Offshore pipelines are exposed to hydrodynamic forces due to the current actions. The forces (drag and lift) have an unsteady nature and oscillate around their average values because of the occurrence of vortex shedding behind the pipeline. Vortex shedding leads to the Vortex-Induced-Vibration (VIV) which is an important source of fatigue damage of pipeline in free spanning sections. Time-averaged drag force can be calculated using drag equation which has a hydrodynamic coefficient. However it does not take the VIV effects into account. To do so and to consider the unsteady nature of the force, the flow pattern and vortex shedding phenomenon around a pipeline should be simulated accurately. The main objective of this study is to simulate the flow field around an offshore pipeline placed on the plane bed. This study investigates the gap ratio effects on the flow pattern and drag force exerted on the pipeline as well. The flow field was simulated by the two dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with a k-ε turbulence closure model. Results indicate that the k-ε turbulence closure can simulate the vortex shedding behind a pipeline and the oscillation of drag force successfully. By considering different values of gap to diameter ratio, it is found that the drag coefficient is increased gradually as the gap ratio rises up and the increase rate reduces as gap ratio increases.

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