## 1 Introduction  
The off-shore oil and gas industry has a growing demand for new and innovative solutions for inspection, maintenance and repair of seabed installations. Currently, these tasks are usually carried out in hostile environments using remotely operated vehicles (ROVs) deployed from a large surface vessel; however, ROV operations require high expenses due to the daily ship cost and the large number of crew members which such operations need. As a result, in order to reduce the inspection cost and deal with the several logistic constraints, the interest in autonomous underwater vehicles (AUVs) has rapidly increased in the last few years. Indeed, AUVs are currently exploited for long-distance monitoring and inspection, as they are provided with an ever-increasing precision navigation system [1, 2]. Moreover, the development of vehicles that can incorporate both the AUV inspection and ROV intervention functionalities is one of the most challenging tasks of the underwater industry as well as the scientific community [3]. A recent example is Hydrone-R, the underwater intervention drone (UID) from Saipem S.p.A., which is a technology that allows accurate inspections on submarine systems, certifies operation, and detects and prevent any malfunctions [4]. With the regard of other innovative AUVs, the basic idea concerns vehicle with many configurations and the ability to autonomously switch between them depending on the main task to accomplish. For instance, the autonomous underwater reconfigurable vehicles (AURV) can select the most appropriate configuration so as to dive with the optimal fluid dynamic efficiency and achieve the minimum power consumption for long distance navigation; furthermore, the robot may be provided with a larger number of degrees of freedom for complex manipulation operations or to perform autonomous docking in a subsea station. As a matter of fact, AURVs do arise as a promising tool for incorporating several reconfigurable modules and accomplishing both the autonomous navigation/inspection and intervention tasks. As far as the AURV state of the art is concerned, one of the most innovative vehicles is made by EELUME [5; 6], an underwater snake robot (USR), developed by Subsea Intervention for underwater inspection and manipulation. This vehicle is basically a self-propelled robotic arm that can move over long distances and carry out maintenance and repair tasks in spaces not usually accessible by conventional underwater vehicles. In greater detail, it is a modular, flexible robot capable of swimming like a snake, or being propelled by conventional thrusters. Additionally, the vehicle can be assembled with an arbitrary number of elements linked each other with a flexible joint. Another example of AURV has been presented by Houston Mechatronics in 2018; specifically, Houston Mechatronics is a subsea service company with deep expertise in robotics and intelligent automation, founded by former NASA roboticists responsible for designing and developing some of the most advanced robots in the world. The conceptual vehicle, named Aquanaut [7], is a robot with two different configurations: in AUV mode, it can cover long distances while accomplishing tasks like seabed mapping, and inspection of large area; in ROV mode, the hull can open its structure, exposing two robotic arms for manipulation operations. In particular, in the closed version, the vehicle appears as a classical UUV with a slender shape; in this configuration, it can move itself with only three motors for long distance, controlling only three degrees of freedoms. In the open version, the upper part of the hull moves away from the main body, exposing other four motors, in order to obtain a vectored configuration of the motors, and two robotics arms.