Dynamics Of Riser Research Articles

Numerical Investigation of Dynamics of the Flexible Riser by Applying Absolute Nodal Coordinate Formulation

Abstract 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.

Time-domain analysis of vortex-induced vibration of a flexible mining riser transporting flow with various velocities and densities

Vortex-induced vibration (VIV) can be inevitably encountered in deep ocean mining. As the internal flow is transported axially in the flexible riser, complicated VIV dynamics occurs undergoing both the internal and external flows. Therefore, the object here is to explore the effect of the internal flow with different velocities and densities on VIV response under various uniform current. In this study, a semi-empirical time domain prediction method for VIV dynamics of flexible risers considering both internal and external flows is introduced and adopted. The governing equations are discretized and solved by using finite element method. Firstly, validations are made for VIV without internal flow based on the numerical results and experimental data. Comparisons prove that the simulation could reproduce the VIV dynamics of a flexible riser. Then with the increase of the internal flow velocities and densities, the effect of the internal flow on VIV response is examined. It is found that the dominating frequency and the root mean square (RMS) displacement in both in-line (IL) and cross-flow (CF) directions are notably influenced by the internal flow velocity and density. Besides, the drag coefficient and IL mean deflection are detected magnified while the internal flow velocity and density are increased under different external flow velocities. It should be noted that the change of the internal flow velocity and density could trigger new mode response of the flexible riser, leading to mode transition for the IL and CF dominating modes. In addition, VIV dynamics shows a similar changing trend with the increase of the internal flow velocity and density when the flexible fluid-conveying riser is subjected to different external flow velocities.

Coupling Effects of A Deep-Water Drilling Riser and the Platform and the Discharging Fluid Column in An Emergency Disconnect Scenario

As drilling operations move into remote locations and extreme water depths, recoil analysis requires more careful considerations and the incidence of emergency disconnect is increased inevitably. To accurately capture the recoil dynamics of a deep-water riser in an emergency disconnect scenario, researchers typically focus on modelling the influential subsystems (e.g., the tensioner, the mud discharge and seawater refilling process) which can be solved in the preprocessing, and then the determined parameters are transmitted into an existing global riser analysis software. Distinctively, the current study devotes efforts into the coupling effects resulting from that the suspended riser reacts the platform heave motion via the tensioner system in the course of recoil and the discharging fluid column follows the oscillation of the riser in the mud discharge process. Four simulation models are established based on lumped mass method employing different formulas for the top boundary condition of the riser and the discharging flow acceleration. It demonstrates that the coupling effects discussed above can significantly affect the recoil behavior during the transition phase from initial disconnect to the final hang-off state. It is recommended to develop a fully-coupled integrated model for recoil analysis and anti-recoil control system design before extreme deep-water applications.

Numerical investigation on VIV suppression of marine riser with triangle groove strips attached on its surface

The effects of Triangle Groove Strips (TGS) on Vortex-induced Vibration (VIV) suppression of marine riser are numerically investigated using Computational Fluid Dynamics (CFD) method. The range of Reynolds number in simulations is 4.0 × 104 < Re < 1.2 × 105. The two-dimensional unsteady Reynolds-Averaged Navier-Stokes (RANS) equations and Shear Stress Transport (SST) k-ω turbulence model are used to calculate the flow around marine riser. The Newmark-β method is employed for evaluating the structure dynamics of marine riser. The effect of the height ratio (ε) of TGS on VIV suppression is evaluated. The amplitude responses, frequency responses, vortex patterns and the flow around the structures are discussed in detail. With the increase of the height ratio of TGS, the suppression effect of TGS on VIV suppression is improved firstly and then weakened. When ε = 0.04, the suppression effect of TGS is the best. Compared with the VIV responses of smooth marine riser, the amplitude ratio is reduced by 38.9%, the peak of the lift coefficient is reduced by 69% and the peak of the drag coefficient is reduced by 40% when Re = 6.0 × 104. With the increase of Reynolds number, the suppression effect of TGS on VIV suppression is improved firstly and then weakened. When the Reynolds number is 7.0 × 104, the amplitude ratio can be reduced by 40.1%. As to the large-amplitude vibration cases, the TGS show nice suppression effect on VIV.

Vortex-induced vibration dynamics of a flexible fluid-conveying marine riser subjected to axial harmonic tension

This study investigates vortex-induced vibration dynamic responses of a marine riser transporting internal flow under the action of axial harmonic tension by means of finite element method. A van der Pol wake oscillator is adopted to represent the fluctuating lift forces, and nonlinear hydrodynamic force is also introduced. The goal of the present work is to discuss VIV dynamic responses of a fluid-conveying riser to guide the design and usage of riser in offshore oil field. The constant tension analyses are first obtained to determine the lock-in region of cross-flow velocity. Then, the influence of excitation frequency and harmonic tension amplitude on the resonance regions, the displacement amplitudes, the maximum stresses for various internal flow velocity, has been investigated. The results show that the dominated resonance region changed from twice the fundamental frequency to the fundamental frequency when harmonic tension amplitudes and cross-flow velocities are simultaneously increased. It is also revealed that the foremost two lock-in regions, respectively, appeared near the fundamental frequency and twice the fundamental frequency for varying cross-flow velocities.

Wake and structure model for simulation of cross-flow/in-line vortex induced vibration of marine risers

Three dimensional responses of riser subjected to Vortex Induced Vibration (VIV) are investigated. Proportionality relations of stress and fatigue damage are mentioned. A computer code has been developed for time domain modeling of VIV of riser accounting for both Cross-Flow (CF) and In-Line (IL) vibration. The wake oscillator model is used to calculate the VIV of each strip. The wake oscillators are coupled to the dynamics of the long riser, while the Newmark-beta method is used for evaluating the structural dynamics of riser. The wake dynamics, including IL and CF vibrations, is represented using a pair of non-linear Van der Pol equations that solved using modified Euler method. The existing experimental and numerical results for stepped and sheared current are used to validate the proposed model and the results show reasonable agreement. The proposed model was implemented on Amir-Kabir semi-submersible riser deployed at the water depth of 713 meters of Caspian Sea. CF/IL VIV of this riser is simulated for various current velocities. The results show that although displacement amplitude of IL direction is lower than CF direction but because of higher curvature, stress values of IL direction for some cases can be higher than CF direction. Also because of higher frequency of IL direction, fatigue damage of this direction can be higher than CF one in some cases. It is shown that with increasing of current velocity; however, variation of displacement amplitude of two directions is low but stress increased and fatigue damage also increased with higher rate. For lower velocities which the modes are controlled with tension, stress and fatigue damage of IL direction is higher than CF direction.

Fatigue damage analysis of the deepwater riser from VIV using pseudo-excitation method

The fatigue damage of a long deepwater riser undergoing in-line and cross-flow vortex-induced vibration (VIV) in deepwater is numerically studied using pseudo-excitation method (PEM) in present paper. For evaluating the fluid–structure interaction problem of vortex-induced vibration of deepwater riser at high Reynolds number, the strip theory is employed in this paper, and the discrete vortex method (DVM) is used to calculate the VIV of each strip to obtain the load spectrum as the pseudo-excitation, while the finite volume method (FVM) is employed to evaluate the structure dynamics of a deepwater riser. The VIV is considered as a stationary random process. The response of riser to vortex induced excitation is calculated using pseudo-excitation method. The DVM model and pseudo-excitation method are both validated by comparing their numerical results with experiments. The fatigue damage of one deepwater riser is evaluated based on the Palmgren–Miner Rule.

Stripwise discrete vortex method for VIV analysis of flexible risers

This paper presents a stripwise discrete vortex method (SDVM) to study the vortex-induced vibration of flexible risers. The discrete vortex method is employed to calculate the vortex-induced vibration (VIV) of each strip, and the finite volume method and increment method are used to compute the 3-D dynamics of the flexible riser. To verify the discrete vortex method for a single cylindrical strip of the entire flexible riser, a cylinder with elastic supports exerted by a 2-D uniform flow with a high Reynolds number is investigated. The 1-D transverse motion, 2-D transverse motion and 2-D stream-wise motion, as well as their influences on the wake shapes, are considered. Comparisons are made between the transverse amplitudes for the single degree of freedom and two degrees of freedom elastic-mounted systems. The relations between the lift coefficients and response amplitudes of the different mass ratios and reduced velocities are further discussed. The characteristics of the motion responses of the cylinder for different mass ratios and degree of freedoms are also discussed. Finally, the present numerical model is employed to calculate a 3-D flexible riser exerted by the uniform current, and the analysis of the numerical results shows the characteristics of the vortex-induced vibration of a flexible riser.

Computational fluid dynamics of riser using kinetic theory of rough spheres

Flow behavior of particles in the riser was simulated using a computational fluid dynamics (CFD). Conservation equations of mass and momentum for solid phase were solved on the basis of kinetic theory of rough spheres (KTRS). The fluctuation kinetic energy of particles is introduced to characterize the random motion of particles as a measure of the translational and rotational velocity fluctuations. The distribution of concentration and velocity of particles are obtained in a riser. The simulated concentration of particles agreed reasonably with the available experimental results. The random-motion kinetic energy of inelastic rough particles is shown to be affected by the particle restitution coefficient and roughness. The effects of the coefficient of normal restitution and roughness on the distribution of solid phase in the riser are studied. In this case, the influence of the coefficient of normal restitution is weak in the riser using KTRS model.

Nonlinear free vibrations and vortex-induced vibrations of fluid-conveying steel catenary riser

This paper presents a model formulation that can be used for analyzing the three-dimensional vibration behaviors of an inclined extensible steel catenary riser (SCR). The virtual work-energy functional, which involves strain energy due to axial stretching and bending rigidity of the riser and virtual work done by the gravitational, inertial and external drag forces, is formulated. The method of Galerkin finite element is used to obtain the mass and stiffness matrices. Then the eigenvalue problem is solved to determine its natural frequencies and corresponding mode shapes. A new nonlinear model capable of analyzing the vortex-induced vibration of SCR in the ocean current was addressed. The unsteady hydrodynamic forces associated with cross-flow vibrations are modeled as distributed van der Pol wake oscillators. Depending on the vortex-excited out-of-plane modes and system fluid-structure parameters, the parametric studies are carried out to determine the maximum response amplitudes of SCR, along with the occurrence of the mode transition phenomenon. The obtained results highlight the effect of internal fluid velocities and top tension on the nonlinear dynamics of riser undergoing vortex-induced vibration.

Reduced-order modelling of vortex-induced vibration of catenary riser

A new reduced-order model capable of analyzing the vortex-induced vibration of catenary riser in the ocean current has been developed. This semi analytical–numerical approach is versatile and allows for a significant reduction in computational effort for the analysis of fluid–riser interactions. The incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser plane of initial equilibrium curvatures. The equations of riser 3-D motion are based on a pinned–pinned, tensioned-beam or flexural cable, modelling which accounts for overall effects of riser bending, extensibility, sag, inclination and structural nonlinearities. The unsteady hydrodynamic forces associated with cross-flow and in-line vibrations are modelled as distributed van der Pol wake oscillators. This hydrodynamic model has been modified in order to capture the effect of varying initial curvatures of the inclined flexible cylinder and to describe the space-time fluctuation of lift and drag forces. Depending on the vortex-excited in-plane/out-of-plane modes and system fluid–structure parameters, the parametric studies are carried out to determine the maximum response amplitudes of catenary risers, along with the occurrence of uni-modal lock-in phenomenon. The obtained results highlight the effect of initial curvatures and geometric nonlinearities on the nonlinear dynamics of riser undergoing vortex-induced vibration.

Dynamics of a vertical riser with weak structural nonlinearity excited by wakes

In this paper we investigate the effect of a weak structural nonlinearity on the dynamical behaviour of a vertical offshore riser subjected to vortex-induced vibration (VIV). Coupling of the riser dynamics with the flow of the surrounding fluid is achieved by attaching a wake oscillator to a reduced model of the structure, which is obtained through the application of the invariant manifold technique for the derivation of nonlinear normal modes. By comparing the free responses of the linear and the nonlinear structure, it was found that the structural nonlinearity has a stiffening effect on the oscillation of the riser, which becomes more pronounced when the internal flow is incorporated into the model. Consequently, in the coupled system, the response is considerably modified for the structure as well as for the fluid variable.

Analytical and numerical analysis of the dynamics of a marine riser connected to a floating platform

Forced dynamics of the system comprised of a riser, connected to a floating platform and conveying fluid, is studied in the presence of ocean waves and ocean currents. Shear effects based on nonlinear elastic theory are included in the formulation. The effects of the structural and hydrodynamic parameters on motion amplitude are evaluated and analytically studied based on an approach developed by Butenin. It is demonstrated that when the nonlinearities are small, the solutions obtained using Butenin's method match the numerical results.

New and efficient high‐temperature processes with circulating fluid bed reactors

AbstractStarting with a brief review of the development of the circulating fluid bed (CFB) reactor principle, its main features as an important tool for efficient and thus environmentally acceptable processing of fine particulate solids, typically less than 150 μm diameter, are discussed. Various new industrial processes and high temperature process options starting with alumina calcination are described. Developing applications in the cement, metallurgical and chemical industries are approaching large‐scale industrial importance. Efficient combustion or pressure gasification of fossil fuels and carbon‐containing residues, fulfilling increasingly strict environmental legislation, is by far the most rapidly expanding field of application. In support of these developments our fundamental understanding of the fluid dynamics of CFB riser and of cyclone precipitation at high solid loadings has to be steadily improved. The reaction behaviour of aggregating fine particulate solids of varying size distributions also remains an open question.