NOTE: These definitions were borrowed by permission from: MarineTech
AUV
AUV's or Autonomous Underwater
Vehicles are underwater robotic vehicles that do not have a tether to
the surface. They are pre-programmed to operate over a particular course
or to respond to sensor data or perhaps acoustic commands.
Applications include surveying, scientific data collection and mine
hunting.
Batteries
The whole spectrum of battery
types used topside can and are used undersea. Factors such as size,
weight, voltage, capacity, and other characteristics are used to make
the proper selection. There are a few factors unique to using batteries
underwater. One is that some batteries can produce gases that are
potentially explosive. In the open air this is usually not a problem
because the small volume of off-gases are quickly diluted. However, in
sealed pressure housing, gas build up can cause problems ranging from
water leaks to explosive failure. Pressure relief valves can safely
vent gas to eliminate these problems.
Unique to using some batteries
and other electronics subsea is pressure balanced, oil-filled
packaging. In this method all air spaces and voids in a housing are
filled with oil or hydraulic fluid. A flexible membrane on the housing
transmits ambient water pressure through the housing. Because the
pressure inside the housing is the same as the outside water pressure,
it is said to be equalized. Since there is no pressure gradient, a
heavy pressure proof housing is not required.
Seawater batteries utilize
seawater as an electrolyte. Most use electrodes of magnesium for
anodes, and oxygen dissolved in the sea water as oxidant. Advantages of
seawater batteries are that they have a very long shelf life while dry,
can be used to very deep depths, have high energy densities, and have
long duty cycles. Disadvantages are that the cell voltages are low
necessitating DC/DC converters and that the output power can be
influenced by factors such as the water circulation, quantity of
dissolved oxygen, temperature, and salinity.
Bottom Samplers
Bottom Samplers and sediment
sampling devices include a box corer that can drop into the mud and
brings back a block of near-surface sediment. A piston corer can return a
cylinder of sediment up to 100 feet (33 meters) long that may
encompass several million years of sedimentary history. For researchers
especially interested in the seawater-seafloor interface, a gravity
corer can return cores up to 20 feet (six meters) long with little
core-top disturbance. Vibracorers use a motor that produces a high
frequency oscillatory vibration to help propel a long core barrel into
unconsolidated sediments.
Buoys
There are two main classes of buoys: drifting and moored.
As the name implies, drifting
buoys are not anchored to the seafloor. They are typically used to
study currents and circulation patterns. A drogue is sometimes added to
have the surface buoy follow a subsurface current.
Subsurface drifting buoys, aka
floats, usually have variable buoyancy engines to descend to a
predetermined depth where they follow this current and possibly collect
environmental data. The buoyancy engine is programmed to occasionally
bring the float to the surface where it can telemeter its data back to
shore. Trajectories of individual floats show how the water moves
horizontally, and trajectories of groups of floats show how the water
is mixed by eddies. This information is important for understanding how
water tracers and pollutants are transported by the ocean. The Argos
program is a large multinational effort to put thousands of floats in
the world’s oceans.
Cameras
Cameras are used underwater
for all sorts of inspections and documentation. Most types of cameras
that can be found topside can be housed and used underwater. They are
used by divers, on subsea vehicles and as stand alone devices. Film
cameras are rarely used any more because of the limited amount of
images that can be stored before surfacing to change film. Thus for
still images, digital cameras are usually preferred. Video camera types
include color, black & white, and low light. For low light imaged
intensified CCD cameras are used as are tube type cameras known as SITs
(silicone intensified target). Dome port optics are used in some
cameras to correct the distortions caused by looking through a flat
window underwater.
Features to consider when
selecting an underwater camera include, fixed vs zoom lens, field of
view, depth capability, resolution, and sensitivity.
Compasses
Magnetic compasses, as we all
know, consists of a small, lightweight magnet balanced on a nearly
frictionless pivot point. Magnetic compasses have several problems when
used on moving platforms like ships. They must be level, they correct
rather slowly when the platform turns, they are influenced by ferrous
metals and they indicates magnetic north rather than true north . For
these reasons, most ships and airplanes use gyroscopic compasses.
Hydrophones
Underwater Hydrophones detect
acoustic signals in the ocean just as Just as microphones collect sound
in the air. Most hydrophones are based on a special property
(piezoelecticity) of certain ceramics that produce a small electrical
current when subjected to pressure changes. When submerged in water, a
ceramic hydrophone produces small-voltage signals over a wide range of
frequencies as it is exposed to underwater sounds propagating from any
direction. By amplifying and recording the electrical signals produced
by a hydrophone, sound in the sea can be measured with great precision.
Although a single hydrophone records sound arriving from any
direction, several hydrophones can be simultaneously deployed in an
array, and the resulting signals can then be manipulated to “listen” in
any direction with even greater sensitivity than a single hydrophone
element. Whether within an array or as a single element, the hydrophone
is the basic sensor of underwater acoustics. Things to consider when
selecting a hydrophone include receiving response, beam width, and
depth rating.
Lighting
Lighting is used underwater to
provide illumination for divers, for submersible pilots and
passengers, and for cameras. Three general classes of lights exist:
incandescent, arc, and LEDs. Incandescents work by heating a filament to
the point where it radiates light. Quartz halogen lights are the most
commonly used incandescent. The name derives from the quartz glass bulb
used that is filled with a halogen gas which is used to redeposit
evaporated filament materials back on the filament. Incandescent lights
are inexpensive but bulbs are fragile and have relatively short life
times.
Pan & Tilts
Pan & Tilts are
positioning devices used to remotely move cameras, lights, sonars and
other devices. Pan refers to movement along the horizontal axis and
tilt refers to movement in the vertical axis. A scanner is a one axis
positioner. Things to consider when selecting a pan & tilt include
type of motor, power requirements, range of motion, payload capacity,
and position feedback.
ROV
ROV's or Remote Operated
Vehicles are underwater robots are used for a wide variety of tasks
ranging from simple inspection to maintenance and repair work. These
tasks are performed on offshore oil rigs, ship hulls, docks, mooring
buoys, dams, bridges, power plants, and in water tanks. ROVs are
lowered on a cable alone or in a protective cage and then operated on a
slack tether that decouples it from the ship's surface motion. Video
cameras serve as "eyes" for the operator who controls the vehicle via
the cable. An ROV can explore, take photographs, collect samples, or
handle instruments, operating around the clock for many consecutive
days. Job specific tool skids are sometimes mounted under larger
vehicles.
Seals
The type of seals discussed
here are not marine mammals or navy commandos. We are interested in
methods for keeping water out of pressure housings. Types of seals used
in the marine environment include gaskets, shaft seals, lip seals,
stuffing tubes and gland seals. The o-ring is the most common type of
seal used underwater is and is probably the most ubiquitous piece of
underwater technology. It is a solid piece of elastomeric material
shaped like a doughnut or torus. When pressed between mating surfaces,
an o-ring blocks the passage of liquids or gases. The o-ring is the
most widely used seal due to its simplicity, low cost, ease of
installation, and small space requirements without supporting
structures. An O-ring can be considered an incompressible viscous fluid
with very high surface tension. This "fluid" is forced by mechanical
or hydraulic pressure to flow into the sealing cavity, blocking the
flow of the less-viscous fluid being sealed. Properly installed, the
O-ring is squeezed about 10 to 15% of its original cross-sectional
diameter. The compression absorbs the tolerance stack up between mating
surfaces (or between shaft and gland in dynamic applications), and
forces the elastomer into microscopic surface grooves on mating parts).
Sonar
Sonar (SOund NAvigation and
Ranging) is used for many purposes. Passive sonars detect noise from
marine objects, such as submarines, ships, and marine animals. Active
sonars emit a pulse of sound or “ping”, into the water and then listen
for an echo when the signal is reflected off an object. To measure the
distance to an object, one measures the time from emission of a pulse
to reception.
Tether
Tethers can be electrical,
fiber optic, or a combination of the two. Underwater electrical cables
date back to the advent of the telegraph the mid 1840’s. Hemp, tar, and
jute was used around conductors. The advent of plastics and other
synthetic materials brought about a variety of new cable designs.
Underwater Positioning & Tracking Systems
Underwater Positioning &
Tracking Systems are used to identify the location of an object or
vehicle underwater. Since radios do not transmit through water, GPS
cannot ne used unless an antenna is cabled to the surface. Several
acoustic methods have been developed for the purpose of underwater
navigation. These include:
Long Base Line (LBL) uses
several transponders on the seafloor. The distance between each
transponder is measured. A transducer on the vessel, ROV, AUV, or
towfish, interrogates each transponder and the ranges to each is
measured. From this information the position of the subsea platform can
be determined by triangulation.
Short Base Line (SBL) is similar to LBL except the array of transponders is spread along the underside of the vessel.
Ultra Short Base Line (USBL)
uses only one transponder and one multi-element hydrophone on the
surface. The measurement of the angle to the transducer is made across
the face of the hydrophone (the ultra short base line). In addition to
the angle, the range is determined by measuring the amount of time it
takes for a pulse to travel from the hydrophone and to be returned by
the transponder. So range and bearing to the target are determined.