We developed an automated cable-laying system operated through a remotely operated vehicle (ROV). This system can automatically install a thin optical-fiber submarine cable at a rate that matches the ground speed of the ROV and adjust the amount of cable slack. The use of an ROV allows accurate and safe deployment of a thin optical-fiber submarine cable on the deep seafloor. The automated cable-laying system has been successfully used in installing the dense oceanfloor network system for earthquakes and tsunamis (DONET2) off the Kii peninsula in Japan. In our previous paper, we described the development process of the automated cable-laying system in detail. The current study focuses on the application in DONET2 installation and shows the practical advantages produced through automation. First, we overview the automated cable-laying system and then report on the successful application in DONET2 installation. The operating time and number of operators were distinctly reduced. Moreover, the physical and mental burden on the operators was relieved. In addition, several lessons learned during DONET2 installation are presented, and the unmanned cable-laying using an autonomous underwater vehicle is briefly discussed.

This paper deals with the motion control of a hovering autonomous underwater vehicle (AUV) with four horizontal and two vertical thrusters, when one or more thrusters are completely malfunctioned. Three thruster fault cases are considered: one horizontal thruster is faulty; two horizontal thrusters are faulty; and one vertical thruster is faulty. Through a series of simulations and an experiment, it is validated that the AUV can track a planned path in a 3-D space with minimally three thrusters (two are horizontal ones); however, some cases require the changes of the vehicle’s preferred direction of motion and its movement manner. Additionally, when the number of active horizontal thrusters is less than the required degree-of-freedom, a continuous state feedback control law does not exist due to the non-holonomic constraint. This paper highlights that the hovering AUV can overcome the non-holonomic constraint if using the feature that their translational and rotational motions can be controlled independently.

This paper deals with the attitude control of a towfish (underwater towed vehicle) with two elevators and a single rudder to improve the image quality of an attached sound navigation ranging (sonar) system. Image distortion can occur if the towfish shakes excessively. Since a towfish is connected to the mother ship through a towing cable and the motion of the towfish is affected not only by the motion of the cable, but also by the position of the center of gravity, towing point, and towing speed, it is necessary to analyze how these factors affect the towfish to appropriately control its attitude. In this study, a method for obtaining a feasible region of the towing point in accordance with the variations in the center of gravity and towing speed is proposed, and the feasible region obtained can ensure that pitch control can be achieved using the installed elevators. In addition, the allowable range of disturbances for yaw control was also investigated. Simulations were conducted using the dynamic models of the towfish and cable to check the obtained feasible region/range, and it was confirmed that there is a region/range where the attitude control can be carried out with relative ease.