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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]

Bend stiffeners are essential components of a flexible riser system, employed to ensure a smooth transition at the upper connection and to protect the riser against over bending and from accumulation of fatigue damage. The highly nonlinear rate dependent behavior of these structures directly affects the integrity assessment of the riser in one of its most critical regions, the top connection. A steady-state formulation (disregarding inertial forces) and numerical solution procedure is developed in this work employing the perturbation method for a nonlinear viscoelastic bend stiffener large deflection beam model subjected to harmonic loading conditions. For stochastic loading conditions, the response is calculated employing the superposition principle by summing up the steady-state result of a number of individual frequency components. A time domain formulation is also derived employing the state-variable approach for the numerical solution of the resulting hereditary integral in the governing equations. A case study is presented for the top connection system of a 4″ ID flexible riser using relaxation and tensile experimental data obtained from a typical class of bend stiffener polyurethane. Harmonic and stochastic input loading conditions are employed for time and frequency domain model comparison/validation and to assess loading history and frequency influence in the curvature response. [ABSTRACT FROM AUTHOR]

An experimental investigation was conducted on a flexible riser with and without various helical strake coverages, with the aim of further improving the understanding of the response performance of the vortex-induced vibration (VIV) of a riser with helical strakes. Two current profiles, including uniform and linearly sheared currents, were simulated. The VIV displacements, VIV strains and hydrodynamic forces were directly measured from the test. Based on the fatigue damage method, the fatigue damage was obtained from the measured strain. Strakes with different coverages were analysed, and response parameters such as the displacement response and fatigue damage were studied. The results of the straked model test indicate that the response characteristics of a bare riser model can be quite distinct from those of a riser with helical strakes, and the response performance closely depends on the coverage of the helical strakes. The results also show that the flexible riser with 75% and 50% strake coverages are the best configurations to suppress VIV fatigue damage in uniform and linearly sheared currents, respectively. [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]

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 effects of different helical strake coverage on the vortex-induced vibration (VIV) of a model flexible riser were studied experimentally, with the aim of further improving the understanding of VIV responses. Uniform and linearly sheared currents were simulated to study response parameters such as non-dimensional displacement, fatigue damage, suppression efficiency, and the comprehensive evaluation is further studied. Test results of the bare model for a uniform current showed that the behavior of both the standing wave and traveling wave dominated VIV displacement. However, for a linearly sheared current, traveling wave behavior dominated VIV displacement in the high-velocity range. The results of the straked model tests indicated that the response was strongly dependent upon the amount of coverage of helical strakes. The flexible riser with 75% strake coverage gave the best comprehensive evaluation in a uniform current, and 50% strake coverage gave the best comprehensive evaluation in a linearly sheared current. [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) 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]