Nonlinear hydrodynamic response of a horizontal cylinder near the free surface based on a time-varying Morison coefficient model

The interaction of waves, currents, and free-surface effects induces complex unsteady flow patterns and nonlinear hydrodynamic loads on horizontal cylindrical elements, which are critical factors governing the fatigue life and stability of marine renewable energy installations. This study investigates the nonlinear interaction between waves, currents, and a horizontal cylinder near the free surface using a Reynolds-averaged Navier–Stokes (RANS) model. A time-varying load coefficient analysis method is proposed within the framework of the Morison equation. Validated against experimental data, simulations examine how varying cylinder submergence depth and wave conditions affect flow patterns and forces. Constant hydrodynamic coefficients derived from conventional least-squares fail to adequately capture the nonlinear loads, whereas time-varying coefficients identified via a Forgetting Factor Least Squares (FF-LS) algorithm provide accurate force predictions. Analysis reveals that the hydrodynamic characteristics, for both constant and time-varying coefficients, are predominantly governed by the relative submergence depth. Variations in relative wave height primarily affect the amplitude of the coefficients while leaving their temporal waveform patterns largely unchanged. The presence of a steady current intensifies vortex shedding and the hydrodynamic response. This work offers an improved methodology for wave load estimation and fatigue assessment of semi-submerged components in floating offshore wind and wave energy systems.