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Abstract Flexible riser is a key enabler for the oil and gas production in ultra-deep water which transports production fluids between floating production systems and subsea wells. As oil and production heads to water depths in excess of 3000 m, high hydrostatic pressure has been one primary challenge facing the riser operators. Excessive hydrostatic pressure may cause collapse failure of flexible risers and thus predicting the critical collapse pressure is of significant importance to their anti-collapse design. Collapse is a complex phenomenon related to the material properties, the geometry of the pipe and its overall surface topography and, therefore, makes the prediction of critical pressure challenging. Related prediction approaches of flexible risers have been developed for decades, yet a comprehensive review of their predictive capabilities, efficiency and drawbacks is lacking. This paper reviews the recent advances on collapse studies of flexible risers and highlights the gaps in existing prediction methods, aiming to facilitate the current anti-collapse design and be a baseline for future utilization of flexible risers in deeper water expansion. [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]

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]

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

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]

Marine risers transporting unsteady multiphase flows may experience internal flow-induced vibrations depending on several flow-structure parameters. This study presents an experimental investigation of large-amplitude intermittent vibrations of an inclined bendable riser transporting an air-water flow that exhibits an unsteady slug flow pattern. The riser has an initial catenary profile with a length over an internal diameter of about 320. By using a non-intrusive measurement with a high-speed motion capture camera, new insights into the riser dynamic features and slug flow-riser interaction phenomena are presented and discussed in the case of varying gas-to-liquid flow rate ratio (R GL). Pressure signals are also recorded at the riser inlet and outlet, measuring the pressure fluctuation caused by the hydrostatic change, flow-induced momentum and frictional forces. For a given liquid flow rate or superficial velocity, the inclined bendable riser experiences a modified mean drift of the equilibrium configuration and a greater vibration amplitude when increasing R GL or the superficial gas velocity. The most critical case has a spatially maximum amplitude of greater than six diameters of the riser pipe. Experimental results of the space-time varying responses and frequencies of the riser are analysed in relation to the associated slug flow features including the slug unit length, translational velocity, frequency and pressure fluctuations. Owing to a coupled slug flow-riser interaction, the liquid slugs and elongated gas bubbles vary spatially and temporally. This entails the highly modulated responses and frequencies, intermittently switching the riser multimodal response, especially when R GL is greater than two. The amplified bending stresses in the riser are reported, highlighting the critical hydrodynamic effect of unsteady multiphase flow. The characteristic slug frequencies associated with the flexible inclined riser dynamics are also evaluated and compared with those predicted by several empirical correlations in the literature based on stationary, straight and rigid pipes. Overall, experimental observations and data are useful for a numerical model development and comparison towards a practical analysis and design of flexible curved/sagged risers transporting multiphase flows. [ABSTRACT FROM AUTHOR]

• This paper proposes new global dynamic analysis methods for flexible risers with consideration of interlayer interactions. • This paper suggest a multi hysteresis modeling method which considers the variation of axisymmetric loads and shear deformation effects. • A comprehensive investigation on the dynamic responses of flexible risers is carried out using the suggested multi hysteresis modeling method. • The interlayer interaction is found to have a significant impact on the global responses and fatigue damage. • A partial hysteresis method is additionally introduced to improve the modeling and computational efficiency of global dynamic analysis. Flexible risers have been widely utilized for the transfer of oil and gas products from the well to the production unit. Continuous interactions of tensile armor layers and anti-friction tapes trigger the slippage of helical wires and a highly nonlinear hysteric behavior in bending. For the precise prediction of the bending behavior of flexible risers, Part I of this series of papers proposed a new analytical model that successfully models the interlayer interaction. In this paper, Part II, the proposed analytical model is extended to the global domain of riser configurations; that is to say, a Multi-Hysteresis Modeling (MHM) method is proposed The proposed method employs accurate and realistic bending hysteresis curves with consideration of shear deformation and axisymmetric loads. Based on the proposed method, three comparative studies are performed to investigate the effects of interlayer interaction. As a result, the interlayer interaction is found to have a considerable impact on the fatigue damage of flexible risers. For a practical global dynamic analysis procedure, this paper also introduces a partial hysteresis modeling (PHM) method, by which only a small number of local regions are modeled with bending hysteresis curves. [ABSTRACT FROM AUTHOR]

w This paper proposes an analytical model for the prediction of nonlinear bending behavior of flexible risers. w The proposed analytical model formulates the stress of tensile armor layers through the equilibrium equations which take into account the shear deformation and radial deformation of polymer layers. w Using a linear differential equation of each component of flexible risers, the analytical model reflects the shear interaction force between layers and contact pressure induced by bending into the equilibrium equations, deriving accurate bending hysteresis curves as a function of curvature and axisymmetric loads. w The validity of the proposed model is examined by comparing the results of the analytical model with the FE analysis results, and a case study on various combined load cases performed for in-depth understanding. An un-bonded, multi-layered assembly of heterogeneous materials in the cross-section of flexible risers enables riser systems to withstand a large radius of curvature while preventing structural damage or buckling of many critical areas such as a touch down zone. The contact surfaces between layers exhibit a continuous sliding of metallic components after a certain level of bending, which lowers the bending stiffness of flexible risers. Such interactions are beneficial in order to reduce the stress of tensile wires, but complicate prediction of the limit state and fatigue damage of riser systems in that the sliding of metal layers involves a complex contact phenomenon that is difficult to solve numerically. This series of papers deals with the development of an improved analysis method for flexible risers using theoretical approaches. Part I, as presented in these pages, develops an analytical model that is capable of estimating the cross-sectional stiffness of flexible risers with consideration of external loads and inter-layer interaction. In Part II, the proposed analytical model is applied to a large-scale riser model to achieve an efficient global dynamic analysis of flexible risers. This paper, the first part of a two-paper series, proposes an analytical model that formulates the stress of tensile armor layers through equilibrium equations that take into account the shear and radial deformation of polymer layers. The equilibrium equations are derived under a linear differential equation for each component of flexible risers, so that not only the shear interaction forces but also the contact pressure originating from the bending of layers is considered in the analytical model. The shear interaction forces between two tensile armor layers are formulated through an equivalent shear stiffness by which the shear stiffness of layers is linearly modeled as a series of springs. The proposed model is verified by comparison with the nonlinear bending stiffness from existing bending models and finite element (FE) analysis. For additional verification, the axial stresses of inner and outer tensile armor also are compared. And for further, more in-depth understanding, a case study of various combined load cases is carried out. [ABSTRACT FROM AUTHOR]