Page 21 - Sayı-08

RISK ASSESSMENT OF UNDERWATER GLIDERS USING 19
UNCERTAINTY ANALYSIS AND FAULT TOLERANT CONTROL

importance of the systems within, and also overviews of fault identification method FMEA and
fault control method FTC with their applications on a part of glider navigation system.

2. Underwater Gliders

Underwater gliders are the vehicles that can operate unmanned and autonomously throughout the
water column. Navigation and glide control are accomplished with usage of sensors together and
in combination. Wetted sensors are used for gathering the physical, chemical and biological
oceanographic data. Sensors on gliders can measure physical variables as pressure, noise
(background, ambient, ships, marine mammal calls, etc.), temperature, salinity, current, biological
variables relevant to the quantity of phytoplankton and zooplankton, and ecologically important
chemical variables such as dissolved oxygen and nitrate.
Buoyancy driven underwater gliders use buoyancy engine to change their volume for propulsion
and movement rather than conventional propeller. This mechanism has low energy consumption
values and gives advantage to gliders operate in long term projects. They have inflatable external
bladders to increase the volume of the glider. When the external bladder filled with liquid,
buoyancy force is increased and initiate the upward motion (Barker, 2012).

Wings are the main components for the underwater gliders to gain horizontal motion and
propulsion in the ocean while profiling. Wings use vertical motion to generate hydrodynamic lift
force for horizontal motion and have an important effect on glide slope and angle (Lippsett &
Carlowicz, 2008). Underwater gliders move with sawtooth pattern in vertical plane
however in horizontal plane follow random route because of the currents and their effects.

Sensors are integral parts of the underwater gliders to collect data. Gliders can be used in different
missions when sensors are replaced with suitable one. According to Kongsberg Underwater
Technology, Inc. CTD sensors, fluorometers, dissolved oxygen sensors, PAR sensors, current
profilers (ADCPs), optical backscatter and other sensors can be mounted to underwater gliders
(2013). With the help of gliders, salinity and temperature profiles versus depth can be obtained,
pollutants in the ocean can be tested and monitored in an automated way. Depending on the
vehicle's configuration scientists can measure the health of the ocean with the help of
phytoplankton. While climate is based on ocean currents, collected data and instant data
transmission by underwater gliders help scientists to refine climate models and improve
forecasting (Dent, 2014). Underwater glider fleets also help to collect data for improvement of
storm intensity forecast during hurricane seasons (Oceanservice.noaa.gov, 2013).

Another usage area of the underwater gliders is military applications. Silence and stealthiness are
important properties for Navies. Usage of buoyancy changing mechanism as propulsion system
makes gliders pretty quiet and hard to detect. Small dimensions and body, easy deployment even
from a small boat also significantly satisfy the Navy’s desires and requirements. Firstly, gliders
are being used in Navy to collect crucial data and guide the fleet. For example, U.S. Navy Lt.
Cmdr. Patrick Cross mentions that “The gliders are a great way to have a persistent sensor out
there to continuously feed us data on what the ocean is doing. Then we can feed that to our shore-
based computer models and get a better picture of the ocean” (Rush, 2005). Secondly, gliders can
be used to spot submarines and underwater mines with their capability of environmental

Sayı 8, 2017 GiDB|DERGi