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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]

The variation in the dynamic characteristics of a flexible riser as the riser transitions from a vertical riser to a catenary-type riser is investigated. It is well known that the straight configuration of a flexible vertical riser conveying fluid destablizes in a divergence-type instability once the velocity of the transporting fluid exceeds a critical speed. As expected, the instability persists if a slight horizontal offset is introduced at the hang-off point. However, as demonstrated in this paper, if a finite horizontal offset is introduced then the instability vanishes and the resulting static configuration of the catenary-type riser is stable regardless of the transport speed of the fluid.

The variation in the dynamic characteristics of a flexible riser as the riser transitions from a vertical riser to a catenary-type riser is investigated. It is well known that the straight configuration of a flexible vertical riser conveying fluid destablizes in a divergence-type instability once the velocity of the transporting fluid exceeds a critical speed. As expected, the instability persists if a slight horizontal offset is introduced at the hang-off point. However, as demonstrated in this paper, if a finite horizontal offset is introduced then the instability vanishes and the resulting static configuration of the catenary-type riser is stable regardless of the transport speed of the fluid.

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]

As water depths for oil and gas exploration and extraction increase, structures such as flexible risers, mooring lines, and umbilical cables are increasingly being used for subsea environments. Compared to conventional fixed-type structures and vertical risers, the dynamics of flexible risers is significantly more complex. In particular, the flexible structures may be prone to dynamic instabilities. The goal of the present paper is to provide a comprehensive study of the dynamics, stability, and vibration of flexible risers. We use Kirchhoff's theory of an extensible, flexible rod that resists torsion to develop a set of nonlinear equations for the dynamics of risers. The resulting model incorporates drag and the effects of the fluid being transported internally. Using a nonlinear stability criterion, our analyses show the nonlinear stability of a simple catenary-type riser modeled either as an inextensible or extensible string. For the more advanced rod models, we use a linear stability analysis to show how the internal fluid being conveyed can destabilize certain static configurations.

As water depths for oil and gas exploration and extraction increase, structures such as flexible risers, mooring lines, and umbilical cables are increasingly being used for subsea environments. Compared to conventional fixed-type structures and vertical risers, the dynamics of flexible risers is significantly more complex. In particular, the flexible structures may be prone to dynamic instabilities. The goal of the present paper is to provide a comprehensive study of the dynamics, stability, and vibration of flexible risers. We use Kirchhoff's theory of an extensible, flexible rod that resists torsion to develop a set of nonlinear equations for the dynamics of risers. The resulting model incorporates drag and the effects of the fluid being transported internally. Using a nonlinear stability criterion, our analyses show the nonlinear stability of a simple catenary-type riser modeled either as an inextensible or extensible string. For the more advanced rod models, we use a linear stability analysis to show how the internal fluid being conveyed can destabilize certain static configurations.

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]

Flexible risers are usually used as conveying systems to bring ocean resources from sea bed up to onshore. Under ocean environments, risers need to bear complex loads and it is crucial to comprehensively examine riser's configurations and to analyze structural dynamic performances under excitation of bottom vehicle motions, to guarantee structural safe operation and required service lives.In this study, considering a saddle-shaped riser, the influences of some important design parameters, including installation position of buoyancy modules, buoyancy ratio and motion of mining vehicle, on riser's configuration and response are carefully examined. Through our FEM simulations, the spatial distributions of structural tensions and curvatures along of riser length, under different configurations, are compared. Then, the impacts of mining vehicle motion on riser dynamic response are discussed, and structural tolerance performance is assessed. The results show that modules installation position and buoyancy ratio have significant impacts on riser configurations. And, an appropriate riser configuration is obtained through comprehensive analysis on the modules positions and buoyancy ratios. Under this proposed configuration, the structural tension and curvature could moderately change with buoyancy modules and bottom-end conditions, in other words, the proposed saddle-shaped riser has a good tolerance performance to various load excitations.

Flexible risers have been used widely in recent years for floating structure in shallow and deep water. Flexible risers have various configurations and each configuration has its specific charectresitics that helps the riser to solve the problems that exist in shallow and deep water. Selecting and designing the best configuration for flexible risers in shallow water, like deep water, present many challenges, some of these challenges can be addressed: high vessel displacement compared to the water depth, minimum riser clearance with the seabed or sea surface or the vessel keel, bending radius limitation and etc. The aim of this paper is to investigate the traditional configurations like lazy wave and pliant wave and to compare them with a new configuration like weight added wave for the riser with 15 inches internal diameter in 45m water depth connected to a turret moored floating storage unit. It is concluded that the traditional configuration can not solve the sea surface clearance problem when the riser is empty condition, in this condition the weight of riser is decreased and the buoyancy force is become higher than riser weight and lift the riser up and the riser become flooded on the sea surface. To preventing the riser from flooding on the sea surface it’s needed to use configuration with added mass. Also concluded that using added mass in touch down zone (TDZ) reduces the tension in PLEM connection point about 89% . Also concluded that the minimum bending radius for weight added wave configuration is increased about 29% in compared with traditional configurations.

[Objectives] As oil and gas exploration enters the field of ultra-deep water,once the outer structure of the flexible riser is damaged,the inner carcass will carry huge external hydrostatic pressure. As one of the geometric imperfections,the gap between the carcass and pressure amour may cause a significant reduction in the collapse strength of flexible risers,which will lead to the so-called wet collapse.[Methods] Aiming at this problem,this paper presents a spring-supported arch model for estimating the collapse strength of a subsea flexible riser with a layer gap,and the wet collapse process of the carcass is divided into pre-contact and post-contact phases. In the pre-contact phase,the instability behavior of the inner carcass is analyzed using the stability theory of single ring structure. When the interlayer gap is closed,the carcass begins to be supported by the pressure armor,and the structure enters the post-contact phase,here the outside pressure amour is considered as springs which support the detached portion of the inner carcass. At the same time,the finite element collapse model is employed to verify the reliability of this spring-supported arch model for a set of gap widths.[Results] The predictions of the proposed models agree very well with the numerical results,and the collapse pressure is very sensitive to the stiffness of the pressure amour and layer gap width. When the initial gap width between layers increases from 0 to 0.5 mm,the collapse pressure of the flexible riser will decrease by about 18%。If the tightly fitted carcass is supported by a stiffer pressure amour,the collapse strength will be greatly improved.[Conclusions] For a flexible riser with a harder pressure amour and a smaller layer gap,the proposed analytical model can effectively predict its hydrostatic crushing strength.

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.

In this paper, the dynamics of the flexible riser are investigated based on the absolute nodal coordinate formulation (ANCF). The stiffness, generalized elastic force, external load, and mass matrixes of the element are deduced based on the principle of energy conversion and assembled with the finite element method. The motion equation of the flexible riser is established. The influence of the environmental load conditions on the flexible riser model is studied in the MATLAB environment. Moreover, the accuracy and reliability of the programs are verified for a beam model with theoretical solutions. Finally, the static and dynamic characteristics of the flexible riser are analyzed, systematically adopting the ANCF method, which in turn proves the effectiveness and feasibility of the ANCF. Therefore, the proposed method is a powerful scheme for investigating the dynamics of flexible structures with large deformation in ocean engineering. [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.

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]

This paper studies the vibration suppression problem of nonlinear flexible riser systems subjected to external disturbances. For suppressing the vibration and improving the control precision, a novel joint control method which includes boundary control and iterative learning control (ILC) is raised, via finite-dimensional backstepping control and Lyapunov's direct method. Iteration term is first designed in discrete domain and then transformed into continuous time domain. Under the action of the proposed boundary-ILC, the uniform boundedness of closed-loop system state and the convergence of the system output are proved by using Lyapunov synthesis. Finally, simulation results are provided to illustrate the effectiveness of the formulated control approach.

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.

The detailed finite element model of a ten-layer flexible riser 2-m long and 218.4-mm outer diameter was constructed in view of the boundary conditions and interlayer contacts at different loads (including tensile force, internal and external pressures). The analysis of the whole pipe was conducted to determine its main bearing layers. Those layers were extracted for local analysis to calculate the fatigue life. The internal and external armors were the main bearing layers in tension. The preparatory and interlocking pressure protection armors were the main bearing layers under external or internal pressures. The fatigue lives were 7.605 ·105 , 1.905 ·105, and 3.723 ·105 cycles, respectively, at cyclic loads. The main bearing layers can endure higher tensile force, external and internal pressures, which indicates that the ten-layer flexible riser can be used for engineering operations in aquatic environments. [ABSTRACT FROM AUTHOR]

As oil and gas exploration moves to deeper areas of the ocean, the weight of flexible risers becomes an important factor in design. To reduce the weight of flexible risers and ease the load on the offshore platform, this paper present a cylindrical tensile armor layer made of composite materials that can replace the helical tensile armor layer made of carbon steel. The ACP (pre) of the workbench is used to model the composite tension armor. Firstly, the composite lamination of the tensile armor is discussed. Then, considering the progressive damage theory of composite material, the whole flexible riser is analyzed mechanically and compared with the original flexible riser. The weight of the flexible riser decreases by 9.73 kg/m, and the axial tensile stiffness decreases by 17.1%, while the axial tensile strength increases by 130%. At the same time, the flexible riser can meet the design strength requirements of torsion and bending. [ABSTRACT FROM AUTHOR]

Flexible riser is a device which transports production fluids between floating vessels and subsea wells. With fewer remaining easy-to-access oil fields nowadays, flexible risers are being required to be installed in a water depth of over 3000m. However, the hydrostatic pressure at such a water depth may cause the collapse of flexible risers, and therefore predicting the critical collapse pressure is of great importance to their design. Riser collapse is a complex phenomenon related to material properties, geometry of the pipe and its overall surface topography and, therefore, makes the prediction of critical pressure challenging. Collapse prediction approaches of flexible risers have been developed for decades, yet a comprehensive review on their predictive capabilities, efficiency and drawbacks is lacking. In this paper, the recent advances on collapse studies of flexible risers are reviewed, which summarizes the methods developed for critical pressure prediction and highlights the related gaps in current research. This review aims to facilitate the current anti-collapse design and be a baseline for future utilization of flexible risers in deeper water expansion.

The creep behaviour of an inner liner, one of the reasons for carcass tearing, may affect the structural integrity of flexible risers. This has been previously discussed without conclusive results owing to complex structure and time-dependent material properties. The present paper proposes a numerical model for predicting creep responses by means of the finite element method. In this model, series coefficient is used to characterise the viscoelastic properties of material. Consequently, the influence of geometric parameters such as span of the carcass layer and thickness of the inner layer on the deformation is observed. Moreover, a threedimensional model assembling the carcass and inner liner was established for mechanical analysis, during which the viscoelasticity of inner liner and the internal friction of the carcass are considered, after which the stress and strain distribution on each layer under the combined external pressure and axial tensile force generated by the inner liner are obtained. Additionally, the effect of external pressures on the stress distribution of the carcass cross-section was found through sensitivity analysis. [ABSTRACT FROM AUTHOR]

This paper presents a numerical algorithm used together with a Finite-Element based Vortex-Induced Vibration (VIV) prediction software VIVANA to obtain a hydrodynamic force coefficient database based on response measurements of flexible riser VIV model test of staggered buoyancy elements. The excitation coefficient database is parameterized; and the representative parameters are systematically varied in VIVANA simulations until the predicted responses in terms of mode number, response frequency, root-mean-square (RMS) response amplitude, and RMS curvature agree well with the experimental data under same flow and boundary conditions. An optimal set of force coefficient database for both the buoyancy element and the bare riser section is obtained and a significant improvement in VIV prediction by VIVANA is achieved. The obtained hydrodynamic force coefficient database can be also applied to other empirical VIV prediction programs. The sensitivity of using the same database to predict the VIV of different configurations has also been investigated. [ABSTRACT FROM AUTHOR]

The microstructure， mechanical properties and hydrogen-induced cracking resistance of high strength steel used for flexible riser after different heat treatments were studied by means of optical microscope(OM)， scanning electron microscope(SEM)， transmission electron microscope(TEM) and electron back-scattered diffraction(EBSD). The results show that compared with the traditional quenching and tempering treatment， multi-stage heat treatment can refine the microstructure， promote the dispersion of fine precipitates and increase the density of grain boundary so that the strength of the tested steel can be improved greatly. Moreover， since multi-stage heat treatment increases the number of hydrogen traps， reduces the size of precipitates and improves the distribution of hydrogen traps and the hydrogen solubility， the hydrogen-induced cracking resistance of the tested steel is also improved. [ABSTRACT FROM AUTHOR]