FeMO2 Dive Cruise 2007
Report Day 11 -- Sunday 21 October 2007 -- On the Remotely Operated Vehicle JASON II
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FeMO2 Dive Cruise 2007 |
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The 12-hour maintenance schedule for Jason allows the Jason team to get the submersible in the water at 4 pm. The dive plan calls for the elevator to be sent down into Pele's Pit and for the Kilo Moana to transit south over a site on the south rift of Lo'ihi called Naha. Jason is to be launched at Naha, so it can work its way up the south rift to the pit. Due to a mix up Jason is launched above Pele's Pit. In response the dive plan sequence is reversed so that Jason will now be worked down to Naha.
Jason is not a submarine or robot. It's a type of submersible called a Remotely Operated Vehicle or ROV. It's not a true submarine because it is not manned, and it is not a true robot because it is attached to the ship via a tether. Through the tether Jason gets electricity to run its thrusters, lights and cameras. The tether returns video images, temperature, depth and salinity information as it happens. It is even possible to put an instrument on the science basket for a dive and send its readout up to the ship. Brian Glazer's electrochemistry probes work this way so that he can read and record the temperature and chemical information in the control van as its being measured 5 km away at the bottom of the Pacific Ocean.
In the good old days instruments were lowered into the dark ocean where the instruments recorded the conditions. There were various tricks - weights, timers and pressure sensors - to get the instrument to start or stop recording their data. Hours later the instruments would be pulled back aboard and partly disassembled to retrieve the data. It was simply not possible to do the fine scale work that is done today with these old instruments. For instance, the sampling of vent water for major chemical analysis (the titanium water samplers) requires Jason to first find the highest temperature water in a vent field and then place the snorkel of the sampler in that particular vent - a task totally impossible if there were no way to measure temperature with a high precision at the centimeter scale.
Not all instruments are connected to Jason and the tether, nor do they need to be. Instruments that record over long stretches of time or collect data for which there is no immediate need, simply record the data for later downloading. The Miniature Temperature Recorders, the Temperature Gradient Recorder and the ISEA lander work this way.
ROV's have a great advantage in convenience and safety over manned submarines. Research submarines are cramped, they get quite chilly when working below the thermocline, you use up the oxygen and you pollute the air with CO2, you pee in a red plastic bottle while two other people look out their view ports. Submarines are dependent on a battery charge for movement, for using manipulators, and for lights so that the observers and cameras can see. Every time a light is turned on or a manipulator is moved the dive is shortened. This brings up another point - dive time. Any deep-sea research submersible may take 2 hours to descend to the seafloor and another 2 hours to surface. For a manned submarine with an 8-hour dive time, half the time with the hatch closed can be consumed going up and down leaving only 4 hours for work on the seafloor. Jason's longest dives have been more than 4 days, 96 hours, long. Said another way, Jason can do in 4 days what would require at least 12 days using a manned submersible. The Jason team is able to operate around the clock, day and night; because they stand 4 hour watches the same way a ship's crew does.
Jason is the second ROV named Jason. The first was smaller and could not carry as much weight. This Jason is sometimes referred to as Jason 2, but now that formality is mostly in record keeping - dives are given numbers like J2-314. Jason 2's newer equipment is also more reliable than the old. Jason is owned by the National Science Foundation and operated by Woods Hole Oceanographic Institution, the same research institution that operates the Alvin submarine. There are many ROV's that are used for science operated by universities and research organizations, and many more that are owned and operated by private companies. The maintenance and repair of floating oil platforms makes the oil industry the largest user of ROV's.
If Jason were just hung on a cable (or "wire") at the stern of the ship it would be jerked up and down as the ship rose and fell on the swell. To solve this Jason's partner Medea is hung on the wire and Jason is tied to her. Medea acts as a depressor weight on the end of the wire and moving up and down with the ship. Jason is isolated from the ship's motion with a slack tether that connects Jason and Medea. Copper strands in the tether carry electrical current to Jason while fiber optics transmits data from Jason. The electricity Jason needs is generated aboard the ship and transmitted down to Medea via copper strands in the steel jacketed winch wire. An electrical connection is then made between the winch wire and the tether at Medea. All this wire and all these connections must be insulated from the salt water. The data makes the return trip to the ship via fiber optics and again the connections between the wire and tether at Medea must be perfect. An additional complication is that the winch wire is attached to a heavy lift winch, which complicates the routing of the electricity and data between the wire and the ship.
The winch and wire lift and lower Medea through the water column. Quite differently, the Jason uses its thrusters to descend and ascend. Despite Medea's weight most of the weight hanging on the wire in deep water is the weight of the wire itself. For example (and these numbers are made up) if Medea weighs 2,000 kg (about 4,000 lbs) and the wire weighs 1 kg/meter (about 2 lbs/3feet) on a Jason dive to 5,000 meters the winch wire will be carrying 7,000 kg of which only 2,000 kg is Medea. To compensate for the increased load on the wire it is tapered - the wire diameter increases as more wire is rolled off the winch.
While Jason is under water, the ship is controlled by the Jason team from the control van. While in control the Jason navigator watches a computer monitor that displays the relative positions of the ship, Medea and Jason. The control van has no windows so the ship maintains a bridge watch to be alert for ship traffic or changes in the wind direction. The bridge communicates with the navigator in the Jason van via telephone. The Kilo Moana has a feature called dynamic positioning where the 2 stern propellers and rudders and the 2 bow thrusters (smaller propellers that push the bow side to side) work together to keep the ship in one place. A computer controls the propellers and rudders to keep the ship on a station defined by GPS coordinates. The Jason team uses the Kilo Moana's dynamic positioning system to hold the ship over Jason's work area. When Jason moves, they must move the ship to keep it over Jason. Jason's horizontal speed is kept below 1 km per hour.
Jason's speed limit means that it cannot move back and forth easily to the elevator once it leaves Pele's Pit. Instead of using the elevator for heavy lifting at the end of the dive, the Jason pilots will have to load what they can on the elevator before leaving the pit. Jason leaves the pit through a low point in the crater wall called the "Spillway." A short time later all the screens go blue in the control van - Jason has stopped transmitting data. The Jason team suspects a failure of the fiber optics in the tether. With no way to see what they are doing the Jason team brings Jason and Medea back aboard. Dive 314 ends at 4 AM Monday morning.
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