Submarine Cabled Real-time Seafloor Observatory and Subsea Engineering ROV for Observatory Construction

Transcription

1 Submarine Cabled Real-time Seafloor Observatory and Subsea Engineering ROV for Observatory Construction Katsuyoshi KAWAGUCHI, Sho Kaneko (Japan Agency for Marine-earth Science and Technology), Takato Nishida (OCC Corporation), Tetsuya Komine (Nichiyu Giken Kogyo Co., Ltd.) > Japan Agency for Marine-earth Science and Technology, 2-15 Natsushima-cho Yokosuka KANAGAWA, JAPAN Abstract: The submarine cabled real-time seafloor observation is one of a suggested engineering approach to be realized the precise real-time and long-term monitoring on seafloor. A development program DONET was started in 2006 to construct a seafloor observatory network to monitor the earthquakes activities and associated Tsunami phenomenon. In-situ observatory construction is a key issue to maintain the system long time and to control the measurement environment on seafloor for precise monitoring. The development of unique tools for subsea engineering ROV are essential to put the seafloor observation network construction into practice. This paper describes the design concept of submarine cabled seafloor observatory network and the functions required for the ROV by the in-situ observatory construction. 1. INTRIDUCTION The DONET (Development of Dense Ocean-floor Network for Earthquakes and Tsunamis) is a distinctive development program of science use of submarine telecomm cable technologies in Japan. This program has aimed to establish a large scale real-time seafloor research and surveillance infrastructure for earthquake, geodetic and tsunami observation and analysis. The initial plan scheduled to install twenty sets of complex cabled monitoring observatories at the interval of 15-20km to cover an active seismogenic zone called Nankai-Trough. A precise earthquakes monitoring capability that performance is equal with the earthquakes observation network on land, can be obtain when the observatory construction round out as original plan as Figure 1 [1], [2]. 2. CABLE SYSTEM DESCRIPTION One of a great challenge of underwater technology is keeping up the large scale system extend over a long period of time in sea-water. In the development of submarine cabled observatory network, the increase of observatory has a large Figure 1. DONET system and observatory arrangement at Nankai Trough. Copyright 2010 SubOptic Page 1 of 7

2 influence on the total system reliability, because of the components of observatory are most tricky part to secure the reliability. The conventional in-line cabled seafloor observatory design is not meets the requirement of large scale system. A novel system design concept is necessary to surely manage a submarine cabled observation network for long period of time (20-30 years). The solution of the requirement is replaceable, maintenanceable and expandable network system configuration and redundancy for the internal or external system failure. The observation network is composed of three system components with different reliability design (Figure 2). There are high reliability backbone cable system, replaceable science node, and expandable observatories. The backbone cable system consists of several types of submarine cables, repeaters, branching units, and termination equipments (science node interface). The constituent of the observation network introduce existing submarine telecomm cable technologies as much as possible to secure the high reliability. The design concept of backbone cable system is not expected any primary system failure in the operation lifetime. The either end of the backbone cable system will be landing and connected to the constant current DC power supplies with different polarity to Figure 2. A design concept for DONET real-time seafloor observatory network. have the redundancy of power feeding channel. The backbone cable system designed to allow loading up to 3kW (3kVDC / 1A) electric power in operation. The independent optical fiber system is allocated from terminal equipment in the science node through the termination equipment respectively (pier to pier connection) by two independent routes (ring topology). The optical amplifiers (repeaters) are prepared every 40-80km optical fiber length interval to achieve the long distance data transmission. These repeaters fit into the coherent optical timedomain reflectometry (C-OTDR) optical fiber trouble location tracking. The branching units (BU) can control the high voltage switching relay to control the power feeding path to the science node and has the ability to electrically separate a science node from backbone cable system when the science node becoming unexpected status. An original double conductor submarine cable was developed in this project for the connection between BU and termination equipment (science node) and was confirmed the performance met the ITU-T recommendations. The science node is a device with the function of hub to connect the observatory to the backbone cable system. The science node can be put up and take off to the backbone cable system with an optical fiber and electrical hybrid high voltage underwater mateable connector. Each science node equipped 8 standard (up to 1KVA connection) hybrid underwater mateable connectors for observatory interface. The power distribution control, data transmission control and time synchronization control are the functions that required for science node. To put these functions on a firm footing is an indispensable solution for scientific use of submarine cable technology. The power distribution system is managing the divergence of power from backbone cable system to observatories. It receives 500watts of constant current DC power Copyright 2010 SubOptic Page 2 of 7

3 supplied from terminal equipment and distributes 40 watts maximum secondary power to each observatory interface as the occasion demands. A constant current DC power system was selected for secondary power output to avoid the general system failure and to get high power transmission efficiency through long submarine cable connection between science node and observatory. The power distribution system has a balanced converter that equalizer the total power consumption of science node constant regardless of the change of the secondary load as Figure 3. This function is valuable to prevent the observation network from the unstable condition of secondary load (such as fluctuation of observatory power consumption, addition / deletion / failure of observatory, problem of power feeding line, etc.). The data transmission control system is managing the distribution of downlink signal from terminal equipment, collection and uplink the data from observatories and sharing precise timing and clock information in the network. A SONET /SDH (Synchronous Digital Hierarchy) network management protocol is used for communicating digital information in this system over optical fiber. Data transmission capacity between terminal equipment on land and a science node on seafloor is approximately 600Mbit/s. A 50Mbit/s bidirectional data transmission capability is secured between science node and each observatory. Precise timing information is embedded in SDH section overhead for time synchronization control. The time synchronization control system can be secure the less than 1 microsecond of precise time synchronization among the component of network system (Figure 4). Figure 3. A block diagram of balanced power converter in science node. Figure 4. A block diagram of DONET telemetry and precise timing control system. Copyright 2010 SubOptic Page 3 of 7

4 The observatory is composed of a sensor package and an extension cable system. The variety of seismometers, pressure gauges and other instruments are put together for sensor package to realize the precise and broadband earthquake observation as Figure 5. The measurement environment control on seafloor is an awaiting solution for the high quality observation on seafloor. Bury the sensor package into the sediment layer on seafloor is expected to reduce the background noise effect from seafloor environment [3], [4]. In preparation for an unexpected system failure, the sensor package can equip the backup data storage and battery for the measures of accident. The data stored in the data storage can be up-load to terminal equipment simultaneously with real-time data when the system failure is restored. In the implementation state, the observatories deployed on the seafloor are connected to one of the five science node Figure 5. Description of DONET standard observatory component. Figure 6. Operation scenario of observatory construction by ROV. in backbone submarine cable system with a point-to-point link as star formed topology (Figure 2). The 10km length of extension cable will be secured the power distribution and communication channel. The standard hybrid underwater mate-able connectors fitted up the both end of the extension cable, make possible the maintenance or replacement of observatories on the seafloor without difficultly. 3. SUBSEA ENGINNERING ROV for OBSERVATORY CONSTRUCTION Figure 6 shows a scenario of subsea observatory construction. The science node and the observatory installed in the seafloor will be connecting by the extension cable system in the sea. The extension cable has to be laying between exact two points away at 10km without any position error. The conventional cable laying method by cable laying ship is not fit for this kind of operation requirement, a ROV (Remotely Operated Vehicle) based thin submarine cable laying method is contrived for seafloor observatory construction. The cable laying ROV (Photo 1) is remodeling of Japanese research ROV Hyper Dolphin to loading 10km length of extension cable and control the cable pay out to seafloor [5]. The cable laying system consists of a tension controlled extension cable management system, a cable bobbin elevator and a VBCS (variable buoyancy control system). The tension controlled extension cable management system (Figure 7) can control the cable payout speed voluntarily to manage the reasonable cable slack correspond to laying course or undulating seafloor terrain. The slip roller and bobbin break mechanically managed the cable payout tension 30kg constant. This tension control is suited for the extension cable breaking strength of 100kg that is a design value to prevent unexpected restrict of ROV with extension cable in the sea. Copyright 2010 SubOptic Page 4 of 7

5 conference & convention The cable bobbin elevator (Figure 8) make possible to equip and release the cable bobbin together with 10km length of extension cable in air and in water. The elevator actuated by the hydraulic power supplied from ROV. It is generate one ton Photo 1. An image engineering ROV. of subsea of pulling torque that is a sufficient power to lift the cable bobbin of 650kgf in air. The cable bobbin is fixed to the chasse of cable laying system with the pair of stab rod when operating. The VBCS is a system can adjust the buoyancy of ROV by pouring or draining seawater to the pressure resist water tank. The Hyper Dolphin has the performance to manage approximately 100kg of buoyant using vertical thrusters. However, this number is not sufficient for the cable laying operation which buoyancy variation is 220kg during the operation. The VBCS equipped a pair of 50L volume pressure resist water tank to compensate the 100kg of buoyancy additionally in water to maintain the mobility of the ROV in cable laying operation (Figure 9). In addition to these main components, cable laying ROV comes to be able to implement cable recover operation by equipping the cable traverse actuator. All additional tools for observatory construction are actuated by the hydraulic pressure supplied by the ROV hyper dolphin hydraulic interfaces for user payload. 4. TRIALS Figure 7. An image of extension cable management system. The development and tests of cable laying ROV has been advanced from 2006 aiming at the DONET seafloor observatory construction scheduled First sea trial is conducted 2006 to confirm the function of three main component of cable laying system and also the launch and recovery operation from mother ship (Photo 2). The system successfully carried out the 5km length of cable pay out and wind up operation on complex terrain seafloor as Photo 3 in The VBCS hardware and software are installed to the hyper dolphin system in 2008 and individually implement the functional test with cable laying system to correct the performance data Figure 8. An image of cable bobbin elevator. Copyright 2010 SubOptic Page 5 of 7

6 Figure 9 A consideration of buoyancy variation of ROV in operation 5. CONCLUDING REMARKS The development and manufacturing of DONET backbone cable system was completed at the end of The backbone cable system installation by cable laying ship was started Jan and will be complete early spring. The development of subsea engineering ROV for DONET observatory construction has been conducted since The performance of the major components of engineering ROV is successfully confirmed by the several sea trials up to the present. Based on the result of the sea trials, two or three ROV working days are essential for construct one observatory. This result means the approximately 50 ROV working days are necessary to construct the twenty sets of observatories according to the initial plan of DONET first phase. The node installation and observatory construction cruise will be scheduled to start from this spring. Photo 2. Cable bobbin elevator operation on deck Photo 3 Cable laying operation on undulated seafloor. 6. REFERENCES [1] K. Kawaguchi, E. Araki, and Y. Kaneda, A Design Concept of Seafloor Observatory Network for Earthquakes and Tsunamis, Proceedings of International Symposium on Underwater Technology 2007 / International Workshop on Scientific Use of Submarine Cables and Related Technologies 2007, April 2007, Tokyo, JAPAN. [2] K. Kawaguchi, Y. Kaneda, and E. Araki, The DONET: A real-time seafloor research infrastructure for the precise earthquake and tsunami monitoring, Proceedings of OCEANS'08 MTS/IEEE KOBE-TECHNO-OCEAN '08 (OTO'08) 2008, April 8-11, Kobe, JAPAN. Copyright 2010 SubOptic Page 6 of 7

7 [3] E. Araki, K. Kawaguchi, S. Kaneko and Y. Kaneda, Design of deep ocean submarine cable observation network for earthquakes and tsunamis, Proceedings of OCEANS'08 MTS/IEEE KOBE- TECHNO-OCEAN '08 (OTO'08) 2008, April 8-11, Kobe, JAPAN. [4] S. Kaneko, E. Araki, K. Kawaguchi, et al. Installation method of high-quality seismic observation in the seafloor, Proceedings of OCEANS'09 IEEE Bremen 2009, May Bremen, Germany [5] K. Kawaguchi, S. Kaneko, T. Nishida and T. Komine, Cable Laying ROV for Real-time Seafloor Observatory Construction, Proceedings of OCEANS'09 IEEE Bremen 2009, May Bremen, Germany Copyright 2010 SubOptic Page 7 of 7