Measurements of wave loads on smooth and rough horizontal cylinders under bimodal sea states

Horizontal cylindrical structures are widely used in maritime engineering, serving various functions such as submarine pipelines or supports for offshore platforms and wind turbines. In real marine environments, these structures are subject to the action of unimodal sea states, characterized by pure wind-generated waves, or bimodal sea states, which consist of a combination of wind waves and swells. This study describes a field experiment aimed at analyzing the influence of mixed sea states on the hydrodynamic forces acting on smooth and rough submerged horizontal cylinders. The experimental campaign was carried out in a designated marine area at the Marine Energy Laboratory (MEL) of the University of Reggio Calabria. The test setup included metal frames anchored to the seabed, supporting two identical and aligned cylinders positioned in an isolated condition. One cylinder was smooth and treated to prevent biofouling, while the other had an artificially roughened surface, achieved by printing barnacle-like textures onto its exterior. For engineering purposes, the hydrodynamic force coefficients of the Morison and transverse equations were derived from the measured free-stream kinematics and the forces acting on the cylinders. The influence of cross-sea states was systematically compared with that of pure wind-wave conditions in terms of force time histories, peak values, and hydrodynamic force coefficients. The results highlight that, for comparable Keulegan–Carpenter numbers in unimodal and bimodal sea states, the spectral shape influences the magnitude of the hydrodynamic loads acting on horizontal cylinders. This effect was observed for the smooth surface and was found to be further amplified under rough conditions, leading to consistently higher force levels and hydrodynamic coefficients in bimodal sea states.