Flexible Riser System Research Articles

Integral BLF-Based Adaptive Dynamic Event-Triggered Boundary Control for a Flexible Riser System.

For the flexible riser systems modeled with partial differential equations (PDEs), this article explores the boundary control problem in depth for the first time using a dynamic event-triggered mechanism (DETM). Given the intrinsic time-space coupling characteristic inherent in PDE computations, implementing a state-dependent DETM for PDE-based flexible risers presents a significant challenge. To overcome this difficulty, a novel dynamic event-triggered control method is introduced for flexible riser systems, focusing on optimizing available control inputs. In order to save computational costs from the controller to the actuator, a dynamic event-triggered adaptive boundary controller is designed to effectively reduce boundary position vibrations. Additionally, considering external disturbances, an adaptive bounded compensation term is incorporated to counteract the influence of external disturbances on the system. Addressing boundary position constraints, a new integral barrier Lyapunov function (iBLF) tailored specifically for flexible riser systems is introduced, thereby alleviating conservatism in the controller design of flexible risers modeled by PDEs. At last, the validity of the proposed method is demonstrated through a simulation example.

Vibration control of 2-D variable-length flexible riser systems with unknown boundary disturbance

The riser system is subject to vibration due to marine environmental loads, which can affect the efficiency of the system or even destroy it. This paper proposes a solution for the vibration control problem of a 2-D variable-length flexible riser based on PDE model. Firstly, a disturbance observer is developed to estimate and offset the unknown disturbance in the feedback loop. Based on the above method, an effective boundary controller is designed to realize the elastic suppression goal of the 2-D variable-length flexible riser. Secondly, when considering the asymmetric output constraints, a time adjustment function is introduced to develop a new controller based on the barrier Lyapunov function, which realizes the control objective of suppressing the boundary vibration of the flexible riser within specified constraints in a finite time. Finally, the effectiveness of the controller is verified by simulation examples.

Active disturbance rejection adaptive fuzzy backstepping control of uncertain flexible marine riser system

In this paper, vibration control of a flexible marine riser with uncertain parameters subjected to distributed disturbance and boundary disturbance is studied. The existing vibration control algorithms have the problems of relying on the accurate mathematical model of the system, complex control laws, and need to measure multiple states of the system, which cannot effectively deal with the uncertainty of model parameters and external disturbances. To effectively suppress the vibration of a marine flexible riser system with uncertain parameters, an adaptive fuzzy backstepping control algorithm was designed. First, the backstepping method and Lyapunov function are used to analyze the subsystem and design the virtual control law. On this basis, an adaptive fuzzy system was designed. The fuzzy system was used to approach the unknown nonlinear term in the design process of the control law and the uniform boundedness of the system was proved by the Lyapunov stability theory. The designed control algorithm has a simple structure, and it can effectively restrain the riser vibration and improve the control rate. Finally, the numerical simulation results show that the designed control algorithm is effective in suppressing the vibration of flexible risers and the control effect is better than previous PID control and robust adaptive control.

Stabilization and decay rate estimation of nonlinear flexible marine riser system with the rotational inertia under nonlinear boundary controls

Stabilization and decay rate estimation of nonlinear flexible marine riser system with the rotational inertia under nonlinear boundary controls

Dynamic Event-Triggered Boundary Control for Cyber-Physical Flexible Riser Systems Subject to Spoofing Attacks

Dynamic Event-Triggered Boundary Control for Cyber-Physical Flexible Riser Systems Subject to Spoofing Attacks

Integral Barrier Lyapunov Function-Based Adaptive Event-Triggered Control of Flexible Riser Systems

Integral Barrier Lyapunov Function-Based Adaptive Event-Triggered Control of Flexible Riser Systems

An insight into the severe slugging characteristics in a catenary flexible riser

This paper reports the experimental results of the severe slugging (SS) flow characteristics in a downward inclined pipeline-catenary flexible riser system. The non-intrusive optical measurement with high-speed cameras was employed to capture the evolution of liquid slugs and the gas–liquid interface. Five SS patterns are identified in the considered flow velocity range. There are two kinds of severe slugging I (SSI-1 and SSI-2) with the essential difference in the occurrence of the fast blockage stage before the slug formation (SF) stage. The severe slugging II (SSII) is characterized with the longest liquid slug less than a riser length and the absence of slug production stage. The liquid slug is further shortened in severe slugging III (SSIII), exhibiting local liquid fallback and accumulation of multiple slugs in the riser. The severe slugging transition (SST) occurs due to the switching between the SSI-1 and SSII. The flow regime partition is plotted in the vsl (liquid superficial velocity)–vsg (gas superficial velocity) diagram, presenting alteration as the inclination angle of upstream pipeline varies. Generally, the SSI is easier to form at a higher inclination angle. The intermittent occurrence of hydrodynamic slug in the upstream pipeline in the SF stage contributes to the appearance of SSI-2 to SST. The SS cycle has approximately a negative exponent relation with vsg, while the riser base pressure changes exponentially with the increase in vsl.

Vibration Control of Marine Flexible Riser with Uncertain Model

When flexible risers in the ocean are subjected to various environmental loads in the ocean, vibration will inevitably occur. Due to the nonlinearity and uncertainty of the actual riser system, it is impossible to obtain an accurate mathematical model of the riser system. The existing control algorithms designed based on the precise model of the riser system cannot meet the actual control requirements. A flexible riser boundary control method based on fuzzy control algorithm is proposed to suppress the vibration of marine flexible riser systems with uncertain models. Firstly, the dynamic models of existing marine flexible risers were studied. In response to the uncertainty of the riser system model, the control experience was transformed into an automatic control strategy in the form of fuzzy language control rules to improve the adaptability of the controller. Then, combining boundary control technology with fuzzy control, a boundary controller is constructed to stabilize the riser in a small neighborhood of its original position and reduce fatigue damage to the riser. Finally, the effectiveness of the designed fuzzy boundary control algorithm was verified through MATLAB simulation results, and the vibration suppression effect of the algorithm on the vertical tube was better than that of traditional PD control.

Boundary Control Design of Three-Dimensional Vibrating Marine Riser System With Barrier Term

Abstract In this paper, a boundary control with barrier term is proposed for a typical three-dimensional flexible riser system by integrating backstepping technique with barrier Lyapunov functions. The proposed boundary control does not depend on the exact value of interference and can effectively restrain the vibration of the riser and constrain the angle between the riser and the platform. The proposed control has a small amount of calculation and strong robustness, and can effectively restrain the riser vibration when the environmental disturbance changes. In addition, based on the proposed boundary control method, the stability of the closed-loop system in space and time is proved by Lyapunov stability theorem when the initial conditions are satisfied. Numerical simulation shows the effectiveness and superiority of the controller.

Dynamic analysis of the subsea production system with lazy-wave risers attached to FPSO.

The lazy-wave riser is an input and output riser for a flexible development system, which is widely used in all the riser and pipeline systems. Because of the influence of various factors, its configuration description, control and motion which have a strong nonlinear character are complex during the running process of the lazy-wave riser. Reference to the specific structure and environmental parameters of a certain lazy-wave risers system with a 300 thousand tons FPSO, with the basis of the specific process of the flexible riser system at work, the lazy-wave risers were discretized into lumped mass models, combined with AQWA, the simplified dynamic model of the whole system at the depth of 2100m has been established by the large hydrodynamic analysis software OrcaFlex. The dynamic response characteristics of the lazy-wave risers have been given by using time domain coupling method. With and without the consideration of the 2nd wave drift load in the simulation process, the effects of the 2nd wave drift load on the results are obtained. The simulation results reveal the difficulty of simulation convergence caused by a large number of risers and flexible components. The 2nd order wave drift loads have a significant effect on the riser system, resulting in the increasement of the effective tension at each end of each riser. To counteract the magnitude of the FPSO response caused by such loads, the number of mooring lines needed to be increased or combined with dynamic positioning techniques to optimize the design.

Boundary Iterative Learning Control of a Flexible Riser With Input Saturation and Output Constraint

For tackling the vibration suppression problem for a flexible riser system with external disturbance, input saturation, and output constraint, this article is constructed. A boundary iterative learning control is constructed together with Lyapunov’s theory and backstepping technology. The output constraint is largely solved by employing a barrier Lyapunov function. External disturbance is handled as the iteration goes on. Under the control of the designed method, the output is limited to a given region, and the system is proved to be closed-loop stable with the assistance of Lyapunov’s theorem of stability. Ultimately, the result of the simulation indicates that the method is effective and has a good control effect.

Numerical investigation of VIV responses of the flexible riser system modelled as tensioned cable subjected to shear flow

The crossflow vortex-induced vibration (VIV) responses of the tensioned cable subjected to the sheared flow velocity profile are investigated numerically using the wake oscillator model. A comparative study is carried out on the VIV response characteristics between the tensioned cable and beam with an aspect ratio (L/D) of 3000. The study was conducted on tensioned cable having different L/D of 2000 and 3000 achieved by varying both diameter and length of the cable. The effect of top tension on the RMS crossflow displacement responses is investigated. The RMS crossflow displacement response is not sensitive to the boundary conditions for the flexible cylinder modeled as a tensioned cable. In contrast, the RMS crossflow displacement response of the lower L/D flexible cylinder modeled as a beam is sensitive to boundary conditions. The standing wave response pattern is observed for the beam with L/D of 500. The standing and traveling wave response patterns are combined for the tensioned cable with L/D = 500. Unlike finite tensioned beams, the higher mode responses are obtained even in the lower aspect ratio for tensioned cables.

Adaptive neural network based boundary control of a flexible marine riser system with output constraints

Adaptive neural network based boundary control of a flexible marine riser system with output constraints

Adaptive fuzzy backstepping control of flexible marine riser with uncertain parameters and input saturation constraint

To suppress the vibration of a flexible riser system with input saturation and uncertain parameters under the influence of unknown external disturbances, an adaptive fuzzy backstepping boundary control algorithm was designed. First, the backstepping method and Lyapunov function are used to analyze the subsystem and design the virtual control law. On this basis, the adaptive fuzzy system was designed to approximate the unknown nonlinear term in the design process, and the smooth hyperbolic tangent function and the auxiliary system based on Nussbaum function were introduced to limit control input. The stability and uniform boundedness of the closed-loop control system are proved by the Lyapunov stability theory without simplifying or discretizing the infinite dimensional dynamic model. Finally, the effectiveness of the proposed control algorithm is verified by comparing the control effect with proportional–integral–derivative control and the previous adaptive backstepping control by MATLAB simulation. The simulation results show that the proposed control algorithm can overcome the uncertainty of system parameters and effectively suppress the riser vibration under input constraints, and its control effect is better than proportional–integral–derivative control and adaptive backstepping control.

PDE Based Adaptive Control of Flexible Riser System With Input Backlash and State Constraints

In this paper, a class of flexible riser systems modeled by partial differential equations (PDEs) with the backlash is considered. The backlash is formulated as the addition of a linear input and a interference-like term, then an new auxiliary item is introduced to compensate for the impact of this backlash. In addition, the constraint problem for the position and the velocity is also taken into consideration. To solve this constrain problem, the logarithmic barrier Lyapunov function is employed. For the flexible riser system, two kinds of adaptive controllers are proposed under the following two cases. One controller is designed when only the parameter of backlash is unknown. On the basis of this result, the other controller is presented when some system parameters cannot be measured through actual measurement. Then, combing the theory of Lyapunov stability, the two controllers can guarantee the boundedness of all signals in the closed-loop flexible riser system. Further, both the position and the velocity satisfy their corresponding constraint condition. Finally, the simulation example verifies that the proposed control method is effective.

Output feedback control of a flexible marine riser with the top tension constraint

In this paper, for the flexible marine riser system with the top tension constraint, the output feedback controller is developed to suppress the vibration of the riser. The dynamic behavior of the flexible riser is expressed by the partial differential equation (PDE). Firstly, a high-gain observer is designed to estimate unmeasurable system states. Secondly, the top tension of the riser is constrained with the aid of the logarithmic barrier Lyapunov function, and the output feedback boundary controller is developed using the direct Lyapunov method. Through rigorous mathematical analysis, the consistently bounded stability of the closed-loop system is achieved. Finally, the simulation is carried out to show that the developed control can stabilize the system with good performance under proper selection of design parameters.

Stabilization control of a flexible marine riser with failed and bounded actuator and time‐varying boundary constraints

AbstractIn this article, we investigate the stabilization control problem of a flexible marine riser system with a failed and bounded control action and time‐varying boundary constraints. The considered system is modeled as a distributed parameter system dominated by a partial differential equation. The involved problems are extended to some more general conditions. The saturation and failures of the actuator make the input and output of the actuator a nonlinear relationship, which leads to the normally designed control input invalid. By applying the Nussbaum gain technique, we report a synthetical control method to tackle the input nonlinearity. A piecewise barrier Lyapunov function (BLF) is introduced to reduce the boundary displacement in an asymmetric open set. Compared with the traditional BLFs, the initial boundary displacement is not forced to retain within the given region by integrating a shifting function into the BLF design. In addition, the adaptive method is combined with the hyperbolic tangent function to provide a smooth control solution to the unknown parameters from nonlinear input and external perturbation. Relying on a Lyapunov‐based analysis and a numerical simulation, it is proven that the actuated riser system is uniformly bounded and the boundary displacement always keeps in the limited set even subject to actuator failures and limitations.

Adaptive robust barrier‐based control of a 3D flexible riser system subject to boundary displacement constraints

AbstractThis article involves an adaptive robust barrier‐based control of a three‐dimensional riser system subject to system uncertainties and output constraints. A barrier‐based Lyapunov function and a dynamical online update technique are merged to develop new adaptive robust control schemes for dampening the vibration, compensating for parametric uncertainties, and dealing with the constraints in the system. A rigorous Lyapunov analysis is exploited to guarantee the uniformly bounded stability in the controlled system. Finally, the efficacy of the designed approach is validated via simulation results.

A procedure to estimate the lateral force in clay-pipe interaction after breakout

The development of new offshore oil and gas fields is continuously expanding to ultra-deep waters. This tendency and the necessity of reducing project costs have been stimulating the development of new technologies as well as the enhancement of floating production systems. In this regard, pipelines and flexible riser systems have been getting more attention due to its low cost of installation and operation. In order to project a pipeline system, it is important to understand the pipe-soil interaction mechanisms and quantify the influence of soil behaviour on pipe response caused by lateral movement such as thermal buckling. The loads that a pipeline is subjected have been a topic of many experimental studies that aim to reproduce those loads in a realistic manner. This present study concerns the analysis of lateral clay-pipe interaction associated with large deformations and berm formation process at the leading edge of the pipe during movement at given burial depths. A series of centrifuge tests was conducted to assess the relationship between horizontal force and lateral pipe displacement. The breakout force experimental results were compared with different literature proposals, showing a good agreement. A procedure was also proposed to evaluate the normalized lateral force through the combination of two different approaches. The results showed a good comparison with the centrifuge experimental data.

Dynamic Flexible Riser Ancillary Equipment—North Sea Asset Integrity Management Experience and Lessons Learned

Abstract Dynamic flexible risers are complex engineered systems, which provide a connection between topside (normally floating) facilities and subsea pipeline infrastructure on offshore oilfields. Such systems require the use of ancillary equipment to ensure the riser’s correct configuration is maintained throughout the service life. Industry experience shows that the integrity management of riser ancillary equipment is not always comprehensive, and failure of such equipment is one of the causes of premature removal of flexible risers from service. This article presents some case studies from the operational experience of dynamic flexible risers by an operator in the UK North Sea covering a period of approximately 20 years. The case studies look at the anomalies identified in service by general visual inspection (GVI) using a remotely operated vehicle (ROV) and the lessons learned. Some of the anomalies, had they not been identified and addressed promptly, could have resulted in costly repairs, which demonstrates the importance of inspecting the ancillary equipment of flexible risers as a part of the riser integrity management strategy. The challenges associated with integrity management of ancillary equipment of dynamic flexible risers are also discussed. The case studies presented in this article demonstrate that ROV GVI is an effective method for identifying installation and in-service anomalies related to flexible riser ancillary equipment. The purpose of this article is to share lessons learned with the wider offshore oil and gas community. It is also believed that the information presented in this article may provide useful information to other users of dynamic flexible riser systems when developing and/or implementing their subsea pipelines integrity management programs.