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Shixiao Fu - One of the best experts on this subject based on the ideXlab platform.

  • time varying hydrodynamics of a Flexible Riser under multi frequency vortex induced vibrations
    Journal of Fluids and Structures, 2018
    Co-Authors: Shixiao Fu, Mengmeng Zhang
    Abstract:

    Abstract This paper proposes the Forgetting Factor Least Squares (FF-LS) method for identification of time-varying hydrodynamics of a Flexible Riser under multi-frequency vortex-induced vibrations (VIV). Differing from the traditional least squares method used to identify hydrodynamics, with equal weights on all sampled data, this method introduces a forgetting factor to give higher weight to data closer to the present moment. FF-LS allows the possibility of handling the time-varying parameters. By following this procedure for all sampled data of a Flexible Riser undergoing multi-frequency VIV, the corresponding time-varying hydrodynamics in the Cross-Flow (CF) direction will be obtained considering multi-frequency coupling. The results show that, under multi-frequency coupling, vortex-induced force coefficients of Flexible Risers change periodically and differ from coefficients at the basic frequency usually used in VIV prediction. This difference is a result of the coupling effect between the basic frequency and high frequency. Time-varying coefficients considering multi-frequency coupling contain steady components and vibration components whose frequencies are the sum of coupling frequencies and the difference of coupling frequencies. To show the advantages of time-varying hydrodynamics, we compare forces reconstructed from different vortex-induced force coefficients, assuming vortex-induced force obtained from the inverse analysis is the real force. The results show that time-varying vortex-induced force coefficients considering a coupling effect between multiple frequencies can accurately reconstruct vortex-induced force, while the sum of vortex-induced force coefficients under multiple frequencies will produce the cross-term that overestimates the vortex-induced force.

  • distribution of drag force coefficient along a Flexible Riser undergoing viv in sheared flow
    Ocean Engineering, 2016
    Co-Authors: Leijian Song, Shixiao Fu, Mengmeng Zhang, Yifan Chen
    Abstract:

    Abstract The drag force coefficients of a Flexible Riser undergoing vortex-induced vibration (VIV) in sheared flow are investigated for Reynolds numbers (Re) up to 1.2×105. Based on the drag forces theoretically calculated by the beam theory using the strains measured in a scale model test, the properties and distribution of the drag coefficients are investigated, and a new empirical model for estimating the drag coefficient on a Flexible Riser undergoing VIV is proposed. The results show that VIV leads to non-uniform distribution of the drag coefficient and amplifies the drag coefficient, and the local drag coefficient can reach up to 3.2. For Re values from 1.0×104–1.2×105, the mean drag coefficient is between 1.3 and 2.0 and decreases as Re increases. Furthermore, the empirical drag coefficient prediction model obtained from experiments under low Re is not suitable for high Re. The corrected empirical prediction model, which accounts for the effect of the flow velocity, the VIV dominant mode number and the dominant frequency, can be used to predict Riser drag coefficients under VIV more accurately at high Re values up to 1.2×105.

  • an investigation into the hydrodynamics of a Flexible Riser undergoing vortex induced vibration
    Journal of Fluids and Structures, 2016
    Co-Authors: Leijian Song, Shixiao Fu, Jianqiao Wu
    Abstract:

    Abstract In this study, a method to obtain the hydrodynamic forces of a Flexible Riser undergoing vortex-induced vibration (VIV) based on measured strain is proposed. The tensioned Riser is approximated as an Euler–Bernoulli beam, and an inverse method is adopted for the calculation of the hydrodynamic forces in the cross flow (CF) and inline (IL) directions. Based on these hydrodynamic forces, and combined with the VIV velocities and accelerations of the Riser, the excitation and added-mass coefficients are obtained through a least-squares method. As an illustration example, the hydrodynamic characteristics of a Flexible Riser model undergoing VIV in a uniform flow are investigated. Results indicate that VIV leads to non-uniform distribution of the drag coefficient and amplifies the drag coefficient along the Riser. Similar distribution of the energy transfer coefficient has been found between the entire Riser and that of the CF direction. For synchronized VIV occurring in both CF and IL directions, the energy transferred from the fluid is all dissipated by the structural damping, and hence leads to the energy balance in both CF and IL directions. It further shows that the excitation coefficients on Flexible Riser undergoing VIV do not agree with those of the forced oscillation tests: excitation coefficients sometimes even become negative within the normally excitation regime, and are related with not only the non-dimensional frequency and amplitude but also the phase angles between the CF and IL vibrations. The added-mass coefficient of Flexible Risers does not keep a constant value of 1.0 anymore, but depends on the non-dimensional frequency and amplitude of vibration.

  • experimental investigation of the response performance of viv on a Flexible Riser with helical strakes
    Ships and Offshore Structures, 2016
    Co-Authors: Shixiao Fu, Yifan Chen
    Abstract:

    Experimental investigations were conducted on a Flexible Riser with and without helical strakes. A uniform current was obtained by towing a Riser model in a tank, and the vortex-induced vibration (VIV) suppression of strakes with different heights and pitches was studied. The experimental results indicate that the response characteristics of a bare Riser can be quite distinct from those of a Riser with helical strakes, and the suppression performance depends on the geometry of the helical strakes. The VIV responses in the cross-flow (CF) and in-line (IL) directions can be coupled via variations in the tensile force. The fatigue damage in the CF direction is of the same order as that in the IL direction for the bare Riser. However, for the Riser fitted with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction. The experimental results also confirmed that the strake height has a greater influence on the VIV response than the strake pitch, and the drag exerted...

  • experimental study on response performance of viv of a Flexible Riser with helical strakes
    China Ocean Engineering, 2015
    Co-Authors: Shixiao Fu, Yifan Chen
    Abstract:

    Laboratory tests were conducted on a Flexible Riser with and without helical strakes. The aim of the present work is to further understand the response performance of the vortex induced vibration (VIV) for a Riser with helical strakes. The experiment was accomplished in the towing tank and the relative current was simulated by towing a Flexible Riser in one direction. Based on the modal analysis method, the displacement responses can be obtained by the measured strain. The strakes with different heights are analyzed here, and the response parameters like strain response and displacement response are studied. The experimental results show that the in-line (IL) response is as important as the cross-flow (CF) response, however, many industrial analysis methods usually ignore the IL response due to VIV. The results also indicate that the response characteristics of a bare Riser can be quite distinct from that of a Riser with helical strakes, and the response performance depends on the geometry on the helical strakes closely. The fatigue damage is further discussed and the results show that the fatigue damage in the CF direction is of the same order as that in the IL direction for the bare Riser. However, for the Riser with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction.

Leijian Song - One of the best experts on this subject based on the ideXlab platform.

  • distribution of drag force coefficient along a Flexible Riser undergoing viv in sheared flow
    Ocean Engineering, 2016
    Co-Authors: Leijian Song, Shixiao Fu, Mengmeng Zhang, Yifan Chen
    Abstract:

    Abstract The drag force coefficients of a Flexible Riser undergoing vortex-induced vibration (VIV) in sheared flow are investigated for Reynolds numbers (Re) up to 1.2×105. Based on the drag forces theoretically calculated by the beam theory using the strains measured in a scale model test, the properties and distribution of the drag coefficients are investigated, and a new empirical model for estimating the drag coefficient on a Flexible Riser undergoing VIV is proposed. The results show that VIV leads to non-uniform distribution of the drag coefficient and amplifies the drag coefficient, and the local drag coefficient can reach up to 3.2. For Re values from 1.0×104–1.2×105, the mean drag coefficient is between 1.3 and 2.0 and decreases as Re increases. Furthermore, the empirical drag coefficient prediction model obtained from experiments under low Re is not suitable for high Re. The corrected empirical prediction model, which accounts for the effect of the flow velocity, the VIV dominant mode number and the dominant frequency, can be used to predict Riser drag coefficients under VIV more accurately at high Re values up to 1.2×105.

  • an investigation into the hydrodynamics of a Flexible Riser undergoing vortex induced vibration
    Journal of Fluids and Structures, 2016
    Co-Authors: Leijian Song, Shixiao Fu, Jianqiao Wu
    Abstract:

    Abstract In this study, a method to obtain the hydrodynamic forces of a Flexible Riser undergoing vortex-induced vibration (VIV) based on measured strain is proposed. The tensioned Riser is approximated as an Euler–Bernoulli beam, and an inverse method is adopted for the calculation of the hydrodynamic forces in the cross flow (CF) and inline (IL) directions. Based on these hydrodynamic forces, and combined with the VIV velocities and accelerations of the Riser, the excitation and added-mass coefficients are obtained through a least-squares method. As an illustration example, the hydrodynamic characteristics of a Flexible Riser model undergoing VIV in a uniform flow are investigated. Results indicate that VIV leads to non-uniform distribution of the drag coefficient and amplifies the drag coefficient along the Riser. Similar distribution of the energy transfer coefficient has been found between the entire Riser and that of the CF direction. For synchronized VIV occurring in both CF and IL directions, the energy transferred from the fluid is all dissipated by the structural damping, and hence leads to the energy balance in both CF and IL directions. It further shows that the excitation coefficients on Flexible Riser undergoing VIV do not agree with those of the forced oscillation tests: excitation coefficients sometimes even become negative within the normally excitation regime, and are related with not only the non-dimensional frequency and amplitude but also the phase angles between the CF and IL vibrations. The added-mass coefficient of Flexible Risers does not keep a constant value of 1.0 anymore, but depends on the non-dimensional frequency and amplitude of vibration.

  • viv response of a long Flexible Riser fitted with strakes in uniform and linearly sheared currents
    Applied Ocean Research, 2015
    Co-Authors: Shixiao Fu, Youming Xiong, Leijian Song
    Abstract:

    Abstract An experimental investigation was conducted on a Flexible Riser with and without various strake arrangements. The aim of the present work was to further improve the understanding of the response performance of the vortex-induced vibration (VIV) of a Riser with helical strakes. Two current profiles, including uniform and linearly sheared flows, were simulated. The uniform current was simulated by towing the Riser model in one direction using the towing carriage, and the linearly sheared current was simulated by fixing one end of the Riser and using a driven cantilever to traverse a circular arc. Based on the modal superposition method, the displacement responses were obtained from the measured strain. Strakes with different heights and pitches were analysed, and response parameters such as the displacement response and fatigue damage were studied. The results of the bare model test show that the lock-in phenomenon displays multi-order characteristics, and the VIV displacement decreases with an increased order of the lock-in regime. The results of the straked model test indicate that the response characteristics of a bare Riser can be quite distinct from those of a Riser with helical strakes, and the response performance depends closely on the geometry of the strake configuration.

  • experimental investigation on the suppression device of viv of a Flexible Riser
    ASME 2014 33rd International Conference on Ocean Offshore and Arctic Engineering, 2014
    Co-Authors: Shixiao Fu, Leijian Song, Tao Peng
    Abstract:

    Experimental investigations were conducted on a Flexible Riser with and without helical strakes. A uniform current was obtained by towing a Riser model in a tank, and the vortex-induced vibration (VIV) suppression of strakes with different heights and pitches was studied. The results of the bare Riser show that the characteristics of the synchronization of the VIV for a Flexible Riser have many orders, and the excited mode jumps from one to another abruptly. During the high order synchronization regime, the VIV response decreases with the increased order of the synchronization. The experimental results also indicate that the response characteristics of a bare Riser can be quite distinct from those of a Riser with helical strakes, and the suppression performance depends on the geometry of the helical strakes. The fatigue damage in the CF direction is of the same order as that in the IL direction for the bare Riser. However, for the Riser fitted with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction. The experimental results also confirmed that strake height has a greater influence on the VIV response than the strake pitch, and the drag exerted on the Riser increases with strake pitch and height.Copyright © 2014 by ASME

Carl M Larsen - One of the best experts on this subject based on the ideXlab platform.

  • Flexible Riser response induced by combined slug flow and wave loads
    ASME 2013 32nd International Conference on Ocean Offshore and Arctic Engineering, 2013
    Co-Authors: Arturo Ortega, Ausberto Rivera, Carl M Larsen
    Abstract:

    Flexible Risers provide optimum solutions for deep water offshore fields. Reliable dynamic analysis of this kind of slender structure is crucial to ensure safety against long time fatigue failure. Beyond the effects from wave loads, the influence from transient internal slug flow on the slender structure dynamics should also be taken into account.In this study two coupled in-house codes were used in order to identify and quantify the effects of an internal slug flow and wave loads on the Flexible Riser dynamics. One code carries out a global dynamic analysis of the slender structure displacements using a finite element formulation. The other program simulates the behaviour of the internal slug flow using a finite volume method. The slug flow is influenced by the dynamic shape of the Riser, while the time varying forces from internal slug flow plus external waves will influence the shape. Hence, a fully coupled analysis is needed in order to solve the coupled problem. By means of the distributed simulation these two programs run synchronously and exchange information during the time integration process.A test case using hydrodynamic forces according to the linear Airy wave theory, coupled with an internal unstable slug flow was analysed and the results show: amplification of the dynamic response due to the interaction between the two load types, effects on the effective tension caused by the internal two-phase flow, and influence on the internal slug flow caused by the wave induced response.Copyright © 2013 by ASME

  • Flexible Riser analysis — comparison of results from computer programs
    Marine Structures, 1992
    Co-Authors: Carl M Larsen
    Abstract:

    Abstract The paper presents results from static and dynamic analyses of standard Flexible Riser systems performed by 11 different institutions using their in-house computer programs. The study was initiated by the International Ship and Offshore Structures Congress (ISSC) Committee V.7 ‘Slender Marine Structures’, and partly included in the 1991 Congress report presented in Wuxi, China. This study presents only the results from the analyses and does not comment on the programs' efficiency or ability to model the various physical effects. The main conclusions from the study are: - static analysis of Flexible Risers is today a straightforward task although it does not represent important uncertainties when dealing with simple Riser systems. In some cases it is possible that significant discrepancies may be obtained if different modelling of tangential drag forces and Riser/seafloor interaction are used. - Results from dynamic analyses show a larger scatter, and a coefficient of variation of 10–15% in dynamic tension must be expected. Hydrodynamic loading and structural damping models are seen to be the main sources for this uncertainty. The paper contains a complete description of the test cases and reports results in such a way that it can provide a ‘benchmark’ test for users and programmers of computer codes for Flexible Riser analysis.

  • Flexible Riser analysis comparison of results from computer programs
    Marine Structures, 1992
    Co-Authors: Carl M Larsen
    Abstract:

    Abstract The paper presents results from static and dynamic analyses of standard Flexible Riser systems performed by 11 different institutions using their in-house computer programs. The study was initiated by the International Ship and Offshore Structures Congress (ISSC) Committee V.7 ‘Slender Marine Structures’, and partly included in the 1991 Congress report presented in Wuxi, China. This study presents only the results from the analyses and does not comment on the programs' efficiency or ability to model the various physical effects. The main conclusions from the study are: - static analysis of Flexible Risers is today a straightforward task although it does not represent important uncertainties when dealing with simple Riser systems. In some cases it is possible that significant discrepancies may be obtained if different modelling of tangential drag forces and Riser/seafloor interaction are used. - Results from dynamic analyses show a larger scatter, and a coefficient of variation of 10–15% in dynamic tension must be expected. Hydrodynamic loading and structural damping models are seen to be the main sources for this uncertainty. The paper contains a complete description of the test cases and reports results in such a way that it can provide a ‘benchmark’ test for users and programmers of computer codes for Flexible Riser analysis.

Giulio Alfano - One of the best experts on this subject based on the ideXlab platform.

  • Experimental and numerical study of structural behavior of a Flexible Riser model
    Applied Ocean Research, 2017
    Co-Authors: M. T. Rahmati, Shayan Norouzi, Hamid Bahai, Giulio Alfano
    Abstract:

    Experimental tests and detailed finite element analyses were carried out on a model of Flexible Riser to evaluate the capability of the finite element method to predict its nonlinear structural response. The Riser consists of four layers, which include two cylindrical polycarbonate tubes and two steel helical layers. One helical layer represents the carcass layer in a Flexible Riser whilst the other represents the Riser tendon armour layer. First, bending load experiments on the model are described which provide some insight regarding the fundamental behavior of Flexible pipe structures. This is followed by the description of the FE models in which all layer components are individually modelled and a surface-to-surface frictional contact model is used to simulate their interaction. Finally, the FE numerical results were compared with the test data to outline the capacity of the numerical method to predict the response of Flexible Riser structures.

  • experimental and numerical study of the bending behavior of a Flexible Riser model
    ASME 2015 34th International Conference on Ocean Offshore and Arctic Engineering, 2015
    Co-Authors: M. T. Rahmati, Shayan Norouzi, Hamid Bahai, Giulio Alfano
    Abstract:

    Unbonded Flexible Risers have become the main means of extracting hydro carbonates from deep waters. So, understanding the complex structural integrity of Flexible Risers has become a crucial issue for the offshore industry. In this paper, an experimental test and a detailed finite element analyses were carried out on a scaled down model of a Flexible Riser pipe in order to understand its bending behavior. The model used consists of four layers which include two cylindrical polycarbonate tubes and two steel helical layers. One helical layer, wounded around the pipe assembly, represents the carcass layer in an actual Flexible Riser whilst the other represents the Riser tendon armour layers. The model was subjected to a three point bending load in order to study its bending-curvature behavior. The test data was then compared with the FE numerical results which predicted a similar nonlinear trend but under predicts the strain at tendon layers.Copyright © 2015 by ASME

Zhijia Zhao - One of the best experts on this subject based on the ideXlab platform.

  • Vibration Suppression and Nonlinearities Solution of a Flexible Riser System
    2019 Chinese Control Conference (CCC), 2019
    Co-Authors: Zhijia Zhao, Xiaoqing Liao, Wei He
    Abstract:

    In this paper, we address the vibration abatement problem of the Flexible Riser system preceded by mixed input nonlinear deadzone-saturation characteristic. By virtue of Lyapunov theory, auxiliary system and dead-zone inverse strategy, an anti-saturation control was put forward to restrain the vibration and remove the input nonlinearity effects. Employing the rigorous analysis, the designed control law is ensured to be uniformly bounded stability of the controlled system. Finally, simulations verify the theoretical result.

  • boundary robust adaptive anti saturation control of vibrating Flexible Riser systems
    Ocean Engineering, 2019
    Co-Authors: Zhijia Zhao, Xiuyu He
    Abstract:

    Abstract In this paper, we address the vibration abatement problem of a Flexible Riser system preceded by nonlinear input saturation characteristic and uncertainties of the system. By virtue of Lyapunov theory, auxiliary system and robust control strategy, a boundary robust adaptive anti-saturation control is developed to restrain the vibration, remove the nonlinear input saturation effects and compensate for the uncertainties of system parameters and boundary disturbance. Applying the rigorous analysis without simplifying or discretizing the partial differential equation dynamics, the controlled system is ensured to be uniformly bounded stable with the designed control law. In the end, simulation results are presented for control performance verification.

  • Boundary Adaptive Robust Control of a Flexible Riser System With Input Nonlinearities
    IEEE Transactions on Systems Man and Cybernetics: Systems, 2019
    Co-Authors: Zhijia Zhao, Xiuyu He
    Abstract:

    This paper focuses on adaptive robust vibration control for Flexible Riser systems affected by input nonlinearities and unknown external disturbances. An auxiliary system is constructed and adjusted to develop a boundary adaptive robust control for restraining the vibrational offset and eliminating the effect of input nonlinearities. Besides, an adaptive law of boundary disturbance upper-bound is constructed together with the vibration control strategy to estimate the magnitude of unknown boundary disturbance. Further, the convergence of states and stability of the system are ensured with the developed control scheme. By choosing the proper control parameters, the control performance is verified with the obtained simulation results.

  • Adaptive control of a Flexible Riser system
    2017 6th Data Driven Control and Learning Systems (DDCLS), 2017
    Co-Authors: Zhijia Zhao
    Abstract:

    An adaptive boundary controller is developed in this article for minimizing the vibration displacement of a Flexible Riser. The dynamics of Riser system is represented by partial-ordinary differential equations. Through combining boundary control, adaptive technique with backstepping technique, an adaptive boundary controller is given for realizing vibration restraint for the Riser system, where an adaptive law is developed for coping with parametric uncertainties and a signum function is introduced for mitigating the boundary disturbance. With the designed adaptive boundary controller, the spillover instability can be successfully avoided. Finally, simulations are displayed to explain that the given controller is valid to realize vibration restraint of a Flexible Riser.

  • Boundary barrier-based control of a Flexible Riser system
    IET Control Theory & Applications, 2017
    Co-Authors: Zhijia Zhao
    Abstract:

    In this study, the vibration control and boundary output constraint problem of a marine Riser system subjected to the disturbances are investigated. The control objective is to ensure the vibration reduction of the marine Riser and the boundary output in the constrained region. A boundary control strategy is put forward to realise this objective through combining with Lyapunov's direct method, barrier-based control and backstepping technique. In addition, the disturbance observer including barrier term is developed to cope with the effects of unknown external disturbance. With the designed control strategy, the closed-loop Riser system is ensured to be uniformly bounded stable through rigorous Lyapunov analysis without violation of the output constraint. Finally, the effectiveness of the designed boundary control strategy is verified via numerical simulations.