Marine: Offshore Pipeline, Hydrodynamic Force

Offshore Pipeline Considering Hydrodynamic Force, ANSYS Fluent CFD Simulation Training

Description

This project simulates seawater flow around an offshore pipeline using ANSYS Fluent.

Offshore pipelines are a core part of marine engineering, carrying oil, gas, and other resources across the seabed between platforms and shore. As seawater waves pass over these pipelines, they generate drag and lift forces on the pipe. To keep the line safe and stable, it must be positioned so that it experiences the lowest possible hydrodynamic loading, which makes this kind of analysis important for offshore pipeline design.

The 2-D model was built in ICEM and consists of a rectangular seawater domain with a circular cross-section representing the pipe. Two key geometric parameters govern the study: the pipe diameter (D) and the gap between the bottom of the pipe and the seafloor (e), expressed through the e/D ratio. The pipe diameter is fixed at 0.4 m, and two cases are considered, e = 0.2 m and e = 0.1 m, giving e/D = 0.5 and e/D = 0.25. The seawater domain is 12 m long and 3.24 m high.

The model was meshed in ICEM using a structured grid of 135,417 elements. To capture the flow accurately, the mesh is refined near the pipe: the circumference of the circular section is split into five segments, and the cells closest to the pipe are smaller and of higher quality.

Simulation Methodology

The main geometric variable in this study is the pipe-to-seafloor gap ratio (e/D). Because the seawater motion is wavy rather than steady, the inlet velocity is defined as a wave-flow equation through a UDF. Likewise, the pressure inside the seawater is measured relative to atmospheric pressure and varies with the wave motion, so the wave (ambient) pressure is also imposed through a UDF. In total, the inlet horizontal velocity, the relative wave pressure, the turbulent kinetic energy, and the turbulence dissipation rate are all defined as UDFs.

The goal is to compare the hydrodynamic forces on the pipeline over one full wave period and identify the optimal configuration. The seawater wavelength (the distance between two wave peaks) is 163.20 m, with a corresponding period of 10.3 s, giving a wave angular frequency of 2π/Tw = 2π/10.3 ≈ 0.61 rad/s. The maximum velocity at a wave peak is 2.729 m/s, and k_m and ε_m denote the maximum turbulent kinetic energy and the maximum turbulence dissipation rate, respectively.

In the wave-pressure equation, H is the wave height and d is the seawater depth. The term −z is the height of the water column at the point where the dynamic pressure is evaluated, and d − (−z) is the distance from that point down to the seabed.

Results & Conclusion

After solving, we obtained two-dimensional contours of velocity and pressure, along with two-dimensional velocity vectors, for both cases (e/D = 0.5 and e/D = 0.25). These results are taken at the final instant of the simulation (10.3 s), i.e., at the end of one complete wave period.

We also obtained time-history graphs of the drag and lift hydrodynamic forces and of the drag and lift coefficients, again for both e/D cases. Comparing the two configurations shows how the pipe's distance from the seabed affects the hydrodynamic loading, which is what determines the optimal placement of the line.