Top Tensioned Research Articles

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.

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.

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.

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.

Vibration Control of Marine Top Tensioned Riser with a Single Tuned Mass Damper

The study of the Tuned Mass Damper (TMD) on Top Tensioned Risers (TTRs) through the application of numerical analysis is of great significance for marine engineering. However, to the best knowledge of the author, neither the in-field riser data nor the ocean current data used in published papers were from engineering design, so the research results provide limited guidance to the actual engineering project. In view of this problem, this study designed a single TMD to suppress the vibration of the engineering TTR under the action of the actual ocean current. First, the dynamic model of a riser-TMD system was established, and the modal superposition method was used to calculate the model. The non-resonant modal method of the flexible structure was used to design the TMD parameters for the engineering riser. Ocean current loading in the South China Sea was then applied to the riser. The vibration of the riser without and with TMD was compared. The result showed that TMD could effectively reduce the vibration response of the riser. When compared without TMD, the maximum value of displacement envelope and the RMS displacement were reduced by 26.70% and 17.83% in the in-line direction, respectively. Moreover, compared to without TMD, the maximum value of displacement envelope and RMS displacement were decreased by 17.01% and 22.05% in the cross-flow direction, respectively. In the in-line direction, the installation position of TMD on the riser was not sensitive to the effect of the displacement response; meanwhile, in the cross-flow direction the installation position of TMD on the riser was more sensitive to the effect of the displacement response.

Vortex-induced vibration prediction of a flexible cylinder by three-dimensional strip model

Numerical simulations on vortex-induced vibration (VIV) of a top tensioned flexible cylinder experiencing the uniform flow and the stepped flow conditions have been carried out by our in-house solver viv3D-FOAM-SJTU in this paper, which is developed by using the open source toolbox OpenFOAM and the three-dimensional strip model. Hydrodynamic forces in each three-dimensional fluid strip are calculated through the PIMPLE algorithm in the OpenFOAM adopting the Reynolds-averaged Navier-Stokes (RANS) method. The flexible cylinder is simplified as an Euler-Bernoulli bending beam, while structural vibrations are computed using the finite element method (FEM) in both the crossflow and the inline directions. The connection between three-dimensional fluid strips and the long flexible cylinder is realized through the interpolation of hydrodynamic forces and vibration displacements in each time step. Detailed comparisons on vibration Root Mean Square (RMS) amplitude, vibration frequency and vibration modes in both flow conditions among present simulation results, experimental results and published data are presented the validity of the viv3D-FOAM-SJTU solver. The fluid strip thickness dependence study shows that three-dimensional features of the wake field begins to appear at specific strip with dramatic vibration responses when the strip thickness reaches around 1/100 L (where L is the axial length of the flexible cylinder). The flow field of all fluid strips will be fully developed when the strip thickness reaches 3/100 L. Considering the cost of computational resources and the computational efficiency, the strip thickness around 1/50 L is recommend for future researches.

Static analysis of interaction between two adjacent top tensioned risers with consideration of wake effects

Collision between adjacent risers has become an important issue as the oil and gas industry moves to deeper waters. In order to estimate the clearance between two marine risers, a static analysis which is mainly concerned with the wake effects is performed in this paper. A new wake model, which is used to predict the wake flow around the downstream riser, is developed based on Prandtl's shearing stress hypothesis. The wake effects with respect to riser interference in uniform current flow are then investigated. In this work, the two risers are both simplified as circular cylinders top-tensioned and pinned at the bottom. A procedure for iteratively predicting the wake velocity distribution and estimating the clearance between two risers is introduced to find the final convergent result by combining the new wake model with the global riser analysis software Riflex. The effects of different factors like riser spacing, top tension force and current velocity are also studied. The results indicate that these factors significantly influence the riser interference. Since the current velocity cannot in general be controlled for a specific site, the riser spacing and the top tension force become the primary design parameters that can be chosen by the operator.

A simplified method to predict fatigue damage of TTR subjected to short-term VIV using artificial neural network

Marine riser is the critical component transporting hydrocarbon and fluid from well to the platform and vice versa. Riser experiences vortex-induced vibration (VIV) caused by current, leading to fatigue damage. Estimation of VIV fatigue damage is essential in designing feasible and operable riser. A simplified approach for predicting fatigue damage is required to reduce the computation time to analyse the fatigue damage. This study aims to propose a simplified approach to predict VIV fatigue damage of top tensioned riser (TTR) using artificial neural network (ANN). A total of 21,532 riser model was generated with different combination of six main input parameters: riser outer diameter, wall thickness, top tension, water depth, surface and bottom current velocity. The modal analysis was performed using OrcaFlex and VIV fatigue damage of the riser was computed using SHEAR7. The six input parameters and corresponding fatigue damage results made up the database for training a 2-layer neural network. Weight and bias values acquired from the training of ANN were used to develop the VIV fatigue damage prediction model of the riser. The hyperparameters of the ANN model were tuned to optimize performance of the model. The results showed the final ANN model predict fatigue damage well with shorter time compared to conventional semi-empirical method. Hence, the proposed approach is suitable to be used for prediction of VIV fatigue damage of TTR at early design stage of TTR.

Modelling and Control of Flow Induced Vibration of Top–Tensioned Marine Risers Using Analytical Methods

For a riser array in deep waters, interference between individual risers in strong ocean current is of operational concern and thereby a key design issue. The lateral deflection is likely to be large, and the risers may experience collision with fatigue or coating damage as a consequence. In this paper, active control of flexible marine riser angle and the reduction of flow induced (forced) vibration under a time varying distributed load were considered using boundary control approach. A torque actuator was introduced in the upper riser package and a boundary control was designed to generate the required signal for riser angle control and vibration reduction with guaranteed closed-loop stability. The design is based on the partial differential equations of the system, which are developed using energy principle. Analytical method of solution was deployed with the aid of a program, developed within the framework of MATLAB, to predict the riser's behaviour by top tensioning. A sensitivity analysis for different values of the control variables was carried out. The results of this work showed that active control of flexible marine riser by top-tensioning reduced flow induced vibration.

Characteristic Analysis of VIV-Induced Fatigue Damage of Top Tensioned Risers Based on Simplified Model

In this study, a simplified empirical model for fatigue analysis of top-tensioned risers due to vortex-induced vibration (VIV) in a nonuniform current is presented. Compared with previous studies on this subject, this model is able to take into account the main intrinsic nature of VIV for low mass ratio structures in lock-in regions, added mass, and nonlinear fluid damping. Based on the simplified model, a closed-form solution of fatigue damage is suggested for the case of a riser with uniform mass and cross-section oscillating in a uniform flow. Fatigue damage characteristics of the top-tensioned risers due to VIV are investigated to determine the effects of various ocean conditions and riser dimensions on fatigue damage.

Stress analysis of top tensioned riser under random waves and vessel motions

The bending stresses of top tensioned riser (TTR) under combined excitations of currents, random waves and vessel motions are presented in this paper, and the effect of the internal flowing fluid on the riser stresses is also considered. The computation programs which are used to solve the differential equations in the time domain are compiled and the principal factors of concern including the angular movements at the upper and lower ends of the riser, lateral displacements and bending stresses are presented. Then the effects of current velocity, random wave, top tension, vessel mean offset, low frequency motion and internal flow velocity on the bending stresses of the riser are analyzed in detail.

Fatigue life assessment of top tensioned risers under vortex-induced vibrations

The fatigue life of top tensioned risers under vortex-induced vibrations (VIVs) with consideration of the effect of internal flowing fluid on the riser is analyzed in the time domain. The long-term stress histories of the riser under VIVs are calculated and the mean stresses, the number of stress cycles and amplitudes are determined by the rainflow counting method. The Palmgren-Miner rule for cumulative damage theory with a specified S-N curve is used to estimate the fatigue life of the riser. The corresponding numerical programs numerical simulation of vortex-induced vibrations (NSVIV) which can be used to calculate the VIV response and fatigue life of the riser are compiled. Finally the influences of the riser’s parameters such as flexural rigidity, top tension and internal flow velocity on the fatigue life of the riser are analyzed in detail and some conclusions are drawn.

Closed-Form Solutions for Vibrations of Nearly Vertical Strings and Beams by Means of Asymptotic Methods

Top tensioned strings and beams are often used in civil and marine applications. Typically these members have constant cross sections, and a pronounced, usually linear, tension variation, due to the effects of gravity. In this paper simple, approximate formulas for the natural frequency of such strings are derived, based on asymptotic techniques, while for the tensioned beam case approximate closed-form results are developed by the Wentzel–Kramers–Brillouin method. Both derivations are shown in reasonable detail. While similar work is known for a beam with varying axial tension this is believed to be the first time that a single analytic expression is developed for the full length of the beam. A simple example in which the bottom tension is only 9% of the top tension is analyzed for cases with and without bending stiffness, and the solutions have been compared to the exact solution for the string case and to the results from three finite-element programs for the beam case. The accuracy was found to be ver...

FEM modelling and automatic control for collision prevention of top tensioned risers

For a riser array in deep waters interference between individual risers in strong ocean current is of operational concern and thereby a key design issue. The lateral deflections are likely to be large, and the risers may experience collision with fatigue or coating damage as a consequence. In this paper a system consisting of a tension leg platform (TLP), a pair of risers, environmental forces and hydrodynamic interaction is mathematically modelled. The risers are modelled by using finite element method (FEM). Case studies show how the introduction of a PI-controller for the top tension affects the horizontal displacement, and hence may prevent collision between the risers. The most effective control objective used in the controller design was the equal effective length, meaning that the sum of the payout and the length of the tensioned riser should be equal for both risers. Introducing this method may reduce the needed spacing between the risers and thereby the size of the well bay.

Overview: Deepwater Exploration and Production (June 2005)

In recent years, the industry has met some amazing deepwater technology challenges. Drilling and production in water depths that just a few years ago would have seemed impossible now are considered routine. Not long ago, a site in water depths greater than 150 m was considered deep, and the Cognac Project—at more than 300 m—was considered a remarkable feat of technology. Today, the feasibility of a project in 2000-m water depth is not an issue. Instead, project efforts focus on topics such as design robustness, effective project-execution strategies, reliability of operations, and cost-effectiveness. It is a tribute to technical innovation that, for most deepwater projects in less than 2000 m of water, several different acceptable methods often are available to solve specific technical challenges. Operators, designers, constructors, and installers continue to improve deepwater technologies and reduce cost and schedule risks. Areas such as well testing, reservoir evaluation with minimal data, riser systems, and flow assurance are being addressed to improve project execution and delivery. Meanwhile, new projects are benefiting from the application of lessons learned from previous efforts. Recent work has achieved promising results in special projects for which water depths reach 3000 m. Today’s challenge is to make the issues at that depth as commonplace as those in less that 2000 m. From a historical perspective, it is truly startling that work on these challenges is proceeding in an almost routine manner, with complete confidence that viable solutions will be found. Additional Deepwater Exploration and Production Technical Papers Available at the SPE eLibrary: www.spe.org SPE 92616 Mooring and Riser Management in Ultradeep Water and Beyond SPE 91012 Incorporating Uncertainties in Well-Count Optimization With Experimental Design for the Deepwater Agbami Field Available at the OTC Library: www.otcnet.org OTC 17292 Holstein Truss Spar and Top Tensioned Riser System: Design Challenges and Innovations OTC 17683 Lessons Learned From the Evolution and Development of Multiple-Line Hybrid Riser Towers for Deepwater Production Applications

Reliable Riser Systems for Spars

The Spar concept as a floating vessel is now recognized as a reliable and economic option for developing deepwater offshore hydrocarbon reservoirs. Many of the advantages of the Spar concept are attributed to the variety of riser systems that can be accommodated. This paper presents a discussion of design considerations for the top tensioned and steel catenary risers deployed from a Spar offshore production vessel. Results from extreme event strength analysis, Spar motion and current-induced fatigue analysis, and a riser-to-riser wake interference analysis are presented. Steel catenary risers are presented, including a comparison of results for risers supported by a Spar and a Semi. These analyses demonstrate that simple and reliable riser systems can be designed for use with the Spar platform concept.