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RISK ASSESSMENT OF UNDERWATER GLIDERS USING 23
UNCERTAINTY ANALYSIS AND FAULT TOLERANT CONTROL

Created chart shows that the power system is the heights probability of failure. This affects the
reliability of the vehicle and need to be improved. Power system is the energy source of the glider
and first parameter need to work perfectly. Any malfunctioning in this system lead to loss of the
underwater glider. Navigation system and leak detection system have the highest failure
probability after the power system. Improvement priority need to be organized according to the
failure probabilities and rates of the subsystems. (Aslansefat, 2014).

Table 1: Failure rate of the subsystems

Sub-System Failure FR(λx10-6) Sub-System Failure FR(λx10-6) Sub-System FR(λx10-6)
Failure
Current Monitor 6,5 Science Sensor 3,62 0,91
Leak Detection 13,59 CTD 0,2 Rudder Broken 0,91
Voltage Detection Iridium 3,62 Wing Broken 0,91
Power/Battery 6,5 1,81 8,5
Battery Detection 8,15 Data Logging 2,72 Argos 13,41
Buoyancy Pump 7,5 Attitude Control 0,91 GPS Host 0,91
Air Bladder Leak 5,44 Command/Control 0,91 Bad GPS 0,91
Collision Vessel 0,91 Onboard Software 1,81 Pitch Motor 3,62
Collision Seabed 3,62 Glider Recovered Roll Motor
1,81 Unknown

4. Fault Tolerant Control

As underwater gliders are autonomous vehicles they benefit from the FTC method a lot in every
aspect of their operation. In the case of a glider, most important operation it does is navigation.
Its data quality, maximum work time and damages to vehicle is all related to navigation efficiency
and success. So navigation related errors are one of the most important possible error types in a
glider. In this part possible navigation errors are defined using FMEA method and FTC method
is used to create a fail-safe system.

4.1 FMEA Method

To determine possible problems and understand their effects on the system FMEA method is a
good choice. Results of FMEA method is also adaptable to FTC system very well. FMEA stands
for “Fault Mode and Effects Analysis”. Carlson states that FMEA is an engineering study that is
done by a team of experts on subject to find and fix the weaknesses early on (2014). In that matter,
FMEAs depend mostly on expert knowledge and error data if available. By applying FMEA, we
can prevent dangerous errors before operation, increase reliability of the system and evaluate the
system from a new point.

FMEA has six steps approach. Those are; determination of failure modes, evaluating severity,
probability and detection numbers, calculating risk priority number and determining the necessary
actions. Severity number tells that how serious the failure is, probability number show how likely
is it for the failure to occur and detection number states is the detection of the failure early on is
easy or not. When those three numbers are multiplied the RPN number occur. If the RPN number
is high, it means that the failure is a dangerous one for our system and has the priority on getting
fixed. Generally, all that information is collected on a table with severity, probability, detection
and RPN numbers; name, position, cause and effect of the failure and also the suggestion on how
to deal with the failure giving the user a good overview.

Sayı 8, 2017 GiDB|DERGi