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Fig. 5

Cx,mean for the monopile foundation versus the number of elements

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Fig. 6

Cx,mean for the hexagonal gravity-based foundation versus the number of elements

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Fig. 7

Cp for the monopile foundation and the hexagonal gravity-based foundation in the xy plane at z/D = 0.1 (left) and z/D = 0.5 (right)

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Fig. 8

Three-dimensional and 2D top views of instantaneous vortical structures around the monopile foundation Q = 5 s−2

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Fig. 9

Three-dimensional view of instantaneous vortical structures around the hexagonal gravity-based foundation. Q = 5 s−2.

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Fig. 10

Time-averaged nondimensional vertical velocity contour in the xy plane at y/D = 0, for the monopile foundation (top) and the hexagonal gravity-based foundation (bottom)

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Fig. 11

Comparison of normalized pressure coefficient (Cp,bed/Cp,bed,toe) along y/D = 0, z/D = 0 and x/D = −2 to −0.5 between the present numerical results (Re = 4 × 106, δ/D = 1), the numerical results by Roulund et al. [4] (Re = 1.8 × 105, δ/D = 1) and the experimental data by Dargahi [5] (Re = 3.9 × 104, δ/D = 4/3)

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Fig. 12

Normalized time-averaged bed shear stress along the centerline y/D = 0 and z/D = 0 for the monopile foundation

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Fig. 13

Normalized time-averaged bed shear stress along the centerline y/D = 0 and z/D = 0 for the hexagonal gravity-based foundation

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Fig. 14

Contours of time-averaged bed shear stress amplification |τ|/τ around: (a) the monopile foundation and (b) the hexagonal gravity-based foundation

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