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Safety and Reliability

Analysis of Submarine Pipeline Scour Using Large-Eddy Simulation of Dense Particle-Liquid Flows

[+] Author and Article Information
Piroz Zamankhan

Department of Mechanical Engineering, University of Kurdistan, P.O. Box 416, Sanandaj, Iran 66177-15175

J. Offshore Mech. Arct. Eng 131(2), 021604 (Mar 30, 2009) (12 pages) doi:10.1115/1.3058705 History: Received March 27, 2008; Revised October 25, 2008; Published March 30, 2009

Using large-eddy simulation technique for dense particle-fluid flows, the current-induced scour is predicted for both the mono- and bidispersed systems below a horizontal submarine pipeline exposed to unidirectional flow. The simulations are four-way coupled, which implies that both solid-liquid and solid-solid interactions are taken into account. Particles are assumed to behave as viscoelastic solids during interactions with their neighboring particles, and their motion are predicted by a Lagrangian method. The interparticle normal and tangential contact forces between particles are calculated using a generalized Hertzian model. The other forces on a particle that are taken into account include gravitational pressure gradient force accounting for the acceleration of the displaced liquid, the drag force resulting from velocity difference with the surrounding liquid, and the Magnus and Saffman lift forces. The predicted scour profiles for monodispersed system are found to compare favorably with the laboratory observations. For the bidispersed system, a seepage flow underneath the pipe (which is a major factor to cause the onset of scour below the pipeline) is found to be weakened using an appropriate size for the sand bed. This fiffnding highlights the importance of the bed particle size distribution on the onset of scour below the pipelines.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

A pipeline laid on the seabed. Inset: The seabed is partially liquefied, and the pipeline floats on the surface of the seabed.

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Figure 2

(a) The old Zamankhan bridge across Zandehrud river (Iran) sitting on andezitic volcanic rock masses. The bridge is about 7 km north of Saman. The left abutment of the bridge is magnified for providing a better visualization. An ingenious design was used in the old Zamankhan bridge for soil stabilization. (b) Scour development around a pier. (c) Scour mechanism.

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Figure 3

Binary collision of monodispersed pairs. (a) Two monosized colliding rough spheres with diameter 2 mm at the beginning of the approach period. (b) The colliding pair in (a) at the end of restitution period. (c) Contours of the local velocity in a cutaway view of particles. (d) Contours of the normal stress at the maximum approach in a cutting yz plane passing through the centers of the particles. (e) Contour plot of the shear stress at the maximum approach in the same plane as in (d). (f) The dimensionless velocity of particles in the x-direction as a function of time. (g) The dimensionless tangential velocity of particles as a function of time. (h) The normalized angular velocity of particles in the z-direction as a function of time.

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Figure 4

Collisions in polydispersed aggregates. (a) The grid. The diameter of ball1, ball2 ball3, ball4, and ball5 is 100 μm, and the diameter of ball6 and ball7 is 1 mm. (b) A perspective view of particles at the end of collisions. The particles are color coded with their local velocities. (c) Contours of the local velocity in a perspective view at t=1.5×10−6 s. (d) Cutaway view of (c). (e) Computed dimensionless velocities of particles in the x-direction as a function of time using FEM. (f) Predictions of the simplified model for the dimensionless velocities of particles in the x-direction as a function of time. (g) The normalized angular velocity of particles in the z-direction as a function of time for the third particle. The solid line represents the model predictions.

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Figure 5

(a) Definition sketch of the pipe position and the original plane sand bed. V0 represents a current relevant to offshore condition. The initial gap between the pipe and the plane bed is e=s−D/2. (b) Periodic boundary conditions in the z-direction. The bottom walls are denoted as gray in color. (c) Side view and some dimensions of flume.

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Figure 6

(a) Perspective view of an instantaneous configuration of sand particles at t=500 s. Here, the whole computational domain is shown. (b) The area beneath the pipe is magnified to provide a better visualization. (c) Contractionlike scour near the corner at the upstream entrance. (d) Contours of volume averaged solids fraction in the whole computational domain.

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Figure 7

Computed velocity field of water at t=500 s. (a) Vector plot of dimensionless velocity V/V0 around the pipe. Here, V0=0.35 m/s. (b) The region downstream the pipe is magnified to provide a better resolution. (c) Contour plot of dimensionless velocity on a yz-plane.

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Figure 8

The contours of the averaged solids fraction in different stages of the scour development: (a) t=0 s, (b) t=50 s, (c) t=90 s, (d)t=120 s, (e) t=500 s, and (f) t=1500 s

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Figure 9

(a) Perspective view of an instantaneous configuration of mixture of fine and coarse sand particles at t=500 s. Here, the whole computational domain is shown. The size of fine and coarse particles are dp(1)=100 μm and dp(2)=560 μm, respectively. (b) The area beneath the pipe is magnified to provide a better visualization. (c) Contractionlike scour, which is formed near the corner at the upstream entrance.

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Figure 10

(a) Contours of volume averaged solids fraction of fine particles in the whole computational domain at t=900 s. (b) Contours of volume averaged solids fraction of coarse particles at t=900 s. Here, the flow is from right to left. (c) Contours of volume averaged solid fraction of fine and coarse particles at t=900 s. (d) Side view of (a). (e) Side view of (b).

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