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A considerable amount of acidic gas penetrates into the annulus between the internal pressure sheath and the outer protective sheath during the service of flexible risers, which will inevitably lead to corrosion of the metal functional layer. Many researchers have modified nanocomposites from the mass transfer perspective to reduce the materials' permeability coefficient. While permeation is an integrated process of heat and mass transfer process, heat distribution directly impacts gas permeation. Herein, the thermal conductivity of flexible riser liner materials is predicted for the first time by a combination of molecular dynamics and experiments, and then the radial temperature distribution under different seawater temperatures and internal fluid temperatures is investigated by finite element analysis. Finally, the effect of temperature distribution on the permeation coefficient was analyzed. The results demonstrate that the thermal conductivity can be predicted by molecular dynamics simulation, and the thermal conductivity of (polyvinylidene difluoride) PVDF/TiO2 decreases, while the thermal conductivity of PVDF/carbon nanotube (CNT) increases compared with pure PVDF. The temperature distribution of the internal pressure sheath material decreases when condensate water is present. As the fluid temperature rises from 30 to 110°C, the maximum increase ratio in the permeability of PVDF/CNT over PVDF increased from 3.6% to 14.8%, and the maximum decrease ratio of PVDF/TiO2 permeability coefficient compared with PVDF is from 1.2% to 4.6%. The results present a new idea to improve the barrier properties of materials by decreasing thermal conductivity. [ABSTRACT FROM AUTHOR]

Abstract Water vapor penetrating an annulus causes condensation and corrosion and endangers the safe operation of the flexible riser. However, in current studies, the condensation of water vapor in the annulus is modeled based on the film condensation hypothesis, which assumes that the metal material is uniformly corroded. This assumption differs from the true corrosion incidents, which are usually initiated by localized corrosion. To further study the condensation law of water vapor in a flexible riser annulus, the evolution of water vapor condensation on the wall surface of a sapphire reactor was evaluated using an annular condensation experimental device. A water vapor condensation growth model was established to analyze the condensation law and its influencing factors. The results show that the maximum radius of condensate droplets on the annular wall surface is approximately 0.48 cm, the average radius of droplets is approximately 0.35 cm, and the volume of condensate that can be accommodated in the 18 × 18 mm simulation area is approximately 0.5 cm3. After the droplet reaches the departure radius and falls off a surface, the time of recondensation is faster than that of the first condensation. The condensation surface coverage increases rapidly in the early stage and then gradually slows down. After stabilization, the maximum coverage can reach 80%. Parameter analysis suggests that the number of fixed nucleation points on the condensation wall affects the time to reach the departure radius and the increasing rate of surface coverage. However, the number of fixed nucleation points has minimal influence on the departure radius and coverage outcomes. The findings of this study establish a theoretical foundation for predicting dropwise condensation behaviors in annulus of pipelines and developing corrosion protection strategies that modify the wall contact angle or structure.

In this paper, we present a new stress calculation method for flexible structures, particularly, tensile armors, and apply it to flexible riser fatigue analysis. The method is based on a 3-dimensional curved bar theory. First, the tensile armor center line was described as a cylindrical helix curve; its bent curve length and bending migration length were derived and studied under different friction scenarios. Second, the tensile and bending stiffness was derived with consideration of more accurate shape parameters and the frictional hysteretic effect, and verified through FEA analysis results. Third, we presented the stress calculation formula for tensile armor under tension and bending load. All stress components were considered, including tensile, bending and shear stresses. Fourth, the method was benchmarked with published experimental results on a flexible prototype tension and bending tests, and comparisons showed general agreements. Fifth, the method was applied to an in-service 8″ flexible riser for fatigue assessment and lifetime extension evaluation, and showed the flexible riser has sufficient remaining fatigue life, and is suitable to continue its service under the current operating conditions. Last, conclusions were drawn. We concluded that the presented tensile armor stress calculation method and modelling techniques are valid for flexible riser fatigue analysis. This method is time efficient, and can be implemented into other multi-scale models for riser dynamic analysis. It is also applicable to other similar helix structure stress analysis, such as wire ropes, submarine hoses, and subsea umbilicals. [ABSTRACT FROM AUTHOR]

Water vapor penetrating an annulus causes condensation and corrosion and endangers the safe operation of the flexible riser. However, in current studies, the condensation of water vapor in the annulus is modeled based on the film condensation hypothesis, which assumes that the metal material is uniformly corroded. This assumption differs from the true corrosion incidents, which are usually initiated by localized corrosion. To further study the condensation law of water vapor in a flexible riser annulus, the evolution of water vapor condensation on the wall surface of a sapphire reactor was evaluated using an annular condensation experimental device. A water vapor condensation growth model was established to analyze the condensation law and its influencing factors. The results show that the maximum radius of condensate droplets on the annular wall surface is approximately 0.48 cm, the average radius of droplets is approximately 0.35 cm, and the volume of condensate that can be accommodated in the 18 × 18 mm simulation area is approximately 0.5 cm3. After the droplet reaches the departure radius and falls off a surface, the time of recondensation is faster than that of the first condensation. The condensation surface coverage increases rapidly in the early stage and then gradually slows down. After stabilization, the maximum coverage can reach 80%. Parameter analysis suggests that the number of fixed nucleation points on the condensation wall affects the time to reach the departure radius and the increasing rate of surface coverage. However, the number of fixed nucleation points has minimal influence on the departure radius and coverage outcomes. The findings of this study establish a theoretical foundation for predicting dropwise condensation behaviors in annulus of pipelines and developing corrosion protection strategies that modify the wall contact angle or structure. [ABSTRACT FROM AUTHOR]

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The vortex-induced vibration (VIV) of flexible risers is a complex fluid–structure interaction (FSI) phenomenon. In this study, we conducted a numerical simulation method based on the slicing method to study the vortex-induced vibration (VIV) of deep-sea flexible risers with different slenderness ratios and uniform flow velocities. The method combines the finite element model of the riser structure with the two-dimensional flow field slices solved by the Fluent solver. The fluid–structure interaction was realized by a self-compiled UDF program and the overset mesh technique. The numerical results were validated by comparing them with experimental data. The VIV characteristics of the riser, such as the vibration track, vibration mode, vibration frequency and wake vortex shedding mode, were analyzed. The article reveals the nonlinear dynamic features of flexible riser vibration, such as multi-frequency vibration, wide-frequency vibration and multi-modal vibration. The article also provides insights into the fluid–structure interaction mechanism of VIV of deep-sea flexible risers. [ABSTRACT FROM AUTHOR]

Previous studies of the marine riser VIV have primarily considered the constant values of the tension, but limited work has been done to consider the variations of the tension with riser vibrations. In addition, previous studies have mainly focused on the effect of the inflow velocity profile on a riser VIV, while a study of a riser VIV at a wide range of velocities is still lacking. This study presents a model that can take into account the variation of the tension with riser vibrations. The VIV characteristics of the riser were investigated based on the fluid-structure coupling methods. It is shown that the model that takes into account the variation of the tension is able to accurately capture the bimodal character of the amplitude-ratio curve and the vortex shedding pattern, and to calculate the vibration frequencies accurately. [ABSTRACT FROM AUTHOR]

As oil and natural gas production continue to go deeper into the ocean, the flexible riser, as a connection to the surface of the marine oil and gas channel, will confront greater problems in its practical application. Composite materials are being considered to replace steel in the unbonded flexible pipe in order to successfully meet the lightweight and high-strength criteria of ultra-deep-water oil and gas production. The carbon-fiber-reinforced material substitutes the steel of the tensile armor layer with a greater strength-to-weight ratio. However, its performance in deep-water environments is less researched. To investigate the mechanical response of a carbon fiber composite flexible riser in the deep sea, this study establishes the ABAQUS quasi-static analysis model to predict the performance of the pipe. Considering the special constitutive relations of composite materials, the tensile stiffness of steel pipe and carbon fiber-reinforced composite flexible pipe are predicted. The results show that the replacement of steel strips with carbon fiber can provide 85.06% tensile stiffness while reducing the weight by 77.7%. Moreover, carbon-fiber-reinforced strips have a lower radial modulus, which may not be sufficient to cause buckling under axial compression, so the instability of the carbon fiber composite armor layer under axial compression is further studied in this paper; furthermore, the characteristics of axial stiffness are analyzed, and the effects of the friction coefficient and hydrostatic pressure are discussed. [ABSTRACT FROM AUTHOR]

The top ends of marine risers are usually hinged under the floating platform, and the vortex-induced vibration due to ocean currents often leads to the potential fatigue damage. The non-intrusive optical measurement (high-speed camera) was employed to monitor the displacements of top-hinged-and-bottom-fixed flexible risers arranged in a circulating water flume. The experimental results indicate that, both the mode order of the riser excited in 3 directions and the dominant frequency increase gradually with the reduced velocity. The maximum out-of-plane root-mean-square vibration amplitude has a first downward and then upward trend in the mode transition cases. The spatial distribution of energy transfer between the fluid and the riser varies with the direction, resulting in the asynchronous mode transition. One of the in-plane vibration frequency coincides with the out-of-plane dominant one, which, if dominating the in-plane response, will make a strong coupling mode, or else a weak coupling mode. [ABSTRACT FROM AUTHOR]

ObjectiveThe viv-FOAM-SJTU solver is used to carry out the numerical simulation of the vortex induced vibrations of a riser under different top oscillation excitation amplitudes. MethodsThe flow field is computed by the strip theory and Reynolds-averaged Navier-Stokes (RANS) method, while the Bernoulli-Euler bending beam theory and finite element method are employed to model the structure of the riser. ResultsThe simulation results show that the cross-flow vibrations transform from multi-mode to single mode with the decrease in oscillation amplitude. At the same time, the inline vibrations maintain single mode, but the amplitude of the mode vibration decreases. ConclusionWith the variation in top oscillation excitation amplitudes, the washing out of the riser by shed vortices affects the characteristics of vortex induced vibrations.

Mixture transport from a seabed mining vehicle to a buffer is a key procedure in deep-sea mining. Dynamic performances of the particle–seawater mixture in a single-peak flexible riser were numerically investigated using computational fluid dynamics and the discrete element method. Both the time-averaged local characteristics and the instantaneous flow behaviors in the riser are presented. The effects of key parameters, such as feeding concentration and mixture transport velocity, were evaluated by a sensitivity analysis. Large local concentration accompanied by small particle velocity occurs in the ascending sections and increases the risk of blockage. The particle–wall contact reaches the maximum value at both the peak and trough of the single-peak riser. A small feeding concentration would reduce both particle–wall contact, and hydraulic gradient, whereas a moderate mixture transport velocity may be appropriate for the transport in terms of operation safety and energy efficiency. In addition, the mixture transport in a double-peak riser was simulated to examine which configuration is better for engineering applications. The lower maximum local concentration, particle–wall contact and hydraulic gradient and the larger minimum particle velocity indicate that a double-peak flexible riser may be more suitable for the mixture transport. [ABSTRACT FROM AUTHOR]

In this article, vibration abatement problems of a riser system with system uncertainty, input deadzone, and output constraint are considered. For obtaining better control precision, a boundary control law is constructed by employing the backstepping method and Lyapunov’s theory. The output constraint is guaranteed by utilizing a barrier Lyapunov function. Adaptive neural networks are designed to cope with the uncertainty of the riser and compensate for the effect caused by the asymmetric deadzone nonlinearity. With the designed controller, the output constraint is satisfied, and the system stability is guaranteed through Lyapunov synthesis. In the end, numerical simulation results are provided to display the performance of the developed adaptive neural network boundary control law. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Acidic gas penetration through the internal pressure sheath of a flexible riser tends to cause a corrosive environment in the annulus, reducing the service life of the flexible riser. Nanoparticles can act as gas barriers in the polymer matrix to slow down the gas permeation. Herein, we prepared PA11/SiO2 composites by the melt blending method. The effect of adding different amounts of SiO2 to PA11 on its gas barrier properties was investigated by conducting CO2 permeation tests between 20 °C and 90 °C. As the temperature increased, the lowest value of the permeability coefficient that could be achieved for the PA11 with different contents of SiO2 increased. The composites PA/0.5% SiO2 and PA/1.5% SiO2 had the lowest permeation coefficients in the glassy state (20 °C) and rubbery state (≥50 °C). We believe that this easy-to-produce industrial PA/SiO2 composite can be used to develop high-performance flexible riser barrier layers. It is crucial for understanding riser permeation behavior and enhancing barrier qualities. [ABSTRACT FROM AUTHOR]

In this paper, the corrosion behavior of low alloy steel in H 2 S-saturated solution is systematically studied through immersion experiment in terms of microstructure morphology, corrosion kinetics, surface morphology, cross-sectional morphology, crystal characteristic and elemental distribution. The experimental results suggest that the corrosion rate decreased gradually with corrosion time. The corrosion products were immediately formed after the steel was immersed in corrosion solution and the structure and thickness became denser and thicker, which improved corrosion resistance. The corrosion products transformed from mackinawite, greigite and molybdenite to mackinawite, greigite, pyrrhotite and molybdenite. For the alloying elements, Cr and Mo tended to be accumulated in corrosion products compared with the steel substrate. A schematic model of corrosion process was proposed according to experimental results. [Display omitted] • The evolution process of iron sulfides on steel surface is systematically analyzed. • The style and surface morphology of iron sulfides changes with prolonging time. • A model of corrosion process of low alloy steel exposed to H 2 S solution is proposed. [ABSTRACT FROM AUTHOR]

In an underwater compressed air energy storage system, fracture of the flexible riser often occurs. To solve this problem, we proposed a static analysis method for a flexible riser based on Cosserat theory. Bending and torsion deformation of a flexible riser were described. Using Euler parameters, static equilibrium differential equations were established. Finite element discretization, finite difference, and an improved Levenberg–Marquardt algorithm were used to perform the numerical calculations. Based on the similarity principle, experiments with three different boundary conditions were performed. Our static model and the OrcaFlex simulations were compared with experiments, and the average error of our static model was 2.9% smaller than that of the simulation. The results showed that the static model presented in this paper was more accurate than the OrcaFlex simulation calculation. This model is useful for estimating the mechanical characteristics and determining the strengths of flexible risers. [ABSTRACT FROM AUTHOR]

In this paper, we present a new stress calculation method for flexible structures, particularly, tensile armors, and apply it to flexible riser fatigue analysis. The method is based on a 3-dimensional curved bar theory. First, the tensile armor center line was described as a cylindrical helix curve; its bent curve length and bending migration length were derived and studied under different friction scenarios. Second, the tensile and bending stiffness was derived with consideration of more accurate shape parameters and the frictional hysteretic effect, and verified through FEA analysis results. Third, we presented the stress calculation formula for tensile armor under tension and bending load. All stress components were considered, including tensile, bending and shear stresses. Fourth, the method was benchmarked with published experimental results on a flexible prototype tension and bending tests, and comparisons showed general agreements. Fifth, the method was applied to an in-service 8″ flexible riser for fatigue assessment and lifetime extension evaluation, and showed the flexible riser has sufficient remaining fatigue life, and is suitable to continue its service under the current operating conditions. Last, conclusions were drawn. We concluded that the presented tensile armor stress calculation method and modelling techniques are valid for flexible riser fatigue analysis. This method is time efficient, and can be implemented into other multi-scale models for riser dynamic analysis. It is also applicable to other similar helix structure stress analysis, such as wire ropes, submarine hoses, and subsea umbilicals.

The vortex-induced vibration (VIV) of flexible risers is a complex fluid–structure interaction (FSI) phenomenon. In this study, we conducted a numerical simulation method based on the slicing method to study the vortex-induced vibration (VIV) of deep-sea flexible risers with different slenderness ratios and uniform flow velocities. The method combines the finite element model of the riser structure with the two-dimensional flow field slices solved by the Fluent solver. The fluid–structure interaction was realized by a self-compiled UDF program and the overset mesh technique. The numerical results were validated by comparing them with experimental data. The VIV characteristics of the riser, such as the vibration track, vibration mode, vibration frequency and wake vortex shedding mode, were analyzed. The article reveals the nonlinear dynamic features of flexible riser vibration, such as multi-frequency vibration, wide-frequency vibration and multi-modal vibration. The article also provides insights into the fluid–structure interaction mechanism of VIV of deep-sea flexible risers.

The increasing energy demand has led to more explorations in the oil and gas industry. To achieve this, marine risers and pipelines are used to convey fluid and other resources to meet the increasing demand. In recent years, hybrid flexible composite risers have become more adaptable. Flexible risers have already proven to be a popular riser solution for various floating production systems in shallow to deepwater in many parts of the world due to their good dynamic behaviour and dependability. The hybrid flexible composite riser is made up of numerous layers of plastics, flexible pipes, composites, and steel. Some innovative monitoring methods, such as Fiber Optics Bragg Gratings (FBG), are based on a clamped composite structure with embedded optical fibre. This study presents characteristics of the monitoring techniques of composite flexible riser technology. The advantages of the monitoring techniques include aiding composite riser measurements, recording data from riser deformation, improving integrity assurance, and dependability of design from stable readings. It also proposes some design approaches as guidelines that are advised, with some policy implications. [ABSTRACT FROM AUTHOR]

As oil and natural gas production continue to go deeper into the ocean, the flexible riser, as a connection to the surface of the marine oil and gas channel, will confront greater problems in its practical application. Composite materials are being considered to replace steel in the unbonded flexible pipe in order to successfully meet the lightweight and high-strength criteria of ultra-deep-water oil and gas production. The carbon-fiber-reinforced material substitutes the steel of the tensile armor layer with a greater strength-to-weight ratio. However, its performance in deep-water environments is less researched. To investigate the mechanical response of a carbon fiber composite flexible riser in the deep sea, this study establishes the ABAQUS quasi-static analysis model to predict the performance of the pipe. Considering the special constitutive relations of composite materials, the tensile stiffness of steel pipe and carbon fiber-reinforced composite flexible pipe are predicted. The results show that the replacement of steel strips with carbon fiber can provide 85.06% tensile stiffness while reducing the weight by 77.7%. Moreover, carbon-fiber-reinforced strips have a lower radial modulus, which may not be sufficient to cause buckling under axial compression, so the instability of the carbon fiber composite armor layer under axial compression is further studied in this paper; furthermore, the characteristics of axial stiffness are analyzed, and the effects of the friction coefficient and hydrostatic pressure are discussed.

This 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. [ABSTRACT FROM AUTHOR]

Mixture transport from a seabed mining vehicle to a buffer is a key procedure in deep-sea mining. Dynamic performances of the particle–seawater mixture in a single-peak flexible riser were numerically investigated using computational fluid dynamics and the discrete element method. Both the time-averaged local characteristics and the instantaneous flow behaviors in the riser are presented. The effects of key parameters, such as feeding concentration and mixture transport velocity, were evaluated by a sensitivity analysis. Large local concentration accompanied by small particle velocity occurs in the ascending sections and increases the risk of blockage. The particle–wall contact reaches the maximum value at both the peak and trough of the single-peak riser. A small feeding concentration would reduce both particle–wall contact, and hydraulic gradient, whereas a moderate mixture transport velocity may be appropriate for the transport in terms of operation safety and energy efficiency. In addition, the mixture transport in a double-peak riser was simulated to examine which configuration is better for engineering applications. The lower maximum local concentration, particle–wall contact and hydraulic gradient and the larger minimum particle velocity indicate that a double-peak flexible riser may be more suitable for the mixture transport.

Acidic gas penetration through the internal pressure sheath of a flexible riser tends to cause a corrosive environment in the annulus, reducing the service life of the flexible riser. Nanoparticles can act as gas barriers in the polymer matrix to slow down the gas permeation. Herein, we prepared PA11/SiO2 composites by the melt blending method. The effect of adding different amounts of SiO2 to PA11 on its gas barrier properties was investigated by conducting CO2 permeation tests between 20 °C and 90 °C. As the temperature increased, the lowest value of the permeability coefficient that could be achieved for the PA11 with different contents of SiO2 increased. The composites PA/0.5% SiO2 and PA/1.5% SiO2 had the lowest permeation coefficients in the glassy state (20 °C) and rubbery state (≥50 °C). We believe that this easy-to-produce industrial PA/SiO2 composite can be used to develop high-performance flexible riser barrier layers. It is crucial for understanding riser permeation behavior and enhancing barrier qualities.

An experimental study is performed for investigating riser responses under flow-induced vibration (FIV) of a flexible riser transporting spiral flow and straight flow in a large-scale simulated transport system for deep-sea mining. A 3D non-contact optical measurement method is employed to examine the characteristics of FIV responses of a 4-m-long flexible riser transporting spiral flow and straight flow in x , y , and z directions. For a flexible riser transporting spiral flow, experimental results show that there is a region of abrupt reduction in amplitude upon reaching the pre-instability region and post-instability region. Also, it is observed that a relatively strong nonlinear disturbance is induced by the circumferential force associated with spiral flow, altering the damping characteristics of riser. When the velocity of circumferential flow of the spiral flow is up to 10 m/s, the amplitude of the flexible riser transporting spiral flow is approximately 58% smaller than that of riser transporting straight flow and also an increase of frequency of about 11% happens. Furthermore, an analysis of vibration stability for flexible riser is made and results show that a significantly converging phase plot is exhibited by the flexible riser transporting spiral flow, resembling elliptical quasi-periodic motion. • A 3D experimental method for vibration responses of deep-sea mining flexible riser transporting spiral flow is presented. • Critical flow velocities and instability regions of a flexible riser transporting spiral flow are investigated. • Vibration characteristics in a flexible riser transporting straight and spiral flows are evaluated. [ABSTRACT FROM AUTHOR]

The increasing energy demand has led to more explorations in the oil and gas industry. To achieve this, marine risers and pipelines are used to convey fluid and other resources to meet the increasing demand. In recent years, hybrid flexible composite risers have become more adaptable. Flexible risers have already proven to be a popular riser solution for various floating production systems in shallow to deepwater in many parts of the world due to their good dynamic behaviour and dependability. The hybrid flexible composite riser is made up of numerous layers of plastics, flexible pipes, composites, and steel. Some innovative monitoring methods, such as Fiber Optics Bragg Gratings (FBG), are based on a clamped composite structure with embedded optical fibre. This study presents characteristics of the monitoring techniques of composite flexible riser technology. The advantages of the monitoring techniques include aiding composite riser measurements, recording data from riser deformation, improving integrity assurance, and dependability of design from stable readings. It also proposes some design approaches as guidelines that are advised, with some policy implications.

To install flexible risers in offshore oil & gas fields, tensioner equipment is employed to apply compression loads to hold and descend the riser to the seabed for connection with subsea equipment. However, when compression loads are higher enough, they can collapse the flexible pipe leading to the necessity to replace the riser. Therefore, in the present work, a modelling technique to obtain and evaluate the suitable and critical loads to be applied to avoid failure in flexible risers during the installation process in offshore fields and the resulting stresses and strains is presented. The modelling methodology incorporated non-linear plastic properties of steel layers of the flexible pipe, and the complex contact conditions between structural and polymeric layers in finite element models of a section of a flexible pipe. A theoretical model was also developed in order to compare finite element results vs analytical modelling results, providing the von Mises stress and strain fields at different load steps. The present modelling strategy was also used to evaluate the maximum displacement to be applied by the tensioner to flexible risers in order to maintain structural integrity, and failure mechanisms in flexible risers subjected to crushing loads. • This paper presents a methodology to predict damage/failure in flexible risers subjected to installation/crushing loads. • The methodology contributes to specify the loads that must be applied to flexible risers for their installation. • Analytical and finite element analyses provide a better understanding of damage and failure in flexible risers. • Finite element modelling and theoretical analyses were verified with experimental data. [ABSTRACT FROM AUTHOR]

Passive control of the vortex-induced vibrations (VIVs) of flexible risers has been typically studied to alleviate their VIV responses by means of breaking the spanwise correlation, modifying the structural characteristic, or disturbing the flow field. However, this paper proposed using innovative suppression devices—unpowered propellers—to mitigate VIVs and output energy through passive rotation under flowing fluids. The effectiveness of three-, four-, and six-blade propellers for VIV suppression was experimentally examined in the Reynolds number range of 2600–10,400. Seven identical propellers were installed uniformly along a riser facing the incoming flow for VIV suppression. The response parameters of the riser models, including the displacement, frequency, trajectory, and fatigue damage, were investigated. Moreover, a comprehensive evaluation of the unpowered propellers, determined by the suppression efficiency and the cost, was also conducted. The experimental results showed that all three types of propellers significantly decreased the VIV amplitudes and inhibited high-order frequency components. Increasing the number of blades effectively enhanced the amplitude suppression of the propellers, whereas it had almost no effect on the vibration frequencies of the controlled risers. It is worth noting that the three-blade propellers with the best comprehensive evaluation may not be a reasonable choice in high-velocity water. • The passively rotating propellers under flows form a counterforce to fluid fields. • The unpowered propellers is capable of suppressing VIV responses significantly. • An increase in the blade quantity enhances suppression performance of propellers. • The hysteresis for risers shows increasingly with increasing the number of blades. [ABSTRACT FROM AUTHOR]

In the deep-sea oil and gas exploitation process, the adjacent risers under the influence of environmental load and shielding effect will produce large amplitude vibration, which may lead to collision and damage to the riser structure. However, there is a lack of study on active collision-preventing control based on vessel flexible riser array system. This paper aims to better prevent the collision between vessel-risers and increase the motion spacing. Preliminarily, we model the vessel-risers system composed of top production vessel, riser array and environment load based on Hamilton's principle and the improved shielding effect model. In the next part, considering the control spillover problem, we design the collision-preventing boundary controllers directly for distributed parameter system (DPS) without discretization or simplification. And the uniform boundedness of the control system is strictly proved by the Lyapunov direct method. Eventually, we verify the effectiveness and feasibility of the collision-preventing controllers by simulating the system in different ocean currents. At the same time, the parameters of the collision-preventing controllers are independent of those of the riser array system, which ensures the robustness of the system. • Modeled vessel-riser array system composed of top production vessel, riser array and environment load based on Hamilton's principle and the shielding effect model. • Under the influence of different current loads, the riser array system will produce different degrees of collisions at different positions. • A set of boundary controllers with strong robustness are designed directly for infinite dimensional systems in order to avoid control spillover. • Effectively prevent riser collision by controlling the top boundary spacing. [ABSTRACT FROM AUTHOR]

The effectiveness of an unconventional configuration of serrated helical strakes (SSs) in alleviating the vortex-induced vibrations (VIVs) of flexible risers was evaluated experimentally in the Reynolds number range of 2600–10,400. Five SSs with varying serration densities and lengths were designed and compared with conventional strakes (CSs) to highlight the unparalleled features of this new geometry. The VIV parameters of the bare and straked risers, including the displacement, frequency, fatigue damage, and suppression efficiency, were systematically investigated. The comprehensive evaluation of the helical strakes was also made considering the factors of amplitude suppression, frequency inhibition, drag force, and cost control. The experimental results confirmed that all the helical strakes were capable of suppressing the VIV responses of flexible riser models. With identical strake areas, the SSs were more effective in inhibiting the VIV amplitude and fatigue damage than the CSs; whereas, the serrated geometry had few effects in improving the reduction of VIV frequency. Moreover, serration density is a critical factor in determining the suppression performance of SSs under the same strake heights and areas. An increase in the strake area enhanced VIV suppression performance; however, excess strakes deteriorated the comprehensive evaluation index due to the increased cost. • Serrated helical strakes behave better in VIV suppression than conventional ones. • Serration density is a key factor in determining the performance of serrated strakes. • An increase in the area of serrated strakes enhances the VIV suppression performance. • Excess strakes deteriorate the comprehensive evaluation in VIV suppression. [ABSTRACT FROM AUTHOR]

Experimental investigations within the Reynolds number range of 2600–9100 were performed to examine the effectiveness of serrated splitter plates in controlling the flow-induced vibration (FIV) of flexible risers. Conventional splitter plates were arranged for contrast to highlight the unparalleled features of the new splitter plates. Bending strains of FIV responses were measured and converted into critical parameters of displacement, dominant frequency, oscillation orbit, suppression efficiency, and fatigue damage for analysis. Test results illustrate that for flexible risers with conventional or serrated splitter plates, a galloping phenomenon occurs at a low reduced velocity, and standing wave characteristics invariably dominate their FIV responses. As the reduced velocity varies increasingly, the conventional splitter plate triggers a continuous monotonous increase in the FIV amplitudes of controlled risers. However, owing to the phase difference in the interaction of the shear layers between proximal plates and distal plates, the FIV amplitudes of the risers fitted with serrated splitter plates start to stay constant virtually and then distinctly decline with appearing chaotic oscillation trajectories. Moreover, the control performance of serrated splitter plates with uniformly distributed serrations in alleviating FIV amplitudes is optimal at high reduced velocities; whereas excessively dense or sparse serrations can reduce FIV inhibition. • Galloping occurs at a low reduced velocity for flexible risers with splitter plates. • Conventional splitter plates trigger a monotonic increase in amplitudes of risers. • FIV amplitudes of risers fitted with serrated plates decline at high flow velocities. • Serrated plates with serration distributed uniformly behave better in FIV control. [ABSTRACT FROM AUTHOR]

The flexible riser top connection with floating production units presents additional tensile armour integrity assessment uncertainties associated to the bend stiffener contact interaction that leads to a non-uniform curvature and contact pressure distribution in a region of large dynamic forces and moments. In this paper, a full-scale 6″ flexible riser and bend stiffener bending-tension experimental campaign is carried out in a horizontal rig. The external tensile armour wires are instrumented with strain gauges for axial strain measurement in several riser cross-sections inside and outside the bend stiffener region. The rig assembly allows the riser rotation in order to measure strains at different circumferential angular positions. The bend stiffener polyurethane hyperelastic mechanical response is obtained by uniaxial tensile tests performed at room temperature. A nonlinear finite element model (FEM), including the riser/bend stiffener contact interaction and interlayer friction mechanisms, is employed to numerically investigate the mechanical behaviour of the flexible riser subjected to the experimental loading conditions. The FEM axial strains on the tensile armour wires are compared with the measured results for pure tension and tension-bending loads. Under axisymmetric loading a relevant experimental axial strain dispersion is observed when compared to the average values. Under tension-bending loading, the interlayer friction coefficient influences on the contact pressure and curvature distribution are initially assessed with the numerical model. The strain gauges located in the cross-sections with highest values of contact pressure and curvature are selected for a detailed numerical-experimental strains comparison. In addition, a parametric study is conducted to calibrate the friction coefficient that yields the minimum mean squared error for all measured data. Generally, good correlations are found between the experimental and numerical results. • Full-scale 6″ flexible riser and bend stiffener bending-tension experimental campaign is carried out. • Relevant experimental axial strain dispersion is observed. • Nonlinear finite element model, including riser/bend stiffener contact and interlayer friction mechanisms, is developed. • A parametric study is conducted to calibrate the friction coefficient that yields the minimum mean squared error. • Generally, good correlations are found between the experimental and numerical results. [ABSTRACT FROM AUTHOR]

One of the measures of suppressing vortex-induced vibration (VIV) for the offshore riser is to install a streamlined fairing device. However, the galloping phenomenon may occur for a non-rotating water-drop fairing, which has a serious influence on the riser operation. An improved meshless discrete vortex method (DVM) in a two-dimensional framework is employed to calculate the fluid forces for flow past an arbitrary geometry. Meanwhile, by coupling the mass-damping-stiffness kinematic equation, VIV responses of a circular cylinder with one degree-of-freedom (DOF) under a wide range of reduced velocities can be determined. Then, systematic simulations are carried out to investigate the effects of physical parameters, including free-stream velocity and trailing edge angle, on the galloping characteristics of fairing structures. At last, a quasi-three-dimensional DVM-finite element method (FEM) coupled strategy is employed to calculate the vibration responses of a flexible riser attached with the water-drop fairing. The results indicate that the non-rotating water-drop fairing can bring about intense galloping instability behaviour. The predicted vibration amplitudes, frequencies and fluid forces for different fairings as well as the circular cylinder are compared. Furthermore, a comparison between flow-induced vibration (FIV) characteristics of the bare riser and flexible risers fitted with various fairing devices is conducted. • Systematic numerical simulations are carried out to investigate the galloping phenomenon of the non-rotating water-drop fairing structure in a 2D framework. • The predicted vibration responses and fluid forces for different fairings as well as the bare circular cylinder are compared. • The large-amplitude flow-induced vibration response of the 38m long riser fitted with the non-rotating fairing device can be observed. • A comparison between flow-induced vibration characteristics of the flexible riser attached with the fairing and the bare riser under various reduced velocities is conducted. [ABSTRACT FROM AUTHOR]

According to Hamilton's principle and a mechanical decomposition model proposed for high-speed spiral flow, vibration governing equations of flexible riser transporting high-speed spiral flow are developed for vortex-induced vibration (VIV) analysis for lifting pipe system in deep-sea mining. The classic van der Pol oscillator is used to simulate the dynamic characteristics of VIV. The governing equations are then discretized and solved by the finite element method and the Runge-Kuta method. Furthermore, dynamic characteristics of VIV of the flexible riser transporting straight flow and spiral flow are investigated and the effects of the spiral flow incidence velocity and angle controlled by the jet device on VIV responses are evaluated. Also, the VIV dynamics considering different damping ratios are examined. The results show that the rotational angular velocity of the spiral flow would amplify the mean deflection of the in-line (IL) direction, and the incidence velocity has a significant nonlinear effect on the dominant frequency of the riser, which would induce the high-order dominant vibration modes when the incidence velocity is large sufficiently. However, the incidence angle has relatively less effects on the VIV responses of flexible riser. Moreover, the locked interval of VIV is less influenced by damping ratio below 0.1. • Vibration governing equations of flexible riser transporting high-speed spiral flow are developed. • A mechanical decomposition model for high-speed spiral flow is proposed. • Dynamic characteristics of flexible riser transporting high-speed spiral flow are investigated. • Impact of incident velocity and angle of spiral flow and damping ratios on the VIV of flexible riser is examined. [ABSTRACT FROM AUTHOR]

A numerical study based on the wake oscillator model has been conducted to determine the vortex-induced vibration (VIV) responses of a flexible riser with an axial time-varying tension. Three different types of flows, viz., linear shear, exponential shear, and real stepped flows, have been considered. The coupling equations of a structural oscillator and a wake oscillator have been solved using a standard central finite difference method of the second order. The VIV response characteristics including the structural displacement, structural frequency, displacement envelope, and displacement evolution for three different flow profiles have been systematically compared. Subsequently, for each flow profile, the effect of the different tension models on the VIV characteristics has been investigated. The numerical results indicated that, both the VIV displacement and VIV frequency in real stepped flow had diverse characteristics from those in linear shear flow. Compared with the corresponding VIV characteristics in real stepped flow, the VIV displacement in exponential shear flow was similar, however, the VIV frequency in exponential shear flow was disparate. The VIV displacement with the constant tension model had larger values than that with the real tension model. • We established a numerical model for studying a long flexible riser with a time-dependent varying tension. • We studied the effect of flow profile on the VIV responses of a long flexible riser. • We further studied the effect of the selected tension model on the VIV responses of a long flexible riser. [ABSTRACT FROM AUTHOR]

• Please find below the highlights of the manuscript "Flexible Riser Tensile Armour Stress Assessment in the Bend Stiffener Region": • A nonlinear finite element model is developed to account for the effect of the bend stiffener on the flexible riser response. • It is particularly investigated the stresses in the tensile armour wires for extreme and fatigue load cases. • The results are compared with the ones obtained from numerical models assuming no bend stiffener interaction and constant curvature. Comparisons are also performed with analytical models. • It is shown that the contact pressure between the bend stiffener and riser affects the stresses in the tensile armour wires. The flexible riser within the bend stiffener in the top connection is a structurally vulnerable region for extreme and fatigue loads as it is subjected to large dynamic tensile forces and curvatures. This paper presents a nonlinear finite element model (FEM) for the flexible riser with bend stiffener taking into account interlayer friction and riser-bend stiffener contact interaction subjected to combined axisymmetric and bending loads. A case study with typical extreme and fatigue load cases is carried out to assess the curvature variation and bend stiffener contact pressure influence in the riser tensile armours stresses. For the extreme load case, detailed finite element results such as contact pressures and friction, normal and transverse bending stresses are compared with a uniform curvature FEM and available analytical models. A fatigue load case is also performed, where the stress amplitudes and cumulative fatigue damages for the internal tensile armour wires are compared with a uniform curvature FEM and analytical models. From the case study results under extreme loading, it can be observed that the model with bend stiffener interaction predicts larger stresses in comparison to the uniform curvature FEM and analytical models. Under typical fatigue loading conditions, significant differences in the cumulative damage are observed between the models, highlighting the importance of an accurate modeling for lifetime integrity assessment in a region of highly variable riser curvature. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) is one of the critical design considerations for the offshore riser and pipeline system. Significant research effort has been devoted to better understanding and predicting riser VIV. Compared to the bare uniform riser configuration, the attached staggered buoyancy modules of a small aspect ratio will extensively alter riser hydrodynamic characters, and hence the riser dynamic response. In the current study, experiments were performed on forced inline (IL)-crossflow (CF) combined vibration of a rigid cylinder with staggered modules (module to bare cylinder diameter and length ratio of 2.5 and 1.0.) as well as self-induced vibration of two flexible models with 100% and 50% buoyancy module coverage ratios. From the rigid model experiment, positive regions for lift coefficient in the phase of velocity (C l v) for both riser and buoyancy module induced vibration are observed. The result indicates potential power-in excitation from both riser and buoyancy modules, which is later confirmed by the bi-frequency and bi-modal vibration from the flexible cylinder experiment. Furthermore, several other phenomena have been reported, including low reduced frequency for buoyancy module induced motion, the phase between CF and IL trajectory, and buoyancy module VIV suppression ability, emphasizing the connection between the rigid and flexible cylinder experiment. [ABSTRACT FROM AUTHOR]

An investigation on the dynamic characteristics of a flexible riser due to the coupling effects of Vortex-Induced Vibration (VIV) and severe slugging is performed in this paper. Firstly, a 2D Computational Fluid Dynamics (CFD) model is developed for capturing the dynamic characteristics of severe slugging. Based on the mass and velocity of internal fluid from the CFD analysis, a classical van der Pol oscillator is employed to simulate the dynamic characteristics of the effect of VIV coupled with severe slugging. The governing equation of the riser with internal flow is discretized by the finite difference method and it is then solved by the Runge-Kutta method. The results show that the effect of severe slugging exhibits quite distinguished dynamics owing to the intermittent feature both in in-line (IL) and cross-flow (CF) directions. The VIV characteristics of the riser with severe slugging, such as frequencies, dominant modes, and root mean square (RMS) displacements, are substantially different from those of the riser without internal flow. In addition, severe slugging may trigger new mode responses of the riser and a multi-frequency vibration phenomenon in IL and CF directions due to severe slugging is observed. • Dynamic characteristics of a flexible riser subjected to VIV and severe slugging are investigated. • A 2D CFD model is developed for capturing the dynamic characteristics of severe slugging. • The responses of the flexible riser in IL and CF directions are simulated by a classical van der Pol oscillator. • Two typical types of severe slugging are considered in the investigation. [ABSTRACT FROM AUTHOR]

This work analyses the three-dimensional dynamic response of flexible catenary risers when subjected to the effects of an internal slug flow. The interaction between the flexible riser and the internal slug flow is a rather complex phenomenon, and this is because the forces due to the acceleration of the internal fluid influence the riser dynamics, which, in turn, influences the slug flow motion in its interior. In order to carry out this analysis, a new computational tool called SLUGFLEX has been developed, which is composed by two computational codes: the code that calculates the structural dynamic response of the flexible riser, and the other one that predicts the development of the internal slug flow. In the structural analysis, the spatial discretization of the riser is performed using spatial Euler beam finite elements, which suffer large displacements, although with small deformations. The tangent stiffness matrix and the internal force vector of the structure are calculated using the corotational approach. The internal slug flow, whose key feature is the intermittence of two different regions: a liquid slug and a large bubble of gas, is mathematically modelled by using a new one-dimensional lagrangian slug tracking method, which has been based on a model developed for straight pipes, and over which the effects of the curvature of the riser and its dynamics were added. The dynamic response of the riser is calculated by using the constant average acceleration method, in combination with the incremental iterative Newton–Raphson scheme for follower loads. The numerical results show good agreement when compared to the experimental ones obtained from the literature. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) may cause severe fatigue damage on deep-sea flexible risers. In many researches on active control of VIV, numerical simulation is widely used because of its suitability for parametric studies and lower cost compared to experiments. However, the existing numerical simulations rarely consider the change of lift during the active control of VIV due to the complexity of the control method. Moreover, the calculation time of numerical simulation is relatively long in the time domain. To solve these problems, the active control proposed in this paper is carried out in the frequency domain. A boundary control method considering the change of lift force is proposed through an active control bending moment is applied to the top of riser. Compared with the experimental and numerical results of the flexible riser model under shear flow, the effectiveness of the proposed method is verified. In addition, the effects of different shear currents and different controlled bending moments on structural fatigue damage are studied. The results demonstrated that the reduction of fatigue damage is smaller when the control bending moment is small. As the control bending moment increases, the reduction of fatigue damage increases. However, when the control bending moment exceeds the critical value, the fatigue damage no longer decreases. From the total power perspective, the control energy and the proportion of energy in the system increase with the growth of the control moment. It is difficult to directly obtain the optimal control bending moment although there is an optimal control bending moment. Trial calculations are used to obtain the optimal control bending moment in this paper. The greater the shear currents, the greater the required control bending moment. [ABSTRACT FROM AUTHOR]