Fault tolerant position mooring control based on structural reliability
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- Institutt for marin teknikk 
Safety of position mooring system is a prime concern in the marine industry and regulations are in force to prevent faults in equipment from causing failure of the whole system. There are many reasons for failures in elements of a mooring system, e.g. material damage, overload, fatigue, brittle fracture, corrosion, abrasion, extreme environment. For mooring cables, one of the typical reasons for failure is overload due to frequent replacement and inspections. Although a design based on knowledge of structural reliability should ensure safety during a prescribed period of use, the risk of line failure still exists. The normal procedure to remedy these failures involves regular maintenance and replacement and is costly. A supplement to avoid failures is to utilize the technique of dynamic positioning, where thrusters assist to balance external forces. For thruster-assisted position mooring, the main objective is to maintain a vessel’s position within a limited region and keep the vessel on the desired heading such that the external environmental load is minimised. The objective of keeping the system in a limited region is to avoid failure of the mooring or riser system. This limited region is normally a circle specified by a centre and radius. A safety factor is used to approximate the margin of environmental effects when designing this circle, and experience from operation with similar systems is also drawn upon. This thesis investigates how the reference position (set point) should be generated within the allowed region, and how control is implemented in the event of failure of part of the mooring system in order to protect the intact parts of the system and of the operation at large. The reference point is generated by minimising a cost function related to mooring line tension or riser angle. A structural reliability index is proposed to involve an on-line short term extreme value evaluation within the system. The magnitude of this index shows the safety level: a higher value means a safer condition and a value lower than a critical magnitude means that the risk is intolerably high. The control target is to avoid that line tension or riser angle exceed critical values. The setpoint chasing algorithm calculates a set-point where the structural index of line tension or riser angle is kept at reasonable levels. After a failure happens, a fault tolerant control strategy is designed to accommodate the effect of the failure. To ensure the safety of the position mooring system, it is necessary to analyse the effects of different faults in advance. These faults include thruster failure, power failure, abrupt signal change, line breakage etc. The focus of this thesis is the failure of mooring lines. A systematic framework for fault diagnosis and fault tolerant control is described. The nonlinear position mooring system is analysed by the structural graph approach and the residuals for line breakage and loss of buoyancy elements are produced to determine the faults from the residual signals. A change detection algorithm is designed to detect the change of the residual signals. Using this change detection algorithm, a line failure is detected and a fault accommodation method is subsequently developed to deal with the particular fault. To avoid other line tensions reaching an unacceptable value, a structural-reliability-based set-point chasing algorithm is proposed to re-generate a new position reference where all remaining lines are kept safe. This work uses diagnosis and fault-tolerant control techniques to utilise available actuators to handle single element failures and prevent the failure from spreading to the entire mooring system. This thesis contributes to ensuring the safety of mooring systems in a certain failure mode. One of the unique features is that the knowledge about structural reliability is combined with the control action by a structural reliability index, which is suggested to evaluate the safety level of the mooring and riser system. In the mooring system, both line breakage and loss of a buoyancy element are discussed in relation to the proposed set-point chasing algorithm. In the top-tensioned riser system, the top and bottom angle is protected from the extreme environmental situation by keeping the index at an acceptable level. In addition, an on-line short term extreme value estimation is incorporated into the estimation of the structural index, which enables this index to be extended to a more general case in the structural response of the system.