is required. At Sites 1150 and 1151, where the water depth is ~2500 m, an oxygen concentration of ~2.7-3.6 ppm is expected based on previous study near these sites (M. Kawabe, pers. comm., 1998).
The power for all the NEREID system is supplied by the SWB system, which consists of three SWB1200 (Kongsberg Simrad, Norway) cells (Hasvold et al., 1997), a DC/DC converter, and an accumulator. The cell is a magnesium/oxygen battery based on a magnesium anode (negative electrode) that uses seawater as the electrolyte and oxygen dissolved in the seawater as the oxidant.
The chemistry of the cell is the dissolution of magnesium at the anode, given as
and consumption of oxygen at the cathode,
which is written in a simplified form
The formation of an alkaline at the cathode surface may lead to the formation of a calcareous deposit as follows:
The alkaline reaction products need to be removed from the cathode surface by sea current because the calcareous formation disturbs the reaction (Equation 2) at the cathode.
The anodes are AZ61 magnesium alloy rods with a diameter of 184 mm and a length of 2200 mm, including the anode connector device. The anode can be replaced by ROV and is surrounded by the cathodes suspended from the titanium frame (Fig. F19). The weight (in air) of each anode is 110 kg and that of the titanium cathode frame is 62 kg. The cathode element consists of a titanium wire core with carbon fibers oriented radially (Fig. F20). The carbon fibers allow rapid material transport and high current density. The cathode collector lead is connected to the titanium frame, which is also part of the cathode. The titanium frame allows seawater to pass easily through the cells so that oxygen-rich seawater is supplied to the cathode and the products of the cell reactions are removed.
The obtainable cell voltage is ~1.6 V, although this depends largely on the conductivity of the seawater, which may vary with temperature and salinity. The catalytic effect of bacteria colonizing on the cathode surface, which was observed on all seawater cells in previous deployments of the system, is another of the many factors affecting the cell voltage. The maximum cell power is limited by the rate of the supply of oxygen to the cathode. The oxygen supply rate is proportional to the oxygen concentration in the seawater and the speed of circulation. To produce the designed output of 6 W for each cell, a minimum circulation of 20 mm/s, oxygen concentration of 3 ppm, and minimum salinity of 20 is required. At Sites 1150 and 1151, where the water depth is ~2500 m, an oxygen concentration of ~2.7-3.6 ppm is expected based on previous study near these sites (M. Kawabe, pers. comm., 1998).
Because the cells have an open structure, the isolation between them is low, which leads to large leakage currents in serially connected cells. The cells are consequently connected in parallel. The DC/DC converter changes the low cell voltage (1.6 V) into the output voltage (42.0 V). The output of the DC/DC converter is fed to the accumulator that averages the power demand on the DC/DC converter and the seawater cells. After deployment of the cells, the DC/DC converter is inactive until the cell voltage becomes >1.54 V. After the cells are activated, the DC/DC converter takes power from the cells and charges the accumulator as long as a sufficient cell voltage (>1.28 V) is available. If the cell voltage becomes lower than that threshold, the DC/DC becomes inactive until the cell voltage is restored to 1.54 V. The low threshold depends on the status of the accumulator cell charge. The lower the cell charge is, the lower the threshold becomes. The lowest threshold is ~1.28 V. The accumulator consists of multiple 2-V Cyclon (Hawker Energy) lead acid cells that form a 5-Ah 36-V cell in total. The accumulator cell is float charged by the DC/DC output. The voltage of the accumulator output is 42.0 V when the accumulator cell is fully charged and has no charging voltage applied by the DC/DC converter. The cell is stored in a 6500-m depth-rated pressure housing that has four-pin GISMA series-10 underwater connectors for the load output and the DC/DC converter. The DC/DC converter is also stored in a similar pressure housing but has two Subconn one-way underwater power connectors for the SWB cells and the GISMA connector for the accumulator.
The SWB system is mounted on the PAT, as shown in Figures F21 and F22. The three SWB cells are stored in concentric positions. The PAT is made of ordinary angle steel that is zinc coated; the base is coated with tar epoxy paint to protect it from corrosion. The titanium frame of the SWB cell and the stainless-steel pressure housings for the DC/DC converter and the accumulator are mounted on the PAT with polyvinyl chloride insulators. The top of the PAT is a white, flat panel made of fiber-reinforced plastic (FRP) drainboard that has access holes for the SWB anodes. The flat panel, which viewed from above is circular, serves as the ROV platform and is supported by a metal frame. The diameter of the top plate is 3200 mm. The bottom structure of the PAT is also circular, and the diameter is 3658 mm, which corresponds to the diameter of the reentry cone. The bottom leg is 240 mm in height. The battery cells are elevated above the reentry cone to improve seawater circulation through the cells. The center bottom part of the PAT contains coaxial rings placed to guide it smoothly over the riser assembly on its installation. The hole in the top center part of the PAT provides space for the MEG frame. The total height of the PAT is 2640 mm to accommodate the SWB cells. The vertical position of MEG on the riser is set to allow ROV service.
The PAT also holds the SAM recorder frame beneath the PAT top panel. The top part of the SAM recorder protrudes from the panel. The SAM recorder can be lowered into the hole of the frame panel so that the UMC at the bottom of the SAM bulkhead is mated by gravity force to a UMC receptacle on the stab-plate placed in the SAM frame. In the lowering operation the hole keeps the SAM canister upright; a key on the bottom bulkhead of the SAM canister aligns with the keyway in the hole to provide correct orientation for mating the UMC. The cable from the UMC receptacle has a T-junction: one branch is connected to the SWB system and the other goes to a receptacle mounted on the FRP panel. An installed cable on the panel is used by the ROV to connect the UMC receptacle to the MEG canister. Initially, the ROV cable is fastened to the top panel by fastening mechanisms and a parking connector for the ROV plug. In September 1999, the ROV removed the fasteners and connected the ROV's UMC plug to the MEG canister. The SAM recorder can be ejected with help from an ROV-operated lever mechanism in the SAM frame (Fig. F23). The lever can be locked at two positions: one at the mated position of the SAM and the other at the released position. By using the locking positions, the ROV can easily replace the SAM recorders.
