## 3 Collaborative AUVs
### Collaborative Missions
#### Intervention Missions  
There is much more in the underwater environment for AUVs beyond survey missions. Manipulating objects, repairing structures or pipes, recovering black-boxes, extracting samples, among other tasks, make it necessary to have a platform with the capacity of autonomously navigate and perform them, since nowadays these are mostly done by manned or remotely operated vehicles. Researchers have worked in recent years in the design and development of such platforms, which results difficult even for a single Intervention AUV (I-AUV) due to the complexity of the vehicle itself plus the manipulator system. The Girona 500 I-AUV is an example of a single vehicle platform for intervention missions. This vehicle is used to autonomously dock into an adapted subsea panel and perform the intervention task of turning a valve and plugging in/unplugging a connector [140]. The same vehicle was also equipped with a three-fingered gripper and an artificial vision system to locate and recover a black-box [141]. Other projects, such as the Italian national project MARIS [142] have been launched to produce theoretical, simulated and experimental results for intervention AUVs either standalone or for collaborative teams. The aim of the MARIS project was the development of technologies that allow the use of teams of AUVs for intervention missions, in particular: reliable guidance and control, stereovision techniques for object recognition, reliable grasp, manipulation and transportation of objects, coordination and control methods for large object grasp and transportation, high-level mission planning techniques, underwater communication, and the design and realization of prototype systems, allowing experimental demonstrations of integrating the results from the previous objectives. The open-frame fully actuated robotic platform R2 ROV/AUV was used for the MARIS project, and the Underwater Modular Manipulator (UMA); none of them developed within the project. A vision system and a gripper were designed for the autonomous execution of manipulation tasks. Tests were performed to assess the correct integration of all the components, with a success rate of a grasping operation of up 70% [143]. This project was an important achievement in terms of autonomous underwater manipulation, and the theoretical studies for multi-vehicle localization and collaborative underwater manipulation systems will be the next step to be demonstrated in field trials. For collaborative I-AUVs, [PERSON] et al. [144; 145] described a novel cooperative control policy for the transportation of large objects in underwater environments using two manipulator vehicles. The cooperative control algorithm takes into account all mission stages: grasping, transportation, and the final positioning of the shared object by two vehicles. The cooperative transportation of the object is carried out to deal with limitations of acoustic communication, this was achieved successfully by exchanging just the tool frame velocities. A simulation was done, with the UwSim dynamic simulator, using two vehicles of 6 degrees of freedom in order to test the control algorithms. The simulation consisted of completing the following tasks: keeping away from joint limits, keeping the manipulability measure above a certain threshold, maintaining the horizontal attitude of the vehicles, maintaining a fixed distance between the vehicles, reaching the desired goal position. The system managed to accomplish the final objective of the mission successfully, by transporting the object to the desired goal position. [PERSON] et al. [146] proposed an innovative decentralized approach for cooperative mobile manipulation of intervention AUVs. The control architecture deals with the simultaneous control of the vehicles and robotic arms, and the underwater localization. Simulations were made in MATLAB Simulink to test the potential of the system. The cooperative mobile manipulation was performed by four AUVs placed at the four corners of the object and obstacles were introduced as spheres.