Numerical Investigation on the Hydrodynamic Coefficients of Subsea Suspended Pipelines Under Unidirectional Currents

Hydrodynamic coefficients of subsea suspended pipelines are crucial for fatigue and stability assessments. The effect of the gap height to diameter ratio e/D (0.1 ≤ e/D ≤ 2.0) and boundary layer thickness to diameter ratio δ/D (0.5 ≤ δ/D ≤ 3.0) on the force coefficients under unidirectional current conditions with the Reynolds numbers Re in the range of 1 × 104 ≤ Re ≤ 1 × 105 are investigated via numerical simulations. The results show that the average drag coefficient increases, whereas the average lift coefficient decreases gradually with the increasing e/D. The vortex shedding is inhibited by the wall for e/D < 0.24, starts at e/D = 0.24, becomes stronger with the increase in e/D in the range from 0.24 to 0.5, and approximates to that behind a wall-free cylinder for e/D > 0.5. The effect of δ/D can be eliminated if the coefficients are normalized by the undisturbed flow velocity at the height of the center of the pipeline. Moreover, empirical prediction formulas are proposed describing the drag and lift coefficients as the function of e/D, which can be applied to engineering designs related to free spans.