Deepwater Steel Catenary Riser Research Articles

Test System Development and Experimental Study on the Fatigue of a Full-Scale Steel Catenary Riser

This paper presents a full-scale deep-water steel catenary riser fatigue test system. The proposed system can carry out fatigue tests on steel catenary risers, hoses, and subsea pipelines up to 21 m in length, ranging from 8 to 24 inches in diameter. The test system was realized by mechanical loading with loading control systems, and could carry out axial tension and compression, bending moment, torsion, and internal pressure to simulate all load types on deep-water steel catenary risers or subsea pipelines. The counterforce was sustained by a counterforce frame. Through mechanical simulation analysis, the authors determined the size of the counterforce frame and designed the connection form of the counterforce frame and loading system. According to the required loading capacity, the appropriate cylinder thickness and diameter were obtained through calculation. After the design and construction of the test system, the authors designed a fatigue test to confirm the loading capacity and accuracy of the test system. The authors performed full-scale testing to assess the fatigue performance of pipe-to-pipe mainline 5G girth welds fabricated to BS 7608. This test was designed according to the stress level of pipelines in the Lingshui 17-2 gas field, and the test results were compared with the calculation results of the S–N curve.

Influence of Boundary Conditions and Internal Fluid Types on Dynamic Characteristics of a Deepwater Riser

Deepwater Steel Catenary Riser (DSCR) is the key structure of oil and gas production system in the ocean. To design an optimized DSCR system, the dynamic characteristics of the DSCR are analyzed by setting some different boundary condi-tions and internal fluid types. The main vibration mode of the DSCR is found orthogonal to both mass and stiffness according to the normalization condition of the main mode, and then the regular mode function and the jth order principal stiffness Kpj are obtained. Under different initial conditions, the transverse responses of DSCR represented by regular coordinates under arbitrary excitation are obtained. Then, a dynamic model in-duced by both axial load and transverse load is reconstructed for comparative study, and the axial load is found to have a great effect on the transverse dynamic characteristics. The effects of internal fluid and boundary conditions on the natural frequency of DSCR are also found at last.

A dynamic model of deep water riser and its global response characteristics induced by ocean internal wave and floating platform

ABSTRACT Because the flow direction of the water above and below the interface of internal wave is opposite, internal wave has a strong shear effect on ocean structures. A dynamic model of internal wave acting on deep water steel catenary riser (DSRC) is built. The global dynamic responses of gas riser and oil riser under the same action of internal wave are both obtained through numerical calculation, and the dynamic response laws of some specified positions are also studied. Then, the maximum value of dynamic response in a certain period of time and its location on the riser is found. At last, the response characteristic of DSCR induced by internal wave is studied in detail when the heave amplitude of floating platform is constant, but the heave frequency changes continuously as well as that the heave frequency is constant, but the heave amplitude changes continuously by some dynamic model experiments.

Nonlinear analysis for ship-generated waves interaction with mooring line/riser systems

A time-domain hybrid finite element-boundary element (FE-BE) method is developed to investigate nonlinear interaction between ship waves and slender structures. The ship waves are modeled in the context of the three-dimensional potential theory including ship steady state motion, and the velocity potential are computed using a higher-order boundary element method (HOBEM). A special global coordinate-based FEM rod model, when using catenary solution as an initial static profile, is employed in the accurate analysis of slender structures. The internal flow is incorporated in the numerical model by adding a new term to the effective tension formulation while both the seabed support and friction effect are performed by using a nonlinear quadratic spring and a Coulomb friction model. The numerical model is verified with the published experimental and numerical results for the ship waves generation and dynamic responses of mooring line/riser. Numerical simulations are executed to determine the dynamic behavior of two example slender structures such as multi-component mooring line and deepwater steel catenary riser under ship waves. Then the coupled analysis program is applied for a truss Spar with its mooring lines subjected to ship waves, wind-generated waves and their combined action, respectively. The dependence of the top node tensions of the slender structures on ship path, ship speed, ship distance, internal flow and seabed friction, is also examined.

Fatigue performance of deepwater steel catenary riser considering nonlinear soil

The touch down zone (TDZ) and top connection point of the vessel are most critical part of fatigue damage in the steel catenary riser (SCR). In general, the linear soil model has been used to evaluate fatigue performance of SCRs because it gives conservative results in the TDZ. However, the conservative linear soil model shows the limitation to accommodate real behavior in the TDZ as water depth is increased. Therefore, the riser behavior on soft clay seabed is investigated using a nonlinear soil model through time domain approach in this study. The numerical analysis considering various important parameters of the nonlinear soil model such as shear strength at mudline, shear strength gradient and suction resistance force is conducted to check the adoptability and applicability of nonlinear soil model for SCR design.

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.

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.

Multiaxial fatigue analysis on reeled deepwater steel catenary risers with girth weld defects

In the present study, we simulated the reel-lay installation process of deepwater steel catenary risers (SCRs) using the finite element method and proposed multiaxial fatigue analysis for reeled SCRs. The reel-lay method is one of the most efficient and economical pipeline installation methods. However, material properties of reeled risers may change, especially in the weld zone, which can affect the fatigue performance. Applying finite element analysis (FEA), we simulated an installation load history through the reel, aligner, and straightener and analyzed the property variations. The impact of weld defects during the installation process, lack of penetration and lack of fusion, was also discussed. Based on the FEA results, we used the Brown-Miller criterion combined with the critical plane approach to predict the fatigue life of reeled and non-reeled models. The results indicated that a weld defect has a significant influence on the material properties of a riser, and the reel-lay method can significantly reduce the fatigue life of SCRs. The analysis conclusion can help designers understand the mechanical performance of welds during reel-lay installation.

Cross-flow VIV-induced fatigue damage of deepwater steel catenary riser at touch-down point

A prediction model of the deepwater steel catenary riser VIV is proposed based on the forced oscillation test data, taking into account the riser-seafloor interaction for the cross-flow VIV-induced fatigue damage at touch-down point (TDP). The model will give more reasonable simulation of SCR response near TDP than the previous pinned truncation model. In the present model, the hysteretic riser-soil interaction model is simplified as the linear spring and damper to simulate the seafloor, and the damping is obtained according to the dissipative power during one periodic riser-soil interaction. In order to validate the model, the comparison with the field measurement and the results predicted by Shear 7 program of a full-scale steel catenary riser is carried out. The main induced modes, mode frequencies and response amplitude are in a good agreement. Furthermore, the parametric studies are carried out to broaden the understanding of the fatigue damage sensitivity to the upper end in-plane offset and seabed characteristics. In addition, the fatigue stress comparison at TDP between the truncation riser model and the present full riser model shows that the existence of touch-down zones is very important for the fatigue damage assessment of steel catenary riser at TDP.

An active truncation method for simulating deep-water riser installation

An active truncation hybrid model test is regarded as the most effective way for deep-water flexible structure test by The International Towing Tank Conference (ITTC). Owing to environmental loads and the movement of vessel, the dynamic boundary conditions at the truncated position related to the active truncation method are hard to be controlled. It has little relevant research which is reported on this testing technology. In this paper, an active truncation method is performed to simulate the process of deep-water steel catenary riser installation. The mechanical model of steel catenary riser (SCR) installation process is developed and corresponding equations for the developed model are derived. At the truncated position, device control principles are fitted from the solutions of the derived equations. 6-DOF (degrees of freedom) platform is used to simulate the motions of vessel. And the software and hardware of control system for this active truncation method are developed. The concept is first proposed, of which, that deep-water steel catenary riser installation test is conducted onshore. The test results show that this active truncation method is capable to simulate the process of steel catenary riser installation well. The test method has also provided the foundation for future works of other deep-water projects.

Fatigue-damage localization in steel catenary risers

Risers used to transport crude oil require Structural Integrity Management plans and programs to allow proper functioning during its design life. Cyclic loading may cause fatigue damage during operation of the riser. Structural Health Monitoring (SHM) is usually applied to detect damages ahead, and be confirmed by non- destructive inspection using remotely operated vehicles. With the information obtained the riser is assessed and if required mitigating measures can be implemented to prevent failure and disasters such as environmental pollution, and human and economic losses. This paper presents a study to locate fatigue damages using signals of the dynamic response. Numerical study cases were defined for a Steel Catenary Riser (SCR) installed in 2000 m water depth. A damage case was considered, decreasing the stiffness value at a specific location of the structure. Dynamic analyses were performed using a commercial software that incorporates nonlinear behavior. The Modal Slope Difference, Modal Slope Difference and Damage Index methods (with two variations) were applied to locate damage. Based on results the MSlD yielded the smallest error values in damage location followed by the Damage Index Method for severity values greater than 6%. Results demonstrate the effectiveness of the proposed methodology to locate fatigue damages in deep-water SCRs.

An Analytical Solution by HAM for Nonlinear Simulation of Deepwater SCR Installation

Steel catenary riser (SCR) is a cost-effective riser system that is widely used in deepwater offshore oilfields development. During SCR J-lay installation, the movement of pull-head must be carefully controlled to ensure riser safety. Since the SCR installation path calculation through numerical simulation software is usually time-consuming, this paper has established a mechanical model for SCR installation by making use of homotopy analysis method (HAM) to simplify its analytical solution, and dimensional analysis was considered in making initial guess solution. Based on this analytical solution, a program within the framework of MATLAB was developed to predict the two-dimensional riser behavior during installation, and a sensitivity analysis for different values of the control variables was carried out. Engineers may efficiently optimize the installation path by the application of this technique.

Multiaxial fatigue analyses of stress joints for deepwater steel catenary risers

In the present study, the dynamic and fatigue characteristics of two types of stress joints are investigated under ocean environmental condition. Connected with the riser and the platform, stress joint at the vessel hang-off position should be one of the main critical design challenges for a steel catenary riser (SCR) in deepwater. When the riser is under a high pressure and deepwater working condition, the stress state for the joint is more complex, and the fatigue damage is easy to occur at this position. Stress joint discussed in this paper includes two types: Tapered Stress Joint (TSJ) and Sleeved Stress Joint (SSJ), and multiaxial fatigue analysis results are given for comparison. Global dynamic analysis for an SCR is performed first, and then the local boundary conditions obtained from the previous analysis are applied to the stress joint FE model for the later dynamic and multiaxial fatigue analysis. Results indicate that the stress level is far lower than the yield limit of material and the damage induced by fatigue needs more attention. Besides, the damage character of the two types of stress joints differs: for TSJ, the place where the stress joint connects with the riser is easy to occur fatigue damage; for SSJ, the most probable position is at the place where the end of the inner sleeve pipe contacts with the riser body. Compared with SSJ, TSJ shows a higher stress level but better fatigue performance, and it will have a higher material cost. In consideration of various factors, designers should choose the most suitable type and also geometric parameters.

Spectral analysis of simulated acceleration records of deepwater SCR for identification of modal parameters

In riser projects it is important to determine modal parameters, such as periods and mode shapes, to evaluate their dynamic behavior. To obtain these parameters of risers, various methods have been developed based on free vibrations theory. These methods do not take into account the variations of the dynamic behavior of the riser in different sea state conditions and also cannot be applied in monitoring activities of risers. In these situations, an alternative method should be investigated because of the need to monitor the structural behavior of future deepwater risers. This work presents a Fourier analysis methodology for the identification of modal parameters; namely natural periods and mode shapes, for a deepwater steel catenary riser (SCR), solely based on acceleration records. A structural model of a riser in 1800m water depth was considered for the theoretical determination of modal parameters. This theoretical data was compared with the modal parameters identified in different sea states through spectral analyses of simulated acceleration records, assuming the presence of accelerometers at 15 discrete locations along the riser. The main advantage of the proposed methodology is that the estimated modal parameters are solely based on the motion (acceleration) records of the riser, which proves to be an efficient way to identify modal parameters that are being exited. This could be considered for structural health monitoring purposes, especially for damage detection and fatigue analysis.

A mechanical model for deepwater steel catenary riser transfer process during installation

Steel catenary risers are widely used in the development of marine oil and gas. Deepwater installation is an important procedure that will affect the service life of steel catenary risers. A comprehensive study of a mechanical model for the deepwater steel catenary riser transfer process during installation is performed in this paper. This model is derived on the basis of large deformation beam theory, the catenary method and the Winkler foundation model. Numerical analysis is also undertaken to investigate the factors influencing the transfer process of deepwater steel catenary risers during installation. The model presented is able to complete the transfer process through controlling the length of the cables from the installation vessel and the platform. The procedure can be applied to the design of deepwater steel catenary riser installations.

Multiaxial fatigue analysis of the stress joint for a deepwater steel catenary riser

The object of this paper is to present a more accurate and useful method for fatigue analysis and life prediction for a stress joint of a deepwater steel catenary riser. The stress joint was used to connect the riser and platform, and it was subjected to ocean environmental loadings and erosion; thus, the fatigue problem is critical. First, global time domain dynamic analysis of the steel catenary riser was performed, and then local analysis was performed for the stress joint. Finally, the fatigue life of the stress joint was evaluated according to the multiaxial fatigue theory, and a preliminary parametric study was also carried out. Results show that taking stress value of the structure as the only criterion is not enough, and fatigue analysis is necessary when the structure is subjected to an alternating load. Compared with uniaxial analysis, multiaxial fatigue analysis can reflect the damage of the structure more realistically. Especially for deepwater steel catenary risers, multiaxial fatigue problems should be given close attention.

Nonlinear analysis of flexible and steel catenary risers with internal flow and seabed interaction effects

Flexible risers and steel catenary risers often provide unique riser solutions for today’s deepwater field development. Accurate analysis of these slender structures, in which there are high-speed HP/HT internal flows, is critical to ensure personnel and asset safety. In this study, a special global coordinate-based FEM rod model was adopted to identify and quantify the effects of internal flow and hydrostatic pressure on both flexible and deepwater steel catenary risers, with emphasis on the latter. By incorporating internal flow induced forces into the model, it was found that the internal flow contributes a new term to the effective tension expression. For flexible risers in shallow water, internal flow and hydrostatic pressure made virtually no change to effective tension by merely altering the riser wall tension. In deep water the internal pressure wielded a dominant role in governing the riser effective tension and furthering the static configuration, while the effect of inflow velocity was negligible. With respect to the riser seabed interaction, both the seabed support and friction effect were considered, with the former modeled by a nonlinear quadratic spring, allowing for a consistent derivation of the tangent stiffness matrix. The presented application examples show that the nonlinear quadratic spring is, when using the catenary solution as an initial static profile, an efficient way to model the quasi-Winkler-type elastic seabed foundation in this finite element scheme.

Nonlinear Dynamic Analysis of Marine Risers under Random Loads for Deepwater Fields in Indian Offshore

The paper presents a numerical Analysis of deep water Steel Catenary Riser (SCR) under random sea loads. A finite element method is implemented in the time domain using Newmark's-Beta method. The response analysis is based on a simulation technique which duly considers the various non-linear effects such as relative velocity squared drag force, variable added mass due to variable submergence and nonlinearity due to large excursions. It also accounts for variable tension in riser due to variable submergence, variable buoyancy and wave forces. Results are presented which illustrates the effects of nonlinearities, long term drift oscillations and current velocity on the bending stress in the marine risers. The bending stress response time histories are obtained and presented in terms of bending stress envelopes and spectra showing contribution of various harmonics which is significant because of a non-linear system.

Review of research on interactions between deepwater steel catenary risers and soft clay seabeds

Steel catenary risers (SCR) have become an enabling technology for deepwater environments. A comprehensive review was conducted on recent research that examined interactions between deepwater steel catenary risers and soft clay seabeds. This included the STRIDE (steel risers in deepwater environments) and CARISIMA (catenary riser soil interaction model for global riser analysis) joint jndustry jrogram’s test data as well as information from existing papers.