Touchdown Zone Research Articles

Dynamic characteristics of pipe-soil interaction for steel catenary riser in touchdown zone

Steel catenary riser (SCR) is one of the most popular and economic risers in the development of deep-water oil and gas field. As a difficulty in the SCR design, the pipe-soil interaction at the touchdown zone is affected by the soil strength, riser diameter, upper floating body, and other factors. Conventionally, the pipe-soil interaction was mainly studied based on the static response of the riser under a linear seabed which was inconsistent with the actual situation. Therefore, a dynamic characteristic model was built in the paper for more realistic simulation of the pipe-soil interaction. And experiments were conducted to verify the reliability of the numerical model. Based on the verified model, the dynamic process of the nonlinear time-domain was analyzed. Different parameters, such as the soil strength and dynamic loads, were changed to analyze their effects on the pipe-soil interaction in the whole process which provided a certain foundation for the SCR design.

Influences of pipe–soil interaction on dynamic behaviour of deepwater S-lay pipeline under random sea states

ABSTRACTThe pipe–soil interactions have an important influence on the dynamic behaviour of offshore pipelines in the touchdown zone (TDZ) during deepwater S-lay. This paper mainly aims to reveal the dynamic behaviour of the pipeline in the TDZ, and its actual dynamic lay effects. A full finite element model for deepwater S-lay systems is primarily developed to simulate the dynamic responses of offshore pipeline from the pipelay vessel via the stinger to the seabed, in which pipelay vessel motions, pipeline hydrodynamics, clashing contacts between the pipeline and the stinger rollers and pipe–seabed interactions are considered particularly. The numerical analysis of an illustrative case for a gas export pipeline being installed to a water depth of 1500 m in the South China Sea is carried out in the time domain using the commercial computer code OrcaFlex. The influences of seabed soil models, soil characteristics, submerged self-weight of the pipe, surface waves, and pipelay vessel motions on the dynamic behaviour of the pipeline in the TDZ have been quantitatively studied in view of non-linear hysteretic soil model. The results indicate that the soil characteristics and the pipelay vessel motions induced by the surface wave have great influence on the bending moment and embedment of the pipeline, and the seabed resistance.

Significance of seabed interaction on fatigue assessment of steel catenary risers in the touchdown zone

The challenges involved with fatigue damage assessment of steel catenary riser (SCR) in the touchdown zone (TDZ) are primarily due to the non-linear behaviour of the SCR-seabed interaction, considerable uncertainty in SCR-seabed interaction modelling and geotechnical parameters. The issue of fatigue damage induced by the cyclic movements of the SCR with the seabed has acquired prominence with the touch down point (TDP) interaction in the TDZ. Therefore, the SCR-seabed response is critical for reliable estimation of fatigue life in the TDZ. Various design approaches pertaining to the lateral pipe-soil resistance model are discussed. These techniques have been applied in the finite element model that can be used to analyse the lateral SCR-seabed interaction under hydrodynamic loading. This study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. In this study, global analyses are performed to assess the influence of vertical linear seabed springs, the lateral seabed model and the non-linear seabed model, including trench evolution into seabed, seabed normalised stiffness, re-penetration offset parameter and soil suction resistance ratio, on the fatigue life of SCRs in the TDZ.

Experimental and numerical investigation of vertical pipe–soil interaction considering pipe velocity

The repeated motion of steel catenary risers (SCR) at the touchdown zone is one of the key factors considered in SCR design. Soil behaviours under different pipe velocities further complicate the accurate assessment of pipe–soil interactions. In the present research, a vertical pipe–soil interaction model test system was developed in this research study to investigate the effect of pipe velocity. The experimental results under soft clay condition indicated a close relationship between soil behaviour and pipe velocity, and the increase of cyclic number was also attributed to the progressive softening and trenching effect of soil until relative stabilisation. A new backbone curve model considering pipe velocity was obtained based on the validated coupled Eulerian–Lagrangianmethod, and the accuracy of the model was verified by the experiment results.

A simple parametric formulation for the seabed trench profile beneath a steel catenary riser

Seabed trench has a profound influence on the fatigue performance of a steel catenary riser (SCR) at the touchdown zone. At present, the most well-regarded approach for simulating the complex trench development process is by applying a nonlinear hysteresis seabed contact model, which is time consuming. Field observations have indicated that the trench depth almost stabilizes after a few months following installation. Hence, for practical fatigue design, it is expedient to specify an initial static trench profile to perform the dynamic simulations. This paper presents a new simple parametric formulation for delineating an initial trench profile, as there appears to be no such approach in the literature. The formulation entails two unknown trench parameters (trench length and global trench position), which can be determined using a new iterative static analysis method proposed herein. However, the analysis involves solving a constrained optimization problem, and is not ideal for practical applications. Thus, a surrogate model is devised, by approximating the trench parameters as multivariate polynomial functions of three dimensionless variables of the SCR. A case study comparing the trenches obtained from seabed contact model, static analysis, and surrogate model, shows that the different trench profiles and the associated maximum fatigue damage are in close agreement.

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.

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.

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.

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.

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.

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.

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.

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.

Local dynamic buckling of FPSO steel catenary riser by coupled time-domain simulations

Steel catenary riser (SCR) is a popular/economical solution for the oil/gas production in deep and ultra-deep water. The behavioral characteristics of SCR have a high correlation with the motion of floating production facility at its survival and operational environments. When large motions of surface floaters occur, such as FPSO in 100-yr storm case, they can cause unacceptable negative tension on SCR near TDZ (touch down zone) and the corresponding elastic deflection can be large due to local dynamic buckling. The generation, propagation, and decay of the elastic wave are also affected by SCR and seabed soil interaction effects. The temporary local dynamic buckling vanishes with the recovery of tension on SCR with the upheaval motion of surface floater. Unlike larger-scale, an-order-of-magnitude longer period global buckling driven by heat and pressure variations in subsea pipelines, the sub-critical local dynamic buckling of SCR is motion-driven and short cycled, which, however, can lead to permanent structural damage when the resulting stress is greatly amplified beyond the elastic limit. The phenomenon is extensively investigated in this paper by using the vessel-mooring-riser coupled dynamic analysis program. It is found that the moment of large downward heave motion at the farthest-horizontal-offset position is the most dangerous for the local dynamic buckling.

Artificial neural network development for stress analysis of steel catenary risers: Sensitivity study and approximation of static stress range

Fatigue design of steel catenary risers (SCRs) is an important challenge especially in the touchdown zone (TDZ). Numerous parameters affect the fatigue damage in the TDZ, including those pertaining to riser motions, riser characteristics and soil properties. So far, only a few sensitivity studies have been published with limited applications, considering small ranges, investigating only a selection of input parameters or failing to examine the interactions between input parameters.This paper aims to test the robustness of previous research and extend the ranges of the input parameters for SCR systems under static loading, by means of numerical simulations. An approximation of the critical stress range in the TDZ defined by the authors previously was refined to assist the sensitivity studies. A large database was created using an automation subroutine coded in Python programming language that links the marine analysis software OrcaFlex and the optimisation software modeFRONTIER. Design of experiment techniques were used for post-processing and quantify the relative effects of the various dimensionless groups and their interactions. An approximation using a series of artificial neural networks is presented; it successfully approximates over 99% of the cases of the database with an accuracy of ±5%.

Trenching effects on structural safety assessment of integrated riser/semisubmersible in cohesive soil

This paper models an integrated riser/semisubmersible system and subjected to irregular waves. The riser’s pipe is suspended from a semisubmersible and smoothly extending down to the cohesive seabed soils at the touchdown point in a catenary shape. Pipe–soil interaction is modelled using a hysteretic non-linear model in the vertical seabed direction and Coulomb friction model in the lateral direction together with an improved model that includes the breakout soil resistance. Initially, this study discusses the significance of pipe–seabed interaction on the riser response for deepwater applications when subjected to random waves on cohesive clay. In the next step, this study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. The influence of the uncertainty in the geotechnical parameters and the development of the trench in the seabed on the dynamic response are determined. The structural behaviour and the uncertainty of fatigue performance in the touchdown zone are presented. It is noted that the confidence in seabed interaction modelling and geotechnical parameter values is needed in order to have structural safety assurance in the final numerical analysis results.

Analytical estimation of static stress range in oscillating steel catenary risers at touchdown areas and its application with dynamic amplification factors

Steel catenary risers (SCRs) are dynamically sensitive structures and their fatigue design in the touchdown zone is challenging. The dynamic response of SCRs is traditionally assessed by performing a series of long time history analyses but a simplifying approach has recently been proposed. The simple method is based on the use of dynamic amplification factors that quantify the dynamic response for a given perturbation at the hang-off point relative to the static response. The determination of the static response of SCRs is therefore a prerequisite to this approach. In this paper, an existing analytical model is extended to accommodate the displacement at the hang-off point of the SCR and predict the static stress range. The results of this analytical model are validated against numerical simulations. Then, using this simple and efficient analytical model, various sensitivity analyses are performed to explore the impact of key dimensionless groups on the static stress range in the touchdown zone.

Enhanced multi-layer fatigue-analysis approach for unbonded flexible risers

This paper proposes an enhanced approach for evaluating the fatigue life of each metallic layer of unbonded flexible risers. Owing to the complex structure of unbonded flexible risers and the nonlinearity of the system, particularly in the critical touchdown zone, the traditional method is insufficient for accurately evaluating the fatigue life of these risers. The main challenge lies in the transposition from global to local analyses, which is a key stage for the fatigue analysis of flexible pipes owing to their complex structure. The new enhanced approach derives a multi-layer stress-decomposition method to meet this challenge. In this study, a numerical model validated experimentally is used to demonstrate the accuracy of the stress-decomposition method. And a numerical case is studied to validate the proposed approach. The results demonstrate that the multi-layer stress-decomposition method is accurate, and the fatigue lives of the metallic layers predicted by the enhanced multi-layer analysis approach are rational. The proposed fatigue-analysis approach provides a practical and reasonable method for predicting fatigue life in the design of unbonded flexible risers.