Development of a Cooperative Multi-Drone System for Offshore Wind Turbine Inspection (SICE FES 2025)

Development of a Cooperative Multi-Drone System for Offshore Wind Turbine Inspection Fuma HOSODA, Yusuke TAMURA, Yasuhisa HIRATA Dept. of Robotics, Tohoku University, Japan SICE Festival 2025 with Annual Conference, September 10, 2025.

Background Global shift to decarbonization - High wind speeds - Remote from residential areas - Vast ocean area available Current capacity: still limited (253MW) Government targets: - 2030: 10GW - 2050: 90GW → Rapid expansion anticipated Cumulative installed capacity [MW] Offshore wind is a key solution in Japan Offshore wind power capacity in Japan 10GW@2030 300 200 100 0 2020 2021 2022 Year 2023 2024 Japan Wind Power Association 2

Inspection & Maintenance Regular inspections required to ensure safety and efficiency Inspection methods: Visual / Non-contact: surface cracks, erosion, lightning damage Internal checks: tap testing, conductivity testing to detect hidden defects Challenges: - High-risk rope-access work at up to 200 m - Harsh offshore conditions: wind, waves, salt corrosion Need for robotic systems to enable safer, faster, and more frequent inspections 3

Related works
Rope-based systems

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関連研究
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Objective of this study To develop an offshore wind maintenance robot system that satisfies these requirements. • Conceptual design of the proposed system • Preliminary experimental evaluation 100~120 m Objective nacelle 80 m Key requirements: - Stable vertical mobility - Must withstand wind disturbances & varying surface conditions - Access to both tower and blades - Enable contact-based blade inspections - Ease of deployment - Simple operation without large-scale installation infrastructure blade tower 5

Conceptual design Key Concept - Multiple drones form a ring array around the tower - stable vertical mobility even in strong winds Inspection drones deployed from array perform blade inspections Drone array acts as mobile base station, enhancing stability for contact and non-contact inspections Inspection Procedure 1. Preparation: Drone array positioned around tower base 2. Deployment: Array ascends; inspection drones move to blade tips 3. Inspection: Visual & contact inspections during stable traversal along blade 6

Scaled Wind Turbine Model for Indoor Testing Purpose: Provide a simplified testbed for dronebased inspection 1.3 Tower Tower: Height: 2.0 m Diameter: 1.0 m (base) -> 0.8 m (top) φ0.8 Blade Blade: 2.0 Simplified rod representation 1.0 φ1.0 7

Tower Elevation System using Multiple Drones Ring array of multiple drones surrounding the tower Each drone has an Outer shell - Protects drone & absorbs contact impacts - Mechanically isolated via bearings & shafts - Ensures at least 2 contact points with tower Inter-drone connections - Linked by carbon-fiber rods - Linear bushings + constant-load springs at joints Enable extension/retraction -> ring diameter adjusts passively to tower profile bearing linear bushing 8

Control Method - Tower Elevation System Key objective - Maintain vertical mobility & structural balance - Synchronized thrust across all drones Each drone: <latexit sha1_base64="+3hLV9zO5/ubIym6y3O1QZEAzfs=">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</latexit> Fi <latexit 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Inspection System using Multicopter blade Ring array of multiple drones surrounding the blade Simpli ed scaled indoor model: - Using one multicopter - Special design: blade inserted through the central square frame fi Coordinates with tower elevation system, but adjusts attitude independently Multicopter: roll & pitch (2 DOF) Coupling mechanism: rotation & extension (2 DOF) model blade (rod) simplification multi-drone system Tower system single drone 10

Control Method - Inspection System Position Estimation Kinematics-based estimation Based on the coupling mechanism’s - length … ToF distance sensor - angle … potentiometer Drone controller - Flight controller with full altitude & position control - Cascaded PID control architecture for trajectory tracking 11

Experiment 1: Tower Elevation Purpose: Verify ascent ability of drone array (3 modules, no inspection drone) Setup: - Indoor scaled wind turbine mockup - 3 interconnected drone modules - Altitude measurement with an RGB-D camera (~3 cm accuracy) Procedure: - Change target altitude multiple times - Observe ring array’s response and steadystate accuracy 12

Experiment 1: Tower Elevation Findings: - Steady-state error within ±10 cm - Meets required accuracy (±20 cm for blade-tip inspections) - Orientation MAE (mean absolute errors): roll = 0.09 rad, pitch = 0.05 rad Steady state Altitude (m) Conclusion: Accurate vertical control, suitable for inspection tasks Target Actual Time (s) 13

Experiment 2: Blade Transition Purpose: Verify transition of inspection multicopter from tower array to blade Setup: - Indoor scaled wind turbine mockup - Coupled system: 3 elevation drones + 1 inspection multicopter Procedure: - Elevation drones ascend to an hold a stable - altitude Inspection multicopter ascends and transitions to the blade region 14

Experiment 2: Blade Transition Target Actual Settling time Transition start Settling time Altitude (m) Altitude (m) Findings: - Inspection drone matched target altitude - Steady-state error of elevation system: ±10 cm - Orientation MAE: roll=0.27 rad, pitch = 0.02 rad - Slight reduction in control accuracy due to vibration during multicopter operation Conclusion: Transition feasible, but further control & hardware improvements needed Time (s) Leader drone Time (s) Inspection drone 15

Conclusions - Cooperative drone-based system for offshore wind turbine maintenance - Concept verification system for Tower Elevation & Inspection - Preliminary experimental results confirmed the system’s potential Future works - Improving control stability during blade transitions - Increasing payload capacity - Outdoor testing in wind-disturbed environments 16