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18 M.O. ŞERİFOĞLU, T. HIZARCI and H. AKYILDIZ

A REPORT ON RISK ASSESSMENT OF UNDERWATER
GLIDERS USING UNCERTAINTY ANALYSIS AND FAULT

TOLERANT CONTROL

Mehmet Ozan ŞERİFOĞLU*, Turgay HIZARCI* and Hakan AKYILDIZ*
*Istanbul Technical University, Department of Ship Building and Ocean Engineering,

akyıldız@itu.edu.tr

ABSTRACT

Underwater gliders are important autonomous underwater vehicles for ocean observations. They are
characterized by long endurance, low speed and high efficiency. Improvement of these vehicles can be
achieved through decreasing uncertainties and increasing reliability in navigation. Uncertainty and
reliability analysis is an efficient way of determining key points that have to be improved. Also, FTC is a
widely used control method in autonomous systems, satisfying system continuity in errors. It is essential to
apply it to underwater glider navigation system. Uncertainty analysis and FTC method can be used together
to enhance system behavior and further determine important design factors.

Key Words: Underwater glider, uncertainty, reliability, FMEA, FTC.

1. Introduction
The significant part of the Earth’s surface is covered by the ocean. Exploration of the ocean
becomes crucial activity to understand the operation of the Earth (Tropea et al., 2007). However,
some factors that prevent the explorations take considerable time. Quantity of the ocean,
inhospitable environment for human being, price that need to be paid, uncertainty and fluxional
situations definitely can be example to these factors. Research and examination of the ocean is
crucial to protect and improve these effects and resources.
Observation, exploration and development are important parameters for improvement of the
ocean observation systems. Stationary and passive systems have many disadvantages against
mobile systems like AUVs, ROVs and AUGs with their high maneuverability and controllability
(Zue, 2015). In addition to that, coordination and communication with multiple underwater
vehicles can perform difficult and time consuming tasks easily. While designing and building
these vehicles mathematical models of the system, the physical parameters of the vehicle, the
loads and the working environments are taking into account. However, parameters like uncertain
current and uncertain density of the seawater creates uncertainty problems in real engineering
(Lei, 2016). So making the necessary uncertainty analyses is really important. Also, it is a lot
more important to design a fault proof system for gliders as their work durations reach months.
To do that, a fault detection and analysis mechanism with a fault control system that can repair
and improve itself is necessary. In this paper there is an analysis of uncertainties of the glider and

GiDB|DERGi Sayı 8, 2017