Experimental investigation of a hull-platform coupled wave motion compensation ship using a nonlinear disturbance observer

High-precision deck motion compensation remains challenging when a large moving platform interacts dynamically with its supporting vessel. This paper presents a wave motion compensation ship (WMCS) comprising a catamaran hull, a redundant parallel mechanism, and an upper platform. Unlike conventional studies that neglect platform reactions, the proposed approach accounts for bidirectional hull–platform coupling induced by platform inertia. A coupled dynamic model and a 12-dimensional affine nonlinear state-space representation are established. Hydrodynamic coefficients are obtained through frequency-domain radiation–diffraction analysis using ANSYS AQWA and assessed through wave-tank tests. A coupling-aware control scheme integrating a nonlinear disturbance observer (NDO) and computed torque control (CTC) is developed to estimate and compensate for lumped disturbances online. Regular-wave experiments on a 1:17.5-scale prototype evaluate the model and controller. Predicted and measured responses show good agreement under the tested conditions. Compared with the coupling-aware CTC baseline using the same nominal model, the NDOCTC scheme achieves compensation rates of 88.11%, 89.90%, and 89.11% for heave, roll, and pitch, respectively. The results demonstrate the feasibility of coupled modeling and disturbance-compensated control for WMCS configurations with non-negligible platform reactions under the tested regular-wave, narrow-band conditions.