Catenary Risers Research Articles

A Review on the Recent Process of Lazy Wave Risers

Lazy wave risers (LWRs) are designed with equidistant buoyancy blocks attached in the lower half of the riser, allowing the riser to take on an arch shape under the buoyancy forces provided by buoyancy blocks. This arch configuration can provide flexibility to the LWR arrangement and effectively isolate the dynamic responses between the offshore floating structure and the riser’s touchdown zone (TDZ). Its design and application aim to address the issues of dynamic response and fatigue damage that traditional steel catenary risers (SCRs) face in deep water and complex marine environments. Given that research on the LWRs in the field of ocean engineering is not sufficiently abundant, the structural characteristics, hydrodynamic loads, global responses, fatigue damage assessment, and structural optimization progress of LWRs are systematically reviewed in this paper to provide references for researchers in related fields. Among these topics, the global response of LWRs is the main point of this review. This section details the theoretical analysis and numerical modeling methods employed in the study of LWRs’ global response, explores the research advancements in the vortex-induced vibration (VIV) related to LWRs, and discusses corresponding experimental studies. Finally, the installation, transfer, and repair processes of LWRs are investigated. Additionally, the importance of leveraging advanced technologies from other fields and combining them with current advanced algorithms is emphasized in efforts to assess fatigue damage and optimize the structures of LWRs, ultimately achieving complementary advantages.

Underwater Line Monitoring Using Optimally Placed Inclinometers

Underwater monitoring presents challenges related to maintaining a continuous power supply and communication, necessitating the use of a smaller number of sensors to effectively cover the entire line. An underwater line tracking method is proposed to evaluate global behaviors and stresses in real time. The method employs angles at several points on the line, as well as displacements and curvatures at both ends. In this method, any line displacement, angle, and curvature are expressed as Fourier series, and Fourier coefficients are obtained by utilizing sensor data. Then, the behavior of any line location is assessed. In addition, to reduce the number of sensors and improve accuracy, optimal inclinometer locations are determined by a genetic algorithm. The proposed line tracking algorithm was validated through two numerical examples; one with an inclined tunnel and one with a marine steel catenary riser attached to a Floating Production Storage and Offloading (FPSO) vessel. Through these examples, the proposed algorithm was proven to capture global behaviors accurately when optimally located sensors are used. In the riser monitoring case, the optimized sensor placement with eight intermediate sensors achieved an average mean distance error of 1.91 m, which is lower than the 2.65 m error obtained with ten intermediate sensors without optimization.

Fatigue testing strategies for the X65 steel catenary riser with small‐scale specimens considering the effect of welding residual stress

AbstractTo accurately evaluate the fatigue performance of a full‐scale deep‐water steel catenary riser (SCR) using small‐size specimens, six fatigue testing strategies were explored, considering the effect of welding residual stress. Through a comparison with the full‐scale resonant bending fatigue testing results, the most equivalent strategy using small‐scale specimens was identified. The results indicate that the test strategies applying constant stress underestimated the fatigue life, compared with that of a full‐scale specimen; meanwhile, fatigue life values obtained from the low‐stress ratio testing strategy were higher, particularly in the low‐stress range region. Comparatively, the fatigue lives obtained for the 100 mm‐wide specimens without cutting were higher in the high‐stress range region. The variable‐applied mean stress strategy using the 25 mm‐wide welded joint specimen was the most suitable for equivalence with the full‐scale fatigue testing, with only a 9.7% difference in the fatigue life testing results. The difference between the applied mean stress and the actual transverse welding residual stress under various fatigue testing strategies is the key factor affecting the equivalence of the fatigue testing results.

Effects of vertical and lateral riser-soil interactions on vortex-induced vibration of a steel catenary riser

A numerical model for computing the vortex-induced vibration (VIV) and fatigue damage of steel catenary risers (SCRs) was developed. The structural dynamics were accurately simulated using an absolute nodal coordinate formulation (ANCF). The Van der Pol wake oscillator is applied to generate the fluctuating lift, which is further transformed into the cross-flow direction by considering the structural deformation. The Randolph-Quiggin (RQ) model and the Coulomb friction ‘bilinear’ model are employed to simulate the vertical and lateral riser-soil interactions, respectively. After case validations, the effects of riser-soil interaction on the VIV amplitude, frequency, mode, and fatigue of the SCR at different current angles are investigated, and a sensitivity study of different seabed model parameters is discussed. The bands of significant VIV frequencies were broadened by riser-soil interactions, accompanied by more frequency components of disturbance and more abundant vibration modes. Severe fatigue damage cannot be captured by the truncated model, and seabed models that require improvement are ignored. It is suggested that vertical and lateral riser-soil interactions should be considered in the evaluation of VIV fatigue damage for SCRs.

Numerical investigation on vortex-induced vibration of catenary riser conveying fluid under top-end heave excitation

ABSTRACT Due to the catenary shape of steel catenary risers (SCR), the heave motion of the floating platform can generate an equivalent oscillatory flow field and excite the vortex-induced vibration (VIV) of SCR. Internal flow conveyed by SCR further complicates the VIV response. A semi-empirical model is improved to simulate the VIV of SCR with internal flow under top-end heave excitation. VIV characteristics such as displacement temporal–spatial distribution and amplitude spectra spatial distribution are analysed and the fatigue damage is calculated. The effects of heave period, heave amplitude, internal flow volumetric flow rate and internal flow density on VIV of SCR are discussed. The heave excitation with short period or large amplitude tends to excite high-order-mode response and enlarge the fatigue damage. The VIV response becomes less regular and the fatigue damage is reduced when the internal flow volumetric flow rate or density increases.

Vortex-induced vibrations of catenary risers in varied flow angles

Vortex-induced vibrations (VIVs) of steel catenary risers (SCR) considering various flow incident angles are modeled and investigated. The dynamics of the flexible cylinder are first described using the Euler-Bernoulli beam theory with the absolute nodal coordinate formulation (ANCF). To capture the nonlinear shear characteristics of the flow, a discrete point vortex theory-based van der Pol wake oscillator is improved by introducing a local reference coordinate system and relative flow velocity. The comparative analysis between the numerical and experimental results demonstrates the numerical model effectively reproduces the multimodal VIVs of curved cylinders with a satisfactory agreement. The VIV response of a real-scale riser is then explored parametrically by varying the flow velocity for incident angles of 0°, 30°, 60°, and 90°. The effects of incident angle on the structural multidirectional response, spatially varying dominant frequency, lock-in position, trajectory, phase evolution, and fluid-structure energy transfer are highlighted. The results of the present study suggest that the three-dimensional excitation coefficients are synchronized and share the same power-in region under oblique flow conditions, which predisposes VIVs of the riser to travelling wave pattern.

A novel three-dimensional SCR motion simulator for modelling the catenary riser–seabed interaction in a centrifuge

Steel catenary risers (SCRs) provide a cost-effective solution for deepwater oil and gas production. However, SCRs are susceptible to potential fatigue failure due to the cyclic motions of floating platforms. Previous studies on the physical modelling of cyclic SCR–seabed interactions have primarily focused on either the continuous cyclic motion of an SCR or a single rest period between two SCR motion packets. However, our understanding of the development of seabed trenches and excess pore pressure and their effects on SCR fatigue during multiple episodes of SCR motion and soil reconsolidation remains limited. This study presents a newly developed model container capable of modelling three-dimensional SCR motions including heave, surge, sway, and vortex-induced vibration in a geotechnical centrifuge. A centrifuge test is conducted to investigate the vertical cyclic SCR–seabed interaction, considering five vertical cyclic motion packets with intervening periods of reconsolidation. The results indicate that ignoring the effects of reconsolidation leads to an overestimation of the fatigue life of an SCR. In this test, the SCR fatigue life is reduced by 18%–23% after five episodic SCR motion packets and intervening reconsolidation.

Dynamic curvature of a steel catenary riser on elastic seabed considering trench shoulder effects: an analytical model

Dynamic curvature of a steel catenary riser on elastic seabed considering trench shoulder effects: an analytical model

Physics-based Data-informed Prediction of Vertical, Catenary, and Stepped Riser Vortex-induced Vibrations

Physics-based Data-informed Prediction of Vertical, Catenary, and Stepped Riser Vortex-induced Vibrations

Incorporation of the riser-seabed-seawater interaction effects into the trench formation and fatigue response of steel catenary risers in the touchdown zone

The existing Steel Catenary Riser (SCR)-seabed interaction models do not account for the contribution of seawater as a key influential factor. The present study has examined the effect of SCR-seabed-seawater interaction on the trench enlargement and consequently on the fatigue performance of the SCR in the touchdown zone (TDZ). Coupled Eulerian-Lagrangian (CEL) analysis with three domains of SCR, seabed soil, and seawater were performed using ABAQUS. A parametric study was conducted to investigate the impact of seawater interaction along with incorporation of the shear strain rate and softening effects. The conventional bearing capacity approach for analysing pipe penetration into the seabed was also reviewed, and modifications were proposed to account for the effects of water entrainment into the trench. The results obtained from CEL analysis were integrated with a global SCR model to incorporate the SCR-seabed-seawater interaction effects into the fatigue analysis. The study showed that the entrainment of seawater into the trench can be coupled with SCR oscillations to further soften the seabed and erode the trench resulting in an enlarged trench profile affecting the fatigue damage in the TDZ.

Assessment of the Void Fraction Correlations for Gas–Liquid Two-Phase Flows in Pipeline-Risers

In the past decades, studies on a void fraction of gas–liquid two-phase flow have mainly focused on straight pipes, while voidage in pipeline-riser flows have been largely neglected. In this work, the void fraction of gas–liquid two-phase flow in pipeline-riser was investigated. The test loop consists of a 12 m horizontal pipe, followed by a 2 m downward inclined pipe, and ended at a riser. Both vertical riser and catenary riser were used in the experiment. Slip in severe slugging and annular flow was more pronounced than in bubbly flow. The transition from the mixed to the separated flow occurs at gas–liquid volumetric flow rate ratio equals to 0.8. However, the separated flow forms when the gas–liquid volumetric flow rate ratio is greater than 0.9 for catenary riser. A total of 28 correlations for predicting the void fractions were evaluated with the experimental data. The Mishima and Hibiki correlation was found to be the most accurate among the studied correlations, predicting 68.5% and 69.3% of the measured data within ±30% for vertical and catenary riser, respectively. A new correlation considering the effect of the elbow is still needed for more accurate prediction of void fraction in pipeline-riser.

Dynamic response of a steel catenary riser at touch-down zone

This paper introduces a numerical model for analysing the dynamic response and fatigue damage of a global steel catenary riser (SCR) at the touch-down zone (TDZ). The model incorporates an innovative pipe-soil interaction model based on a new effective-stress framework to evaluate the changing soil strength induced by the cyclic motion of the SCR, thereby allowing for a more accurate assessment of the effects of soil remoulding and trenching on the dynamic responses of the SCR. The global SCR model is implemented in a commercial finite element software – ABAQUS, with the pipe-soil interaction model integrated through a user-defined element subroutine. A comprehensive series of time-domain simulations is conducted to investigate the evolution of seabed and the subsequent development of trenching along the seabed pipeline. These simulations provide valuable insights into the structural performance of the SCR at the TDZ. Additionally, this research explores the effects of water entrainment, changing seabed strength & stiffness, and the heaving motion of the floater on the fatigue damage of the global SCR. Understanding these factors is essential for obtaining an extensive understanding of the SCR's behaviour in the TDZ. The findings provide significant insights that can be leveraged to optimise existing design practices and enhance the efficiency and safety of offshore infrastructure.

Experimental investigation on the coupling between the platform motion and the catenary flexible riser response

The vortex-induced vibration of a catenary flexible riser hinged underneath a two-degree-of-freedom horizontally oscillating platform is experimentally investigated in this work. The catenary riser with an aspect ratio of 125 was arranged in the water tank with its concave facing the oncoming flow. The dynamic behavior of the riser was measured in the depth-averaged Reynolds number range of 150–1880 using the non-intrusive optical technique with high-speed cameras. Meanwhile, the sway and surge of the platform were monitored with two laser displacement sensors. The experimental results indicate that the out-of-plane response of the riser is less affected by the sway of the platform in spite of the top-linked motion. There is no distinct alteration in both the response amplitude and frequency in comparison with a top fixed riser. In contrast, the surge of the platform has a great influence on the in-plane response of the riser. Four types of relative motion between the platform and flexible riser are identified. The relative motion and mode competition are the two main reasons for that. In addition, strong coupling and weak coupling between the platform motion and riser vibration are identified. A coupling length is proposed to quantify the interaction between the platform and the riser, which is closely related to the spatial mode competition. As the reduced velocity grows, the enhanced oscillation of the platform contributes to the elongated coupling length of the riser. However, the influence is gradually attenuated at higher depth-averaged reduced velocities, especially in the out-of-plane response. The coupling also exists between the in-plane and out-of-plane responses of the flexible riser. The platform motion has a suppressing effect on this coupling, especially at high-depth-averaged reduced velocities.

Research on the Dynamic Response of a Steel Catenary Riser in the Touchdown Zone under Pigging Conditions

A periodic pigging operation performed to clean off sediment and provide operators with detailed health information for a pipeline is mandatorily required. The research on pigging-induced issues for the steel catenary riser (SCR), one of the key parts in offshore hydrocarbon recovery pipelines between the floating production system and the seabed, has been scarce until now. As a result, there is an urgent need for theories to guide the pigging operation to ensure safe pigging is achieved in deepwater risers. In this paper, a study aiming to determine the effects of the pigging impact load and the pigging-induced slugging load on the dynamic response of the riser is reported. A SCR pigging model was established and proposed based on the finite element analysis (FEA) method. The stress distribution and displacement of the SCR were investigated under the pigging conditions, with the consideration of the effects of waves, currents, and floating platform movements. It was found that the pigging load has large effects on the stress and displacement of the touchdown zone (TDZ), especially the touchdown point (TDP). The displacement of the TDZ in the Y (vertical) direction is more significant than that in the X (horizontal) direction under pigging conditions, and the maximum displacement of the TDZ in the Y direction is proportional to the weight of the pig, as well as the length of the pigging-induced slugging.

Distributions of nonlinear dynamics, stress and fatigue damage along a steel catenary riser in current affected by top-end surge

A numerical model for computing the complex nonlinear dynamics and fatigue damage of steel catenary riser (SCR) system is proposed in this paper. The structural model of SCR is described by the absolute nodal coordinate formulation (ANCF), which is further coupled with Morrison equation to incorporate the hydrodynamic loads. The hysteretic riser-soil interaction model and Coulomb friction ‘bilinear’ model are applied to simulate the seabed reactions. After validation, the distributions of nonlinear motions along the SCR, the response characteristics of touchdown point (TDP) and the corresponding stress and fatigue are fully discussed. It is found that the motion trajectory in x-z plane presents various regimes along the SCR, relating to the phase difference between x- and z-motions induced by the variations of primary and harmonic frequencies. The secondary uplift of z-motion, accompanying by the sharp decline of vertical seabed force, is captured for TDP. The values of secondary uplift and seabed force decline increase or decrease with growing surge amplitude or period in step style. Both the amplitude and period of top-end surge affect the features of axial tension, bending moment, stress and fatigue damage, where the former plays a more significant role in the concerned parameter range.

Investigation of trench effect on fatigue response of steel catenary risers using an effective stress analysis

The design of Steel Catenary Risers (SCR) heavily relies on the interaction between the riser and seabed in the touchdown zone (TDZ). To accurately estimate the SCR-seabed stiffness, the soil behaviour must be assessed in two main phases: (1) during cyclic SCR motions, which generate excess pore pressure, leading to soil softening and remoulding in the TDZ, and (2) during inactivity periods throughout the SCR’s lifespan, causing dissipation of excess pore pressure associated with the consolidation state. The latter was neglected by the existing non-linear hysteretic soil models, which are based on the total stress approach. In the current study, a global riser analysis was conducted and the consolidation effect was incorporated using an effective stress framework. Long-term soil stiffness was determined by capturing both the remoulding and consolidation effects, which are respectively associated with the damage accumulation during the SCR cyclic motions and soil strength recovery during the intervening pause period. The constructed model was then combined with a new methodology named Hybrid Trench Model to investigate how the consolidation effect contributes to the fatigue performance in different trench depths. Stochastic fatigue analysis showed that the consolidation effect might increase the damage by around 10%-40% over the long-term assessment.

Fatigue reliability assessment of offshore catenary risers conveying internal slug flow

AbstractThis paper proposes a fatigue reliability assessment method of offshore steel catenary risers (SCRs) caused by internal slug flow. Offshore risers subject to slug flow are modeled first by employing a finite element method scheme. Subsequently, a combination of Kriging metamodel and Monte Carlo simulation (MCS) is established to assess the fatigue reliability of SCRs conveying internal slug flow. An explanatory example of a 16‐inch SCR is presented to validate the performance of the proposed method and the results indicate that the failure rates of the SCR are significantly affected by the frequency of slug flow. Moreover, the accuracy and feasibility of the present method are validated by a comprehensive comparison with the existing experiments and numerical results. The present method contributes to the design and maintenance of offshore risers with internal slug flow.

Prediction of unsteady slug flow in a long curved inclined riser with a slug tracking model

An improved one-dimensional mechanistic model is presented for the prediction of unsteady gas-liquid slug flows in inclined curved pipes, using the slug tracking approach. The equations for mass and momentum conservation are applied to the slug body, liquid film, and elongated bubble regions constituting a slug unit cell. The proposed model can be applied to horizontal or inclined upward flows. Statements of mass conservation result in axial changes of the liquid and gas velocities in the liquid film and elongated bubble. The slug initiation at the inlet is modelled as a random process with slug length variations. Closure relationships for the bubble nose velocity, modified by the wake effect, and the slug frequency for slug initiation are employed. The discretized governing equations are solved fully implicitly, introducing numerical treatments associated with the outlet boundary conditions and the merging of slug units. Of practical interest is an upward gas-liquid slug flow in a catenary riser with a high aspect ratio (length over diameter) being an order of a thousand representing an offshore subsea pipe for the oil and gas production. By considering the pipe initially fully filled with the traveling liquid, the dynamic scenario of the pipe transporting successive slug units is simulated, capturing the continuing evolution of slug flow patterns along the pipe exhibiting the disappearances of liquid slugs due to the bubble coalescences. Spatio-temporal variations of the liquid holdup, the pressure and its gradient, the film and slug lengths, the slug frequency, the velocities of the slug front, bubble nose, liquid in the slug body and film, and of the gas in the elongated bubble are evaluated. The backward flow occurrence in the film zone near the outlet is also predicted due to the pipe inclination. Parametric investigations are performed by specifying the superficial liquid and gas velocities, and comparing the cases of catenary pipes (with variable inclinations) versus inclined and horizontal straight pipes (with fixed inclinations). Results highlight the important effect of gas-to-oil superficial velocity ratio (GOR) in combination with the pipe inclination and curvature effects. Fluctuations of slug flow properties appear to be considerably amplified and more intermittent when increasing the GOR. This observation is important towards regulating the practical flow rates for subsea oil and gas productions as well as designing flexible pipes subject to slug flow-induced vibrations.

การสั่นอิสระในระนาบและนอกระนาบของท่อลำเลียงของไหลแบบคาร์ทีนารีในทะเลลึก

การสั่นอิสระในระนาบและนอกระนาบของท่อลำเลียงของไหลแบบคาร์ทีนารีในทะเลลึก

Dynamic Properties of Steel Catenary Riser near Touchdown Point Under Coplanar Vessel Heave and Vortex Induced Vibration

Dynamic Properties of Steel Catenary Riser near Touchdown Point Under Coplanar Vessel Heave and Vortex Induced Vibration