Abstract Early diagnosis and fault-tolerant control are essential for safe operation of floating platforms where mooring systems maintain vessel position and must withstand environmental loads。 This paper considers two critical faults, line breakage and loss of a buoyancy element and employs vector statistical change detection for timely diagnosis of faults。 Diagnosis design is scrutinized and a procedure is proposed based on specified false alarm probability and estimation of the distribution of the test statistics on which change detection is based。 A structural reliability index is applied for monitoring the safety level of each mooring line and, a set-point chasing algorithm accommodates the effects of line failure, as an integral part of the reliability- based set-point chasing control algorithm。 The feasibility of the diagnosis and of the fault-tolerant control strategy is verified in model basin tests。82241

Keywords: Fault diagnosis, Fault-Tolerant Control, Position Mooring, Structural Reliability, Change Detection, Optimal Set-point Chasing

Nomenclature

DP Dynamic  Positioning

FPSO Floating production storage and offloading

LF Low frequency

WF Wave frequency

MSS Marine system simulator

IMO International  Maritime  Organisation PM Position mooring with

thruster assistance

COT Centre of turret

COG Centre of gravity

HRS Hydro-acoustic  reference system

MRU Motion reference units

EFF Earth-fixed frame

BFF Body-fixed  frame

TP Terminal point

MLE Maximum  likelihood estimation

1。

Introduction

Increasing safety and efficiency of the work-over operation on floating platforms is attracting more and more attention。 In offshore operations, marine vessels are often required to be kept at a position by thrusters, known as a DP system。  References to DP systems can be found in [1], [2] and [3] etc。 For dynamic positioning of surface vessels moored to the seabed, this is referred to as a PM system。 The main objective of the PM system is to keep the vessel at a certain position, while the second- ary objective is to prevent line breakage by keeping the vessel within a limited region。 Several studies have appeared about thruster-assisted PM systems, including [4], [5], [6] and [7], and reliability issues were addressed together with the DP control as- pects in [8]。 The reactions in the event of faults and the ability to maintain safe control even in case of failure of part of a mooring system has only been sparsely dealt with in earlier studies and this will be the main subject of this paper。

Fault-tolerant control of marine vessels presents

Preprint Control Engineering Practice 25th November 2014

a challenging issue。 Regulations normally define different system levels by hardware redundancies and prevent system failure by replacing faulty hard- ware [9]。 Risk analysis is also performed to evalu- ate the effect of faults,which is traditionally based on the reliability characteristics of mechanical com- ponents, with studies of risk of fatigue damage or line breakage under extreme conditions [10], [11], [8]。 Human interactions are still the significant factors to handle instantaneous faults。 However, the uncertainty of human interaction is high。 A re- view of the accident databases shows that human error is the dominant factor in maritime accidents [12]。 To prevent risk and to reduce costs, faults should be isolated and handled automatically   by a fault-tolerant control capability of the position mooring control system。  One of the  approaches to detect faults is the structural analysis technique [13]。 Once a fault has been found in a component of the system, the controller is to be re-designed such that the faulty effect can be attenuated。 The control re-configuration strategy may be designed dependent on the type of the fault and thus imple- mented as needed。