# A Survey on Unmanned Surface Vehicles for Disaster Robotics: Main Challenges and Directions
## 4 Use of USVs in Environmental Disasters with Contaminants
### Chemical Hazards
#### 4.1.3 Oil Spill Ensnaring and Removing  
When an oil spill occurs, the very first action is to stop pollution at its source to mitigate the adverse environmental effects of an oil spill. Subsequent actions can be containment, recovery, and disposal of the oil. Research efforts are being focused on the development of technologies to remove the oil in situ, minimize operational time, and protect the health and safety of the cleaning crew [123]. A variety of strategies have been developed to cleanup the pollutant and thus, minimize the extent of the environmental impact of oil spills. [PERSON] et al. [139] divide conventional cleanup strategies into three groups: natural degradation, where no action is performed apart from monitoring the movement of the spill; mechanical cleanup methods, which include containment and recovery of oil that remains on the sea surface by using barriers/booms or skimmers; and chemical methods, which use dispersants to reduce the interfacial tension between oil and water or other agents such as emulsion breakers, gelling, and sinking agent. We refer the reader to a thorough description of oil spill response methods (e.g., mechanical, chemical, etc.) and corresponding oil response equipment (e.g., booms, skimmers, etc.) in [139]. An application of USVs in _mechanical cleanup methods_ includes a team of unmanned ships towing a boom, creating a mechanical barrier capable of controlling the motion and spreading of the floating substance. The idea is that once the operation starts, the USVs tow the boom towards the objective, minimizing the towing effort. Near the objective, the USVs deploy the boom in the water and advance. As they get closer, the fleet of USVs moves in formation to confine the spill, which may then be transported by the formation towards a given destination [125]. Such an approach requires resolving many challenging technical problems such as planning, coordination, communication, cooperation, and navigation of both vehicles. [PERSON] et al. [118; 119] study the cooperative control of two USVs performing a caging and transport mission on the water surface. A flexible floating rope connects the two vehicles with the purpose of capturing the floating oil from a given location and transporting it to a designated position. Experiments to test the coordination control strategy of the USVs to accomplish the caging mission were executed in the Echo Park Lake in Los Angeles. [PERSON] et al. [121] also perform oil spill confinement using two USVs towing a boom. Their approach includes a mathematical model of the system and a navigation system based on the Null Space Based behavioral control. The focus of their work is to study the navigation tasks that need to be imposed for a proper towing of an oil spill confinement boom. Later, [PERSON] et al. [124; 125; 140] study the control and coordination needs for the use USVs with autonomous control when automatically towing a boom. The core idea is to provide the algorithm with the area where the oil spill occurred, and the vehicles would automatically try to contain the oil spill. Experiments were performed using two scaled USVs and a ground station, in a lake near Madrid, Spain, to test a parallel formation and the performance of USVs when towing a boom. The results show this method is feasible, provided that some specific criteria were satisfied concerning feedback control and geometry (V-shape) of the boom. [PERSON] et al. [95] present a robotic system for environmental disaster response that incrementally forms a chain around the floating contaminant. The proposed system uses several USVs to surround and contain the pollutant by docking on each other, in a similar way containment booms would behave. These USVs are connected, floating on the water surface to form a physical barrier to contain the floating contaminant. The system has the potential to work autonomously or being controlled by a human operator. They test the effectiveness of the docking method and initial tests involving the containment of the contaminant.