Cabled Seafloor Observatories Research Articles

A deep-sea in-situ chemical laboratory for cabled seafloor observatories

A deep-sea in-situ chemical laboratory (DICL) is proposed for new insight into ocean chemical evolution. The DICL integrates a deep-sea in-situ anion analyzer (DIAA) and several chemical sensors connected to cabled seafloor observatories via a science instrument interface module (SIIM). It can measure the concentrations of seawater anions (e.g., Fˉ, Clˉ, Brˉ, NO3ˉ, SO4 2ˉ, PO4 3ˉ), chlorophyll-a, dissolved oxygen, and methane, as well as pH value. The system operation status can be monitored remotely on the shore, and the science data can be acquired in real-time. After water pressure tests and shallow sea trials, the DICL was tested in the Monterey Accelerated Research System (MARS) deep-sea observatory and collected high-resolution time-series chemical parameters. The experimental results demonstrated that the DICL's mechatronics integration design is reasonable and feasible.

Design of a Highly Compatible Underwater Wireless Power Transfer Station for Seafloor Observation Equipment

Conventional cabled seafloor observatories (CSOs) power in-situ instruments via wet-mated or dry-mated direct electrical contact (DEC) connectors to achieve long-term and real-time observation. However, the DEC connectors have high risks of water leakage and short circuits in power feeding, especially under high water pressure. This paper proposes a highly compatible underwater station based on inductive wireless power transfer (IPT) technology to address the above reliability issue. A novel energy transmitter with runway-structure coils is applied to the proposed underwater station to cover a sufficient power feeding area so that various in-situ equipment can be powered with desirable misalignment tolerance. The magnetic field distribution is analyzed by both derivation and finite element analysis (FEA) methods, and the principal parameters of the transmitter are further optimized and compared with both the mixed-integer sequential quadratic programming (MISQP) algorithm and the evolutionary algorithm (EA) for better performance. The same results show a reliable optimization process. The WPT circuit characteristics are also investigated to power different loads and improve the power transmission efficiency. The output power decreases, and the transmission efficiency rises and then decreases as the load increases. In addition, receivers with higher mutual inductance have better transmission performance. A prototype of the underwater station has been tested both in air and in water, and the experimental results have proven it can power multiple seafloor observation instruments stably and achieve compatibility requirements. The maximum output power of the station prototype for testing is 100 W, and the maximum overall transmission efficiency is 61%.

Cable Laying Using an ROV

AbstractA remotely operated vehicle (ROV) can install submarine cables along a planned route accurately, thereby deploying cabled seafloor observatories at the intended locations. In this study, we developed an ROV-based cable-laying system. Its manual and automated versions have been successfully used for the installation of the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET). The automated cable-laying system shows many advantages; for example, the cable-laying speed increased by approximately 0.1 knot from 0.5 knot, and the number of operators was reduced by half. Thus, the physical and mental burdens on the shipboard members including ROV operators were relieved. In this paper, we share our experiences when laying cables using an ROV during DONET installation. First, the issues that should be addressed to complete a successful cable-laying operation are discussed; subsequently, our manual and automated cable-laying systems are introduced. Then, the observations made during field operations for DONET installation are presented, which provide useful tips for developing an ROV-based cable-laying system.

A Novel Fault Location Approach for Scientific Cabled Seafloor Observatories

The maintenance of scientific cabled seafloor observatories (CSOs) is not only extremely difficult but also of high cost for their subsea location. Therefore, the cable fault detection and location are essential and must be carried out accurately. For this purpose, a novel on-line fault location approach based on robust state estimation is proposed, considering state data gross errors in sensor measurements and the influence of temperature on system parameter variation. The circuit theory is used to build state estimation equations and identify the power system topology of faulty CSOs. This method can increase the accuracy of fault location, and reduce the lose form shutting down a faulty CSO in traditional fault location methods. It is verified by computer simulation and the laboratory prototype of a planned CSO in the East China Sea, and the fault location error is proved to be less than 1 km.

Large-Signal Stability Analysis of the Undersea Direct Current Power System for Scientific Cabled Seafloor Observatories

A large number of power electronic converters and long-distance submarine cables are an important part of the undersea direct current (DC) power system of the scientific cabled seafloor observatories (CSOs). However, the constant power load (CPL) characteristics of the converters and the distributed parameter characteristics of long-distance submarine cables greatly affect the stability of the CSO DC power system. This paper analyzes the large-signal stability of the CSO DC power system, and the equivalent circuits of long-distance submarine cables are established by theoretical analysis and computer simulation. A simplified computer simulation model and an equivalent experimental prototype model of a single-node CSO DC power system was built in the laboratory to study this issue. The mixed potential function method is used to analyze the large-signal stability of the CSO DC power system, and the large-signal stability criterion is obtained theoretically. The validity of the large-signal stability criterion is proved by simulations and experiments. The conclusion is that reducing the inductance of the submarine cable, increasing the capacitance of the submarine cable, increasing the output voltage of the shore station power feeding equipment (PFE) or reducing the power consumption of the undersea station, are beneficial to improve the large-signal stability of the CSO DC power system.

A Current-to-Voltage DC-DC Converter for Powering Backbone Devices of Scientific Cabled Seafloor Observatories

In scientific cabled seafloor observatories, branching units and optical repeaters are essential backbone devices, in which zener diodes are commonly used in their power supply circuits. However, the low efficiency of zener-diode-based power feeding modules under large currents makes for a significant threat to the long-term reliability of backbone devices. In this paper, a novel full-bridge DC-DC converter with a duty-cycle-overlap control (DCOC) strategy is proposed to achieve high-efficiency current-to-voltage conversion. The circuit design of the converter and the principle of the DCOC strategy are analyzed. A prototype of the converter is implemented to demonstrate the feasibility of the proposed power feeding approach for large-scale cabled seafloor observatories.

Use of a coded voltage signal for cable switching and fault isolation in cabled seafloor observatories

Cabled seafloor observatories play an important role in ocean exploration for its long-term, real-time, and in-situ observation characteristics. In establishing a permanent, reliable, and robust seafloor observatory, a highly reliable cable switching and fault isolation method is essential. After reviewing the advantages and disadvantages of existing switching methods, we propose a novel active switching method for network configuration. Without additional communication path requirements, the switching method provides a way to communicate with a shore station through an existing power transmission path. A coded voltage signal with a distinct sequence is employed as the communication medium to transmit commands. The analysis of the maximum bit frequency of the voltage signals guarantees the accuracy of command recognition. A prototype based on the switching method is built and tested in a laboratory environment, which validated the functionality and reliability of the method.

A Sensor Control Model for Cabled Seafloor Observatories in the East China Sea.

Seafloor observatories enable real-time, continuous and long-term observations that promise major breakthroughs in ocean sciences. The effort to dynamically control in situ sensor systems performing individual and cooperative observation tasks is both a challenge and a guarantee for the stable operations of functional observatories. However, current sensor control systems for seafloor observatories are application-oriented and focus on specific engineering requirements rather than general model research. In this paper, a sensor control model was proposed to provide a theoretical specification for designing, developing and deploying a sensor control system for cabled seafloor observatories. The model abstracted the sensor control as a standardized bidirectional information flow process and accordingly structured the control system into the application layer, the service layer, the networking layer, and the sensing layer. The layered functions and interactions between adjacent layers in return realized this information process. A prototype control system was designed and developed, the monitoring architecture of which was derived from one of the deployment scenarios of the sensor control model. The prototype system was tested for its plug-and-play enablement through a series of trials such as tank tests and shallow sea trials. It was put into service for the operational Xiaoqushan Seafloor Observatory and was consistently functioning and presenting satisfactory practical performance in accordance with all the requirements laid for the project.

Switching High Voltage DC Power in Branching Units of Large-scale Cabled Seafloor Observatories

Cabled Seafloor Observatories(CSOs) are powered by shore stations with DC voltage levels up to -10kV. When a backbone or spur cable fault occurs, the power system must isolate the fault by disconnecting the faulty segment quickly. So the BUs installed in the backbone cable need to carry out the switching operation under high voltages(HV). However, DC circuits have no zero-current points and the fault currents will rise rapidly, bringing a big challenge to the switching on/off operation. In this paper, we have purposed an HVDC circuit breaker based on series-connected IGBTs with voltage balance. Under the backbone voltage of -10kV and a peak current of around 14A, the simulation results show that the proposed design is feasible, with the switching-off time about 1ms.

Smart IEEE–1588 GPS Clock Emulator for Cabled Ocean Sensors

Today, cabled seafloor observatories are installed at many sites around the globe, gathering different types of sensors in the marine environment where a Global Positioning System (GPS) signal is not accessible. Accurate time marking of ocean sensor data is highly important in many marine applications. This paper presents a smart GPS emulator based on the IEEE-1588 Precision Time Protocol (PTP). The device was designed and implemented to be able to provide accurate timing data (trigger + time code) to any ocean sensor as a broadband seismometer. In this case, accurate location and magnitude of a detected earthquake are dependent on the accuracy of the data time marks. The performance of time synchronization is tested, using a commercial broadband seismometer, and the results are presented. These tests are based on a comparison of the synchronization trigger between master and slave clocks as well as the analysis of the data acquired by the seismometer. The work presented here leads to an improved performance of the ocean-bottom seismometers as well as tsunami warning systems.

Surface-sediment bioturbation quantified with cameras on the NEPTUNE Canada cabled observatory

The mixing of deep-sea sediments by benthic megafauna is an important ecological service that influences biogeochemical processes. Quantifying the contribution of individual spe- cies to bioturbation and their responses to environmental variations requires experimental manip- ulation or direct observation, both of which are logistically challenging in the deep sea. Emerging cabled seafloor observatories now permit real-time data transfer to shore and interactive sam- pling, providing a new tool for long-term studies of the benthos at high temporal resolutions. We report on the development of a methodological approach to study surficial bioturbation by megafauna in a submarine canyon by using video cameras remotely operated over the internet, through the NEPTUNE Canada observatory. Observation protocols and image analysis tech- niques were developed to quantify organism size, locomotion and appearance rates for 2 flatfishes (Dover sole Microstomus pacificus and Pacific halibut Hippoglossus stenolepis) and the fragile pink sea urchin Allocentrotus fragilis. Application of a Bayesian model to extrapolate megafaunal locomotion patterns and appearance rates yielded sediment-surface reworking rates on the order of 26.0 to 35.1 cm 2 yr −1 . Future observations can be directly incorporated into the model to improve accuracy. We propose that this combined observation and Bayesian modeling approach could become a useful component of a long-term program for monitoring ecological processes on the seafloor.

Temperature correction and usefulness of ocean bottom pressure data from cabled seafloor observatories around Japan for analyses of tsunamis, ocean tides, and low-frequency geophysical phenomena

Ocean bottom pressure (OBP) data obtained by cabled seafloor observatories deployed around Japan, are known to be significantly affected by temperature changes. This paper examines the relationship between the OBP and temperature records of six OBP gauges in terms of a regression coefficient and lag at a wide range of frequencies. No significant temperature dependency is recognized in secular variations, while substantial increases, at rates of the order of 1 hPa/year, are commonly evident in the OBP records. Strong temperature dependencies are apparent for periods of hours to days, and we correct the OBP data based on the estimated OBP-temperature relationship. At periods longer than days, the temperature corrections work well for extracting geophysical signals for OBP data at a station off Hokkaido (KPG2), while other corrected data show insufficient signal-to-noise ratios. At a tsunami frequency, the correction can reduce OBP fluctuations, due to rapid temperature changes, by as much as millimeters, and is especially effective for data at a station off Shikoku (MPG2) at which rapid temperature changes most frequently occur. A tidal analysis shows that OBP data at a station off Honshu (TM1), and at KPG2, are useful for studies on the long-term variations of tidal constituents.