Hydrodynamic forces on subsea cables immersed in wave boundary layers

Abstract

This paper investigates the hydrodynamic forces on small-diameter cables (50 mm ≤ D ≤ 200 mm) under wave conditions. A total of 87 experimental tests are conducted in the parameter ranges of 20 ≤ KC ≤ 2000, 10⁴ ≤ Re_p ≤ 10⁵, 10 ≤ β ≤ 1 000 and 0.001 < k_s/D ≤ 2.6, where KC is the Keulegan–Carpenter number, Re_p is the Reynolds number defined with the cable diameter and peak freestream velocity, β is the Stokes number and β = Re_p/KC, and k_s/D is the ratio between the seabed roughness and cable diameter. The results show that wave boundary layers significantly affect the forces on cables in contact with the seabed. The variations in the force coefficients with the governing parameters of KC and k_s/D are interpreted based on the characteristic wave boundary layer features, namely, velocity deficit (reduction) and wall turbulence. Two counteracting mechanisms influence the force coefficients: velocity reduction in the wave boundary layer decreases the force coefficients, whereas strong wall turbulence from the seabed increases the force coefficients. Empirical formulas for evaluating the force coefficients of an on-bottom cable immersed in a wave boundary layer are proposed based on the present results.

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