Jin-Kyu Choi

Thruster fault-tolerant control of a hovering AUV with four horizontal and two vertical thrusters

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. Graphical Abstract

Development of an automated cable-laying system for DONET construction

This paper describes an automated cable-laying system that can automatically pay out a submarine cable on the seafloor while keeping balance with the ground speed. We have installed DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) at Kumanonada and a ROV (Remotely Operated Vehicle) was successfully used to lay the cables which connect observatories to the nodes of the DONET. However, such a cable-laying work was regarded as one of the hardest works since it takes approximately 10 hours (about 10km distance) and is manually carried out by a ROV operator(s). The automated cable-laying system can contribute to reduce the physical and mental burdens on the ROV operator and speed up the construction of DONET2 (the second phase of DONET). In this paper, the components and cable-laying control strategies are described and some experimental results are presented. In addition, we report on the first mission to connect the borehole observatory C0002 to the Node D of the DONET. This achievement has made it possible to receive the data of the borehole observatory in real-time. The paid out cable is 8470m and the travelled distance is approximately 8000m.

Automated Cable-Laying System for Thin Optical-Fiber Submarine Cable Installation

We have installed the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) that includes 20 observatories at Kumanonada (called DONET1) for the purpose of monitoring the seismogenic zone around the Nankai Trough. During the construction of DONET1, a remotely operated vehicle (ROV) was successfully used to lay the optical-fiber submarine cables that connect the observatories with the science nodes; however, the cable-laying operation was onerous because it takes approximately 10 h and is manually performed by ROV operators. For this reason, the cable-laying operation is usually not performed on successive days, which makes it difficult to schedule the construction work. In this paper, we present an automated cable-laying system that can automatically pay out the optical-fiber submarine cable on the seafloor at a rate that keeps pace with the ground speed and also adjusts the cable tautness. The components and control strategies are first described, and the results of some experiments and the first mission are then presented. The experiments and the first mission demonstrate that the automated cable-laying system can reduce the physical and mental burdens on the ROV operator and speed up the installation of DONET2 in 31 observatories. DONET2 is planned to be deployed off the Kii Peninsula (on the western side of DONET1).

Advanced Real Time Monitoring System and Simulation Researches for Earthquakes and Tsunamis in Japan

Mega thrust earthquakes generated large tsunamis quite often. Based on lessons learned from the 2004 Sumatra and the 2011 East Japan Earthquakes/Tsunamis, we recognized the importance of real time monitoring on the natural hazards. Monitoring systems using multi kinds of sensors such as the accelerometer, broadband seismometer, pressure gauge, difference pressure gauge, hydrophone and thermometer is indispensable not only for mitigation of damage from earthquakes and tsunamis, but also for understanding of broadband crustal activities around mega thrust earthquake seismogenic zones. Therefore, we have developed the Dense Ocean floor Network for Earthquakes and Tsunamis (DONET) to acquire the seafloor data in real time around the Nankai trough seismogenic zone, southwestern Japan. The first phase of deployment (DONET1) was completed and the second phase (DONET2) is being developed at the time of writing of the manuscript. At the 2011 East Japan Earthquake, DONET1 observatories detected offshore tsunamis 15 min earlier than onshore stations. Furthermore, DONET1 and DONET2 will be expected to monitor silent phenomena such as low frequency tremors and slow earthquakes for the estimation of seismic stage which would occur in the inter-seismic or pre-seismic stage. The recurrence cycle of mega thrust earthquakes, modeling of tsunami inundation and seismic response on buildings and cities are also important in the disaster mitigation programs and related measures. Real-time monitoring data should be integrated with the advanced simulations for precise earthquake and/or tsunami early warnings and rapid estimation of the damages.

Decision-making on seafloor surveillance infrastructure site for Earthquake and Tsunami monitoring in Western Japan

DONET (Dense Ocean-floor Observatory Network for Earthquakes and Tsunamis) is a submarine cabled real-time seafloor surveillance infrastructure for earthquake activity at assumed focus region of mega-thrust earthquake around Japan. The original system DONET1 was constructed in To-Nankai earthquake focus region and twenty seafloor observatories are working in operation beginning in 2011 to contribute the earthquake and tsunami early warning program in Japan. Development of second seafloor network DONET2 was planned in 2010 to target Nankai earthquake focus region. This paper describes a decision making approach of DONET2 observation site arrangement based on the knowledge of DONET1 development and construction.

First sea trial of underwater observation system using autonomous towed vehicle

This paper discusses an autonomous towed vehicle for underwater inspection in a port area, in which a sea current is so fast and complex. The autonomous towed vehicle has three different navigation modes; towed mode, autonomous mode and kite mode, to assure safe and reliable inspection in such a port area. In this paper, we describe the results of a tank experiment and a real sea trial using this prototype model to verify the effectiveness about our proposed system. The towing test was executed and the suitable values of the control parameter in the proposed system were decided. Next, the stability of the vehicle in the towed mode was con finned and a depth constant, a heave and pitch control was performed successfully in the real sea area experiments. And it was confirmed that the way point control was possible in an untethered mode

ROV-Based Automated Cable-Laying System: Application to DONET2 Installation

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.

Automated cable-laying system: Contribution to DONET2 installation

We have developed an automated cable-laying system that is operated with a remotely operated vehicle (ROV) and can automatically dispense a thin optical-fiber submarine cable at a rate that keeps pace with the ROVs ground speed. The use of an ROV allows to install a thin optical-fiber submarine cable on the deep seafloor accurately and safely. The ROV-based automated cable-laying system is being used successfully for the installation of the dense oceanfloor network system for earthquakes and tsunamis at off the Kii peninsula, Japan (called DONET2). In this paper, the contribution of the automated cable-laying system to the installation of DONET2 is described and some data to demonstrate this are presented. We first overview the automated cable-laying system and then present and discuss the results associated with the main issue. Lastly, some experimental knowledges obtained during DONET2 installation are described.

Advanced technology and application of the ocean floor observatory around the Nankai trough, Southwestern Japan

The Nankai trough is one of the largest and hazardous seismogenic zones in the world. Around the Nankai Trough, there are 3 mega thrust earthquake seismogenic zones such as the Tokai, Tonankai and Nakai seismogenic zones. Especially, the estimation of seismic linkage between the Tonankai and Nankai seismogenic zone is very important and indispensable for the reduction of earthquake and tsunami damages. Actually, the Tonankai and Nankai earthquake occurred in 1944/ 1946, 1854 as the seismic linkage events. Therefore, for understandings of early warning and prediction of erathquakes and tsunamis, the ocean floor network equipped with multi kinds of sensors such as seismometers and pressure gauges are very important and significant tool to monitor the broad band phenomena in the Nankai seismogenic zones. In the Tonankai seismogenic zone, we constructed DONET (Dense Ocean floor Network for Earthquakes and Tsunamis) with 20 observatories for early warning and improvement of prediction reliability. Multi kinds of sensors such as an accelerometer, a broad band seismometer, a precise pressure gauge, a differential pressure gauge and a precise thermometer are equipped in each observatory. Furthermore, the long term borehole observatory system with the seismometer, strainmeter, tiltmeter etc. is already installed in the drilled hole by IOPD NanTroSEIZE project, finally, this system will connect to DONET for the 3 dimensional real time monitoring. Now, we are already developing DONET2 with 31 observatories around the Nankai seismogenic zone and will complete deployments within 3 years. DONET2 system is more powerful rather than DONET system to monitor the large seismogenic zone areas. Especially, these offshore real time data will be applied to estimate of tsunami scales. We have to learn the lesson from tsunami damages of East Japan Earthquake 2011, Sumatra Earthquake 2004 etc. So, we are developing the advanced tsunami early estimation system using not only real time monitoring but also advanced simulation.