Catenary Risers Research Articles

Extreme response based reliability analysis of composite risers for applications in deepwater

As current oil reserves start to deplete, companies are looking to exploit deeper deposits. At these greater depths composite risers, with their high strength-to-weight ratio, reduce the effective tensions and bending moments compared to steel risers. However, there is still limited research into their behaviour, with one key missing element being a comparison with traditional riser designs which accounts for variances in material properties and wave loads. This paper therefore conducts a strength-based reliability analysis of composite catenary risers operating between 1,500 m and 4,000 m. A static global catenary model is combined with Classical Laminate Theory to determine the extreme response and its performance is verified against FEA. This response is evaluated with the Tsai-Wu failure criterion to determine first-ply failure. The effect of laminate moisture absorption on the long-term reliability of submerged composite-based risers is also investigated as it can cause a significant reduction in the strength of composite risers. The reliability analysis is conducted using the Monte Carlo Method, revealing that the composite risers perform well at 4000 m. The degradation in performance from moisture absorption becomes increasingly important at greater depths and needs further investigation for these applications.

Residual Stress Study of High Manganese Steel Riser Pipe Manufactured by Longitudinal Butt Welding (2) : Equation for Predicting Welding Induced Residual Stresses Established through Parametric Study

In recent years, the deep and ultra-deep seas are recognized as the last remaining regions on the planet where oil and natural gas resources are discovered and produced. In order to develop such natural resources in an environmentally safe and cost-effective method, various engineering systems and technologies are being developed. In keeping with this direction of development, there is significant worldwide interest among major offshore structural and system operators in examining the possibility of using high-strength steel for long-distance gas transmission pipelines and risers. In particular, riser pipes using high manganese steel are often made by longitudinal butt welding, but studies related to the prediction of residual stresses for this are insufficient.In general, it is known that in a cylindrical structure such as steel catenary risers (Hereafter, ‘SCRs’), residual stress in the through-thickness direction can be decomposed into a membrane component and a bending component. In this paper, a parametric study through nonlinear finite element analysis was performed to predict the residual stress for each direction of a longitudinal butt-welded riser by considering the main parameters (r/t) of the pipe that can affect the actual residual stress. A new predicting equation for estimating the residual stress profile was proposed using the results of a number of parametric precision numerical analysis derived through this process. Key words: Riser, High manganese steel, Longitudinal butt welding, Residual stress, FEA, Parametric study, Through-thickness, Membrane stress, Bending stress, Self-equilibrium stress, Empirical formulae

Corrosion-Fatigue Crack Growth Performance of Titanium Grade 29 Welds in Tapered Stress Joints

Abstract Design of a steel catenary riser requires the use of connection hardware to decouple the large bending moments induced by the host floater at the hang-off location. Reliability of this connection hardware is essential, particularly in applications involving high pressure and high temperature fluids. One option for this connection hardware is the metallic tapered stress joint. Titanium (Ti) Grade 29 has been identified as an attractive material candidate for demanding stress joint applications due to its “high strength, low weight, superior fatigue performance and innate corrosion resistance”.2 Titanium stress joints for deepwater applications are typically not fabricated as a single piece due to titanium ingot volume limitations, thus making an intermediate girth weld necessary to satisfy length requirements. As with steel, the potential effect of hydrogen embrittlement induced by cathodic and galvanic potentials must be assessed to ensure long-term weld integrity. This paper describes testing from a joint industry project (JIP) conducted to qualify titanium stress joint (TSJ) welds for ultra-deepwater applications under harsh service and environmental conditions. Corrosion-fatigue crack growth rate (CFCGR) results for Ti Grade 29 flat welding-groove weld (1G/PA) gas tungsten arc welding (GTAW) specimens in seawater under cathodic potential and sour brine under galvanic potential are presented and compared to vendor recommended design curves.

Numerical Modeling Approach for Steel Catenary Riser Behavior at Touchdown Zone

AbstractThis paper introduces a new numerical approach in the analysis of the dynamic interaction between the steel catenary riser (SCR) and seabed in the vicinity of the touchdown zone (TDZ). The ...

Characterisation of LCF performance of X100 weld-joints: Mechanistic yield strength modelling, finite element analyses and DIC testing

This paper is concerned with the effect of welding on the fatigue behaviour of X100 material for steel catenary risers. The methodology includes both modelling and experimental characterisation. The modelling combines (i) a physically-based yield strength model to capture the thermally-induced microstructural heterogeneity and associated spatial variations in relative contributions of the key strengthening mechanisms due to welding, and (ii) a five-material cyclic plasticity model with a Coffin-Manson strain-life fatigue model for prediction of cross-weld heterogeneity in cyclic plasticity and fatigue response. The combined non-linear isotropic-kinematic cyclic plasticity behaviour of the five weld joint constituent materials (PM, weld metal (WM) and heat-affected zone (HAZ) subregions) is implemented via a user material (UMAT) subroutine, including Kocks-Mecking monotonic-cyclic evolution of yield stress. The experimental methodology consists of tensile tests with digital image correlation (DIC) for X100 PM and cross-weld samples. The results indicate that the primary phenomenon driving the detrimental effect of welding on fatigue is the evolution of cyclic strain localisation in the inter-critical heat-affected zone (ICHAZ), leading to predicted ICHAZ failure.

Nonlinear riser-seabed interaction response among touchdown zone of a steel catenary riser in consideration of vortex-induced vibration

Steel catenary riser (SCR) is a preferred solution for deepwater oil and gas exploitation under harsh marine environment. Its riser-seabed interaction (RSI) among touchdown zone (TDZ) in consideration of the vortex-induced vibration (VIV) of sag bend is at present a research frontier in ocean engineering. This paper integrates Randolph-Quiggin model into a published global numerical riser model, to investigate the nonlinear RSI response among TDZ of an SCR with VIV. The adopted numerical model is validated against some published experimental measurements for VIV and RSI respectively. Three combined cases with top-end platform heave motion and VIV are simulated, and the trench development as well as the SCR's response characteristics among TDZ is analyzed. VIV makes the trench develop more smoothly, and the trench length as well as depth turns smaller. The VIV effect on RSI is relatively significant with violent heave excitation, under which the P-z curves present clearer interaction mode transitions. VIV enlarges the maximum bending moment visibly, and makes the sag bend of SCR dominated by higher-order modes, which are both detrimental to the structural safety. The coupling effect between RSI and VIV on the riser's dynamic response needs to be considered for the design of full-scale SCRs.

A complete analytical solution for axial pipeline walking considering seabed resistance as rigid plastic behaviour

Offshore pipelines are important infrastructure for transporting oil and gas. They are subjected to cycles of high temperature and pressure variation during operation and shutdown periods, and are therefore susceptible to the stability problem of axial walking due to the asymmetry between the expansion and contraction. Axial walking can cause damage to subsea pipeline systems, for instance to the connected manifolds and steel catenary risers. This paper provides a complete analytical solution to examine the process of pipeline walking based on underlying principles of mechanics, considering the pipeline–seabed sliding response as rigid plastic. The analytical solution allows calculation of the cumulative displacement, strain, axial force and soil resistance along the pipeline under arbitrary temperature or internal pressure change histories, accounting for the coupled contributions from seabed slopes and end tension forces. A dimensionless parameter η is proposed to explain the triggering mechanism for walking by comparing the activating force to the soil resistance. New criteria are established to judge whether pipeline axial walking is possible. Calculation examples are presented in the final part of this paper to demonstrate the analytical solution. This study can be used as a rigorous benchmark and a simple practical tool to evaluate pipeline walking under arbitrary loading histories of temperature and internal pressure variations.

Experimental investigation on vortex-induced force of a Steel Catenary Riser under in-plane vessel motion

A method to identify vortex-induced forces and coefficients from measured strains of a Steel Catenary Riser (SCR) undergoing vessel motion-induced Vortex-induced Vibration (VIV) is proposed. Euler–Bernoulli beam vibration equations with time-varying tension is adopted to describe the out-of-plane VIV responses. Vortex-induced forces are reconstructed via inverse analysis method, and the Forgetting Factor Least Squares (FF-LS) method is employed to identify time-varying vortex-induced force coefficients, including excitation coefficients and added mass coefficients. The method is verified via a finite element analysis procedure in commercial software Orcaflex. The time-varying excitation coefficients and added mass coefficients of an SCR undergoing vessel motion-induced VIV are investigated. Results show that vessel motion-induced VIV is excited at the middle or lower part of the SCR and in the acceleration period of in-plane velocity, where most of the excitation coefficients are positive, while during the deceleration period, the excitation coefficients becomes too small to excite VIVs. Parameter γ [1] has strong correlation with excitation coefficients. In addition, time-varying tensions contribute significantly to the variations of added mass coefficients under the condition that the ratio of dynamic top tension to pretension exceeds the range of 0.7–1.3. Moreover, chaotic behaviors are observed in vortex-induced force coefficients and are more evident with the increase of vessel motion velocity. This behavior may attribute to the randomness existing in in-plane velocity and its coupling with out-of-plane vibrations.

МЕТОД РОЗРАХУНКУ СТРИЖНЕВИХ КОНСТРУКЦІЙ НА КОЛИВАННЯ З ВРАХУВАННЯМ ВЛАСНОЇ ВАГИ

Abstract. The work is devoted to the study of free vibrations taking into account the self-weight of vertical rod structures, which in the design scheme are reduced to a clamped-pinned rod. The research used the author's method, which is based on the exact solution of the corresponding partial differential equation of vibration with variable coefficients. For the given boundary conditions the frequency equation is written in dimensionless form and the method of finding its roots is determined. Determining the frequencies of natural vibration is reduced to finding the corresponding dimensionless coefficients from the frequency equations. Representing the vibration coefficient as a variable function of the dimensionless parameter that characterizes the mechanical system, and calculating the data set on the allowable range of parameter values, the vibration coefficient was approximated by a polynomial. As a result, the final formulas for calculating the vibration coefficient and, consequently, the natural frequency for a rod with any physical and mechanical characteristics are obtained. The calculation of the natural frequency of the steel catenary riser (SCR) reduced to a given design scheme is performed. When designing risers, among other things, the characteristics of the internal fluid flow, vibration of the equipment are taken into account. However, it is mandatory in the practice of dynamic calculations of structures to study the frequency spectrum of their nature vibrations. Because these constructions, as a rule, have quite large height, the question of the account of self-weight at their designing is rather important aspect. An example in which the author's method calculates the first three oscillation frequencies is considered. Comparing them with the known formulas for weightless rods, it was found that taking into account the self-weight decrease the value of the corresponding frequencies. The error of values for the first frequency is the largest and is 15%. The greatest difference between the corresponding natural frequencies, taking into account the self-weight, is achieved at the critical value of the longitudinal load, in terms of stability.

Nonlinear formulation and free vibration of a large-sag extensible catenary riser

The nonlinear formulation using the principle of virtual work-energy for free vibration of a large-sag extensible catenary riser in two dimensions is presented in this paper. A support at one end is hinged and the other is a free-sliding roller in the horizontal direction. The catenary riser has a large-sag configuration in the static equilibrium state and is assumed to displace with large amplitude to the motion state. The total virtual work of the catenary riser system involves the virtual strain energy due to bending, the virtual strain energy due to axial deformation, the virtual work done by the effective weight, and the inertia forces. The nonlinear equations of motion for two-dimensional free vibration in the Cartesian coordinate system is developed based on the difference between the Euler's equations in the static state and the displaced state. The linear and nonlinear stiffness matrices of the catenary riser are obtained and the eigenvalue problem is solved using the Galerkin finite element procedure. The natural frequencies and mode shapes are obtained. The results are validated with regard to the reference research addressing the accuracy and efficiency of the proposed nonlinear formulation. The numerical results for free vibration and the effect of the nonlinear behavior for catenary riser are presented.

Real-time estimation of riser's deformed shape using inclinometers and Extended Kalman Filter

The real-time monitoring of underwater risers, cables, and mooring lines by multiple sensors is in great demand but still very challenging. In this study, a new real-time riser monitoring method based on an Extended Kalman Filter (EKF) is proposed. It estimates the overall shape of riser in real-time utilizing the measured signals from multiple bi-axial (inclination and heading) inclinometers along the riser. The novel EKF algorithm is shown to be robust against sensor noises and successfully reproduces the actual riser profiles at each time step, which has been verified by multiple tests through numerical simulations. For verification, a turret-moored FPSO (Floating Production Storage and Offloading) with a SCR (Steel Catenary Riser) is employed in four different random waves and currents. Subsequent algorithms are also developed so that the corresponding bending and axial stresses along the riser can also be estimated in real time from the obtained riser shape, which can further be used for the real-time estimation of fatigue-damage accumulation.

Dynamic response of deep-water catenary risers made of functionally graded materials

This paper examines the structural behavior of a steel catenary riser (SCR) with a segment composed of a functionally graded material (FGM) at the touchdown zone (TDZ). A co-rotational beam formulation is employed to evaluate the influence of material gradation on the static and dynamic nonlinear behavior of the riser. A finite element model is developed to predict the riser response considering environmental and operational conditions such as self-weight, buoyancy, current, soil contact, waves, buoys, and ship motion. Moreover, a comparison of SCR configurations is made between those constructed from FGM and those constructed from standard homogeneous materials. The study indicates that an SCR configuration with an incorporated FGM segment improves the curvatures near the TDZ, where designing to mitigate fatigue damage is critical.

Impact of seabed slope on steel catenary riser touchdown zone response

Several factors can affect the response of steel catenary risers (SCR) around its touch down zone (TDZ). These include the stiffness of the soil, the soil suction force on the riser TDZ, the soil degradation with cyclic TDZ loading, etc. Riser strength and fatigue response computation are usually performed considering flat seabed and with the use of rigid or linear (spring) riser soil interaction model. However, bathymetric information obtained for the SCR lay path on the seabed reveal complex seabed profile variation, indicating that the seabed is far from being flat around the SCR TDZ. This paper presents findings from an investigation conducted on the influence of seabed slopes on the strength and fatigue response of SCRs, using a non-linear (NL) riser soil interaction model. The responses of SCRs on positively and negatively sloped seabed (rotated about the static touch down point on flat seabed) are compared with responses of SCRs on flat seabed. From the results, it can be deduced that the SCR dynamic and fatigue responses may be overpredicted or underpredicted in magnitudes dependent on the slope deviation of actual seabed from a flat seabed.

A tabular optimisation technique for steel lazy wave riser

Steel lazy wave riser (SLWR) is derived from the simple catenary riser (SCR) by the installation of buoyancy modules on its section. Infinite SLWR configurations are possible, and this poses difficulties in determining the best configuration. However, it is possible to capture some suitable configurations which satisfy some given design criteria specific to a project. We referred to these as the optimum configurations for the problem. Several advanced optimization tools and techniques for engineering optimization are available. In this paper, we present a 2D tabular optimization method for SLWR, which is an index-based optimization technique. The approach reduces a multidimensional problem to a 2D type providing a significant reduction in the required computational resources. It combines the design variables in pairs and assigns indices to the resulting design points (configurations) for each combination. The optimum design points are then tracked through index matching using techniques such as data sorting and intersection operations. In the application of the technique to SLWR design, we set the number of design variable for the problem to three. This results in three pair of combinations of the design variables. The design variables are the apparent mass ratio, the sag bend elevation, and the arc height. The output variables of interest to be optimized include the SLWR hanging length, the smeared buoyancy section length, the smeared buoyancy thickness, the riser hang-off tension, the stress utilization and fatigue damage around the bends. Selected optimum SLWR configurations from the optimization process are subjected to an irregular wave simulation to demonstrate the suitability of the approach for such optimisation problems.

Mode switching and standing-travelling waves in slug flow-induced vibration of catenary riser

An experiment in a small-scale air-water test loop is performed to investigate the flow-induced vibration behaviors of a flexible catenary riser conveying slug flow over a wide range of the gas-to-liquid flow rate ratios from 1.0 to 4.5. The clamped-clamped riser model has an inner diameter of 4 mm and an aspect ratio with the length over the inner diameter of about 360. The non-intrusive high-speed cameras are employed to simultaneously capture the catenary riser dynamics and the internal unsteady slug flow characteristics. The space-time varying features of the slug flow-induced vibration are presented and discussed in terms of the response amplitudes, frequencies, modal components, and associated flow-induced forces. Slug flow properties including the translational velocities, lengths, fluctuation frequencies and pressure changes are reported. When increasing the ratio of the gas-to-liquid flow rates, experimental results highlight how a variation of the slug length and frequency contributes to a mode-switching phenomenon over time. This switching from the dominant first-mode frequency to the second-mode frequency leads to the associated transition from the standing wave to the travelling wave of the catenary riser response. The response amplitudes are amplified by the increased translational velocity and slug length, entailing the increased flow-induced momentum and gravitational forces, as the gas-to-liquid flow rate ratio is increased. Consequently, higher vibration modes participate into the riser dynamics exhibiting a strong multi-mode competition.

Influence of trench geometry on fatigue response of steel catenary risers by using a boundary layer solution on a sloped seabed

Previous studies have shown that the geometry of the trench affects the fatigue response of steel catenary risers (SCR). Fatigue performance of SCR in the touchdown zone (TDZ) has been investigated in several studies, sometimes leading to contradictory results, with no coherent agreement regarding if the effect of the trench on fatigue performance of SCR would be beneficial or detrimental. This paper aims at developing a model based on the well-known boundary layer method (BLM) to capture the trench effect on the fatigue performance of SCR. Using analytical and numerical approaches, a meaningful relationship was observed between the slope of trench shoulders, the magnitude of the peak fatigue damage, and its location in the trench. It was also observed that the effect of trench formation on the fatigue performance of SCR could be predicted by using TDP oscillations on a sloped seabed. The study contributes to achieving a coherent agreement about the trench effect on fatigue.

Efficient methods to mitigate SCR-induced walking of short subsea flowlines

Steel catenary riser is a long-established option for subsea projects in deep-water regions. Sustained pulling force of steel catenary risers on subsea flowlines in combination with cyclic thermal load throughout the system lifetime may lead to progressive global axial displacement of subsea pipelines which has been termed as ‘walking’. One of the challenges in the deep-water industry is long-term walking of subsea flowlines in a cumulative manner. Common practice methods for walking mitigation are quite expensive operations. State-of-the-art mitigation strategies are proposed in the paper by means of modifying pipe pieces before the installation operation. Bowed pipe pieces and miter joints are two recommended approaches for walking mitigation. The presented mitigation strategies are relatively cost-effective solutions for the pipe-walking challenge and they are able to considerably cease the potential cyclic walking. Comprehensive FE analyses in ABAQUS software are performed to evaluate the proposed deformed pipelines response subject to two loading conditions. Through-life integrity of the suggested pre-deformed pipeline is assessed in terms of effective axial force, local buckles and excessive axial strains. A comparison of the presented method with conventional techniques shows the effectiveness of the proposed configuration. The proposed methods can significantly reduce effective axial force throughout the subsea pipeline by means of artificially introduced deformations. The cumulative walking of the presented method is practically zero. In addition, the influence of combined triggering mechanisms to the walking phenomenon is assessed when the pipeline is located on a sloping seabed and it is subject to non-uniform thermal loads. A parametric study is performed to improve confidence in design and provide a reasonably practical technique with an optimal shape.

Centrifuge modelling of pipe-soil interaction in clay with crust layer

Seabed in regions, such as the Gulf of Guinea and North West Shelf of Australia, may exhibit a crust layer where the undrained shear strength can be an order of magnitude higher than that of the immediately underlying sediment. This can complicate design of steel catenary risers, where fatigue depends on the cyclic vertical stiffness of the pipe-soil interaction. Potential punch-through of the riser into the underlying soft soil may invalidate design assumptions based on the pipe-soil stiffness within the crust layer. The long-term evolution of pipe-soil stiffness within the crust layer, which exhibits similar properties to an over-consolidated soil, is also poorly understood. This paper describes centrifuge model tests undertaken in a clay sample with a crust layer, simulating the punch-through process of a pipe under load control and investigating the pipe-soil stiffness during long-term cyclic loading tests under displacement control. Results confirm that the potential for punching-through the crust layer depends strongly on the relative ratio of pipe diameter to crust layer thickness. The long-term evolution of pipe-soil stiffness showed a steady increase after an initial remoulding stage in contractile soils (normally consolidated and lightly over-consolidated), but a steady reduction in the heavily over-consolidated, more dilatant, crust. The magnitude of pipe-soil stiffness changes (during both remoulding and reconsolidation) is governed by the over-consolidation ratio of the soil and the amplitude of the cyclic displacements. This study provides insights on the relevant cyclic stiffness to consider when assessing SCR fatigue life in over-consolidated soils and soils exhibiting a superficial crust layer.

Research on the pipeline walking caused by cyclic increasing soil friction for free deep-sea submarine pipelines laid on even seabed

Pipelines are important to offshore oil and gas development, but suffers from the pipeline walking phenomenon due to cyclic temperature variations—where large axial walking distances threaten the safety of pipeline systems. Current research indicates that pipeline walking is triggered by steel catenary riser (SCR) tension, seabed slopes, or thermal transients. This paper proposes a new driving mechanism for the pipeline walking phenomenon, involving cyclic hardening soil strength. The finite element analysis method was adopted to analyse the soil friction difference induced walking phenomenon, and the influence of key parameters on the gain in soil friction on walking distance was studied. Pipeline walking distances under different drainage conditions in the heating and cooling processes were also calculated, and the impact of the degree of drainage in the heating process was determined. To better understand the new pipeline walking mechanism, theoretical analysis of the walking behaviour under different cyclic soil friction conditions was carried out. Analytical solutions for estimating the pipeline walking distance were also provided, based on the simplified theoretical analysis.

An experiment study of vortex induced vibration of a steel catenary riser under steady current

The vortex induced vibration (VIV) of marine risers has been investigated by many researchers in experimental studies of a straight flexible riser model as well as a rigid cylinder to reveal the dynamic response characteristic and the mechanics behind it. However, due to the limitation of experimental apparatus, very few studies are about the VIV of a steel catenary riser (SCR) which is with a complex geometry. To investigate the VIV features and to further develop the corresponding numerical predictions of a SCR under steady current, a large-scale model test of a SCR was towed in an ocean basin at various speeds. Fiber Bragg grating strain sensors are instrumented on the riser model to measure both in-plane and out-of-plane responses. The characteristics of oscillating amplitude and dominating frequency response, the phenomenon of mode competition and travelling wave and the fatigue damage of the steel catenary riser in inline and cross-flow direction under steady current are analyzed.