Spray Gliders Seaglider Slocum

Autonomous platforms/underwater vehicles Buoyancy driven by battery (or thermal) powered hydraulic pumps Vary their volume to change boyancy and glide forward Dive and climb through the ocean Have wings and/or rudders to turn Forward pitch and roll angle controlled by shifting and rotation of internal mass (such as a battery) Roll-rotation around the axis of the glider, changes the orientation of the wings

Ascent/Descent Ascent/descent angle & heading of Spray are controlled by moving batteries internally to alter the pitch and roll of the glider

Components Wings Outer Hull (2 types) Batteries Hydraulic System http://spray.ucsd.edu/ Wings Outer Hull (2 types) Batteries Hydraulic System Compass Altitude Sensor GPS receiver Iridium transceiver Microprocessor controller Aft Flooded Bay - External bladders - Sensors selected for mission

Hull Types Simple aluminum (Spray and Slocum) Compressibility of hull is less than that of seawater Compound hull (Seaglider) Flooded fiberglass fairing Streamlined laminar-flow shape Interior aluminum hull Carbon fiber (new deep Slocum version) Batteries- stored energy for bouyancy control & electronics Lithium thionyl chloride Twice the energy/unit mass (compared to Alkaline) Longer shelf life Alkaline Safer (less probability for explosive failure) Less expensive than Li 60-70% of bat. used for propulsion, the rest for comm. & onboard functions

Electric Buoyancy Engines Single-stroke hydraulic pumps Immune to 'vapor lock' Pump cylinder fills with gas, compression ratio of pump affected Used on Slocum Shallow & coastal operation, more powerful motor needed Reciprocating (multi-stroke) pumps Smaller and Lighter Sensitive to 'vapor lock' Seaglider & Spray

Sensors CTDs (some w/seawater pump) Backscatter Sensor Acoustic Altimeter ADCP Chlorophyll Fluorometer Optical properties and radiation Sensors Oxygen Sensors Now also nutrient sensors (optical) there are constraints to the types of sensors that can be used, in terms of size, mass, power consumption

Webb Research (WRC) also makes a deep electric glider now, 1000m max. And is still working on a thermal engine glider with following specs: New battery technologies are currently being explored (seawater batteries and laptop fuel cells) which might increase endurance by factor 2-3.

Gliders – Slocum Thermal 60-80% of the energy goes into propulsion, so a thermal-powered glider may have a range 3-4 times that of a similar eletric-powered vehicle. a b c d

Slocum thermal glider flying:

Commercial availability, updated costs: Sprays only built for in-house projects/collaborators. No serious commercial vendor yet. Cost still 45-50k$ for basic CTD glider. Webb gliders cost 70-90k (coastal vs. deep version), plus extra sensors Seaglider is in-house cost of 90k, the 6000m version projected to be 120k. Commercial availability limited, cost 120k$ and up. Rule of thumb for operating costs: Need about 30k$ for each deployment (currently 3-6months each), for deployment/recovery (man-power, travel, boat-rental), telemetry costs, new batteries, service work, piloting (easy if nothing happens, but time-sink when problems), testing. Have to allow also for wear and loss, probably need to budget a new glider every 3-5 years.

Velocity estimates (averaged over diving depth): “Dead reckoning”  Measurement of vertically averaged currents from the difference between dead reckoning position and GPS position fix.  Distance, time and resistance are known  velocity

2 ways to get velocity profiles: Use density profiles between separate dives to get geostrophic current profile, but only relative shear - can be combined with depth-average current from dead reckoning to make flow profile absolute (only component perpendicular to path) ADCP on glider can give small overlapping segments of vertical shear which can be integrated to give a profile. In general, the 2 methods agree reasonably well.

Virtual Mooring mode: As long as currents are not stronger than glider speed, a glider can be programmed to perform repeated profiles while holding horizontal position nearly constant.

Advantages No external moving parts or motors (remotely) controlled horizontal position (profiling floats lack this) Deployment from small boats Reuseable Post-calibration of sensors possible Operational Cost of making a section as low as $2/km “Gliders can be operated for a year for the cost of a single day of research vessel operation” (Eriksen 2003), BUT…. See next slide Antennas in wings of Spray Eliminates unnecessary drag Communication still possible with 1 antenna damaged Can provide sustained sampling of the subsurface ocean, over long ranges and times, even in adverse weather conditions

Disadvantages Slow speeds  may get carried away, not good in strong currents  confuse temporal variability with spatial structure?  may alias flow in presence of variability/eddies (not synoptic) Max depth only to ~1500m (but SeaGlider being developed for 6000m now) Can't take water samples Not produced in large numbers, can't have many like drifters & floats Sensor constraints Must be small & included internally Low weight Low in power consumption: O(0.1J)  sustained O(1m) depth res. Bulky sensors increase drag, decrease mission length Operational cost often underestimated: need order 25k$ per deployment (batteries, telemetry, shipping, manpower for deployment/recovery, control) Impact on daily routine/work/life…. Biofouling, increase of drag, decrease of range, loss of sensor accuracy May get caught in a fishing net May get run over by a ship while transmitting data (or attacked by sharks)

Biological Hazards

Telemetry costs (more later in class): Typically (for Spray) transmit 2kb data per dive, and 4 dives per day (if 1000m). Total 8kb/day. Iridium SBD (email) service costs approx 1$/kb, so about 300$/month, or $1000-$2000 per deployment (2x that for shallow dives). If more data are wanted it get VERY expensive. Will then need to move to direct dial-up Iridium which costs about $0.1/kb. Impact of speed: Dynamic drag is limiting factor, so power needed goes with square of speed. Going very slow requires very little pumping  very long endurance. Typical lifetimes quotes are at standard speed of 20-25cm/s. If needed, can pump a lot and go up/down steeply, to achieve speeds of 40-45cm/s, but reduces mission A LOT. Try to exploit sheared current or slower depth-average flow (that’s what displaces glider), or even tidal flows.

Some example applications of gliders….

S and v on CalCOFI lines 80 and 90

Chlorophyll in a subsurface eddy Line 93, contour intervals S 0.1, σθ 0.25 Warm salty eddy from south

Mean flow Cores of poleward flow inshore and at depth ADP Geo Cores of poleward flow inshore and at depth Equatorward California Current offshore and shallow

Rossel Isl Gizo Honiara

New developments: Acoustic modem data shuttle (prototype at SIO) Under-ice glider (with RAFOS navigation), prototype at APL/UW 6000m glider with 18month endurance, 10000km range (Seaglider) N.B.: increasing endurance cannot be done by just adding batteries: the increase in inertia/mass, and increase in size/drag offset the energy to a large extent. Really requires more efficient pumps, compressibility compensation (make it like seawater), reducing drag, monitoring energy use to run batteries to 5%, or better energy sources.

Data shuttle glider….

diving below strong currents.... (?)        Navigating acoustically below the surface 

Requirements Checklist  a) Long Range/Remote Regions b) Lightweight compared to other inst. c) Can't take samples d) i. Measurements to 1500m ii. Not to sea floor e) Vertical sections- sawtooth path f) Continuous observations g) Scalable to situation h) No remote sensing i) Not a stable platform j) Higher accuracy, not expandable or cheap inst. k) Data telemetry possible l) Controlled buoyancy, do not follow water mass motions            

Sources SIO IDG Spray home page http://spray.ucsd.edu APL Seaglider home page http://seaglider.washington.edu/ and www.apl.washington.edu/projects/seaglider/summary.html Slocum home page www.webbresearch.com/slocumglider.aspx Publications (all on class server) Davis, R.E., C.E. Eriksen and C.P. Jones, 2002. Autonomous buoyancy-driven underwater gliders. Pp 37-58 in The Technology and Applications of Autonomous Underwater Vehicles.G. Griffiths, ed, Taylor and Francis, London. 324 pp. Rudnick, Daniel L., Davis, Russ E., Eriksen, Charles C., Fratantoni, David M., Perry, Mary Jane, 2004. Underwater Gliders for Ocean Research, Marine Technology Society Journal., 48-59. Autonomous Underwater Gliders. ALPS Workshop. La Jolla, CA, April 2003. OceanObs09 white paper draft

Other websites with missions, operations, data, plots: www.locean-ipsl.upmc.fr/gliders/EGO www.noc.soton.ac.uk/omf/lmm/glider/index.php www.ifremer.fr/lpo/gliders/welcome.htm http://spray.ucsd.edu http://marine.rutgers.edu/ (go to “research” and “coastal ocean observation lab”)