# Development of Modular Bio-Inspired Autonomous Underwater Vehicle for Close Subsea Asset Inspection
## 1 Introduction  
The use of offshore wind power will play an essential role in our future electricity generation. It is forecast that by 2050, 12 percent of the world's primary energy supply will come from wind energy, and 20 percent of this will come from offshore wind [1, 2]. However, ongoing wear and corrosion from the harsh sea environment drives up cost and introduces downtime to this renewable and clean energy source [3]. To ensure reliable production, regular inspection tasks during high seas up to 100 m depth need to be performed in a cost-effective and safe manner [4]. These tasks are currently being conducted largely using Remotely Operated Vehicles (ROVs), which generally need tethers and a human operator, or using Autonomous Underwater Vehicles (AUVs), which are limited in their accessibility and maneuverability [5, 6]. To extend AUV intervention ability and perform critical inspection tasks, they need to be efficient and flexible in operation. A fish-like AUV with a bending body of a spinal column design that is able to mimic propulsion techniques of living fish can provide efficient thrust at minimum swimming velocities, and higher maneuverability in limited spaces during sensor data acquisition. RoboFish was created by the project \"Autonomous Biomimetic Robot-fish for Offshore Wind Farm Inspection\" supported by the EPSRC Supergen Renewable Energy Hub and \"Innovating the Future of Bio-Inspired Autonomous, Robots for Offshore Renewable Energy Inspection\" supported by the White Rose University Consortium. It was specifically aimed at investigating and exploiting bio-inspired mobility features to facilitate autonomous inspection of offshore infrastructure and is an agile and efficient biomimetic AUV that will in the near future be able to continuously inspect the foundations of offshore wind turbines and drastically reduce potential risks to divers, maintenance costs, and operational constraints. RoboFish replicates the full-body movement of an eel, allowing agility and energy efficiency in close proximity to structures. The understanding of fish swimming behaviours and the exploration of their benefits and application in engineering designs is an interdisciplinary research field of significant and ongoing interest [7; 8; 9; 10]. Swimming robots that mimic the techniques of natural swimmers promise to provide an increase in overall swimming performance over conventional thruster propelled systems. Reference [11] shows that thrusters waste energy by generating a vortex perpendicular to the desired thrust direction. On the other hand, aquatic animals are able to efficiently produce a jet in the desired direction through actively and passively controlled body motion. Based on the modular assembly of identical body modules and the resulting equal mass distribution, a swimming gait resembling that of an eel is anticipated. Research into eel locomotion in Reference [12] predicts swimming efficiencies of 0.5 to 0.87, depending on choice of calculation, compared to thruster efficiencies of up to 0.4 in Reference [11]. Among the two main categories of fish swimming, propulsion employing displacement of the centre line of the fish, the so-called Body Caudal Fin (BCF), is suggested to have advantages in speed and long-distance travel over flapping fin propulsion of Median Paired Fin [13]. Given that the target application of RoboFish is wind farm inspection, the slender body design of a BCF swimmer is beneficial for the anticipated long-distance travel between wind turbines, maintaining a high level of maneuverability through its body flexibility. This also makes more complex routes available that can potentially reduce travel distance. Low noise and mitigated risk of entanglement of continuously rotating parts suggests lower environmental disturbance. Furthermore, the multi-actuated system allows flexibility and adaptability in entering tight spaces and maneuvering in complex environments.