Tracking Mooring Lines of Floating Structures by an Autonomous Underwater Vehicle

This study presents a novel method for tracking mooring lines of Floating Offshore Wind Turbines (FOWTs) using an Autonomous Underwater Vehicle (AUV) equipped with a tilt‐controlled Multibeam Imaging Sonar (MBS). The proposed approach enables the AUV to estimate the 3D positions of mooring lines and safely track them in real‐time, overcoming the limitations of traditional Remotely Operated Vehicle (ROV)‐based inspections. By utilizing the tilt‐controlled MBS and a pre‐trained You Only Look Once (YOLO) model, the AUV identifies the mooring lines within sonar imagery and dynamically adjusts its velocities to maintain a safe distance during the inspection. A re‐navigation method using the Rauch‐Tung‐Striebel (RTS) smoother enhances the AUV's localization accuracy by correcting its trajectory using post‐processed data from sensors such as the Doppler Velocity Log (DVL), Super Short Baseline (SSBL) system, and Global Navigation Satellite System (GNSS). Additionally, reconstruction with catenary curve fitting is employed to estimate the mooring line's catenary parameters, offering insights into its potential deformation. The approach was validated using a hovering‐type AUV, Tri‐TON through both tank experiments and a sea experiment at an FOWT Hibiki in Kitakyushu, Japan. In the sea experiment, the AUV successfully tracked the mooring lines for 423 s, demonstrating its ability to estimate the position and catenary parameters of the mooring lines. The experimental results highlight areas for future improvement, particularly in enhancing localization accuracy, developing robust control algorithms, and expanding the analysis of mooring line conditions. This method lays the groundwork for future advancements in automated mooring line inspections and enables the integration of additional techniques, such as visual inspection.