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Abstract Ocean mining emerges as a way of meeting future demands for valuable minerals. Risers conveying ore from the seabed to the surface must withstand large wave and current loads. In addition, vortex-induced vibrations (VIV) are an important source of fatigue. However, the VIV of mining risers may be affected by particles flowing inside. In this paper, a novel framework for coupled analysis of risers with internal flow is utilized to study VIV of a mining riser. Focus is on the effect of time-varying internal slurry flows, and the associated density waves traveling along the riser. Simulated VIV response and fatigue damage are reported in detail, to illuminate the effect of the period/wavelength of the internal flow. It is found that the internal flow produces significant changes to the VIV fatigue damage, if the wavelength of the internal density wave is close to λ c r = L / n , where L is the riser length, and n is the dominating VIV mode. The results suggest that careful analysis is necessary when designing a mining riser where internal density waves are expected to occur with a wavelength near or above the critical value. In such cases, the numerical method presented here is a useful tool. [ABSTRACT FROM AUTHOR]

Vortex-induced motions (VIM) is becoming a noteworthy issue for column-stabilized floating platforms, mainly due to its substantial fatigue damage to risers and mooring system. The VIM of deep-draft semi-submersible is more complex than single column floaters because of the wake interference between columns, as well as the considerable yaw motions. In the present work, a numerical approach for simulating VIM of deep-draft semi-submersible is proposed. Specifically, detached-eddy simulation is used for turbulence modeling and dynamic overset grid technique is used for moving objects. Simulations for stationary drag and VIM of a model-scale paired-column semi-submersible are conducted with the proposed approach. The numerical results are compared with experimental data. Transverse, in-line and yaw motions are allowed during VIM simulations and are further analyzed in frequency domain by Fast Fourier Transform (FFT). Different VIM characteristics are observed at different current velocities. The work done by each component of the structure is also discussed. Flow visualizations are presented for better understanding of the wake interferences during VIM. The accuracy and reliability of the current numerical approach is assessed. [ABSTRACT FROM AUTHOR]

The fatigue life of steel catenary risers is significantly influenced by cyclic riser–seabed–water interaction in the touchdown zone. In this study, the penetration and extraction of a shallow embedded riser section, subjected to cyclic vertical loading, are simulated using a computational fluid dynamics approach with ANSYS CFX. An empirical strength degradation model is proposed for soil softening due to undrained remoulding and clay–water interaction in the highly sheared interface, termed ‘shear wetting.’ The combined effects of strain rate and softening on the mobilized shear strength of deepwater offshore clay are implemented in CFX. A sufficiently large number of loading cycles is simulated using this computationally efficient numerical technique to achieve a stable response. A significantly large reduction of vertical resistance of a shallowly embedded riser section due to cyclic loading, as observed in physical model tests, is obtained using the proposed strength degradation model with shear wetting, which cannot be explained simply by undrained remoulding. [ABSTRACT FROM AUTHOR]

Flexible risers are being required to be installed in a water depth of over 3000 m for fewer remaining easy-to-access oil fields nowadays. Their innermost carcass layers are designed for external pressure resistance since the hydrostatic pressure at such a water depth may cause the collapse failure of flexible risers. Determining a critical collapse pressure for the carcass is of great importance to the whole structural safety of flexible risers. However, the complexity of the carcass profile always makes FE analysis computational intensive. To overcome that problem, the treatment of the interlocked carcass as an equivalent layer is adopted by researchers to accelerate the anti-collapse analyses. This paper presents an equivalent layer method to enable that treatment, which obtains the equivalent properties for the layer through strain energy and membrane stiffness equivalences. The strain energy of the carcass was obtained through FE models and then used in a derived equation set to calculate the geometric and material properties for the equivalent layer. After all the equivalent properties have been determined, the FE model of the equivalent layer was developed to predict the critical pressure of the carcass. The result of prediction was compared with that of the full 3D carcass model as well as the equivalent models that built based on other existing equivalent methods, which showed that the proposed equivalent layer method performs better on predicting the critical pressure of the carcass. [ABSTRACT FROM AUTHOR]

Unstable hydrodynamic slug flow is extensively encountered in marine risers, resulting in internal flow-induced vibration, which is quite different with the vortex-induced vibration caused by a cross flow. Experiments were performed in an air-water test loop to study the internal slug flow-induced vibration of a free-hanging flexible riser with aspect ratio (the ratio of the length to the diameter of a riser) of 158. A non-intrusive technique of high speed imaging method was employed to record the vibration displacements of the riser model and the flow regime of two-phase mixture in the riser simultaneously. The results show that an in-plane vibration is created by the internal slug flow, and the responses along the two directions in the plane are interdependent to each other. In the considered gas-liquid ratio (the ratio of air flow rate to water flow rate), the second-order mode dominates the response. The hydrodynamic behaviors of slug flow, including the gas-liquid ratio, liquid slug length and superficial velocity, affect the local pressure fluctuation and hence influence the vibration amplitude. The fluid-structure interaction is reflected from the same frequencies presenting in the vibration and pressure fluctuation. [ABSTRACT FROM AUTHOR]

The present paper provides a method to estimate the clearance between two flexible risers in tandem arrangement by taking into account the hydrodynamic wake interference when subjected to a current flow and a combined current plus waves flow. Modelling of the wake interference in the near wake field is based on the modification of the mean wake velocity deficit in the far wake field. Two modified wake models are introduced in the presented paper. The wake effects in steady current and in current plus waves are investigated by using both wake models. For the current-only flow, the deflection shapes of a pair of tandem risers in steep-wave configuration are presented, highlighting the importance of the wake effect on clearance estimation. For combined current plus waves, the wake interference is investigated by considering the position-dependent drag force, with different combinations of riser natural frequency and wave frequency. The collision failure probability is predicted when the risers are subjected to current plus random waves. [ABSTRACT FROM AUTHOR]

A modal space based direct method is developed for vortex-induced vibration (VIV) prediction of flexible risers. A VIV response solution model is established in modal space, consisting of a hydrodynamic force equation and a dynamic response equation. Adopting a non-iterative root-search method in modal space, the equation set gets solved directly without power-balance iterations. This direct method is compared against conventional empirical models, with its validity and applicability verified. This method entirely gets rid of convergence problems during predictions, which is conducive to practical engineering applications and puts forward fresh ideas for further development of empirical models for VIV prediction. [ABSTRACT FROM AUTHOR]

This paper presents an improvised coupling technique for stress impact and dynamic response studies of a deepwater flowline-riser pipe system exposed to cyclic loadings resulting from severe slug flow. It involved partial coupling of transient multiphase flow simulator OLGA and structural finite element code ABAQUS. In this analysis, a parametric study was carried out where mass flow rates of 9.8 kg/s, 13 kg/s, and 16 kg/s were sampled in order to reflect the corresponding marginal flow rates of 6000 (950.4 m 3 /day); 8000 (1267.2 m 3 /day); and 10000 (1584 m 3 /day) barrels of oil per day (bbl/day) for a typically aged offshore oilfield presumed to be piped with a 10 inch (0.254 m) diameter subsea production pipe system. Numerical results obtained, have shown that the 6000 bbl/day flow rate imposed predominant stress at the riser base by producing maximum stress values of 64 MPa, 63 MPa, and 68 MPa in the axial, transversal and lateral global directions of the flowline-riser cross-section, respectively. This observation has further validated the severity of severe slug flow to low flow rate of reservoir fluids. With this and other data provided in this paper will enhance the fatigue design optimisation of deepwater production risers. [ABSTRACT FROM AUTHOR]

In this numerical study, it was mainly intended to investigate the flow and flow control for a practical drilling riser system with auxiliary lines at various incidence angles. The flow past the riser system was simulated through solving the Reynolds averaged Navier–Stokes equations and k − ω turbulence model. The cases about fluid flow past single cylinder, two staggered cylinders and eight even-distributed auxiliary lines around main line were simulated first, and the results match well with previous researches. The Reynolds number ( Re ) used here was 35,000, which belongs to the subcritical Reynolds region, and the flow in this Reynolds region can shows the typical flow characteristics in practical drilling operator. The effects of the incidence angle, which was in the range of 0–360° and at 30° intervals, on the drag and lift coefficients, the pressure distribution around the main line, the vortex shedding frequencies and the flow structure were investigated. The mean and RMS values of the drag and lift coefficients of the main line vary with the incidence angle irregularly due to the complexity of the geometry of the riser system. The flow patterns can be classified into six types based on the time-averaged streamlines. The effects of the auxiliary lines on mean force coefficients of the main line in different flow patterns were discussed in detail based on the distributions of the pressure on the main line. The auxiliary lines can suppress the vortex shedding on the main line at all incidence angles. Especially, in the CVP (clamped vortices pair, α = 210 ° and α = 330 ° ) pattern, the vortex shedding behind the main line is suppressed most effectively by multiple downstream auxiliary lines. The mean value of the drag coefficient is just about 45.5% of that of the single cylinder, and the RMS value of the lift coefficient is about 1.5% of that of the single cylinder. This is the favorable condition for drilling operation. The worst effect of suppression occurs in the SLA (shear layers afflux, α = 90 ° ) pattern, and the RMS value of the lift coefficient is about 76.8% of that of the single cylinder. Due to the interaction between the vortices shed from different lines in riser system, the frequencies of the lift coefficients of the lines in riser system change largely, and some higher frequencies appear in the lift coefficients of the main line. As α = 60 ° and α = 120 ° , the Strouhal number of the main line is about 0.75. [ABSTRACT FROM AUTHOR]

Exploration of deeper oceans for oil and gas requires increasingly lightweight solutions. A key enabler in this aspect is the use of fiber-reinforced composite materials to replace metals in risers. However, design synthesis and analyses of composite risers are more challenging than for conventional metals due to the complex behavior and damage mechanisms which composite materials exhibit. Composite risers are predicted to be a high-impact technology that will be mainstream in the medium term but there is still relatively little literature pertaining directly to the behavior of these materials under the complex loading scenarios arising from their use in deep water structures. Therefore there is a need to perform a review and assessment of the available technologies and methodologies in the literature to gain a good understanding of their predictive capabilities, efficiency and drawbacks. This article provides a comprehensive review of published research on manufacture, experimental investigations and numerical analyses of composite risers in deepwater conditions determining the gaps and key challenges for the future to increase their application. [ABSTRACT FROM AUTHOR]

Active control of axial dynamic stress response of deepwater risers with Linear Quadratic Gaussian controllers is presented in this paper. The risers are installed between the subsea wellhead and the Tension Leg Deck located in the middle of the moon-pool in the Floating Production, Drilling, Storage and Offloading unit. The inevitable heave motion of the floating hull causes a time-varying axial tension in the risers. As a result, the frequent occurrence of axial dynamic stress response of relatively large amplitude may cause fatigue damage to steel risers. This paper thus explores the possibility of using a winch to reduce axial dynamic stress response of deepwater risers. The equations of motion of actively controlled risers against waves are first established. Axial dynamic stress response of the system is then determined in terms of the complex dynamic characteristics and the pseudo-excitation method. Based on the derived formulae, the performance of the winch for reducing axial dynamic stress response of risers is numerically assessed. The investigations show that the winch not only provides great flexibility for selecting system parameters but also significantly reduces the axial dynamic stress response of risers under wave excitation. [ABSTRACT FROM AUTHOR]

Abstract: This paper models a slender, flexible structure used as a drill string or riser in the offshore oil and gas industry that connects the well-head with a floating control vessel. These systems are used in deep-water drilling applications and present considerable design challenges due to their extreme flexibility and susceptibility to buckling and vibration. Two typical configurations are used (), with a common feature involving the attachment of a buoy designed to relieve some of the axial forces acting on the riser, especially at the attachment points. Previous work by the authors studied numerical results of small-amplitude vibrations and two other equilibrium configurations using parameter values that closely resemble the full-scale application (). Here, two new configurations are considered, and experiments are designed and conducted to verify these equilibrium results. [Copyright &y& Elsevier]

Abstract: The paper deals with the reliability-based design optimization (RBDO) of a riser support installed on a floating production storage and offloading (FPSO) unit under operation, extreme, damaged, and one line failure cases and installation loading conditions. The optimization problem is formulated such that probabilistic thickness variables described with random characteristics are determined by minimizing the weight of the riser support structure subjected to stress constraints for the given target reliability. The initial design model is generated based on actual FPSO riser support specifications. The finite element analysis is conducted using NASTRAN, and the probabilistic optimal solutions are obtained via the moving least squares method in the context of RBDO using a response surface meta-model. For the meta-modeling of the inequality constraint functions of stresses, a constraint-feasible moving least squares method (CF-MLSM) is adopted in the present study. The CF-MLSM has been shown to ensure constraint feasibility regardless of the nonlinearity of the constraint function, the feasible bounds, and the random characteristics during the meta-model-based RBDO process. The solution results from the proposed RBDO strategy present improved design performances under various riser operating conditions. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Nonlinear multi-contact coupling between vertical risers and guide frames inside the spar moon-pool is studied. The existing numerical model for hull/mooring/riser coupled dynamics analysis treats riser as an elastic rod truncated at the keel (truncated riser model), and the risers are free to slide in vertical direction with constant tension, while restricted in horizontal direction. The truncated riser model neglects the portion of the riser inside the moon-pool and tends to overestimate the spar pitch motion. In the newly developed model, the risers are extended through the moon-pool with realistic boundary conditions at multiple guide frames, and thus additional contact forces and moments on the spar hull are considered. The gap effects between the riser buoyancy-cans and riser guide frames are also modeled using three different types of gap springs. Their different dynamic characteristics are extensively studied. The new riser model also considers the Coulomb damping between buoyancy-cans and riser guide frames and also allows the impact-like contact force calculation on risers for ensuring fatigue analysis. [Copyright &y& Elsevier]