Position Mooring System Research Articles

Structural reliability based energy-efficient arctic position mooring control of moored offshore structures under ice loads

This work develops an energy-efficient robust control for arctic position mooring systems subject to discontinuous ice loads and system uncertainties. To provide a safe and energy-efficient reference position for moored offshore structures, an improved structural reliability-based setpoint chasing algorithm is proposed. An ice-load disturbance observer is constructed to handle the arctic environmental disturbances, relax the continuity, differentiability, and boundedness requirements of disturbances. The system uncertainties are addressed by an adaptive fuzzy estimator. With the reference position and disturbance estimation, an improved dynamic surface control law is derived by introducing a novel filter to improve the convergence rate of the filter error, it enhances also the control performance of the arctic position mooring with fast response. Theoretical analysis proves the stability of the resulting closed-loop system and the boundedness of all its signals with the invariant set mechanism and Lyapunov theory. Comparative numerical simulations are carried out on a turret-moored floating production storage and offloading (FPSO) vessel to demonstrate the performance and effectiveness of the proposed control scheme.

A survey on modeling and control of thruster-assisted position mooring systems

This review presents a systematic summary of the state-of-the-art development of technological solutions, modeling, and control strategies of thruster-assisted position mooring (TAPM) systems. The survey serves as a starting point for exploring automatic control and real-time monitoring solutions proposed for TAPM systems. A brief historical background of the mooring systems is given. The kinematics and a simplified kinetic control-design model of a TAPM system are derived in accordance with established control methods, including a quasistatic linearized model for the restoring and damping forces based on low-frequency horizontal motions of the vessel. In addition, another two mooring line models, i.e., the catenary equation and the finite element method model, are presented for the purpose of higher-fidelity simulations. The basic TAPM control strategies are reviewed, including heading control, surge-sway damping, roll–pitch damping (for semisubmersibles), and line break detection and compensation. Details on the concepts of setpoint chasing for optimal positioning of a vessel at the equilibrium position are discussed based on balancing the mooring forces with the environmental loads and avoiding mooring line failure modes. One method for setpoint chasing is the use of a structural reliability index, accounting for both mean mooring line tensions and dynamic effects. Another method is the use of a lowpass filter on the position of the vessel itself, to provide a reference position. The most advanced method seems to be the use of a fault-tolerant control framework that, in addition to direct fault detection and isolation in the mooring system, incorporates minimization of either the low-frequency tensions in the mooring lines or minimization of the reliability indices for the mooring lines to select the optimal directions for the setpoint to move. A hybrid (or supervisory switching) control method is also presented, where a best-fit control law and observer law are automatically selected among a bank of control and observer algorithms based on the supervision of the sea-state and automatic switching logic.

Energy-efficient control of a thruster-assisted position mooring system using neural Q-learning techniques

ABSTRACT Thruster-assisted position mooring (PM) systems use both mooring lines and thrusters to maintain the position and heading of marine structures in ocean environments. In order to operate in an energy-efficient manner in moderate sea conditions, appropriate setpoints need to be found for the feedback controller, where the mooring system counteracts the main environmental loads and the thrusters reduce the oscillatory motion of the marine structure. The theory of reinforcement learning (RL) provides powerful and effective tools for designing decision making agents, which can optimise their behaviours based on the interactions with the environment. We propose several designs of decision making agent based on state-of-art neural Q-learning techniques. The simulation results show that the RL agents can successfully identify the optimal setpoints through the interaction with an unknown and stochastic environment, and double Q-learning and prioritised replay techniques prove to be fairly effective in shortening the learning time of the agent.

Optimal setpoint learning of a thruster-assisted position mooring system using a deep deterministic policy gradient approach

Thruster-assisted position mooring (PM) systems use both mooring lines and thrusters for station keeping of marine structures in ocean environments. To operate in an energy-efficient manner in moderate sea conditions, setpoints need to be appropriately chosen for the setpoint controller, so that the mooring system counteracts main environmental loads, while the thrusters reduce oscillatory motions of the marine structure. In this paper, reinforcement learning is used to design a decision-making agent for setpoint selection. In particular, a deep deterministic policy gradient (DDPG) approach is adopted with the powerful actor–critic architecture to continuously modify the setpoint setting at an optimal position. Extensive numerical experiments demonstrated that with the DDPG-based PM system, the intelligent agent is able to successfully identify the optimal positioning region in an unknown and stochastic environment, and the power consumption of the thrusters is maintained at a considerably low level.

Detection of mooring line failures using Dynamic Hypothesis Testing

This article proposes a novel methodology for the detection of mooring line breakage in thruster assisted position mooring (PM) systems, when no measurements of the tensions on the mooring lines are available. For dynamic positioning (DP) of marine vessels moored to the seabed via a turret-based spread mooring system, thrusters provide only complementary assistance to the mooring system, which is responsible for generating a large part of the forces and moments required for station keeping. However, in extreme weather conditions thruster assistance is essential to avoid mooring line failure. Once a mooring line is parted, the remaining lines must withstand an increase in the tension forces required to compensate for the lost tension in the ruptured line. This in turn may lead to a cascade breakage of the mooring lines. Hence, it is of paramount importance to detect any line breakage as soon as it occurs to compensate for the lost tension by proper use of DP thruster assistance. As a contribution to solving this problem, in this paper we propose a methodology that builds on Dynamic Hypothesis Testing (DHT) whereby a set of hypotheses are assessed, at each sampling time, using the measured inputs and outputs of the thruster assisted position mooring system. While the first hypothesis corresponds to the assumption that all mooring lines are intact, the remaining hypotheses are built assuming that a single, or multiple line breakage events have taken place. At each sampling time, the inputs and outputs to the system are used to generate the conditional probability of each hypothesis being true. The conditional probabilities are then used to evaluate which hypothesis is more probable to be compatible with the collected measurements. In addition, we find conditions for any pair of hypothesis to be distinguishable. Numerical simulations, carried out using a high fidelity nonlinear PM simulator, illustrate the efficiency of the proposed methodology.

Numerical and Experimental Study on the Influence of the Set Point on the Operation of a Thruster-Assisted Position Mooring System

Numerical and Experimental Study on the Influence of the Set Point on the Operation of a Thruster-Assisted Position Mooring System

Vessel motion control using rope tension control strategy

In this paper, the problem of designing a Position Mooring (PM) system for a barge ship is addressed. A mathematical model of a system comprising a barge ship and mooring system is derived. Hydrodynamic coefficients of the low speed model for PM ship are identified by suitable experiments. A PID control scheme is implemented to achieve PM for the vessel by changing the tension of the mooring ropes. The proposed strategy is finally tested on station keeping, and desired positions of a 1/50 scale model ship are obtained.

A Study on Position Mooring System Design for the Vessel Moored by Mooring Lines

This paper presents the experimental results about position mooring (PM) system applied to the barge ship. The aim of the PM is to maintain the position and motion of the ship's surge and sway directions as desired. In this paper, a system consisting of a barge vessel and mooring lines is mathematically modeled. The position and orientation of the vessel is controlled by changing the tensions of the mooring lines with the subwinch. A comparison of the newly designed mathematical model with the existing PID control method and the results of experiments indicated that the proposed designing method is more efficient than the traditional method.

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.

Robust adaptive control of a thruster assisted position mooring system

In this paper, robust adaptive control is developed for a thruster assisted position mooring system in the transverse direction. To provide an accurate and concise representation for the dynamic behavior of the mooring system, the flexible mooring lines are modeled as a distributed parameter system of partial differential equations (PDEs). The proposed control is applied at the top boundary of the mooring lines for station keeping via Lyapunov’s direct method. Adaptive control is designed to handle the system parametric uncertainties. With the proposed robust adaptive control, uniform boundedness of the system under the ocean current disturbance is achieved. The proposed control is implementable with actual instrumentations since all the signals in the control can be measured by sensors or calculated by using a backward difference algorithm. The effectiveness of the proposed control is verified by numerical simulations.

A Positioning Mooring System Design for Barge Ship Based on PID Control Approach

This paper presents some experimental results about Position Mooring (PM) system applied to the barge ship. In PM operation, the station keeping in surge, sway of vessel is provided by the mooring system. In this paper, a system, consisting of a barge vessel and mooring lines, is mathematically modeled. The position and orientation of vessel is controlled by changing the tensions in the mooring lines. The PID control strategy is applied to evaluate the efficiency of proposed system. Experimental result which corresponds to the applied control strategy is presented and discussed.

Robust Adaptive Position Mooring Control for Marine Vessels

In this paper, robust adaptive control with dynamic control allocation is proposed for the positioning of marine vessels equipped with a thruster assisted mooring system, in the presence of parametric uncertainties, unknown disturbances and input nonlinearities. Using neural network approximation and variable structure based techniques in combination with backstepping and Lyapunov synthesis, the positioning control is developed to handle the uncertainties, input saturation and dead-zone characteristics of the mooring lines and thrusters. Full state feedback with all states measurable and output feedback using high gain observer to estimate unmeasurable states are considered. Dynamic control allocation is presented for actuation of the position mooring system. Under the proposed robust adaptive control, semi-global uniform boundedness of the closed-loop signals are guaranteed. Numerical simulations are carried out to show the effectiveness of the proposed control.

Position mooring control based on a structural reliability criterion

To prevent failure of mooring lines in modern position mooring (PM) systems, position moored vessels are kept within a small distance from a desired reference position. A safe position within such region is where stress in all mooring lines are kept well below tensile strength. To prevent several mooring lines simultaneously from exceeding a stress threshold, this paper suggests a new algorithm to determine the reference position and an associated control system. The safety of each line is assessed through a structural reliability index. A reference position where all mooring lines are safe is achieved using structural reliability indices in a cost function, where both the mean mooring-line tension and dynamic effects are considered. An optimal set-point is automatically produced without need for manual interaction. The parameters of the extreme value distribution are calculated on-line thereby adapting the set-point calculations to the prevailing environment. In contrast to earlier approaches, several mooring line are simultaneously accounted for by the algorithm, not only the most critical one. Detailed simulations illustrate the features of the new method and it is shown that the structural reliability criterion based algorithm ensures the safety of mooring lines in a variety of external environmental conditions and also in situations of failure of a single line.

Setpoint Chasing for Thruster-Assisted Position Mooring

In the present industrial thruster-assisted position mooring (PM) systems, thrusters are used to damp the vessel's dynamical motions and to provide compensation of any line breakage. To increase robustness and extend the weather window for PM systems, this paper proposes a new concept of setpoint (SP) chasing for moderate and extreme conditions. The SP in moderate conditions is the vessel's equilibrium position where the environmental loads are balanced by the mooring forces to avoid the conflict between the control action of thruster assistance and the mooring system. To chase this SP, the thruster assistance provides additional damping and restoring forces to compensate the dynamic environmental loads while the mooring system compensates the mean environmental loads. The simulations and experiments suggested that the proposed SP chasing for moderate conditions improved the vessel performance while the utilization of the mooring system was maximized. Due to risk for mooring line breakage, the SP in extreme conditions is the position closer to the field zero point than the equilibrium position. To chase this SP, the thruster assistance provides damping, restoring force and mean force controls to compensate both the dynamic and a part of the mean environmental loads while the mooring system compensates the rest of the mean environmental loads. The simulations indicated that the proposed SP chasing for extreme conditions reduced the vessel's drift and tension in the most loaded line.

Rigid moorings in shallow water: A wave power application. Part I: Experimental verification of methods

Experimental work carried out at 1:60 scale in a wave flume assessed the pitch motion and anchor loading of 3 articulated tower installations in 50 m water depth while being exposed to north Atlantic storms with H s of 15.2 m and T p of 18.4 s. The three installations differ only in that their mass and buoyancy characteristics provide a natural period in pitch at equilibrium of 13 s, 20 s and 34 s respectively. It is verified that the dominant behaviour can be simulated by a relatively simple mathematical model, allowing the critical parameters of peak anchor loads and pitch angles to be calculated and extrapolated to full scale. It is demonstrated from the experimental and simulation results that the mass characteristics of a non surface piercing tower can be used to offset some of the challenges of moving to shallow water. If done correctly, it is possible to keep horizontal anchor loads under control and reduce vortex-induced transverse loading at the expense of increased pitch motions. Overall, the use of articulated tower installations in water depths of 50 m would appear to be technically feasible, even in exposed areas. The limitations on the size of such structures and the consequences of the resulting pitch accelerations and induced anchor loads are the subject of further study. It is proposed that the model verified herein can be used to further assess their potential at delivering viable wave power position mooring systems.

Switching control for thruster-assisted position mooring

The position mooring (PM) for marine vessels contains several controllers, such as heading, damping, restoring and mean force controllers which could be activated or deactivated depending on the environmental and operational conditions. This paper proposes to integrate these controllers into a switching control system. The supervisory control is adopted to facilitate the automatic switching. The supervisory switching control involves the designs of a set of models to track the existing operational condition and the switching logic. Simulations and experiments will be provided for validation. It is shown that the switching control could extend the weather window of the PM system.

Thruster assisted position mooring based on structural reliability

This paper addresses dynamic positioning of surface vessels moored to the seabed via a spread mooring system, referred to as position mooring. In normal weather conditions, the mooring system constrains the vessel and the controller applies thruster force for motion damping and heading control, only. When environmental loads due to wind, waves, and current increase, thruster assistance is required also for positioning in order to avoid damage to the mooring lines. While traditional position mooring systems apply thruster force based on constraining the vessel to lie within a predefined geographical region, the controller designed in this paper instead employs structural reliability measures for the mooring lines to restrict movement. These structural reliability measures become an intrinsic part of the controller, enabling it to automatically adjust the admissible region for the vessel in the presence of changing winds, currents, and waves.

DIAGNOSIS AND FAULT-TOLERANT CONTROL FOR THRUSTER-ASSISTED POSITION MOORING

Development of fault-tolerant control systems is crucial to maintain safe operation of offshore installations. The objective of this paper is to develop a fault- tolerant control for thruster-assisted position mooring (PM) system with faults occurring in the mooring lines. Faults in line's pretension or line breaks will degrade the performance of the positioning of the vessel. Faults are detected and isolated through fault diagnosis procedures. When faults are detected, they are accommodated through the control action in which only parameters of the controlled plant have to be updated to cope with the faulty condition. Simulations are carried out to verify the advantages of the fault-tolerant control strategy for a PM system.

Ventilation of Controllable Pitch Thrusters

In oil and gas exploration and exploitation, many ships and rigs conduct station-keeping operations by the use of dynamic positioning (DP) systems or thruster-assisted position mooring (PM) systems. Such systems use thrusters to control the ship or rig position and heading. In severe weather conditions, thrusters may experience large and rapid changes in the propeller loading due to ventilation and in and out of water effects. To reduce the negative effects of this undesired dynamic loading on the mechanical components and the electrical power plant, the thruster controllers can locally compensate for the disturbances. To be able to design and verify such controllers, a model of the losses is needed. In this paper, experiments with ventilating controllable pitch propellers are presented. The results are used to develop a ventilation model for use in simulations. The model is verified by comparing simulations with time series from experiments. Scaling laws for ventilating propellers are also discussed.

Modelling and Control of Thruster Assisted Position Mooring Systems for Ships

This paper adresses the mathematical modelling and control design of an automatic thruster assisted position mooring (POSMOOR) system. Such control systems are applied to anchored floating production offloading and storage vessels (FPSO's) and semi-subs. The POSMOOR controller is designed using model based control with a LQG feedback controller in conjunction with a Kalman filter. The controller design is in addition to the environmental loads accounting for the mooring forces acting on the vessel. This is reflected in the model structure and in the inclusion of new functionality.