Hydroelastic interaction of waves with an isolated concentric monopile–flexible cage integrated structure

In a concentric monopile–flexible cage structure, the effects of annular-region wave interaction on the complex pressure distribution, total horizontal loading, and local three-component net deformation remain insufficiently understood. A frequency-domain semi-analytical model couples outer and annular wave fields to a porous cylindrical shell membrane with axial, circumferential, and radial displacements using linear potential-flow theory, matched eigenfunction expansions, and weighted modal projection. Limiting-case comparisons assess consistency with the rigid-cylinder and isolated-flexible-cage responses. Spatial integration converts the pressure difference into the side-net force; complex addition with the central-cylinder force determines the total force, and shell-membrane dynamics governs deformation. Reference parameters are constructed using empirical porous-admittance relations and an idealized unit-cell tensile–mass equivalence. Over 1≤k0h≤6, pressure-difference and side-net-force maxima occur at the low-frequency boundary, whereas the sampled maxima of normalized total force and radial displacement occur at k0h=2.530 and 1.065, respectively. Near k0h=4.235, spatial cancelation yields a nearly zero side-net horizontal resultant despite finite local pressure and deformation. The monopile dominates total loading, while geometry and porous admittance modify the side-net contribution and its phase-dependent effect on the total. Across a 20-fold tensile-stiffness range at fixed areal mass and porous admittance, the total-force maximum changes by <0.2%, while maximum normalized radial displacement at k0h=1 decreases by 95.80%. Total horizontal force and local shell deformation thus require separate measures because pressure transmission, phase, spatial integration, and dynamic flexibility affect them differently.