Experimental study on hydrodynamic characteristics of offshore seabed-fixed cylinder with truncated porous shells

The impermeable shells in collision protection systems installed on seabed-mounted offshore structures alter the hydrodynamic profile and may amplify wave loadings, resulting in hydrodynamic penalties. Surface-pierced porous designs are proposed as potential solutions, yet their efficacy in reducing these penalties remains insufficiently explored. To bridge this gap, model tests are conducted for a seabed-fixed cylinder with truncated porous shells. The shells feature various parameters including diameter, porosity, submerged depth, and opening shape to investigate their influence on the cylinder's hydrodynamic characteristics. Regular and irregular wave conditions are tested, with base shear forces, moments, and wave surfaces recorded. The results indicate that non-porous shells introduce hydrodynamic penalties to the model, increasing base shear by 24%–27% and base moment by 21%–25%. Larger shell diameters yield more severe amplifications, particularly under high wave heights. Porous shells significantly minimize these penalties, with merely 5% porosity reducing the increases to 7% for base shear and 13% for base moment. For practical engineering applications, shells with 10% porosity offer a potential balance between protective functionality and hydrodynamic performance. Furthermore, increasing porosity beyond 10% shows gradually converged hydrodynamic performance approaching that of the original cylinder without shells. The submerged depth of porous shells plays a secondary role in affecting hydrodynamic performance, while the opening shape exhibits even more minimal influence. Analysis of diffraction waves demonstrates that porous shells interfere with diffraction by diminishing diffraction wave amplitude and exacerbating phase lag between incident and diffracted waves, suggesting that porous shells effectively suppress wave diffraction forces.