Top Tensioned Risers Research Articles

Three-dimensional nonlinear vortex-induced vibrations of risers under internal gas-liquid and external shear flows

A novel three-dimensional (3D) dynamic model is proposed to explore the nonlinear vortex-induced vibration (VIV) characteristics of top-tension risers under the combined action of internal gas-liquid two-phase flow and external shear flow. The van der Pol equation is used to evaluate the interaction between the riser and the external shear flow. The nonlinear governing equations are established through Hamilton's principle, and they are discretized by the Galerkin method and solved via the Runge-Kutta method. The VIV responses calculated by the proposed nonlinear dynamic model are compared with the previously experimental and computational fluid dynamics (CFD) results, demonstrating the effectiveness and accuracy of the present theoretical method. The influences of the internal flow effect, the tension and the shear parameter of the external flow on the VIV responses of the riser are analyzed. The results show that when the internal flow effect is not considered, the transition of the external flow from uniform flow to shear flow leads to an increase in excited modes and a decrease in resonance responses of the riser in the cross-flow (CF) direction. Furthermore, with the increase of the external shear flow velocity, resonances at different positions of the riser are not synchronized, which is different from that of the riser in uniform flow. In the case of the riser conveying gas-liquid two-phase flow and being subjected to external uniform or shear flow, the increase in the velocity of the liquid phase leads to an increase in the maximum response amplitudes of the riser in the in-line (IL) and axial directions. When the internal flow changes from single-phase to gas-liquid two-phase, the maximum VIV responses of the riser in the IL and axial directions tend to decrease regardless of whether the riser is in uniform flow or shear flow.

Combined vibration analysis of a top tensioned riser with the geometrical nonlinearity

A coupling model representing the interactions between the riser and vortices is proposed, of which the time-varying top tension and the internal nonlinear axial force due to the bending of the riser are considered. In addition, the van der Pol oscillator is introduced to simulate the time-varying characteristic of the vortices. The motion equations are derived and the first-order mode approximation is obtained with the Galerkin approach. The multiple scale method is applied to study the steady-state solutions of the system. Effects of system parameters (the structural damping, nonlinearity, the amplitude ratio of the varying-tension, the shedding frequency) on the responses are investigated in detail. The synchronous and non-synchronous motions between the structure and wake oscillator are studied. The bifurcation diagrams for the responses in terms of the amplitude ratio and the nonlinear parameter are obtained in the large parameter ranges. The results show that responses with different topological properties including quasi-periodic, periodic and chaotic solutions can occur under different parameter conditions. A pair of chaotic attractors can be found for the large parameter range of the amplitude ratio. The joint effects of the amplitude ratio and the nonlinear parameter on the responses in the large parameter range are studied as well, which are further verified by the phase projections and Poincaré sections. The results show that the topological properties of responses can be controlled by the amplitude ratio. These results can be helpful to the dynamic design of the riser in practice.

Cross-flow vortex-induced vibrations of a top tensioned riser subjected to boundary disturbances with frequencies close to vortex shedding

In the present paper, the effects of external disturbances on cross-flow vortex-induced vibrations (VIVs) of a top tensioned riser are investigated based on a wake oscillator model. Two different flow conditions, namely uniform and linearly sheared profiles, and different combinations of disturbance frequency and amplitude were considered. Simulation results showed that synchronization between VIV and disturbance can occur when the disturbance frequency approaches the Strouhal frequency or when the disturbance amplitude is significant. This synchronization leads to a significant increase in riser response and the commonly used industry practice of simulating VIV and disturbance independently and summing both could underestimate the actual fatigue rate in such condition. However, in certain cases where VIV did not lock onto the disturbance, it was found that the disturbance could suppress the VIV and the industry practice tends to overestimate the fatigue rate.

An experimental investigation on flow-induced motion and fluid force of a tension leg platform in complex flows

Investigating the dynamic response of tension leg platform (TLP) in complex flows could better reduce or avoid its influence on the fatigue of mooring system and riser system. In this study, full-water-depth tendons and top-tension risers (TTRs) model were adopted, while linear springs arranged horizontally were adopted to instead of mooring system in traditional models, and different types of shear flow with uneven distribution along the water depth were set up to conduct an in-depth study of TLP response at 45° heading. Significant FIM and lock-in phenomena were found in the research, with the maximum amplitude occurring at Vr = 7. The law of response of TLP when local flow accelerates around the column or pontoon was discussed, the reasons for the increase of response outside the lock-in region was clarified and the non-linear effect of the mooring system demonstrated experimentally. To deeply understand the forces during the motion of TLP, this analysis puts forward a method to reconstruct the fluid force, which is divided into two parts: the disturbing force and the radiation force. The proposed method may provide reference for further studies of the deep mechanical rules of flow-induced motion (FIM).

Dynamic responses of the active hydraulic pneumatic tensioner for a TLP in internal solitary waves

Tensioner system is the key connection device used to connect the platform and the top tensioned riser. An internal solitary wave (ISW) can cause serious damage to the tensioner system due to rough drift or broken mooring lines. In this paper, an active control tensioner system is proposed under the action of the ISW. By using the coupling model of the Tension Leg Platform (TLP), the dynamic response of the platform with the active tensioner and the passive tensioner under different control modes are studied. It is concluded that the surge/sway and heave motions of the platform are significantly affected by the ISW. Compared with the passive tensioner, the active tensioner obviously reduces the amplitude of the piston rod motion at low frequency and is more stable at high frequency.

Vortex-induced vibrations of a free spanning submarine power cable

Vortex-induced vibrations (VIV) of a flexible structure occur when the vortex shedding frequency is close to the natural frequency of the structure. Submarine power cables exhibit more complex vortex-induced dynamics than the top-tensioned risers and submarine pipelines, because their modal properties such as the frequencies and modal shapes are affected by the structural sags. In order to better understand this problem and improve the frequency-domain analyses of vortex-induced cable fatigue damage risks, a laboratory study on VIV of a free spanning submarine power cable in steady currents with four sags is conducted. The scaled laboratory model is designed based on the gravity similarity and elastic similarity. With the geometric similarity ratio of 1:7, it represents in prototype a 500 kV self-contained oil-filling submarine power cable. The measured data indicates that the lock-in of the first antisymmetric, first symmetric and second symmetric modes all take place within the flow ranges in the experiments. The analysis further shows that the VIV features of the submarine power cable are significantly affected by the sags. At sag d = 0.23m, the cable mainly exhibits a single-mode VIV response. As the sag decreases from 0.23m to 0.09m, the natural frequencies of the cable are more closely spaced and its VIV tends towards a multi-mode response. During the multi-mode response, the first antisymmetric, first symmetric and second symmetric modes are observed to coexist. Among the excited modes, because the natural frequency of the first symmetric mode is the intermediate one, its vibration is apparently suppressed. The maximum root-mean-square value of the modal weight of the first symmetric mode decreases from 0.26 to 0.086 as the sag decreases from 0.23m to 0.09m.

Wave Effects on Vortex-Induced Vibrations of a Top-Tensioned Riser

Abstract Offshore drilling risers and many production risers are top-tensioned risers (TTRs), connecting the vessel and seabed via joints. External loads such as currents, waves, and vessel motions introduce cyclic loads and motions on riser sections and associated components, such as universal joint, flex joints, and well head at the bottom, which may shorten the service life due to accumulated fatigue damage. Dynamic responses under combined currents and waves are more complicated than vortex-induced vibrations (VIVs) caused by pure currents, and it is not fully understood. Several model test campaigns on TTR have been carried out at SINTEF Ocean (former MARINTEK) during the past decades. Currents, waves, and vessel motions were modeled, and the riser model responses were measured. In this study, selected cases from such model tests are analyzed, and used to validate a semi-empirical time domain VIV prediction tool—VIVANA-TD. A better understanding of the dynamic responses of TTR under combined currents and waves has been achieved. By comparing the results from numerical simulation using VIVANA-TD and model test measurements, validity and limitation of the time domain tool have been investigated. Important features that need to be considered are discussed. The experience gained from the present study establishes a good basis for VIV and wave load prediction of full-scale TTRs under combined currents and waves where the uncertainty of combined wave and VIV prediction is further reduced.

Fatigue damage prediction of top tensioned riser subjected to vortex-induced vibrations using artificial neural networks

Several marine structures are subjected to critical loads resulting in their fatigue failure. Among these structures, the riser is an essential component that transports the hydrocarbons and fluids from the platform to the subsea well and vice versa. Risers are subjected to vortex-induced vibrations (VIV) caused by current, leading to fatigue damage (Lekkala et al., 2022a; 2022b). Therefore, developing a simplified and valid approach to estimate fatigue behaviour and damage is necessary. Though semi-empirical methods are available for fatigue analysis, their deployments demand immense computational time and costs. In this study, we propose a novel framework based on data-driven analysis utilising artificial neural networks (ANN) to predict the VIV-induced fatigue damage of the top-tensioned riser (TTR). To build a reliable database, OrcaFlex, and SHEAR7, are used for the riser's modal and fatigue damage analysis due to VIV, respectively. A total of 24,686 riser models based on combinations of eight input parameters (outer diameter of riser, wall thickness of riser, water depth, surface, and bottom velocities, top tension, buoyancy modules thickness, and coverage ratio) and corresponding output as fatigue damage results are used for training a multi-layered ANN. The fatigue damage prediction surrogate model has been achieved by tuning the hyperparameters to improve the model's prediction accuracy. The findings of the ANN-based riser fatigue damage approach confirm that the ANN model exhibits a mean absolute percentage error (MAPE) between the predictions and truths from SHEAR7 is 9.29%. Also, the proposed framework can reduce computational time and cost compared to conventional semi-empirical methods. Finally, detailed parametric investigations are carried out using an optimised ANN model to evaluate the accuracy with the semi-empirical tool. These metrics suggest that the proposed ANN method could be used to determine the fatigue damage of risers in real-world field conditions.

Vortex-induced vibrations of a top tensioned riser subjected to flows with spanwise varying directions

Vortex-induced vibrations (VIVs) of a top tensioned riser (TTR) subjected to flows with spanwise varying directions have been simulated and studied in this paper. The riser is simplified as an Euler–Bernoulli beam and its dynamics is described using the finite element method with the Absolute Nodal Coordinate Formulation (ANCF). The hydrodynamic forces due to vortex shedding are modelled with wake oscillators attached to nodes along the riser. The model has been firstly used to simulate an experiment and acceptable model results have been obtained in comparison with the measurements. Then, the VIVs of the TTR under two flow conditions, one with uniform and the other with linearly sheared velocity, have been simulated. In both cases, the direction of the flow varies linearly along the riser. Simulation results have shown different response characteristics at small and high flow velocities with respect to the frequency, amplitude and vibrating direction of the riser. In general, at a small flow velocity where the response of the riser is dominated by the standing wave, the variation in the flow direction has a major influence on the VIV of the riser. However, when the travelling wave is predominant at a high flow velocity, the effect of the spanwise varying flow direction is insignificant. The energy transfer between the structure and flow has been studied to reveal the possible mechanism that leads to these differences in the responses of the riser at small and high flow velocities. The results of the present paper suggest that the engineering simplification of ignoring the influence of flow direction variation on VIV may only be applicable at high flow velocities when the response of the riser is dominated by the travelling wave.

Structural nonlinearity of the tensioner model affects parametric instability prediction of deep-water top tensioned risers

TTRs (Top Tensioned Risers) may experience parametric instability ascribed to the time-varying tension exerted by the top floating platform. As ocean exploration steps into deep-waters, ocean environment becomes severe and the flexibility of risers is enhanced due to the increased aspect ratio. Therefore, it requires to figure out the new influential factors for the dynamic prediction with accuracy of TTRs. In parametric stability analysis, it typically adopts a linear model and an engineering model for the tensioner. The structural nonlinearity of the tensioner is commonly neglected or simplified. In this study, a nonlinear model of a direct-acting tensioner system is derived in a rigorous way. The parametric stability of a deep-water TTR is predicted based on Floquet theory employing three tensioner models (i.e., a linear model, an engineering model, and the nonlinear model). It has demonstrated that the structural nonlinearity of the tensioner has a significant effect on the calculations of the top tension and natural frequencies of the riser, and thus affects parametric stability predictions. The instability region adopting the nonlinear model is the largest, and the structural nonlinearity effect becomes more profound with the increase of high-pressure gas volume and low-pressure nitrogen pressure. Moreover, an improved linear model and an improved engineering model have been proposed.

Three-dimensional nonlinear vortex-induced vibrations of top-tension risers considering platform motion

A novel three-dimensional (3-D) dynamic model considering nonlinearity of tension is proposed to explore in-line (IL), cross-flow (CF) and axial (AX) vortex-induced vibration (VIV) responses of riser under platform motion. The interaction between riser and external flow is simulated by the van der Pol equation, and the nonlinear equations of motion are established by Hamilton's principle and the Galerkin method, and solved by Runge-Kutta method. The influences of heave and swing motions of platform on VIV responses of the riser are investigated. It is found that for large velocity of external flow, the axial displacement of riser results in an increase in IL excitation mode and also an increase in IL oscillating displacement. When the riser is excited by swing motion of platform, with the increase of excitation amplitude, the maximum response amplitudes will also increase in all the three directions, and the amplitudes in high frequency domain increase more obviously. Combined effect of heave and swing motions of the platform results in the increase of VIV amplitudes in IL and AX directions, and combined action of forced and parametric excitations has little effect on response amplitude of riser in CF direction.

Multi-Mode Motion Dynamics of a Top Tensioned Riser Excited by Vortices and Varying Tension

A coupled model of a top tensioned riser (TTR) under combined excitations from vortices and time-varying tension is proposed. And the van der Pol oscillator is introduced to simulate the time-varying characteristics of vortex shedding. The first three mode approximations as a six degree-of-freedom (DOF) system of vibrations of the fluid-structure interaction system are derived. Two cases of resonances are analyzed by using the numerical method. One case is on the vortex induced vibration (VIV) of the six DOF system, where the 1-to-1 internal resonance (also named “lock-in” phenomenon) between frequencies of the first modes of structure and the wake oscillator are investigated. Bifurcation curves between dynamic responses in terms of the shedding frequency are derived. Comparing to the vibrations of the first order mode, the “lock-in” phenomenon for vibrations of the higher order modes can occur in the wider parameter range with large values. The second case is on the combined resonance between the 1-to-1 internal and 1-to-2 parametric resonance. Response curves versus the shedding frequency under different amplitude ratios are calculated by the Runge–Kutta method. Effects of system parameters including the amplitude ratio of the parametric excitation, shedding frequency on the response curves are investigated. Comparing to the case when only considering the 1-to-1 internal resonance in the VIV, results of the combined resonance show those large-amplitude responses of the structure and wake oscillator can be induced. Responses will increase rapidly as the amplitude of the varying tension increases. Unstable responses can occur within a large frequency band of nondimensional shedding frequency. Furthermore, the dynamic characteristics of the responses of the six DOF system including the bifurcation diagrams, time histories, phase-plane projection and Poincaré sections are computed for different amplitude ratios. Results indicate that complex dynamics such as the quasi-periodic and chaotic solutions can occur for different amplitude ratios. The analysis can be helpful for the parameter design of deep-water risers.

Vortex-induced vibration response characteristics of catenary riser conveying two-phase internal flow

Vortex-induced vibration (VIV) of the riser considering both external flow and multiphase internal flow is a complex fluid-structure interaction problem. Due to its flexible feature, steel catenary riser (SCR) may be more affected by internal flow than top-tensioned riser (TTR). Firstly, the semi-empirical time domain VIV model of riser with internal flow is introduced, and the theories of gas-liquid flow and solid-liquid flow are applied to modify the model. Then the VIV of TTR and SCR with internal flow are compared. Subsequently, the VIV of SCR with gas-liquid flow and solid-liquid flow are calculated, and their different effects are discussed. The results show that internal flow triggers higher-order modes of VIV and enlarges the fatigue damage in general. With similar length, internal flow has more significant impact on SCR than TTR. The increase of liquid volumetric flow rate, solid production and gas volumetric quality enlarges the modal order and fatigue damage. The increase of solid volumetric quality and solid density reduces the modal order and fatigue damage. Compared with gas-liquid flow, solid-liquid flow may decrease the VIV amplitude and reduce the fatigue damage.

Numerical Simulation of Vortex-Induced Vibration of TTR and SCR

Top tensioned risers (TTRs) and steel catenary risers (SCRs) have been widely used in the field of marine engineering. They are commonly used to transport fluids from subsea wells to surface platforms. Under the action of ocean currents, these risers are often subjected to vortex-induced vibrations (VIVs), which might lead to serious fatigue damage. In this study, VIV around TTR and SCR were numerically simulated using the computational fluid dynamics software FLUENT when the Reynolds number was 4000. In the calculations, the full hexahedron grid and large eddy simulation were used to ensure calculation accuracy from the boundary conditions, as well as solution control. The shape, frequency, and amplitude of VIV produced by TTRs and SCRs at different times and depths were simulated.

An experimental investigation on vortex-induced motion (VIM) of a tension leg platform in irregular waves combined with a uniform flow

Vortex-induced motion (VIM) of a tension leg platform (TLP) is critical to avoid structural and fatigue damages to the tendons and risers. Many previous studies have conducted model tests for TLPs in towing tanks. However, the models were usually established using linear springs to replicate the underwater moorings. In this study, a TLP model with full-water-depth tendons and top tension risers (TTRs) was developed. A series of model tests were conducted in a large-scale wave tank at three incidence angles (θ=0°, 22.5°, and 45°) for the reduced velocity ranging from 4 to 11.5 and two extra cases of flow combined with irregular waves. The maximum transverse amplitude of 0.32D was observed at a reduced velocity of 7.0 when θ=0°. On the other hand, the in-line and yaw motion experienced increasing amplitudes with reduced velocities and no lock-in region was identified. The most significant responses of transverse and yaw motion appeared at 0° and 22.5° headings, respectively, while no critical heading was recognized for surge motion. The competing mechanics between VIM and the wave-induced motion were revealed in the cases of combined flows and irregular waves. The tensions in tendons and TTRs that result from VIM were found as significant as those induced by wave loads, which highlights the necessity of considering VIM in the design of TLP. Furthermore, the result showed that the VIM amplitudes were considerably suppressed under the nonlinear stiffness and larger damping provided by the tendons and TTRs.

The Instability and Response Studies of a Top-Tensioned Riser under Parametric Excitations Using the Differential Quadrature Method

The differential quadrature method (DQM) is a numerical technique widely applied in structure mechanics problems. In this work, a top-tensioned riser conveying fluid is considered. The governing equation of this riser under parametric excitations is deduced. Through Galerkin’s method, the partial differential governing equation with respect to time t and vertical coordinate z is reduced into a 1D differential equation with respect only to time. Moreover, the DQM is applied to discretize the governing equation to give solution schemes for the risers’ parametric vibration problem. Furthermore, the instability region of Mathieu equation is studied by both the DQM and the Floquet theory to verify the effectiveness of the DQM, and the solutions of both methods show good consistency. After that, the influences of some factors such as damping coefficient, internal flow velocity, and wet-weight coefficient on the parametric instability of a top-tensioned riser are discussed through investigating the instability regions solved by the DQM solution scheme. Hence, conclusions are obtained that the increase of damping coefficient will save the riser from parametric resonance while increasing internal flow velocity, or the wet-weight coefficient will deteriorate the parametric instability of the riser. Finally, the time-domain responses of several specific cases in both stable region and unstable region are presented.

Numerical investigation of Vortex-Induced Vibration response for a full-scale riser with staggered buoyancy modules

Vortex-Induced Vibration (VIV) caused by ocean current is one of the main sources of the long-term fatigue damage of marine riser. Buoyancy module is an essential equipment for deep-water riser system, which can reduce the top tension remand. Based on the hydrodynamic force decomposition model and forced vibration test data, a time domain prediction model of VIV response for top-tensioned riser with buoyancy modules is established in this paper. Firstly, the numerical model is verified with the experimental measurements of an 11.48 m riser with buoyancy modules under uniform flow. Then, a 610 m riser is chosen as the research object, and five staggered buoyancy module configurations with different coverage ratios are simulated, to reveal the VIV response characteristics of full-scale riser with buoyancy modules. Some interesting phenomena are reasonably explained with VIV mechanism analysis. In terms of frequency response, displacement and modal order, the influence of coverage ratio on riser's VIV is analyzed. Finally, the optimal coverage of buoyancy modules for VIV suppression is discussed, which is of great significance in engineering application.

A surrogate model for estimating uncertainty in marine riser fatigue damage resulting from vortex-induced vibration

Of interest in this work is the quantification of uncertainty in fatigue damage of a top-tensioned riser (TTR) resulting from vortex-induced vibration (VIV). Using a distributed wake oscillator model in time-domain simulations of a riser in uniform current, followed by rainflow cycle-counting, fatigue damage is computed. A polynomial chaos expansion (PCE) representation of damage is derived that treats three wake oscillator model variables as direct stochastic input: (i) motion amplitude; (ii) the ratio of vortex-shedding frequency to riser natural frequency; and (iii) current velocity. Damage at different locations along the selected riser is assessed. Damage predictions, obtained using the efficient PCE framework, are compared against the more computationally expensive alternative that involves Monte Carlo Simulation (MCS). This study suggests that PCE offers a tractable, versatile and accurate option for VIV-related fatigue damage prediction.

Fatigue damage assessment of offshore riser subjected to vortex-induced vibrations by SHEAR7

Marine risers are a critical component in transporting the produced hydrocarbons from the seabed to the topside facility and vice versa. These risers are subjected to VIV when encountered with ocean currents, leading to the accumulation of fatigue damage, which reduces the risers' service life. Estimation of fatigue damage is an important aspect to assure reliable and operable conditions. SHEAR7 is used in this analysis to estimate the fatigue damage of the Top Tensioned Riser (TTR). SHEAR7 is widely used semi-empirical software in the offshore industry to estimate the fatigue damage induced by VIV. Recently, SHEAR7 has been revised to account for the higher harmonic corrections based on experimental and field data. The key pertaining to the revision of adding higher harmonics can contribute 50% higher strain along the length of the riser at some locations (Jhingran and Vandiver, 2007). VIV fatigue damage analysis was conducted using SHEAR7 v4.7b and v4.9b to investigate the effect of the higher harmonics considered in the later version. The TTR model used in this analysis was modelled in OrcaFlex and validated with available references in the literature. TTR with different model scenarios was developed, and comparisons were made between the predicated responses from SHEAR7 v4.b and v4.9b for the riser. It was observed that higher harmonic correction factors have a great impact on the performance of the riser. Finally, sensitivity analyses were carried out to check the effect of individual parameters and higher harmonic correction factors on predicted responses between the two versions of SHEAR7. • The fatigue damage of the top tensioned riser subjected to vortex-induced vibrations was analyzed. • The effect of higher harmonic frequencies on the fatigue performance of the riser was numerically investigated. • The difference between the riser responses for with and without higher harmonic frequencies was statistically evaluated. • The parametric study was carried out to estimate the effect of individual design parameters on the riser performance.

Frequency Response Function and Design Parameter Effects of Hydro-Pneumatic Tensioner for Top-Tensioned Riser

The top-tensioned riser is an important equipment in offshore oil and gas development. The hydro-pneumatic tensioner is an essential device to ensure the safety of the top-tensioned riser. To investigate the dynamic performance of the marine platform hydro-pneumatic tensioner, this paper proposed a first-order Taylor approximation method and created the frequency response function of the hydro-pneumatic tensioner. According to the frequency response function, the hydro-pneumatic tensioner is a first-order spring-mass system. With the given parameters, the system stiffness coefficient is 66.1 kN/m, the natural annular frequency is 20.99 rad/s and the damping ratio is 2.23 × 10−4. The effects of the high-pressure accumulator, low-pressure accumulator, hydraulic cylinder and pipeline design parameters on the stiffness coefficient, natural annular frequency and damping ratio are analyzed. The stiffness coefficient can be increased by (1) increasing the high-pressure accumulator pressure and reducing the high-pressure accumulator volume; (2) increasing the pressure of the low-pressure accumulator and reducing the low-pressure accumulator volume; (3) increasing the piston diameter; and vice versa. The natural annular frequency can be increased by: (1) increasing the high-pressure accumulator pressure and reducing the high-pressure accumulator volume; (2) increasing the pressure of the low-pressure accumulator and reducing the low-pressure accumulator volume; (3) increasing the piston diameter; and vice versa. The damping ratio can be increased by increasing the pipeline length and reducing the pipeline inner diameter.