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www.sea-technology.com February 2017 / st 25 communication to the surface telem- etry buoy. Telemetry Gateway Buoy For relaying the subsea data via sat- ellite, a develogic SB.600 sensor-and- telemetry buoy was selected. This buoy features a spar-type design with 600- mm diameter and an overall length (including mast) of 12 m. Due to the lack of external antennas and cabling and the hermetically sealed design with a pressure rating of 10 bar, it provides maximum robustness under severe conditions. Due to a small buoyancy cross-section, it is very stable in rough seas, minimizing mooring load. The buoy is equipped with multiple com- munication interfaces, as well as exten- sive monitoring and safety subsystems. To address the possibility of damage to external cabling, the subsea acous- tic modem is directly integrated into the lower bulkhead. The steel mooring wire acts as a carrier for the inductive telemetry. An AIS aids-to-navigation (AtoN) transponder, active X/S-band radar re- flector, high-intensity navigation light and backup GPS/Iridium beacon pro- vide maximum safety with regard to collision avoidance. The integrated solar power system with a rechargeable battery capacity of 7,200 Wh, supplemented with redun- dant hydrogen monitoring and a cata- lytic conversion safety system, together with the optimized power consumption of the entire setup, is able to support continuous long-term deployments, even in regions with low insolation. Telemetry Network The challenge of this project was to design a robust multinode subsea com- munication network around the opera- tion area that takes minimal installa- tion effort. As counterfilling installation work had to be stopped in case no real- time monitoring data were available and sampling periods were as short as 2 min., the highest communication reli- ability was mandatory to minimize idle time of the two on-site vessels, one of which contained the materials for the counterfill and the flexible fallpipe to distribute them. With the available ex- pertise in acoustic communication and the ease of deployment of systems with a wireless subsea data link, the choice of acoustic modems was natural. But as it was clear from the beginning that the disturbance resulting from the counter- fill rock and sediment dumping would cause high wide-band noise levels of unpredictable sound pressure, induc- tive communication through steel wire was selected as a second means of communication to minimize the risk of downtime. All seafloor landers and the surface buoy were equipped with develogic HAM.Base acoustic modems, allowing on-site baseline measurements imme- diately after lander deployment, prior to the start of counterfilling. During the baseline measurement period, the ring-type inductive network interconnection was made by ROV. Every lander was equipped with an ad- ditional inductive modem and an ROV- transportable reel with a capacity of up to 1,300 m of 2-mm coated steel wire cable. The ROV picked up the reel from one lander, laid out the cable while traveling to the next lander's position and made the connection there via a contactless interface. To make an ROV operation for recovery obsolete, the in- ductive connector was designed so that the reels together with the cable were to be ejected upon triggering the ballast release of the landers. During the active counterfill instal- lation, data from the seafloor landers connected to the inductive network were relayed via the mooring to the sur- face gateway buoy. Data from a remote seafloor lander responsible for collect- ing reference data were transmitted via the acoustic link. Additional landers sampling backup data were installed at a later time, relying solely on acoustic communication with the surface buoy. Sonar Systems firstname.lastname@example.org USA 1.508.291.0057 CHOOSE THE LEADER IN UNDERWATER TECHNOLOGY Acoustic Releases and Transponders USBL Positioning and Tracking "The subsea part of the monitoring system was designed around develogic CSL.1000 compact sea- floor landers equipped with inductive and acoustic telemetry."