Mooring System Diagnosis and Structural Reliability Control for Position Moored Vessels

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

Fault Monitoring and Fault Recovery Control for Position Moored Tanker

Fault monitoring and fault recovery control for position-moored vesselsThis paper addresses fault-tolerant control for position mooring of a shuttle or floating production storage and offloading vessels. A complete framework for fault diagnosis is presented. A loss of a sub-sea mooring line buoyancy element and line breakage are given particular attention, since such failures might cause high-risk abortion of an oil-loading operation. With significant drift forces from waves, non-Gaussian elements dominate forces and the residuals designed for fault diagnosis. Hypothesis testing is designed using dedicated change detection for the type of distribution encountered. A new position recovery algorithm is proposed as a means of fault accommodation in order to keep the mooring system in a safe state, despite faults. The position control is shown to be capable of accommodating serious failures and preventing breakage of a mooring line, or a loss of a buoyancy element, from causing subsequent failures. Properties of the detection and fault-tolerant control algorithms are demonstrated by high fidelity simulations.</jats:p

Supervisory Control of Line Breakage for Thruster-Assisted Position Mooring System

Thruster-assisted position mooring (TAPM) is an energy-efficient and reliable stationkeeping method for deep water structures. Mooring line breakage can significantly influence the control system, and ultimately reduce the reliability and safety during operation and production. Therefore, line break detection is a crucial issue for TAPM systems. Tension measurement units are useful tools to detect line failures. However, these units increase the building cost of the system, and in a large portion of existing units in operation line tension sensors are not installed. This paper presents a fault-tolerant control scheme based on estimator-based supervisory control methodology to detect and isolate a line failure with only position measurements. After detecting a line break, a supervisor switches automatically a new controller into the feedback loop to keep the vessel within the safety region. Numerical simulations are conducted to verify the performance of the proposed technique, for a turret-based mooring system.© 2015, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. This is the authors’ accepted and refereed manuscript to the articl

Development of new station-keeping strategy for DP assisted mooring system in Arctic ocean

극지방용 부유식 해양구조물의 위치유지시스템은 일반적인 환경과 다르게 유빙의 특성을 고려한 위치유지 전략이 필요하다. DP 보조계류시스템은 위치유지를 위해 가장 널리 사용되는 계류와 DP 시스템이 결합된 형태로 이러한 극한의 환경에 적합한 솔루션으로 주목받고 있다. 하지만, 쇄빙이 선행되어도 구조물 주변에 축적되는 빙 특성으로 인해 구조물의 과도한 표류가 발생할 수 있고 계류시스템의 파단 위험이 상승할 수 있다. 따라서, 극지용 DP 시스템은 해역의 빙 특성을 고려하여 구조물의 안전을 보장할 수 있는 제어 전략을 수립해야한다. 본 논문에서는 구조물에 유발되는 빙의 부하를 최소화 하고 안정적인 운용을 가능하게 하는 DP 보조계류시스템의 새로운 제어 전략을 제시하고자 한다. 새로운 제어전략은 계류선 장력을 이용한 구조물의 위치제어와 빙 표류방향에 대한 방향제어로 구성하였다. 선행적으로 구조물의 위치와 계류선 장력을 지속적으로 안전범위에 유지시킬 수 있는 DP 보조계류시스템용 위치제어인 Set point (SP)제어법을 제시하였다. 그 후, 다양한 빙 조건에서 DP 시스템의 정적해석을 수행하여 대빙 위치유지성능의 특성을 파악하였다. 본 결과를 토대로 빙의 부하를 최소화하기 위한 방향제어를 개발하였으며 최종적으로 극지방용 DP 보조계류시스템의 제어전략을 수립하였다. |The control strategy for a station keeping of floating offshore platform in Arctic ocean should consider characteristics of ice load unlike normal sea. The Dynamic Positioning (DP) assisted mooring system which is a combination of mooring and DP system has been noted as an appropriate solution for severe environmental condition like Arctic ocean. However, even if the breaking of ice is preceded, accumulation of ice floes around platforms can increase the risks such as the excessive drift and failure of mooring system. As a result, DP system in this condition should establish a proper control strategy which can ensure the safety of platform considering the characteristics of ice load. In this paper, a new station keeping strategy to minimize the load of platform caused by ice loads and enable safety operation is suggested. The new strategy is consisted of the position control using mooring tension and heading control considering the direction of ice drift. Firstly, Position control named Set point(SP) control is suggested which can keep the position and mooring tension in permissible range. Subsequently, DP capability analysis in various ice condition is conducted to investigate characteristics of station keeping quality against ice loads. Using the result of DP capability analysis, the heading control to minimize the load from ice is developed. As a result, a new control strategy for DP assisted mooring system under ice condition is established.1. Introduction 1.1 Background 1 1.2 Literature Review 2 1.3 Contents of Thesis 3 2. Position control 2.1 Overview 4 2.2 Local tension estimation 4 2.2.1 Objective 4 2.2.2 Generation of grid 5 2.2.3 Estimation process 5 2.2.4 FPSO with mooring system for a simulation 8 2.2.5 Local tension calculation 8 2.3 Global tension generation 12 2.3.1 Objective 12 2.3.2 Calculation of global tension 13 2.4 Determination of SP 15 2.4.1 Superposition of local and global tension 15 2.4.2 Result of SP 17 2.5 Result of position control 20 3. DP capability analysis under ice condition 3.1 Overview 26 3.2 DP capability plot without ice loads 26 3.2.1 Wind envelop 28 3.2.2 Thrust envelop 29 3.3 DP capability plot under ice condition 30 3.3.1 Overview 30 3.3.2 Characteristics of ice load 31 3.3.2.1 GEM simulator 31 3.3.2.2 Ice parameters 32 3.3.2.3 GEM simulation for ice load 33 3.3.3 Environmental condition 35 3.3.4 Generation of DP capability plot 36 3.4 Discussion 40 4. Heading control 4.1 Overview 43 4.2 Strategy for heading control 44 4.3 Simulation with heading control 46 4.3.1 Cases 46 4.3.2 Ice loads 47 4.3.3 Procedure for heading control 48 4.4 Result of heading control 49 4.4.1 Case 1 49 4.4.2 Case 2 51 4.4.3 Case 3 52 4.4.4 Case 4 53 5. Conclusion Reference 57Maste

Structural Reliability Based Dynamic Positioning of Turret-Moored FPSOs in Extreme Seas

FPSO is widely used during the deep-sea oil and gas exploration operations, for which it is an effective way to keep their position by means of positioning mooring (PM) technology to ensure the long-term reliability of operations, even in extreme seas. Here, a kind of dynamic positioning (DP) controller in terms of structural reliability is presented for the single-point turret-moored FPSOs. Firstly, the mathematical model of the moored FPSO in terms of kinematics and dynamics is established. Secondly, the catenary method is applied to analyze the mooring line dynamics, and mathematical model of one single mooring line is set up based on the catenary equation. Thereafter, mathematical model for the whole turret mooring system is established. Thirdly, a structural reliability index is defined to evaluate the breaking strength of each mooring line. At the same time, control constraints are also considered to design a state feedback controller using the backstepping technique. Finally, a series of simulation tests are carried out for a certain turret-moored FPSO with eight mooring lines. It is shown in the simulation results that the moored FPSO can keep its position well in extreme seas. Besides, the FPSO mooring line tension is reduced effectively to ensure mooring lines safety to a large extent in harsh sea environment

Modeling and Control of Marine Flexible Systems

Ph.DDOCTOR OF PHILOSOPH

Eco–Friendly Dynamic Positioning Algorithm Development

This research used the penalty method to develop a dynamic positioning control algorithm object for the purpose of minimizing the fuel consumption and CO2 gas emissions of an offshore platform. The performance of the penalty method was evaluated by comparing it with other conventional methods such as pseudo-inverse, quadratic programming, and genetic algorithm methods. The optimal performance of the penalty method in minimizing fuel consumption and CO2 emissions in both Gulf of Mexico (GOM) 100-year and one-year storm conditions was compared to pseudo-inverse and quadratic-programming methods. A feed-forward control using second-order wave force direct integration was newly applied in this research. The feed-forward control improved both the position maintenance performance and fuel consumption in Gulf of Mexico 100-year and one-year storm conditions. Global motion performance was compared after placing turrets in two locations (mid-ship and bow) and by using a hull-mooring-riser, fully coupled simulation. The results indicated that the mid-turret design reduces heave motion, even though its horizontal motion is unstable. In addition, the dynamic positioning control enhanced the horizontal motion of the mid-ship turret design. To reduce fish-tailing motion in a tandem offloading operation, the dynamic positioning control was employed. Separated Matrix Method based simulations were conducted on a fully coupled hull, mooring, riser, hawser, and thrusters

Control Stategy for Marine Riser of Position-Moored Vessel in Open and Level Ice-Covered Sea

Ph.DDOCTOR OF PHILOSOPH