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# Offshore Pipelines Considering Hydrodynamic Forces

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The present issue simulates the flow of seawater around offshore pipelines to investigate lift and drag forces.

This ANSYS Fluent project includes CFD simulation files and a training movie.

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## Offshore Pipeline Project Description

The present issue simulates the flow of seawater around offshore pipelines. The wave motion of the seawater on the pipelines creates drag and lift forces. Therefore, the location of these transmission pipelines must be in optimal condition to withstand less hydrodynamic forces. One of the geometric parameters measured in the present problem is the ratio of the distance from the pipe floor to the seafloor. Given that the movement of seawater is wavy, it is necessary to define the input velocity as an equation of a wave flow using the UDF.

Also, since the pressure inside the seawater has a pressure relative to the atmospheric pressure, and the pressure changes due to the wave motion of the seawater, the equation of the ambient pressure in the wave is used in the form of the UDF. All functions related to the input velocity of the incoming water flow in the horizontal direction, the relative wave pressure, the turbulent kinetic energy, and the turbulence dissipation rate all are defined as the UDF in the software. The aim of this project is to compare the amount of hydrodynamic forces acting on the pipeline over one period of the sea wave to achieve the optimal state.

## UDF

Hydrodynamic forces have been studied over time; The simulation was performed at a time of 10.3 s (equivalent to a full-wave period) with a time step of 0.02 s. In the present study, the wavelength of seawater (the distance between the two peaks of the wave) is considered to be 163.20 m, and the corresponding duration (period) is 10.3 s. Therefore, the dimensionless frequency number of the wave angle, ie 2π/Tw, is equal to 2π/10.3=0.61. Also, the maximum velocity at a wave peak is assumed to be 2,729 m.s-1. At the same time, km and Ԑm, indicate the maximum turbulence kinetic energy and the maximum turbulence dissipation rate, respectively.

In the equation of the sea wave pressure, H is equal to the height of the sea wave, d is equal to the depth of the seawater, -z is equal to the height of the water column at the point for which the dynamic pressure is calculated, and d-(-z) is equal to the distance from the point for which the dynamic pressure is calculated to the bottom of the sea.

## Geometry & Mesh

The present 2-D model is drawn using ICEM software. The present model consists of a rectangular space for seawater flow and a circular section as a pipe. The two important longitudinal parameters in the model are the pipe diameter (D) and the distance between the bottom of the pipe and the seafloor (e), which is measured by the e/D ratio. The diameter of the pipe has a constant value of 0.4 m, and the value of e in the two different states is 0.2 m and 0.1 m. In addition, the length and height of the space for seawater are 12 m and 3.24 m, respectively. The figure below shows a view of the geometry. Meshing is done using ICEM software and the mesh type is structured. The element number is 135417 and the cells adjacent to the circular section are smaller and more accurate. In fact, the circumference of the circular section is divided into five different parts, and the mesh is done in such a way that in the space close to the circle, the quality and accuracy of the mesh is higher. The figure below shows a view of the Mesh. ## Offshore Pipeline CFD Simulation

To simulate the present model, we consider several assumptions, which are:

• The solver is pressure-based.
• The simulation is transient Since the purpose of the problem is to study the amount of hydrodynamic forces over time.
• The effect of gravity on the fluid is considered to be -9.81 m.s-2.

The following is a summary of the steps for defining the problem and its solution:

 Models (offshore pipeline) Viscous model k-epsilon k-epsilon model standard near-wall treatment standard wall function Boundary conditions (offshore pipeline) Inlet Velocity inlet x-velocity UDF y-velocity 0 m.s-1 Outlet Outflow flow rate weighting 1 Wall of tube and bottom wall Wall Wall motion stationary wall Solution Methods (offshore pipeline) Pressure-velocity coupling SIMPLE Spatial discretization pressure second-order momentum second-order upwind turbulent kinetic energy first-order upwind turbulent dissipation rate first-order upwind Initialization (offshore pipeline) Initialization method Standard gauge pressure 561065.4 pascal x-velocity, y-velocity 0 m.s-1

## Results

After the solution process is complete, we obtain the two-dimensional velocity and pressure contours and the two-dimensional velocity vectors. These contours are related to two different modes (e/D=0.5 and e/D=0.25) at the last second of the simulation process (10.3 s), ie at the end of a period of a complete alternation. Also, we achieve graphs of changes in drag and lift hydrodynamic forces and drag and lift coefficients over time. These diagrams are also related to the two different modes (e/D=0.5 and e/D=0.25).

All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.

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