A decoupled prediction model for hydrodynamic forces on near-wall cylinders in combined wave-current boundary layers

• A decoupled model is developed to predict hydrodynamic forces on near-wall cylinders under wave-current boundary layer flows. • The model separates hydrodynamic coefficients from local boundary layer velocity. • Model performance is evaluated across various flow conditions and gap ratios. The prediction of hydrodynamic forces on subsea structures, such as pipelines and power cables laid near the seabed, is critical for ensuring their on-bottom stability and long-term integrity in marine environments. Traditionally, the Morison equation has been used to estimate these forces, which decomposes forces into inertial and viscous components. However, this approach does not fully capture the influences of the boundary layer and gap between the structures and the bottom wall. A novel decoupled model is proposed in the present study to predict the hydrodynamic forces on a near-wall cylinder subjected to a combined current-wave induced boundary layer flow, representative of subsea cables and pipelines in coastal and offshore environments. This model explicitly separates the effects of the boundary layer on the hydrodynamic coefficient and local flow velocity, treating them as two independent components. The hydrodynamic coefficients are obtained through numerical simulations of a time-dependent, wall-normal uniform flow over a friction-free bottom wall, thereby eliminating boundary layer effects. These coefficients are time-dependent and are governed by the cylinder-wall gap. Meanwhile, the local flow velocity is determined using an analytical velocity profile which accounts for velocity reduction due to the boundary layer effects. The performance of the predictive model is systematically evaluated across different flow conditions and gap ratios. Comparisons with numerical simulations for a near-wall cylinder subjected to a combined current-wave boundary layer flow demonstrate that the decoupled model can predict the drag forces with satisfactory agreement. While the lift forces tend to be overpredicted in some cases, this conservative estimation remains acceptable for engineering design. The proposed model offers an efficient and practical approach for estimating hydrodynamic forces on power cables, pipelines and other coastal and offshore infrastructure subjected under boundary layer effects.