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Networked predictive vibration control for offshore platforms with random time delays, packet dropouts and disordering
... (i) In contrast with some existing literatures concerning networked control of the jacket structure [26,27], the influences of DoS attacks and network-induced delays are considered simultaneously. The networked jacket structure system (NJSS) is then viewed as a switched delay system. ...
... Then, from (13)-(15), one can rewrite (8) as Remark 2. In fact, the network-based modelling for the structure has been proposed [26,27]. ...
... Similar to [22,[25][26][27][28], the time scheduling in Fig. 4 is a simplified form, in which the encodingdecoding techniques are not mainly studied. Now, substituting (18) into (3) and (7), respectively, we havė ...
This article focuses on observer-based state feedback H∞ control for a jacket structure against DoS attacks and external wave loads. First, a networked model of the structure is formulated as a switched delay system, in which DoS attacks and network-induced delays are considered simultaneously. A matching switched observer is developed for estimating states of the networked jacket structure system. Then, some new sufficient conditions are provided for the observer-based networked H∞ controller for the resultant switched system. Finally, it is shown from several case studies that the provided mechanism can maintain desired performance of the jacket structure against attacks and wave loads. In addition, the developed control schemes can save the control cost significantly.
... To explore the possible modification of the TLD-FB for a large water storage tank, the formulation is further generalized for the MTLD-FB system. In doing so, the random wave load is generated using the JONSWAP wave spectrum (Ma et al., 2019) to simulate the realistic oceanic environment. Four different sea states of irregular waves with return periods of 100, 10, 5, and 2 years have been considered to study the robustness of the proposed system under different oceanic conditions. ...
... In consequence, for jacket offshore platforms, the first mode dominates the response and consideration of the first mode is adequate for vibration control purposes (Li et al., 2003;Xu et al., 2022). Most of the previous studies have also targeted the first modal response control of offshore platforms (Lu et al., 2012;Ma et al., 2019;Cao et al., 2021;Xu et al., 2022). Further, to study the contribution of each mode, a modal time history analysis is performed for a particular time history loading. ...
The applications of various conventional passive control devices to mitigate wave-induced vibration of offshore structures are remarkable. However, the same is not true for useable liquid storage tanks to utilize as passive vibration control devices for offshore structures due to the low inherent damping of deep tanks. The present study explores the prospect of a usual deep-water storage tank performing as a tuned liquid damper with a floating base (TLD-FB) that overcomes the detuning and liquid level fluctuation effect of a deep tank. The proposed system ensures constant shallow liquid depth over the floating base so that it can act as a usual TLD tuned to the targeted frequency of the structure. To explore the possible modification of the TLD-FB for a large water storage tank, the formulation is further generalized for multiple TLD with floating-based (MTLD-FB). The effectiveness of the TLD-FB and MTLD-FB systems are studied both in the frequency and time domains under irregular waves considering an SDOF system and an MDOF system. The results of the numerical study show that the system is effective in wave-induced vibration mitigation.
... Each term on the right side of the above inequality is constant, which means there is an upper bound for (34). With the proper reference matrix D m , the adaptive law satisfies the premise (32). Third, the error system between (3) and (26) iṡ ...
... We conduct several simulation experiments on the offshore platform located in Gulf of Mexico described in Table 1 [32,33]. All the numerical tests use MATLAB as the simulating tool on CPU AMD Ryzen 7 4700U. ...
Adaptive control methods are suitable for offshore steel structures subject to harmful vibrations, as they employ reference models to adapt to coastal and nearshore physics. To decrease the dependence on the accurate characteristics of the offshore platform, a compensating measure containing the ocean environment is proposed in the adaptive control scheme. With incomplete states as the driving input, external loads are approximated using a wavelet neural network frame. Numerical experiments are conducted on a platform model with varying parameters to test the performance of the proposed adaptive controller. It is shown that the adaptive weights derived from the chosen Lyapunov function are qualified both theoretically and practically. The system-output-based adaptive controller overcomes the disadvantage of state loss. The compensated disturbance environment guarantees the reliability of the restored reference system based on mismatched physics. The designed estimator as a part of the adaptive controller compensates for the deviations of the environment between the reference and the practical, resulting in a desirable reduction in the excessive vibration.
... A modified-transformationbased networked controller is presented to depress the vibrations of acceleration and displacement of the jacket platform subject to current forces and sea waves . By taking the large communication delays, packet losses, and packet disorders into account, the predictive feedforward and feedback controller is used to a networked platform system against irregular wave forces (Ma, Hu, et al., 2019). ...
... (i) Different from some existing results about the networked control of jacket platforms (Ma, Hu, et al., 2019;B.-L. Zhang et al., 2020), the networked model of the concerned platform incorporates the simultaneous presence of earthquakes and stochastic actuator faults. ...
This paper deals with the problems of networked modelling and memory-event-triggering H ∞ reliable control for steel jacket-type platforms in network environments. First, a networked model of the jacket platform against earthquakes and probabilistic actuator faults is established. Based on this model, a memory-based event-triggering communication scheme is introduced to account for the constrained communication resources. A distinct feature of the proposed triggering scheme is that it has great potential to identify and trigger the significantly changed data than the existing ones without ‘memory’. Then, by formulating the network-based closed-loop platform system as a stochastic delay system, some sufficient conditions are derived for co-designing the desired controller and the triggering scheme. Finally, comparative simulation results are provided to demonstrate the effectiveness and merits of the proposed triggering and control co-design method. It is shown that compared with some existing event-triggering control methods, the proposed memory-event-triggering H ∞ reliable controller is effective to mitigate vibrations of the jacket platform and is potentially advantageous to save more network bandwidth and control cost.
... As basic infrastructures for the development of ocean resources, offshore structures are unavoidably affected by several external loadings besides waves [1][2][3][4]. In general, the unwanted loadings cause excessive vibration of the platform [5][6][7]. Notice that reducing oscillation amplitude of jacket platforms to 15% can prolong the service life significantly, thereby saving the cost of inspection and maintenance of the platform [8]. Therefore, it is important to find effective control schemes besides passive, active and semi-active control to attenuate the oscillation level of structures; one can see [9][10][11], and the references therein. ...
... In the presented model, the time-varying mass of the offshore structure is considered. If the mass of the offshore structure is taken as a constant, the dynamic model described by (7) and (8) reduces into the one in [5,7,14,21]. ...
Vibration damping of jacket platforms is among the significant issues in marine science and engineering, and the design of active vibration control schemes is very important to ensure the stability and safety of the jacket platforms against external loadings. This paper provides three fuzzy output feedback H ∞ controllers of the jacket platforms for irregular wave forces. By considering time-varying masses of jacket platforms, a Takagi–Sugeno (T–S) fuzzy dynamic model of the structure is established. Then fuzzy output feedback H ∞ control schemes are developed via using output signals of the platform with current and/or are delayed. Several existence conditions of fuzzy output feedback H ∞ controllers are derived. Simulation results demonstrate that the fuzzy output feedback H ∞ control strategies are remarkable to suppress the vibration of structure. Moreover, by choosing proper delayed output information of the system, the presented delayed fuzzy output feedback H ∞ control schemes outperform the conventional fuzzy output feedback H ∞ control approach.
... When dealing with unknown and energy-bounded disturbances, as well as other sources of uncertainty mentioned above, it may be more practical to focus on finding feasible solutions with a high tolerance, rather than trying to derive an optimal solution directly [6]. To solve these uncertainty-induced problems, fuzzy-logic control and predictive networked control theories are utilized [12][13][14][15]. For certain constrained systems with any defined functional, synthesis event-triggered strategies [16], fault-tolerant robust strategies with qualified dynamic output feedback scheme [17], and delay-dependent stabilizing criteria have been proposed in recent years [18]. ...
In this work, an adaptive learning robust controller is proposed to suppress the vibration of offshore platforms, which are subject to waves, winds, varying control delays and parametric perturbations. To realize nonlinear uncertainty approximation under the bounded H∞ performance, the H∞ controller incorporates both an online adaptive part and an offline fixed part. The adaptive part constructed by neural networks adjusts online, while the fixed part is obtained by regulating the H∞ performance. Importantly, adaptive updating strategy does not require accurate values or upper bounds for real‐time control delay or uncertainty. Several comparable experiments demonstrate the feasibility and effectiveness in vibration‐suppression of the designed adaptive controller in shallow/deep water. This scheme significantly reduces system response variations due to structural and hydrodynamic uncertainty, as well as additional random environmental forces caused by winds.
... • By embedding the mode of platform heave motion into the control channel, an augmented riser-tensioner recoil control system is presented. Then, by adopting the feedforward mechanism (Tang et al., 2007;Ma et al., 2019) and the internal model principle (Liang et al., 2016) jointly, a dynamic optimal recoil controller is designed. ...
... Both active and semi-active systems need to identify the structural modes and external excitation frequencies, and there has a time delay. The time delay compensation control algorithms for offshore platforms have been studied such as deriving and predicting irregular wave forces [15,16]. However, the self-excited platform of the actuator would occur due to inaccurate prediction and power supply instability [17]. ...
A tuned liquid multicolumn damper (TLMCD) composed of three liquid columns and bottom connecting pipes is proposed to reduce the pitch motion of a semisubmersible substructure for wind turbine. The pitch damping of the semisubmersible floating offshore wind turbine (FOWT) substructure scale model with TLMCD in regular waves is studied experimentally. The TLMCD with the optimized mass ratio and tuning ratio provides effective pitch motion control especially near resonance period. The OpenFOAM is further developed to simulate dynamic response of the FOWT substructure under internal sloshing and external wave excitations, where the wave elevation and the FOWT substructure response have been validated against the experimental data. The design natural frequency of the TLMCD is equal to that of the FOWT substructure, and the analytical solutions and numerical results of liquid column decay are in good agreements. The motion response of the FOWT substructure, velocity distribution and the wall pressure are used to analyze the damping mechanism of the TLMCD. Based on the hydrodynamic moment, the energy dissipation characteristics of the TLMCD are analyzed quantitatively, where the damping and exciting moment are distinguished from each other. The analyses show that the TLMCD has the best damping effect near resonance frequency, where the 2.0% mass ratio of the liquid can reduce the maximum pitch motion by 10.84% to 18.53%.
... For example, in Huang et al. (2017), a sampled-data H infinity controller was developed for dealing with irregular wave force and actuator saturation. Furthermore, many works are based on networked control due to its merits in easy installment and high reliability (Cao et al., 2020;Ma et al., 2019a;Zhang et al., 2016a;Ma et al., 2019b). Unlike the methods * Corresponding author. ...
In this article, reinforcement learning based fault-tolerant control for the nonlinear offshore platform is studied. First, the dynamic model of the nonlinear steel-jacket offshore platform with active mass damper and actuator fault is investigated then an affine nonlinear representation for the offshore platform is obtained. By estimating actuator fault and irregular wave force and inserting their estimation into the cost function, the disturbance control and fault-tolerant control problem (FTC) are converted into optimal control problem. Online policy iteration is used to minimize the performance index. Finally, simulation results show the effectiveness of our method.
Marine structures are inevitably influenced by parametric perturbations as well as multiple external loadings. Among these loadings, earthquake is generally more destructive and unpredictable than others. It is significant to develop effective active control schemes to guarantee the safety, stability, and integrity of marine structures subject to earthquakes and parametric perturbations. In this paper, the problem of networked [Formula: see text] robust damping control is addressed to stabilize a marine structure subject to earthquakes. First, in consideration of perturbations of the structure parameters, an uncertain model of the networked marine structure under earthquakes is presented. Second, a robust networked [Formula: see text] control scheme is presented to suppress seismic responses of the structure. By using stability theory of time-delay systems, several sufficient conditions on robust stability of the networked marine structure system are obtained, and the linear matrix inequality methods are utilized to solve the gain matrix of the controller. Finally, simulation indicates that compared with the traditional robust [Formula: see text] control and the proposed networked [Formula: see text] control, the seismic responses amplitudes of the marine structure under the two controllers are almost the same, while the latter is more economic than the former.
In this paper, we investigate the vibration control problem in an offshore platform control structure. A model predictive controller is designed under internal model principle (IMP) and model predictive control (MPC) on the basis of linear quadratic optimal theory, where a rolling-optimized observer is taken to observe and estimate mixed external disturbance. Firstly, a steel jacket offshore platform is modelled as a single-degree-of-freedom (SDOF) vibration system subjected to varying waves and winds containing sensing delay. And the process of finding a vibration suppression controller is summarized as a global optimization problem. Secondly, an optimal quadratic regulator is proposed to attenuate the structure vibration, which naturally adopts IMP considering the varying dynamics of marine disturbance. Data-driven MPC method is then adopted to deal with leading disturbance items. Thirdly, a rolling-horizon optimal algorithm is applied to the proposed disturbance observer so that desired predictive states in deriving the optimal control law are obtained regardless of disturbance sensing delay. Lyapunov stability of the proposed control strategy is proved and comparable simulation experiments are conducted with other controllers.
The vibration control problem of offshore jacket platforms is studied. The model of offshore platforms with nonlinear interactions of the pile-soil affected by the wave load disturbances is established. The wave loads are determined by using the Morison equation and are considered as disturbances with known dynamic characteristics but unknown initial conditions. Based on the successive approximation approach (SAA), we propose an approximate optimal vibration control scheme for the nonlinear systems of the offshore jacket platforms. Numerical simulations illustrate the effectiveness of the proposed approach.
This paper is concerned with trajectory tracking control for an underactuated unmanned surface vehicle (USV) subject to external disturbance and error constraints. Firstly, a disturbance observer is constructed to observe the unknown external disturbance induced by wind, waves, and currents. Secondly, in order to avoid the “explosion of complexity” problem caused by virtual control variable differentiation, a first order filter and a time-varying tan-type barrier Lyapunov function (BLF) are developed. Then, by introducing the backstepping and dynamic surface control techniques, feedback control laws ensuring trajectory tracking are designed such that the errors of position and heading angle between the unmanned surface vehicle and a virtual leader converge to a bounded region. Finally, the effectiveness of the designed control scheme is verified by a simulation example.
This paper addresses the dynamic modeling and active controller design of offshore structures subject to control delays and earthquakes. First, by considering the time-varying control delays and earthquakes, a delayed dynamic model of the marine structure subject to earthquake is established. Then, the asymptotic stability are ensured by some sufficient conditions and the performance index of closed-loop system are derived, and the design scheme of an active damping controller for the offshore structure is presented. The simulation results show that the vibration of marine structures caused by earthquakes can be effectively reduced through the designed control scheme.
In recent decades, different control strategies (i.e., passive, active, semi active or hybrid) have been extensively employed to suppress the response of offshore structures subjected to dynamic lateral loads Jacket platforms are vital structures for oil-rich countries which are frequently subjected to dynamic harsh sea waves. Therefore, the vibration control of these structures is substantial to increase productivity, safety, serviceability and prevent premature possible fatigue failure. In this study, the behavior of the Ressalat platform located in the Persian Gulf is evaluated under wave loads with two different return periods of 50 and 100 years. A simplified equivalent seven degrees of freedom lumped mass linear model is adopted for the Ressalat platform. In the Persian Gulf, wave loads are the dominant load in designing procedure. Due to the stochastic nature of wave loads, random wave and constrained new-wave theories are utilized in generation of the wave records. Then, Passive Tuned Mass Damper (PTMD) and Active Tuned Mass Damper (ATMD) are employed to suppress the platform vibration, and subsequently, their control performances are compared. Various calculated normalized performance criteria (response of controlled to uncontrolled structure) for passive and active controlled platform, including normalized maximum and root mean square (RMS) of displacement, velocity and acceleration of the deck level are calculated and compared. Modeling and analyzing the uncontrolled and controlled platforms are performed in MATLAB and SIMULINK software. Moreover, the Fuzzy intelligent algorithm with triangular membership function is implemented to calculate the control force, and the Harmony Search Algorithm (HAS) is examined to optimize the actuator power. Also, the Fluid-Structure Interaction (FSI), the effect of added mass due to the accelerated motion of the body in the fluid and the saturation of the actuator are considered. The results show the proposed control system effectiveness by reducing 20% and 50% of the maximum and RMS of the platform deck acceleration respectively over time.
A neural network-based fuzzy controller is proposed to attenuate the irregular wave-induced vibration of a steel-jacket offshore platform. Firstly, the offshore platform is modeled as a system with time-varying control delay under random wave forces. Secondly, disturbance rejection measures are taken in designing an optimal controller containing delayed states. Thirdly, neural networks are adopted to observe and restore the controlled system, in order to learn the delayed control law using instant state. Finally, fuzzy models are constructed for reducing the complexity in data collecting and neural network training. Trained with sample data from fuzzy models, the neuro-fuzzy observation system is able to reconstruct the control system, and the generalized neural network-based controller works efficiently in different delayed cases. It achieves better vibration-attenuating performance under uncertain control delay and random waves, when compared to existed optimal control laws and fuzzy controllers without neural networks. The main contributions of this paper are: 1) obtaining a neural network-based observer in state approximation; 2) designing a neural network-based controller based on fuzzy rules to cope with random control delay.
In the paper, we study the problem of vibration control for offshore platform in nonideal network environment under wave and current forces based on the modified transformation. We consider the wave and current synchronously and give a dynamical system for the outside loads. The problem of time delay, packet dropouts and disordering due to nonideal network are taken into account. In the networked control model, two buffers output the received packets according to time stamps and upper bound of time delay. Based on a transformation, we reduce the time delay system into an equivalent delay-free system. Taking the nonideal network communication conditions into account, we design a modified transformation based on the past time data, wave and current loads, and received packets through network at current time. The networked controller is presented to depress the vibration of structure in nonideal network communication environment based on the modified transformation. An algorithm is given to simplify the computation of control law by recurrence. The system stability under presented networked controller is analyzed based on Lyapunov functional. Through the simulation results and comparison with other networked controllers, we can see that the presented controller can reduce the oscillation of offshore platform effectively.
Offshore platforms are widely used to explore, drill, produce, storage, and transport ocean resources and are usually subject to environmental loading, such as waves, winds, ice, and currents, which may lead to failure of deck facilities, fatigue failure of platforms, inefficiency of operation, and even discomfort of crews. In order to ensure reliability and safety of offshore platforms, it is of great significance to explore a proper way of suppressing vibration of offshore platforms. There are mainly three types of control schemes, i.e., passive control schemes, semi-active control schemes, and active control schemes, to deal with vibration of offshore platforms. This paper provides an overview of these schemes. Firstly, passive control schemes and several semi-active control schemes are briefly summarized. Secondly, some classical active control approaches, such as optimal control, robust control, and intelligent control, are briefly reviewed. Thirdly, recent advances of active control schemes with delayed feedback control, sliding model control, sampled-data control, and network-based control are deeply analyzed. Finally, some challenging issues are provided to guide future research directions.
The development of offshore wind energy in Taiwan is moving toward the establishment of a demonstration wind farm before the end of 2016. The aim of this study is to investigate the dynamic motion of a floating platform in a wind-wavecurrent flume through the use of a 1:50 Froude-scale model. Compared to the target-scaling properties, the errors in the geometric dimensions and weight of the scale model are less than 2%. A series of experiments were conducted, including free decay tests and complex operating conditions, which combined the irregular waves, current, and wind. The drift and drag force was measured when the floating platform was subjected to the waves, wind, and current. © by The International Society of Offshore and Polar Engineers.
Advances in geological studies, have identified increased seismic activity in the world's ocean once believed to be far from seismic hazards. The increase in demand of oil and other hydrocarbons leaves no option but to install a suitable offshore platform on these seismically sensitive offshore basins. Therefore, earthquake based design criteria for offshore structures are essential. The focus of the present review is on various computational techniques involved for seismic response study. The structural and load modeling approaches, the disturbed fluid-structure and soil-structure interaction as well as hydrodynamic damping due to earthquake excitation are also discussed. A brief description on the reliability-based seismic design approach is also presented.
The articles in this special section focus on advancements in theories and industrial applications of networked control systems in the industrial informatics industry.
This paper is concerned with the problem of data-driven predictive control for networked control systems (NCSs), which is designed by applying the subspace matrices technique, obtained directly from the input/output data transferred from networks. The networked predictive control consists of the control prediction generator and network delay compensator. The control prediction generator provides a set of future control predictions to make the closed-loop system achieve the desired control performance and the network delay compensator eliminates the effects of the network transmission delay. The effectiveness and superiority of the proposed method is demonstrated in simulation as well as experiment study.
In this paper, the stochastic optimal control of linear networked control system (NCS) with uncertain system dynamics and in the presence of network imperfections such as random delays and packet losses is derived. The proposed stochastic optimal control method uses an adaptive estimator (AE) and ideas from Q-learning to solve the infinite horizon optimal regulation of unknown NCS with time-varying system matrices. Next, a stochastic suboptimal control scheme which uses AE and Q-learning is introduced for the regulation of unknown linear time-invariant NCS that is derived using certainty equivalence property. Update laws for online tuning the unknown parameters of the AE to obtain the Q-function are derived. Lyapunov theory is used to show that all signals are asymptotically stable (AS) and that the estimated control signals converge to optimal or suboptimal control inputs. Simulation results are included to show the effectiveness of the proposed schemes. The result is an optimal control scheme that operates forward-in-time manner for unknown linear systems in contrast with standard Riccati equation-based schemes which function backward-in-time.
A new method for real-time prediction of uncertain network transmission time delays and a method for closed-loop control of
manufacturing and industrial plants through networks are introduced. The proposed delay prediction method is based on the
multilayer perceptron neural model. In order to minimize the number of neurons in the first layer of the network and hence
reducing the computational burden in a real-time implementation, a method for determination of the Markov order of the time
delay sequence is presented. Using the predicted delay, and a zero-order hold equivalent discrete-time model of the plant,
a time varying state feedback control algorithm with a real-time gain updating strategy is proposed. A sufficient condition
for closed-loop stability is also derived using the switching theorem for linear systems. The proposed method is shown, through
two industrial networked case studies, namely, a DC motor driving a transportation roller for paper sheets and a milling machine.
Simulation studies depict the efficacy of the proposed method in controlling such challenging problems.
Congestion in a networked control system (NCS) has many undesirable effects that can make a control system unstable if severe enough. These include delays and packet drops. Therefore, reducing network traffic, including traffic generated by the NCS itself is a necessity. In event based control (EBC) a control signal is generated when a specific event is triggered such as the control error exceeding a predetermined threshold. When the rate of change of a variable is bounded, event triggering reduces the effective frequency in which control signals have to be generated when compared to a system with periodic sampling. This paper proposes a new method of event triggering called event triggered adaptive differential modulation (ETADM) that combines the bandwidth reduction strategies of event triggering and human speech coding techniques. The proposed method can be applied to nonlinear systems with a stabilizing feedback. In addition, it can be shown to be robust to packet drops. Stability for this method can be analyzed in terms of input to state stability (ISS) for a given bound of the signal reconstruction error.
This paper studies the vibration control of a jacket-type offshore platform with an active mass damper (AMD) and presents a feedforward and feedback optimal control (FFOC) law. The linearized Morison equation is employed to estimate the wave load. The offshore structure is simplified into a single degree of freedom (SDOF) system. The original vibration control is formulated as the optimal control for a linear discrete-time system affected by external disturbances with known dynamic characteristics but unknown initial conditions. We give the existence and uniqueness conditions of the FFOC law. Simulation results show that, compared with the classical state feedback optimal control (CFOC) law, the presented control scheme is more efficient in reducing the displacement and velocity of the offshore structure subjected to irregular wave forces.
This note is concerned with stability and controller design of networked control systems (NCSs) with packet dropouts. New NCS models are provided considering both single- and multiple-packet transmissions. Both sensor-to-controller (S/C) and controller-to-actuator (C/A) packet dropouts are modeled and their history behavior is described by different independent Markov chains. In term of the given models, sufficient conditions for stochastic stability are derived in the form of linear matrix inequalities (LMIs) and corresponding control laws are given. Numerical examples illustrate the effectiveness of the results.
This paper aims to study on feedback control for a networked system with both uncertain delays and, packet dropouts and disturbances. Here, a so-called robust predictive control (RPC) approach is designed as follows: 1) delays and packet dropouts are accurately detected online by a network problem detector; 2) a so-called proportional-integral-based neural network grey model (PINNGM) is developed in a general form to be capable of forecasting accurately in advance the network problems and the effects of disturbances on the system performance; 3) using the PINNGM outputs, a small adaptive buffer (SAB) is optimally generated on the remote side to deal with the large delays and/or packet dropouts and, therefore, simplify the control design; 4) based on the PINNGM and SAB, an adaptive sampling-based integral state feedback controller is simply constructed to compensate the small delays and disturbances. Thus, the steady-state control performance is achieved with fast response, high adaptability, and robustness. Case studies are finally provided to evaluate the effectiveness of the proposed approach.
The objective of the present research is to examine the effectiveness of the lateral vibration control of wave-excited response of offshore platforms with magneto-rheological (MR) damper. In this study, the offshore platform is simplified to be a single-degree-of-freedom (SDOF) system by extracting the first vibration mode of the structure. The external generalized wave force is determined with a white noise via a designed filter. A semi-active control method based on optimal control theory is proposed considering that the yield stress of the MR damper can be varied continuously within a certain range. The dynamics of SDOF structure coupled with the MR damper is investigated. Numerical simulation demonstrates that the MR damper with this control strategy can significantly reduce the maximum responses and the root-mean-square (RMS) values.
The objective of this paper is to mitigate the wave-induced vibration of offshore platform with magneto-rheological (MR) damper. The model of the platform coupled with MR damper is established where the external wave force is approximated with a white noise via a designed filter. Based on Linear Quadratic Gaussian (LQG) method, the optimal control force is determined when taking the measurement noise into account. Semi-active control algorithm is applied to generate the MR damping force by comparing with the optimal control force. Numerical example demonstrates that the semi-active control strategy based on LQG method can reduce the responses of the platform effectively.
An optimal active sliding mode controller with specified decay rate design is proposed to control the wave-induced offshore jacket-type platform with active mass damper (AMD). Irregular wave loading is approximated by finite sums of Fourier series, and its dynamic characteristics are governed by an exosystem derived from the linearized Morison equation. The offshore platform system is first decomposed into two virtual subsystems based on a linear transformation. By considering the velocity of AMD as a virtual control force for the first subsystem, the optimal virtual controller is derived. Furthermore, an optimal sliding mode surface with specified decay rate is proposed. Then, the active sliding mode controller is designed to ensure that the state trajectories reach the sliding surface in finite time and remain on it thereafter. Numerical simulation is employed to verify the effectiveness of the proposed approach.
This paper addresses the data-based networked control problem for a class of nonlinear systems. Network communication constraints, such as network-induced delay, packet disorder, and packet dropout in both the feedback and forward channels, are considered and further treated as the round-trip time (RTT) delay that is redefined. By using the packet-based transmission mechanism and the model-free adaptive control algorithm, a data-based networked predictive control method is proposed to actively compensate for the random RTT delay. The proposed method does not require any information on the plant model and depends only on the input and output data of the plant. A simple and explicit sufficient condition, which is related to the upper bound of the RTT delays, is derived for the stability of the closed-loop system. Additionally, a zero steady-state output tracking error can be achieved for a step reference input. The effectiveness of the proposed method is demonstrated via simulation and experimental results.
The development of offshore wind power in Taiwan is moving toward to the next step: to establish the demonstration wind farm before 2016. The aim of this study is to investigate the dynamic motion of the floating platform (semi-submersible type) in a wind-wave-current flume using a 1:50 scale model. The thrust force on the wind turbine was simulated by a solid disk with a diameter of 0.66m at present stage. Compared to the prototype, the elaborate scale model has only 2% error in geometric dimension, total weights and gravity position. The natural frequency and damping coefficient of floating offshore wind turbine (FOWT) was obtained from the free decay test. Different wave directions, wave environments, drafts, wind intensities, current speeds and combined wind/wave/current were applied to study the instability of FOWT and operating conditions. The six-degree-of freedom (DOF), accelerations and drift forces were measured to compare with model result and improve the existing design. Copyright © 2015 by the International Society of Offshore and Polar Engineers (ISOPE).
This paper is concerned with the design and performance analysis of networked control systems with network-induced delay, packet disorder, and packet dropout. Based on the incremental form of the plant input-output model and an incremental error feedback control strategy, an incremental networked predictive control (INPC) scheme is proposed to actively compensate for the round-trip time delay resulting from the above communication constraints. The output tracking performance and closed-loop stability of the resulting INPC system are considered for two cases: 1) plant-model match case and 2) plant-model mismatch case. For the former case, the INPC system can achieve the same output tracking performance and closed-loop stability as those of the corresponding local control system. For the latter case, a sufficient condition for the stability of the closed-loop INPC system is derived using the switched system theory. Furthermore, for both cases, the INPC system can achieve a zero steady-state output tracking error for step commands. Finally, both numerical simulations and practical experiments on an Internet-based servo motor system illustrate the effectiveness of the proposed method.
This paper presents a robust mixed control method for wave-excited offshore jacket platforms. Its objective was to design a controller that minimizes the upper bound of the performance measure on platform dynamics satisfying some norm bound constraint simultaneously. Based on mixed control theory and linear matrix inequality techniques, a novel approach to stabilize offshore platform vibration with constrained performances is proposed. Uncertainties of the wave excitation are considered in dynamic performance analysis of offshore platforms. A reduced mode offshore platform structure under wave excitation is analyzed, and simulations are used to verify the effectiveness of the proposed approach. Compared with existing control methods, the proposed approach makes a significant improvement for dynamic performances of offshore platforms under random wave excitation.
A networked control system (NCS) is a control system in which plants, sensors, controllers, and actuators are connected through communication networks. In this paper, the optimal sampled-data control problem of linear systems with multiple time delays, which is one of the fundamental problems in NCSs, is considered, where multiple time delays and the sampling period are uncertain. First, the problem is transformed into the optimal control problem of discrete-time uncertain linear systems, where the intersample behavior is considered in the cost function. Next, under a certain assumption, the obtained problem is further transformed into a convex quadratic programming problem. Finally, a numerical simulation is presented.
This paper presents the optimal tracking control methodology for an offshore steel jacket platform subject to external wave force. Based on a dynamic model of an offshore steel jacket platform with an active mass damper mechanism and a linear exogenous system model of the external wave force on the offshore platform, an optimal tracking control scheme with feedforward compensation is proposed to attenuate the wave-induced vibration of the offshore platform. A feedforward and feedback optimal tracking controller (FFOTC) can be obtained by solving an algebraic Riccati equation and a Sylvester equation, respectively. It is demonstrated that the wave-induced vibration amplitudes of the offshore platform under the FFOTC are much smaller than the ones under the feedback optimal tracking controller (FOTC) and the feedforward and feedback optimal controller (FFOC). Furthermore, the required control force under the FFOTC is smaller than the ones under the FOTC and the FFOC.
The jacket platforms are strong enough to resist extreme ice forces in Bohai Sea, China. However, ice-induced vibration acceleration has been observed and caused non-structure failure such as human discomfort and gas leakage due to flange loosing of pipes in winters. As a method of reducing ice-induced vibrations, this paper proposed an isolation cone system composed of cones, springs and dampers, which is located on the pile of jacket platforms at sea level. In order to examine the ice-induced vibration acceleration reduction effectiveness of the isolation cone system for jacket platforms, a numerical simulation is conducted to JZ20-2MUQ jacket platform as an example. It is shown from the numerical study and observed data that the isolation cone system is effective in reducing the ice-induced vibration acceleration responses and avoiding non-structure failure of jacket platform structures. Crown Copyright
In this article, we consider the robust reliable sample-data control problem for an offshore steel jacket platform with input time-varying delay and possible occurrence of actuator faults subject to nonlinear self-exited hydrodynamic forces. The main objective of this work is to design a state feedback reliable sample-data controller such that for all admissible uncertainties as well as actuator failure cases, the resulting closed-loop system is robustly exponentially stable. By constructing an appropriate Lyapunov–Krasovskii functional and using linear matrix inequality (LMI) approach, a new set of sufficient condition is derived in terms of LMIs for the existence of robust reliable sample-data control law. In particular, the uncertainty under consideration in system parameters includes linear fractional norm-bounded uncertainty. Further, Schur complement and Jenson’s integral inequality are used to substantially simplify the derivation in the main results. More precisely, the controller gain matrix for the nonlinear offshore steel jacket platform can be achieved by solving the LMIs, which can be easily facilitated by using some standard numerical packages. Finally, a numerical example with simulation result is provided to illustrate the applicability and effectiveness of the proposed reliable sampled-data control scheme.
The network-based modelling and active control for an offshore steel jacket platform with an active tuned mass damper mechanism is investigated. A network-based dynamic model of the offshore platform is first established. A network-based state feedback control scheme is developed. Under this scheme, the corresponding closed-loop system is modelled by a system with an artificial interval time-varying delay. Then, a delay-dependent stability criterion for the corresponding closed-loop system is derived. Based on this stability criterion, a sufficient condition on the existence of the network-based controller is obtained. It is found through simulation results that (i) both the oscillation amplitudes of the offshore platform and the required control force under the network-based state feedback controller are smaller than those under the nonlinear controller and the dynamic output feedback controller; (ii) the oscillation amplitudes of the offshore steel jacket platform under the network-based feedback controller are almost the same as the ones under the integral sliding mode controller, while the required control force by the former is smaller than the one by the latter.
This paper is concerned with active control for an offshore steel jacket platform subjected to wave-induced force and parameter perturbations. An uncertain dynamic model for the offshore platform is first established, where uncertainties not only on the natural frequency and the damping ratio of both the offshore platform and the active tuned mass damper (TMD) but also on the damping and stiffness of the TMD are considered. Then, by intentionally introducing a proper time delay into the control channel, a novel sliding mode control scheme is proposed. This scheme uses information about mixed current and delayed states. It is shown through simulation results that this scheme is more effective in both improving the control performance and reducing control force of the offshore platform than some existing ones, such as delay-free sliding mode control, nonlinear control, dynamic output feedback control, and delayed dynamic output feedback control. Furthermore, it is shown that the introduced time delay in this scheme can take values in different ranges while the corresponding control performance of the offshore platform is almost at the same level.
In this paper, we present a novel H"~ control scheme for a networked control system (NCS) with multiple data packet dropouts. Multiple data packet dropouts occur randomly in both control channel and measurement channel. The NCS with both measurement and control packet dropouts is modeled as a stochastic parameter system which contains two independent Bernoulli distributed white sequences. An H"~ dynamic output controller is designed to exponentially stabilize the networked system in the sense of mean square, and also to achieve the prescribed H"~ disturbance attenuation level. An iterative algorithm is developed to compute the optimal H"~ disturbance attenuation and the controller parameters by solving the semi-definite programming problem via an interior-point approach. Two illustrative examples are provided to show the applicability of the proposed method.
The performance of networked control systems is strongly affected by time-varying transmission delays. A traditional solution to this problem consists of storing arriving packets in a buffer which smooths delay jitter at the cost of an increased constant delay. The size of the buffer is based on either a long-term or worst case analysis of network behavior leading to poor performance when the instantaneous network behavior is different. To overcome this problem, this paper proposes the following: 1) to adapt the buffer size according to the actual delay variation; 2) to resize buffer content by using cubic spline smoothing which also reduces the signal noise; and 3) to use a Smith predictor at the controller side. Simulation results show that the adaptive buffering strategy reduces delay and packet loss probability while the spline smoothing process improves control performance even in case of constant-size buffers.
This paper is concerned with delayed H∞ control for an
offshore steel jacket platforms subject to external wave force. By
artificially introducing a proper time-delay into control channel, a
delayed H∞ controller is designed to attenuate the
wave-induced vibration of the offshore platform and thereby improve the
control performance of the system. The problem of the controller design
is transformed into a nonlinear minimization problem. It is shown
through simulation results that compared with the delay-free
H∞ control scheme, the vibration amplitudes of the
offshore steel jacket platform with the delayed H∞
control scheme are in the same level as the ones with the delay-free
H∞ control scheme, while the required control force by
the delayed H∞ control scheme is much smaller than that
by the delay-free H∞ control scheme. Compared with the
feedforward and feedback optimal control scheme, both the vibration
amplitudes of the offshore platform and the required control force with
the delayed H∞ control scheme are smaller than the ones
with the feedforward and feedback optimal control scheme.
This study deals with a robust trajectory tracking control problem of discrete-time networked control systems. The systems have time-varying interval-bounded uncertainties, random network-induced delays and data dropouts in both sensor-to-controller channel and controller-to-buffer/actuator channel. A dynamic predictive feedback linearisation controller is proposed so that the system dynamics and delays caused by networked communication time and data dropouts are compensated, and the system output can perfectly track the reference trajectory when uncertainties are absent. Tracking errors caused by the time-varying parameter uncertainties are suppressed in the H∞ sense. Finally, the proposed method is illustrated by two numerical examples which show the application procedure and effectiveness of the proposed control scheme.
A well-designed software framework is important for the rapid implementation of reliable and evolvable networked control applications and to facilitate the proliferation of networked control by enhancing its ease of deployment. In this brief, we address the problem of developing such a framework for networked control that is both real-time and extensible. We enhance Etherware, a middleware developed at the University of Illinois, so that it is suitable for time-critical networked control applications. We introduce a notion of quality of service (QoS) for the execution of a component. We also propose a real-time scheduling mechanism so that the execution of components can not only be concurrent but also be prioritized based on the specified QoS of each execution. We have implemented this framework in Etherware. We illustrate the applicability of this software framework by deploying it for the control of an unstable system, namely, a networked version of an inverted pendulum control system, and verify the performance of the enhanced Etherware. We also exhibit sophisticated runtime functionalities, such as runtime controller upgrade and migration, to demonstrate the flexible and temporally predictable capabilities of the enhanced Etherware. Overall, Etherware thus facilitates rapid development of control system applications with temporally predictable behavior so that physical properties such as stability are maintained.
This paper is concerned with sliding mode H∞H∞ control for an offshore steel jacket platform subject to nonlinear self-excited wave force and external disturbance. A sliding mode H∞H∞ controller is designed to reduce the oscillation amplitudes of the offshore platform. In the case that the dynamic model of the offshore platform is subject to parameter perturbations, a robust sliding mode H∞H∞ control scheme is proposed. It is found through simulation results that (i) compared with an H∞H∞ controller and a sliding mode controller, the sliding mode H∞H∞ controller requires much less control force, and (ii) the oscillation amplitudes of the offshore platform under the sliding mode H∞H∞ controller are less than those under the sliding mode controller.
This paper studies an event-triggered communication scheme and an H∞H∞ control co-design method for networked control systems (NCSs) with communication delay and packet loss. First, an event-triggered communication scheme and a sampled-state-error dependent model for NCSs are presented. In this scheme and model, (a) the sensor takes samples in a periodic manner; (b) a triggering condition is applied to the sampled signal to determine whether a signal is transmitted to the controller or not; and (c) the closed-loop system with a networked state feedback controller is modeled as a time-delay system. Secondly, by constructing a novel Lyapunov–Krasovskii functional, three theorems for the system asymptotical stability subject to imperfect communications are derived. Thirdly, a new algorithm is developed for the triggering condition and the controller feedback gain to meet the specified performance. This design algorithm is based on the two permissible limits on the signal transfer. These limits are: the maximum allowable communication delay bound and the maximum allowable number of successive packet losses, respectively. Finally, the proposed co-design method is demonstrated by two numerical examples.
This paper presents a sum of squares (SOS) approach to the stability analysis of networked control systems (NCSs) incorporating bounded time-varying delays, bounded time-varying transmission intervals and a shared communication medium. A shared communication medium imposes that per transmission only one node, which consists of several actuators or sensors, can access the network and transmit its corresponding data. Which node obtains access is determined by a network protocol. We will provide mathematical models that describe these NCSs and transform them into suitable hybrid systems formulations. Based on these hybrid systems formulations we construct Lyapunov functions using SOS techniques that can be solved using LMI-based computations. This leads to several beneficial features: (i) we can deal with plants and controllers which are described by nonlinear (piecewise) polynomial differential equations, (ii) we can allow for non-zero lower bounds on the delays and transmission intervals in contrast with various existing approaches, (iii) we allow more flexibility in the Lyapunov functions thereby obtaining less conservative estimates of the maximal allowable transmission intervals (MATI) and maximal allowable delay (MAD), and finally (iv) it provides an automated method to address stability analysis problems in NCS. Several numerical examples illustrate the strengths of our approach.
The proposed networked control and monitoring system based on industrial Ethernet has been successfully designed, installed, commissioned and completed in this paper. The system focused on reliable real-time communication transmission, intelligent management, automatic field & integrated control and data acquisition to support supervision of comprehensive information analysis. The hardware architecture and system philology design were introduced. Windows sockets and multi-threading technique to realize real-time and reliable transmission then were utilized in the system. This paper also emphasized the method that application accesses Windows system shared resources by utilizing I/O address redirection technique through Microsoft Network Client Software (MS- Client) under Disk Operating System (DOS) platform. The global production process status of the plant was supplied through monitoring system for managers to make better decision. Real- time, stability, reliability, safety of the whole system has been proved in practice.
This paper is concerned with sliding mode control of offshore steel jacket platforms subject to nonlinear wave-induced forces. Two sliding mode control schemes are proposed to reduce the oscillation amplitudes of the platforms. One scheme is that a conventional sliding mode controller (SMC) is designed, under the SMC the oscillation amplitudes of the platform are effectively reduced; however, a very large control force is required. The other scheme is that by intentionally introducing a time-delay into the control channel, a delayed sliding mode controller (DSMC) is designed. Compared with the SMC, the DSMC improves the control performances in two aspects: (i) the oscillation amplitudes of the first, second and third floors under the control of the DSMC are less than those under the control of the SMC; (ii) the control force required by the DSMC is much less than that by the SMC, which are confirmed by simulation results.
Offshore platforms are usually located in hostile environments. These platforms undergo excessive vibrations due to wave loads for both normal operating and extreme conditions. To ensure safety, the displacements of the platforms need to be limited, whereas for the comfort of people who work at the structures, accelerations also need to be restricted. This article is devoted to developing a proper procedure on applying H2 control algorithm for controlling the lateral vibration of a jacket-type offshore platform by using an active mass damper. In comparison with earlier studies, a number of improvements in problem formulation, wave force filter design, and control algorithm implementation are made. The present paper also numerically demonstrated the effectiveness of H2 active control. As expected, it significantly outperforms the corresponding passive control that uses a tuned mass damper.
This paper studies the vibration suppression of actively controlled jacket-type offshore platforms with fixed delay in the control. Based on the wave theory and Morison equation, an exosystem is designed to describe the irregular wave forces. Through a particular transformation, the original delay system is reduced into a non-delay system. Based on the reduced system, the paper develops a feedforward and feedback optimal control law with memory (FFOCLM). The memory terms in FFOCLM compensate the time-delay in control input. The feedforward term of the controller includes the information of the irregular wave forces. The feedback loop incorporates the displacement and velocity of structure into the control law. The FFOCLM is proved to be existent and unique, and able to stabilize the time-delay system. The feasibility and effectiveness of the presented control law is demonstrated by a numerical example of a jacket-type offshore structure.
Networked control systems (NCSs) are digital control systems in which the functionality of the sensor, control, and actuator reside in physically different computer nodes communicating over a network. However, random delays and data loss of the communication network can endanger the stability of an NCS. We have proposed model-based predictive NCSs (MBPNCSs) that compensate for the aforementioned problems and avoid performance loss using a predictive control scheme based on a model of the plant. There are three main contributions of this paper to existing methods: an NCS that can work under random network delay and data loss with realistic structural assumptions, an explicit mechanism for reducing the effects of network delay and data loss on the deviation of plant state estimates from actual plant states, and an architecture where upstream nodes can work without receiving acknowledge information about the status of previously sent data packets from downstream nodes. In this paper, we describe MBPNCS and then introduce a stability criterion. This is followed by computer simulations and experiments involving the speed control of a dc motor. The results show that considerable improvement over performance is achieved with respect to an event-based NCS.
This brief studies the predictive controller design of networked systems with communication delay and data loss. A networked predictive control scheme is employed to compensate for communication delay and data loss actively rather than passively. Based on analysis of the closed-loop networked predictive control systems, a design strategy of the predictive controller is proposed. The designed predictive controller can achieve the desired control performance and also guarantee the system stability. A numerical example demonstrates the compensation for communication delay and data loss in networked systems using the proposed predictive controller design strategy.
There are many communication imperfections in networked control systems (NCS) such as varying transmission delays, varying sampling/transmission intervals, packet loss, communication constraints and quantization effects. Most of the available literature on NCS focuses on only some of these aspects, while ignoring the others. In this paper we present a general framework that incorporates communication constraints, varying transmission intervals and varying delays. Based on a newly developed NCS model including all these network phenomena, we will provide an explicit construction of a continuum of Lyapunov functions. Based on this continuum of Lyapunov functions we will derive bounds on the maximally allowable transmission interval (MATI) and the maximally allowable delay (MAD) that guarantee stability of the NCS in the presence of communication constraints. The developed theory includes recently improved results for delay-free NCS as a special case. After considering stability, we also study semi-global practical stability (under weaker conditions) and performance of the NCS in terms of Lp gains from disturbance inputs to controlled outputs. The developed results lead to tradeoff curves between MATI, MAD and performance gains that depend on the used protocol. These tradeoff curves provide quantitative information that supports the network designer when selecting appropriate networks and protocols guaranteeing stability and a desirable level of performance, while being robust to specified variations in delays and transmission intervals. The complete design procedure will be illustrated using a benchmark example.
In this paper, adaptive filtering schemes are proposed for state estimation in sensor networks and/or networked control systems with mixed uncertainties of random measurement delays, packet dropouts and missing measurements. That is, all three uncertainties in the measurement have certain probability of occurrence in the network. The filter gains can be derived by solving a set of recursive discrete-time Riccati equations. Examples are presented to demonstrate the applicability and performances of the proposed schemes.
The wave-induced dynamic force is one of the most important excitations to be dealt with in the design of offshore structures. In order to perform a reliable design of an offshore structure, it is important to obtain an exact evaluation of its dynamic response but also to examine the ways of reducing the response. This paper presents the response of offshore jacket platforms installed with energy dissipation devices such as viscoelastic, viscous and friction dampers under wave loading. The offshore jacket platforms are modeled as multi-degrees-of-freedom system provided with dampers at each floor location. The wave forces are modeled as per Morison's equation. The governing equations of motion of the jacket platform with dampers are derived and their solution in the frequency domain is presented. The uni-directional random wave loading is expressed by the Pierson-Muskowitz spectrum. The response of the jacket platform with viscoelastic, viscous and friction dampers is compared with the corresponding response without dampers in order to investigate the effectiveness of the passive control systems. It is observed that the additional dampers add substantial damping to structure and thus favorably control the response of platform structure. Among the various energy dissipation devices used for study, the viscoelastic dampers perform better in comparison to the other dampers. This is due to the fact that the added viscoelastic dampers contribute to increased viscous damping as well as lateral stiffness which reduces the response of the offshore jacket platforms significantly.
This paper addresses the problem of output feedback control for networked control systems (NCSs) with limited communication capacity. Firstly, we propose a new model to describe the non-ideal network conditions and the input/output state quantization of the NCSs in a unified framework. Secondly, based on our newly proposed model and an improved separation lemma, the observer-based controller is developed for the asymptotical stabilization of the NCSs, which are shown in terms of nonlinear matrices inequalities. The nonlinear problems can be computed through solving a convex optimization problems, and the observed and controller gains could be derived by solving a set of linear matrix inequalities. Thirdly, two simulation examples are given to demonstrate the effectiveness of the proposed method.
In this paper, the output-feedback control problem is considered for networked systems involving in signal quantization and data packet dropout. The states of the controlled system are unavailable and the output signals are quantized before being communicated. An estimation method is introduced to cope with the effect of random packet loss that is modelled as a Bernoulli process. The quantized measurement signals are dealt with by utilizing the sector bound method, in which the quantization error is treated as sector-bounded uncertainty. The output-feedback controller is designed which guarantees the closed-loop system is exponentially mean-square stable. The simulation example is given to illustrate the proposed method.
Several traffic control systems are simply introduced, and based on the networked control presented is a new intelligent intersection control system. It is characterized by the agent-based control and the local simple remote complex (LSRC) design principle. On one hand the intelligent control of intersection is guaranteed, and on the other hand the cost of the control system is decreased. The structure and function of the whole system are described in detail. According to the characteristic of city traffic, the intersection signal is controlled by fuzzy neural network, which fully uses the advantage of this system.
The defining characteristic of a networked control system (NCS) is
having one or more control loops closed via a serial communication
channel. Typically, when the words networking and control are used
together, the focus is on the control of networks, but in this article
our intent is nearly inverse, not control of networks but control
through networks. NCS design objectives revolve around the performance
and stability of a target physical device rather than of the network.
The problem of stabilizing queue lengths, for example, is of secondary
importance. Integrating computer networks into control systems to
replace the traditional point-to-point wiring has enormous advantages,
including lower cost, reduced weight and power, simpler installation and
maintenance, and higher reliability. In this article, in addition to
introducing networked control systems, we demonstrate how dispensing
with queues and dynamically scheduling control traffic improves
closed-loop performance