Experimental and numerical investigation of hydrodynamic forces on a submerged horizontal cylinder near the free surface under focused waves

Extreme waves pose a significant threat to marine structures, yet research on hydrodynamic forces acting on submerged horizontal cylinders near the free surface (representing jacket platform braces, pipelines, and submerged floating tunnels) remains scarce. This study systematically investigates the spatiotemporal distribution of wave forces on a submerged horizontal cylinder under focused wave action, combining physical model experiments and numerical simulations. By analyzing key parameters such as water depth, peak frequency, effective wave height, and cylinder radius, we reveal the asymmetry of structural loading and its variation patterns. High spatio-temporal resolution surface pressure distributions obtained from 12 pressure sensors demonstrate the significant influence of local flow separation and impact effects on wave force response. Frequency-domain analysis indicates that horizontal wave forces are primarily controlled by dominant frequency and quasi-static components, while vertical forces exhibit strong nonlinearity and low-frequency responses. Notably, under shallow submergence conditions, wave group modulation intensifies, leading to a significant amplification of low-frequency forces, which become a crucial load component for structural safety design. The study also explores the wave force prediction capabilities of an improved Morison equation under focused wave conditions. Our findings provide essential experimental support and theoretical references for the hydrodynamic response analysis and structural design of near-surface structures like floating tunnels and offshore pipelines in extreme wave environments.