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Comparing Controllers for Dynamic Positioning of Ships in Extreme Seas

Rabanal, Ole Maurice Røste
Master thesis
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http://hdl.handle.net/11250/2352520
Utgivelsesdato
2015
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  • Institutt for teknisk kybernetikk [2251]
Sammendrag
The primary purpose of this study is to develop two different dynamic position-

ing (DP) controllers for a model-scale supply vessel Cybership III, and determine

which control strategy provides a safer, smarter and greener control when the ves-

sel is exposed to extreme seas. DP has since the 1960s contributed to the safety

and efficiency of oil-related and other operations for marine vessels. With this

computer-controlled system, vessels such as research vessels, supply vessels and

cruise ships can automatically maintain position and heading by using propellers

when the vessel is exposed to wind, waves and current.

A nonlinear passive observer (NPO), proportional-integral-derivative with acceler-

ation feedback (PID-AFB) controller, sliding-mode control (SMC) and a reference

model for Cybership III were derived. Test cases were first performed with the

Marine Cybernetics Simulator (MCSim) in calm, harsh and extreme seas. This

simulator consists of a high fidelity process plant model of Cybership III in Mat-

lab/Simulink together with models for generation of environmental forces. Sec-

ondly, tests in calm and harsh seas were performed on the model vessel in the

Marine Cybernetics Laboratory (MCLab) at MARINTEK. The performance for

tests in MCSim was evaluated by integral squared error (ISE), integral absolute

error (IAE) and integral time-weighted absolute error (ITAE). The measures of

performance for tests in the MCLab were performed by using ISE, IAE and ITAE

in combination with the energy consumption of the thrusters on Cybership III in

order to generate a cost function. This cost function is a new method used to eval-

uate the performance of the controllers with the aim of achieving a safer operation

meaning good accuracy, a greener operation by minimizing energy consumption and

a smarter operation by achieving the two latter simultaneously. Other measures

of performance used to evaluate controllers on Cybership III and similar vessels

in MCLab have as far as the knowledge of the author, only been performed with

respect to the error of the positions and heading. This thesis uses a new measure of

performance not only evaluating the error, but also the energy consumption during

tests, thus evaluating the controller with more realistic performance measures of

performance.

The PID-AFB-controller provided the best performance when the vessel was tested

in extreme seas in MCSim and was evaluated only by ISE, IAE and ITAE, while

the SMC achieved the best performance when tested in calm and harsh seas both

in the MCLab and MCSim. The tests performed in MCSim were not comparable

with the tests performed in the MCLab and the reasons for this may be inaccurate

model parameters, the lack of a proper thruster model, 3D effects of the waves

in the basin and reflection on the tank wall. In retrospect of the experiments,

Cybership III was found to weight 14% more than modeled in MCSim and hence the test cases in MCLab was considered to be more credible.
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