Catenary Risers Research Articles

Dynamic behaviors of conventional SCR and lazy-wave SCR for FPSOs in deepwater

The general and detailed structural/dynamic performances of steel catenary risers (SCRs) and lazy-wave steel catenary risers (LWSCRs) are directly compared under the same storm and floater (deepwater turret-moored FPSO) conditions. Their global performances are simulated by using the hull/mooring/riser fully-coupled dynamic analysis program developed by authors. This study proves that conventional SCRs connected to the FPSO hull exhibit significant dynamic responses and negative tensions, which may induce large structural stress amplification, local dynamic buckling, and serious fatigue damage at the members nearby TD (touch-down) region. The generation, propagation, and decay of the elastic waves generated by the temporal dynamic buckling are investigated in detail. On the other hand, LWSCRs can eliminate the local dynamic buckling for the same floater/storm condition and significantly reduce the stress level and fatigue damage near TDZ (touch-down zone) due to the motion isolation effect. It is because the modified shape with intermediate sag and arch portions can effectively isolate and absorb the downward riser motion. Therefore, LWSCRs can effectively be used for FPSO under very harsh environmental conditions. Despite the big advantage in dynamic characteristics, LWSCR is generally more expensive, more difficult to design, and may have slugging problem for internal flows.

The coupling VIV analysis of SCRs with rigid swing

With the development of deepwater oil and gas exploration, Steel Catenary Risers (SCRs) become preferred risers for resource production, import and export. Vortex induced vibration (VIV) is the key problem encountered in the design of SCRs. In this study, a new model, the rigid swing model, is proposed based on the consideration of large curvature of SCRs. The sag bend of SCRs is assumed as a rigid swing system around the axis from the hanging point to the touch down point (TDP) in the model. The torque, produced by the lift force and the swing vector, provides the driving torque for the swing system, and the weight of SCRs provides the restoring torque. The simulated response of rigid swing is coupled with bending vibration, and then the coupling VIV model of SCRs is studied in consideration of bending vibration and rigid motion. The calculated results indicate that the rigid swing has a magnitude equal to that of bending vibration, and the rigid motion affects the dynamic response of SCRs and can not be neglected in the VIV analysis.

파랑하중에 의한 Steel Catenary Riser 피로손상 평가 방법의 비교검토

The purpose of this study is to suggest guidelines for riser fatigue analysis in terms of selection of reasonable analysis method. Three analysis methods (spectral, regular wave, rain-flow counting) are introduced and compared. As the riser systems give non-linear response, the time-domain analysis method is more preferred than frequency-domain analysis method. The spectral fatigue analysis method, however, is still useful for identifying fatigue prone areas. Once stress RAO is established, fatigue damage can be calculated very quickly. The regular wave method and the rain-flow counting method are more time consuming but give more exact results compare to spectral method. In case of regular wave method, a set of regular waves which represent random sea states is considered for dynamic analysis. The rain-flow counting method is the most intuitive and exact method because it refers time history stresses containing most of non-linear effects of the riser system. However, it is not common for early design stage to use rain-flow counting method because of its high cost. In this study, it was confirmed that the regular wave method is the most cost effective way in specific cases. However, if the system is highly non-linear, it seems that the regular wave method gives less accurate results than rain-flow counting method. Therefore, it is imperative that the engineers select appropriate analysis method based on design stage and given engineering period. This paper also discusses the theoretical background of each calculation method and hydrodynamic aspects of marine riser systems. A steel catenary riser (SCR) line on FPSO was considered and marine dynamic program (OrcaFlex) was used for static and dynamic analysis.

Numerical Modeling of Suction and Trench Formation at the Touchdown Zone of Steel Catenary Riser

Steel catenary risers (SCR) are widely used in deepwater oil and gas production. The riser-seabed-water interaction near the touchdown zone is one of the main concerns in the design of fatigue life of SCR. During upward displacement, suction develops under the riser and a trench might be formed when it separates from the seabed near the touchdown point. In the subsequent downward movement, the riser penetrates through this trench to the seabed. Therefore, modeling of suction and trench formation is very important. In the existing models available in the literature for uplift resistance, these factors are incorporated using empirical relationships. It is also recognized that the available finite-element (FE) modeling techniques for this large-deformation problem are computationally very expensive, although penetration behavior can be simulated. In the present research program, both penetration and uplift behavior are simulated using FE and computational fluid dynamics (CFD) approaches. The simulation results for penetration are presented in Hawlader et al. (2014). In this paper, CFD simulations of uplift resistance, suction and trench formation using ANSYS CFX are discussed. A new model for undrained shear strength of soft clay is proposed that is applicable to a wide range of shear strain rates. The effects of strain rate and strength degradation are incorporated properly in ANSYS CFX and simulations are performed for one penetration-uplift cycle. Comparing with empirical models developed from experimental results and also with FE results for idealized conditions, it is shown that the present CFX model can simulate the suction and uplift resistance. Moreover, the CFX model developed in this study using the subdomain approach is computationally very efficient. The suction under the riser is the main source of uplift resistance for shallow embedments. The parametric study shows that the maximum uplift resistance and depth of trench depend on uplift velocity and the undrained shear strength of clay.

Analysis on J lay of SCR based on catenary and large deflection beam theory

This paper mainly focuses on J lay of Steel Catenary Riser, and proposes a new model using Sectional Mechanics Model, by iterating and composing catenary method and large deflection method. First the initial deformation and reaction force are evaluated by simple catenary model. For the pipe section with large deformation, asymptotic expansion in singular perturbation techniques pursues the solutions of second-order nonlinear governing equation. Comparison between these three methods shows that the new model displays very close results with high-accuracy large deflection model. This new model is also performed to evaluate the sensitivity of top tension, wet weight, and stiffness on the riser.

Finite memory quadratic Volterra model for the response prediction of a slender marine structure under a Morison load

A quadratic Volterra model with a finite nonlinear memory effect was introduced and applied to the time series prediction of a slender marine structure exposed to the Morison load. First, the unknown nonlinear single-input–single-output dynamic system was identified using the nonlinear autoregressive with exogenous input (NARX) technique based on the prepared datasets of the wave elevation and system response, which was obtained by running nonlinear time domain analysis for a certain short term sea state. The structure of NARX was designed in such a way that the linear part had infinite memory, whereas the nonlinear part had finite memory of a certain length. Second, the frequency domain Volterra kernels, both linear and quadratic, were derived analytically by applying the harmonic probing method to the identified system. To derive the frequency response functions, the sigmoidal function used in NARX to realize the nonlinear relationship between the input and output was expanded to polynomials based on the Taylor series expansion, so that the harmonics of same frequencies were easily matched between the input and output. Finally, the time series of the system response under arbitrarily given short term sea states were predicted using the quadratic Volterra series. The proposed methodology was used to predict the nonlinear dynamic response of a 2-dimentional free standing catenary riser exposed to a random ocean wave load, and the comparison between the prediction and simulation results was made on the probability distribution of the maximum excursion of riser top. The results show that the proposed methodology can successfully capture the nonlinear effects of the dynamic response of a slender marine structure induced by the quadratic term of the Morison formula.

Penetration of Steel Catenary Riser in Soft Clay Seabed: Finite-Element and Finite-Volume Methods

AbstractThe penetration of steel catenary risers and other cylindrical objects, such as offshore pipelines or T-bar penetrometers, in a soft clay seabed is of practical importance in deepwater oil and gas development. Finite-element (FE) analyses of these large-deformation problems are computationally very expensive. Water can also play a significant role through development of suction behind the riser. The main objective of the present study is to develop an advanced numerical modeling technique to simulate riser–seabed–water interaction near the touchdown zone. Keeping in mind two critical issues, namely the computational cost and modeling of suction, two different numerical modeling techniques are developed. In the first one, the computational fluid dynamics (CFD) approach is used. The CFD modeling is performed using ANSYS CFX 13.0 software. Among the three different types of CFX models developed in the present study, the subdomain modeling technique is found to be the most efficient. In the second num...

Approximation of the maximum dynamic stress range in steel catenary risers using artificial neural networks

Adequate assessment of fatigue damage in steel catenary risers (SCRs) is essential, and usually evaluated with time consuming numerical analyses. Simplified design strategies would improve the efficiency of the screening tasks in the early design stages.As part of on-going research aiming to define a simplified fatigue design procedure for SCRs in the touchdown zone (TDZ), the sensitivity of fatigue damage to various parameters is explored using a large database (>40,000 cases). An approximation of the maximum stress range in the TDZ is established using several artificial neural networks and predicts well the fatigue life of selected example SCRs.

Time domain approach for coupled cross-flow and in-line VIV induced fatigue damage of steel catenary riser at touchdown zone

Existing VIV prediction approaches for steel catenary riser (SCR) typically employ truncation model without considering the interaction between the SCR and soil, and only allow for cross-flow (CF) VIV. In this study, a time domain approach accounting for the SCR-soil interaction is proposed to predict the CF and in-line (IL) VIV induced fatigue damage of a SCR at touchdown zone (TDZ). The hydrodynamic force resulting from the vortex shedding is modeled using the forced oscillation test data of a rigid cylinder and an empirical damping model, which are defined as functions of the non-dimensional dominant frequency and amplitude of the SCR response. Due to the coupling effect, the IL VIV force is magnified based on the CF VIV amplitude. By combining a linear hysteretic interaction model with a trench shape model, some particular phenomena during the vertical SCR-soil interaction are captured and qualitatively discussed, while for the horizontal direction, the seabed is simplified as nonlinear spring model. Based on these models, parametric studies are conducted to broaden the understanding of the sensitivity of VIV induced fatigue damage to the seabed characteristic. The results indicate trench depth, vertical and lateral stiffness, and clay suction are significantly affect the VIV induced maximum fatigue damage at TDZ.

Fatigue performance of deepwater SCR under short-term VIV considering various S-N curves

In this study, a method for fatigue performance estimation of deepwater steel catenary riser (SCR) under short-term vortex-induced vibration was investigated for selected S-N curves. General tendency between S-N curve capacity and fatigue performance was analysed. SCRs are generally used to transport produced oil and gas or to export separated oil and gas, and are exposed to various environmental loads in terms of current, wave, wind and others. Current is closely related with VIV and it affects fatigue life of riser structures significantly. In this regards, the process of appropriate S-N curve selection was performed in the initial design stage based on the scale of fabrication-related initial imperfections such as welding, hot spot, crack, stress concentration factor, and others. To draw the general tendency, the effects of stress concentration factor (SCF), S-N curve type, current profile, and three different sizes of SCRs were considered, and the relationship between S-N curve capacity and short-term VIV fatigue performance of SCR was derived. In case of S-N curve selection, DNV (2012) guideline was adopted and four different current profiles of the Gulf of Mexico (normal condition and Hurricane condition) and Brazil (Amazon basin and Campos basin) were considered. The obtained results will be useful to select the S-N curve for deepwater SCRs and also to understand the relationship between S-N curve capacity and short-term VIV fatigue performance of deepwater SCRs.

Catenary riser sliding and rolling on seabed during induced lateral movement

Catenary risers have an interaction zone with the seabed, usually referenced as flowline. Movements in this region can be induced by sea currents and large offsets in floating unit, leading to touchdown position changes and affecting internal loads along riser length. In this work the contact flowline-seabed is modeled including sliding and rolling friction. Case studies involving large offsets in floating unit and lateral sea currents are solved to better understand the consequences of possible rolling and large sliding. The riser is modeled using a geometrically-exact finite element beam model. The contact is addressed with a new technique to include rotation movements from underlying beam models. This leads to global riser models including complex kinematics, being able to represent scenarios with alternating sliding/rolling and its consequences on internal loads of riser structure. A parametric study is performed to measure the influence of the friction coefficient in tension and torsion along typical flexible pipe and steel pipe catenary risers.

OrcaFlex를 이용한 심해 SCR 구조 해석

The design challenges when attempting to obtain sufficient strength for a deepwater steel catenary riser (SCR) include high stress near the hang-off location, an elevated beam-column buckling load due to the effective compression in the touchdown zone (TDZ), and increased stress and low-cycle fatigue damage in the TDZ. Therefore, a systematic strength analysis is required for the proper design of an SCR. However, deepwater SCR analysis is a new research area. Thus, the objective of this study was to develop an overall analysis procedure for a deepwater SCR. The structural behavior of a deepwater SCR under various enviro nmental loading conditions was investigated, and a sensitivity analysis was conducted with respect to various parameters such as the SCR weight, weight of the internal contents, hang-off angle (HOA), and vertical soil stiffness. Based on a deepwater SCR design example, it was foun d that the maximum stress of an SCR occurred at a hang-off location under parallel loading direction with respect to the riser plane, except for a wave dominant dynamic survival loading condition. Furthermore, the tensile stress governed the total stress of the SCRs, whereas the bending stress governed the total stress at the TDZ. The weight of the SCR and internal contents affected the maximum stress of the SCR more than the H OA and vertical soil stiffness, because the weight of the SCR, including the internal contents, was directly related to its tensile stress.

Synergetic analysis and possible control of vortex-induced vibrations in a fluid-conveying steel catenary riser

This work aimed to demonstrate possibilities for both active and passive control of the vortex-induced vibration and fatigue life of steel catenary risers via an analysis of the self-organization and evolution of the structural vibration based on synergetic theory. An analysis of the complex interrelated and synergistic relationship between the order parameter and the fast variable was performed, and the master equation of the nodal displacements was established as the order parameter for the evolution of the riser’s structural vibration. Passive control methods include modifying the structure’s elastic modulus, the internal fluid velocity, the top tension and the structural damping ratio, while an active control involves adjusting the external flow rate. Optimized parameters were obtained by analyzing the non-steady state solution of the master equation. The results show that the fatigue life greatly increases as the riser’s elastic modulus decreases. In contrast, the fatigue life decreases with an increase of the internal fluid velocity. With an increase of the top tension, the vibration amplitudes and the number of modes may decrease, resulting in fewer bending stress cycles and a longer fatigue life. Furthermore, the structural damping ratio should be as large as possible. Finally, an active and passive control of the riser structure’s response to vortex-induced vibration and its fatigue life can be achieved by carefully modifying the parameters mentioned above. The results may provide a theoretical framework for engineering practice concerning the design and control of steel catenary riser structures which are affected by vortex-induced vibration.

Dynamic Plastic Deformation of Deepwater Steel Catenary Risers Under Extreme Cyclic Compressive Loading

Steel catenary risers (SCRs) on a large-heave-motion vessel are susceptible to compression in the riser touchdown zone (TDZ). Dynamic compression can lead to overstress under extreme or abnormal weather conditions. The response of an SCR under compression is highly nonlinear and sensitive to various factors. However, the current available industry design codes and practices do not provide a clear guidance to address the acceptability of compression, overstress, and the resulting plastic strains. In addition, the current analysis method used in industry common practice cannot capture accurately the nonlinear behavior of an SCR involving accumulated plastic deformation, hysteresis effects, and local buckling. In this paper, a finite-element-analysis modeling method that uses combined beam and solid elements is presented. This method enables simulation of large plastic deformation, pipe ovality, and local pipe buckling in the TDZ of a deeppwater SCR. The model is developed with Abaqus (Dassault Systèmes 2009). The SCR nonlinear response is examined through dynamic analysis of a deepwater SCR that is hung from a semisubmersible. The key analysis results are compared with a nonlinear beam-element model. Moreover, dynamic-ratcheting analysis under multiple plastic-strain cycles by use of the proposed solid-riser model is conducted to understand the plastic-strain accumulation and to check the acceptability of the survival response of a deepwater SCR under a series of sever hurricanes in its service life. This paper presents the methodology for evaluating the compression and plastic deformation that could be experienced by deepwater SCRs, including the modeling approach, analysis results, possible failure modes, and conclusions. The impact of the study findings on the robustness and suitability of SCRs for deepwater application is discussed.

Sensitivity studies of SCR fatigue damage in the touchdown zone using an efficient simplified framework for stress range evaluation

Steel catenary risers (SCRs) are widely used in deep water. Several sources of nonlinearities make SCR fatigue design challenging. Limited understanding of the influence of the various input parameters on the structural response of SCRs leads to unnecessarily high conservatism in design. Also, time consuming numerical simulations are usually performed to assess SCR fatigue damage which is inefficient, especially for early design stages.A simplified framework for fatigue analysis of SCRs in the touchdown zone (TDZ) has been developed previously, using artificial neural networks. The approach may be used to efficiently estimate maximum static and dynamic stress ranges in the TDZ, from which the fatigue damage can be deduced. Comparison of the maximum static and dynamic stress changes for a given input motion allows quantification of the dynamic amplification factor (DAF). This paper explores the sensitivity of the maximum dynamic stress ranges and DAF to the key dimensionless groups of input parameters and also certain individual input parameters. The study illustrates the usefulness of the proposed framework in understanding SCR behaviour in the TDZ, providing guidance on optimisation of SCR design from a fatigue perspective. The paper also reflects on the potential benefits of using DAFs for SCR fatigue design.

Fatigue analysis of steel catenary risers based on a plasticity model

The most critical issue in the steel catenary riser design is to evaluate the fatigue damage in the touchdown zone accurately. Appropriate modeling of the riser-soil resistance in the touchdown zone can lead to significant cost reduction by optimizing design. This paper presents a plasticity model that can be applied to numerically simulate riser-soil interaction and evaluate dynamic responses and the fatigue damage of a steel catenary riser in the touchdown zone. Utilizing the model, numerous riser-soil elements are attached to the steel catenary riser finite elements, in which each simulates local foundation restraint along the riser touchdown zone. The riser-soil interaction plasticity model accounts for the behavior within an allowable combined loading surface. The model will be represented in this paper, allowing simple numerical implementation. More importantly, it can be incorporated within the structural analysis of a steel catenary riser with the finite element method. The applicability of the model is interpreted theoretically and the results are shown through application to an offshore 8.625″ steel catenary riser example. The fatigue analysis results of the liner elastic riser-soil model are also shown. According to the comparison results of the two models, the fatigue life analysis results of the plasticity framework are reasonable and the horizontal effects of the riser-soil interaction can be included.

A new method for strake configuration design of Steel Catenary Risers

A number of studies of vortex-induced vibration (VIV) having important effect on the fatigue performance of riser have performed analytically as well as experimentally in the last decade. However, there are few research results on the configuration design of VIV suppression devices such as fairings, strakes, and etc. Therefore, a systematic design procedure of strake configuration (i.e., location and length) using the modal approach for steel catenary riser (SCR) is proposed in this study. The proposed method considers the dominant mode for VIV suppression through modal analysis of bare riser to find appropriate location and various design parameters are introduced to calculate appropriate length of VIV suppression devices. From the numerical example of SCR, it is shown that the required strake length from the optimal location for VIV suppression of SCR is much shorter than that of the conventional method. This is achieved by staggered strake installation at the effective location. Furthermore, no additional calculations or engineering cost are required for the proposed method, because the proposed method follows exact same analysis procedure of the conventional one except using the modal information of bare SCR obtained from initial analysis of bare SCR.

Riser-soil interaction model effects on the dynamic behavior of a steel catenary riser

A mathematical model employed to analyze the global dynamics of a Steel Catenary Riser (SCR) taking into account the interaction with the seafloor and the effect of the soil reaction forces is established. The choice of soil model plays an important role for the dynamic behavior of SCRs. The riser has been modeled using flexible beam with large curvature and elastic foundation beam to describe the riser-soil interaction by means of realistic nonlinear load-deflection (P–y) curves. The study is made to improve an existing finite element numerical code for dynamic analysis of mooring lines and risers, known as CABLE 3D, which is based on a slender rod assumption. Effects of nonlinear seabed model on the dynamic behavior of SCRs under vessel cyclic perturbation have further been investigated and discussed using a realistic P–y curve to simulate soil deformation and resistance forces. The interaction model depends on several factors, such as soil strength, penetration depth and riser characteristics. The dynamic responses of the riser Touchdown Point (TDP) excited by vessel periodic heave motion are studied and the results are compared with those from the linear spring model. SCR has been perturbed by 10 regular sinusoidal cycles and the responses calculated by improved code show a number of features such as suction force mobilization, gradual increasing penetration depth, and gradual reduction of soil resistance at maximum penetration. The riser behavior at the touchdown zone (TDZ) depends on the riser top motion amplitude, nonlinear soil stiffness and suction force. The impact of the riser-soil interaction model on the dynamic behavior in the TDZ has been thoroughly studied in this paper.

A nonlinear model for deepwater steel lazy-wave riser configuration with ocean current and internal flow

Steel lazy-wave riser (SLWR) has gained a viable solution as a kind of advanced steel catenary riser (SCR) to meet the increasing challenges of simple catenary riser on large hang-off tension and fatigue damage of touchdown point (TDP) in deepwater applications. The configuration and static performance of SLWR are essential during its design stage and varies a lot when subjected to loading from ocean current and internal flow. The governing equations which consist of conventional small deformation beam theory for the portion of pipeline lying on the seabed, coupled with a large deformation beam theory for the suspended section are established. The proposed model with appropriate boundary conditions can simulate the nonlinear mechanical behavior of SLWR with the effect of pipe–soil interaction, ocean current and internal flow. The finite difference method is adopted to obtain the numerical results, and comprehensive parametric analysis is carried out to investigate the influences of ocean current and internal flow on the riser’s static performance. The proposed model can be an important reference for the design and analysis of deepwater SLWR.

Critical top tension for static equilibrium configuration of a steel catenary riser

This paper aims to present the critical top tension for static equilibrium configurations of a steel catenary riser (SCR) by using the finite element method. The critical top tension is the minimum top tension that can maintain the equilibrium of the SCR. If the top tension is smaller than the critical value, the equilibrium of the SCR does not exist. If the top tension is larger than the critical value, there are two possible equilibrium configurations. These two configurations exhibit the nonlinear large displacement. The configuration with the smaller displacement is stable, while the one with larger displacement is unstable. The numerical results show that the increases in the riser’s vertical distances, horizontal offsets, riser’s weights, internal flow velocities, and current velocities increase the critical top tensions of the SCR. In addition, the parametric studies are also performed in order to investigate the limit states for the analysis and design of the SCR.