Riser Response Research Articles

Post-failure behavior of lazy-wave risers

This paper addresses the modeling and post-failure analysis of flexible lazy-wave risers to understand their behavior after the rupture since the impact of falling risers can damage neighboring equipment and structures. Effects of important parameters, such as integration algorithm, time step, longitudinal drag coefficient, riser slenderness, moment–curvature relationship, soil stiffness, failure point, and current direction, are assessed. A methodology for the definition of a safe zone to avoid impact with the falling riser is presented. It is shown that longitudinal drag coefficient and riser slenderness significantly influence the physical response of lazy-wave risers after failure. Furthermore, use of nonlinear moment–curvature relationship is necessary to obtain more realistic results, and consideration of currents in different directions is important to properly determine the safe zone. Methods and results presented in this paper may assist in reducing the occurrence of damage in elements of offshore systems due to impact with falling risers.

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.

Interaction of wave motion and water hammer on the response of the risers in deep-sea mining

The expending global economy is expected to result in a shortage of mineral resources. Deep-sea mining offers a promising solution to this problem by using the hydraulic lifting technique. However, the riser system within this hydraulic system is associated with technical challenges: (a) heave motion induced by ocean waves causes longitudinal stress waves within the riser wall, and (b) blockages at the riser bottom arising from the clogging of mining nodules. The interaction between these phenomena as well as the interaction between the fluid and the riser wall pose technical challenges that remain unaddressed. In response to these challenges, a partitioned shell-based water hammer model with two-way fluid-pipe interaction was developed in this study, aimed at predicting the longitudinal response of water-filled risers. A parametric study is then conducted to investigate the influences of the ocean wave, structural and fluid characteristics on the dynamic response of the riser. The model predicts the spatial and temporal distributions of the pressure and velocity within the fluid inside the riser, in addition to the displacements, velocities, and stresses within the riser wall under both normal operating conditions and end-blocked conditions. The study then provides practical recommendations for engineering hydraulic lifting techniques.

Numerical Analysis of Mechanical Behaviors of Composite Tensile Armored Flexible Risers in Deep-Sea Oil and Gas

As oil and natural gas production continue to go deeper into the ocean, the flexible riser, as a connection to the surface of the marine oil and gas channel, will confront greater problems in its practical application. Composite materials are being considered to replace steel in the unbonded flexible pipe in order to successfully meet the lightweight and high-strength criteria of ultra-deep-water oil and gas production. The carbon-fiber-reinforced material substitutes the steel of the tensile armor layer with a greater strength-to-weight ratio. However, its performance in deep-water environments is less researched. To investigate the mechanical response of a carbon fiber composite flexible riser in the deep sea, this study establishes the ABAQUS quasi-static analysis model to predict the performance of the pipe. Considering the special constitutive relations of composite materials, the tensile stiffness of steel pipe and carbon fiber-reinforced composite flexible pipe are predicted. The results show that the replacement of steel strips with carbon fiber can provide 85.06% tensile stiffness while reducing the weight by 77.7%. Moreover, carbon-fiber-reinforced strips have a lower radial modulus, which may not be sufficient to cause buckling under axial compression, so the instability of the carbon fiber composite armor layer under axial compression is further studied in this paper; furthermore, the characteristics of axial stiffness are analyzed, and the effects of the friction coefficient and hydrostatic pressure are discussed.

Dynamic instability mechanisms of offshore pipelines and risers

Dynamic response of offshore pipelines and risers under cross-flow (e.g. ocean currents, vessel motion etc.) has successfully been analysed. Although the pipelines are used for fluid transportation, sufficient attention has not been paid to the dynamic instability under axial flow. In this study, axial flow-induced instability mechanisms of offshore pipelines and risers are investigated. To this end, the interaction of the axial flow forces of a fluid element with the forces of the corresponding pipe element during pipeline flexural motion is analysed. Interpretation of the terms of the motion equation is provided and analogies with the model of dynamic buckling of a Euler beam are detected. Original transfer matrices are developed for the dynamic control of pipelines under axial flow. The critical values of the flow velocity causing dynamic instability and the corresponding axial flow-induced buckling modes are determined. Implementation of the method to a representative case study is provided and discussed.

Prediction model for multidirectional vortex-induced vibrations of catenary riser in convex/concave and perpendicular flows

This study presents an advanced numerical prediction model based on nonlinear wake oscillators for the investigation of multidirectional vortex-induced vibration (VIV) responses of a long catenary riser depending on the structural curved configuration versus the incoming flow direction. By considering a uniform free-stream flow aligned with the initial curvature plane of the catenary riser in a convex or concave shape, the normal flow velocity component is nonlinearly sheared owing to the spanwise variation of inclination angles. This is different from the perpendicular flow case in which the normal flow velocity is spatially constant. To capture the influence of flow direction on the three-dimensionally coupled cross-flow and in-line VIV, distributed wake oscillators are introduced and applied to simulate the fluctuating hydrodynamic lift and drag forces that account for the important effects of cylinder inclination, variable vortex excitation frequencies, global–local​ displacement relationships, and relative flow-cylinder velocities. The amplified mean drag force and the axial force depending on the tangential flow velocity are also considered. These empirical hydrodynamic effects are nonlinearly coupled with the equations of riser three-dimensional motion, and the overall governing equations are numerically solved to assess the multimode, multi-frequency and multi-degree-of-freedom responses. Validation of the model is carried out for VIV of flexible straight and curved cylinders based on experimental results in the literature. Parametric investigations are performed by varying the incoming flow velocities in perpendicular, convex and concave configurations for a long catenary riser with a low mass-damping ratio. In-plane (horizontal and vertical) and out-of-plane VIV features are presented and discussed in terms of space–time varying amplitudes, drag-amplified shape reconfigurations, oscillation frequencies, lock-in occurrences, dual resonances, orbital motion trajectories, fluid–structure energy transfer, and multimodal contributions. Overall prediction results highlight the influence of the cylinder geometric configuration versus the incoming flow orientation on multidirectional VIV characteristics that should be recognised and incorporated into practical analysis tools.

Synthesis and Soft Implementation of Supervisory Control Scheme in an Industrial Fluid Catalytic Cracking (FCC) Unit of a Nigerian Refinery

In this paper, a self-tuning hierarchical controller in which a Fuzzy logic controller supervises the control actions of a conventional PID has been proposed, implemented and presented. The controller has been applied to a control study of Fluid Catalytic Cracking (FCC) unit riser temperature, and regenerator temperature respectively. Performance comparison of the proposed Fuzzy-PID controller and the conventional PID was made in simulation studies of regulatory and servo performances of the two controller types. Six performance measures: Percent overshoot (OS), settling time (ST), integral absolute error (IAE), integral square error (ISE), integral time absolute error (ITAE) and integral time square error (ITSE) were employed as the tools for performance comparison between the conventional PID and the Fuzzy-PID controller. For the tracking of riser temperature with a set point at 524oC, the performance indicators under PID control gave the following results overshoot (14.5%); settling time (40 seconds) Integral absolute error (8.246), integral square error (3.3); integral time absolute error(1762);integral time square error (43.77) while for the same indicators under Fuzzy-PID control the following values: overshoot (3.3%); settling time (40 seconds) ;Integral absolute error (8.811); integral square error (14.5); integral time absolute error(280),;integral time square error (31.11) .The results allude to the superiority of the fuzzy-PID scheme over the PID scheme in tracking the optimal set point of riser temperature. More so, for tracking the regenerator set point temperature of 746oC, comparative study of step response under the two schemes gave the following results in six performance indicators: overshoot (PID(12.6%)/Fuzzy-PID(6%)); settling time (PID(80 seconds)/Fuzzy-PID(20seconds));Integral absolute error(PID(14.29)/Fuzzy-PID(8.63)); integral square error(PID(6.713).Fuzzy-PID(4.506)); integral time absolute error(PID(2858)/Fuzzy-PID(305.9)), integral time square error (PID(77.55)/Fuzzy-PID(33.05)).Moreover, the fuzzy-PID controller also showed superior performance over the conventional PID controller in terms disturbance rejection (regulatory response) of both riser and regenerator temperature. The results from this study suggest that the application of fuzzy-PID scheme to temperature control offers good promise of improved fluid catalytic cracking unit (FCCU) operations.

Effect of internal solitary wave on the dynamic response of a flexible riser

Dynamic responses can arise when a flexible riser encounters the internal solitary wave (ISW) in the ocean. It is, thus, necessary to explore its dynamical behaviors induced by ISW. First, the governing equations of the flexible riser, ISW, and hydrodynamic force are introduced and interpreted. The accordant validations are performed so as to examine the accuracy of the applied models. Then, the dynamic response of the flexible riser is studied with the increase of the nondimensional ISW amplitude. Representative characteristics, such as vortex-induced vibration (VIV), top tension, and stress along the flexible riser, are mainly analyzed and discussed. The results prove that with the propagation of ISW, VIV in both inline (IL) and crossflow (CF) directions can be incurred. The maximum values of IL and CF oscillation frequencies, top tension, inline deflection, as well as riser stress can be detected while the ISW trough reaches to the flexible riser. Furthermore, all these four dynamic characteristics show an increasing trend with the increase of the nondimensional ISW amplitude. Note that the maximum values of oscillation frequencies, inline deflection, and stress on the upper section of the flexible riser tend to be larger compared with those on the lower section. In addition, the maximum inline deflection, top tension, and stress along the flexible riser can be affected by VIV responses.

Dynamic Analysis of Marine Drilling Risers Considering Steady Current, Surface Wave, Internal Solitary Wave, and Vessel Motion

In this paper, considering combined excitations of the steady current, surface wave, internal solitary wave as well as, the vessel’s surge and heave motions, a time‐domain model is established to investigate the lateral dynamic response of marine drilling risers. The finite element and Newmark‐β methods are used to solve the nonlinear partial differential equation in the time‐domain. The entire numerical solution process is realized by a self‐developed program based on MATLAB. The reliability of the numerical solution program is verified by comparing it with published numerical results. The lateral dynamic responses of a real‐scale marine drilling riser, usually used in the deep water oil/gas industry in the South China Sea, are studied. The impact level of each factor in the riser’s lateral dynamic response is discussed.

Analysis of full-scale riser responses in field conditions based on Gaussian mixture model

Offshore slender marine structures experience complex and combined load conditions from waves, current and vessel motions that may result in both wave frequency and vortex shedding response patterns. Field measurements often consist of records of environmental conditions and riser responses, typically with 30 min intervals. These data can be represented in a high-dimensional parameter space. However, it is difficult to visualize and understand the structural responses, as they are affected by many of these parameters. It becomes easier to identify trends and key parameters if the measurements with the same characteristics can be grouped together. Cluster analysis is an unsupervised learning method, which groups the data based on their relative distance, density of the data space, intervals, or statistical distributions. In the present study, a Gaussian mixture model guided by domain knowledge has been applied to analyze field measurements. Using the 242 measurement events of the Helland-Hansen riser, it is demonstrated that riser responses can be grouped into 12 clusters by the identification of key environmental parameters. This results in an improved understanding of complex structure responses. Furthermore, the cluster results are valuable for evaluating the riser response prediction accuracy.

Influence of structural parameters of unbonded flexible pipes on bending performance

Unbonded flexible pipes have complex structural forms and numerous structural parameters. There are different degrees of contact and slip between the structural layers under bending loads, which brings challenges to the structural design and bending characteristics analysis of flexible pipes. In this paper, according to the law of energy conservation and deformation geometry theory, the mechanical equilibrium equation of each layer of unbonded flexible riser under bending load is established and the contact condition between layers is introduced in the first step. According to the slip state of the spiral strip, the response of the flexible riser under bending load was divided into three stages: no slip, partial slip and complete slip. The expressions of bending moment and bending stiffness were derived. The influence of structural parameters of tensile armor layer of unbonded flexible pipes on its bending performance under bending load is studied by using MATLAB software to compile the corresponding calculation program. The results show that the increase of friction coefficient between layers of tensile armor will improve the bending resistance of flexible pipes. The increase of helix angle will reduce the bending stiffness of flexible pipes in the initial stage. The increase of the number of spiral strips will increase the bending stiffness of flexible pipes in the initial stage. In the process of structural design of flexible pipes, the influence of the above structural parameters on the bending characteristics of flexible pipes should be taken into account.

An insight into the interaction between the in-plane and out-of-plane responses of a catenary flexible riser

Nonlinear dynamic response of catenary flexible risers excited by ocean currents is the main reason for fatigue failures. In this work, a series of experiments are conducted to improve the understanding of the intrinsic relationship between the in-plane and out-of-plane responses of a catenary flexible riser subjected to a sheared current in the depth-averaged Reynold number range of 150–1880. It is found that the interaction between the in-plane and out-of-plane responses is sensitive to the reduced velocity. Four patterns are identified, including the weak coupling, the partial coupling, the strong coupling, and the strong coupling+. The interaction is negligible at low reduced velocities. In contrast, when the reduced velocity is sufficiently large, the in-plane dominant frequency coincides with the out-of-plane one, signifying a strong coupling. As the reduced velocity increases, only a part of the riser possesses the same dominant frequency in the in-plane and out-of-plane directions, termed partial coupling. At higher reduced velocities, the coupled pipe section is shortened and the interaction is finally attenuated.

A study on the dynamic responses of a steel catenary riser transporting varying-density flow and subjected to regular waves

In order to transport crude oil and natural gas from seabed wells to surface floating platforms, the steel catenary risers have been widely used, which are long, flexible and inclined with geometric and structural nonlinearities. These risers are inevitably subjected to waves and vibrate with time. Meanwhile, they may transport oil and natural gas at the same time, in which the total fluid density may change with time and space. In this paper, a nonlinear dynamic model of a steel catenary riser excited by the internal varying-density flow and the external regular waves is theoretically modelled. The present model is numerically solved and validated with the comparisons to experiments and simulation results. The dynamic responses of the riser are analysed and the results demonstrate that when the fluid density inside the riser is fluctuating with a small circular frequency, the vibrations of the riser will be dominated by the internal varying-density flow, however for a large circular frequency, the vibrations of the riser will be dominated by the external regular waves. The dynamic responses of the riser in the region dominated by the external regular waves will be seldom affected by the internal varying-density flow and vice versa.

Energy relationships in transient pipe flow with fluid–structural interaction

During water hammer wave propagation, pressure and velocity fields vary over time due to transformations between strain and kinetic energies. In the literature, some energy expressions have been applied to explore energy transformations during water hammers, which account for various effects including friction stress, leaks, blockages, air pockets, and side flows. However, there are no energy expressions for water hammers that consider the fluid–structural interaction (FSI) effect, such as a buffer blocked riser subjected to heave motion. Therefore, in this study, the energy expressions for transient flow with FSI are obtained from the governing equations of the extended water hammer model. Moreover, based on continuum mechanics theory, general energy expressions are developed for the classical, the extended, and the partitioned shell–based water hammer models. Using the partitioned shell–based water hammer model, a buffer–blocked riser subjected to heave motion was investigated for energy transformation. Accounting for the fluid–structural interaction, there are three resonances in the dynamic response of riser while the heave motion frequency varies in the range of ocean wave frequencies. These three resonant frequencies are correlated to the frequency of pressure wave, the fundamental frequencies of fluid–filled riser that only accounted for fluid mass and the empty riser.

Three-dimensional nonlinear vortex-induced vibrations of top-tension risers considering platform motion

A novel three-dimensional (3-D) dynamic model considering nonlinearity of tension is proposed to explore in-line (IL), cross-flow (CF) and axial (AX) vortex-induced vibration (VIV) responses of riser under platform motion. The interaction between riser and external flow is simulated by the van der Pol equation, and the nonlinear equations of motion are established by Hamilton's principle and the Galerkin method, and solved by Runge-Kutta method. The influences of heave and swing motions of platform on VIV responses of the riser are investigated. It is found that for large velocity of external flow, the axial displacement of riser results in an increase in IL excitation mode and also an increase in IL oscillating displacement. When the riser is excited by swing motion of platform, with the increase of excitation amplitude, the maximum response amplitudes will also increase in all the three directions, and the amplitudes in high frequency domain increase more obviously. Combined effect of heave and swing motions of the platform results in the increase of VIV amplitudes in IL and AX directions, and combined action of forced and parametric excitations has little effect on response amplitude of riser in CF direction.

An Efficient Methodology of Estimating the Extreme Response of Risers from Floaters in Multidirectional Sea States

An Efficient Methodology of Estimating the Extreme Response of Risers from Floaters in Multidirectional Sea States

Analysis of flow-induced vibration characteristics for flexible riser attached with non-rotating water-drop fairing

One of the measures of suppressing vortex-induced vibration (VIV) for the offshore riser is to install a streamlined fairing device. However, the galloping phenomenon may occur for a non-rotating water-drop fairing, which has a serious influence on the riser operation. An improved meshless discrete vortex method (DVM) in a two-dimensional framework is employed to calculate the fluid forces for flow past an arbitrary geometry. Meanwhile, by coupling the mass-damping-stiffness kinematic equation, VIV responses of a circular cylinder with one degree-of-freedom (DOF) under a wide range of reduced velocities can be determined. Then, systematic simulations are carried out to investigate the effects of physical parameters, including free-stream velocity and trailing edge angle, on the galloping characteristics of fairing structures. At last, a quasi-three-dimensional DVM-finite element method (FEM) coupled strategy is employed to calculate the vibration responses of a flexible riser attached with the water-drop fairing. The results indicate that the non-rotating water-drop fairing can bring about intense galloping instability behaviour. The predicted vibration amplitudes, frequencies and fluid forces for different fairings as well as the circular cylinder are compared. Furthermore, a comparison between flow-induced vibration (FIV) characteristics of the bare riser and flexible risers fitted with various fairing devices is conducted.

Vortex-induced vibration of flexible riser transporting high-speed spiral flow in deep-sea mining

According to Hamilton's principle and a mechanical decomposition model proposed for high-speed spiral flow, vibration governing equations of flexible riser transporting high-speed spiral flow are developed for vortex-induced vibration (VIV) analysis for lifting pipe system in deep-sea mining. The classic van der Pol oscillator is used to simulate the dynamic characteristics of VIV. The governing equations are then discretized and solved by the finite element method and the Runge-Kuta method. Furthermore, dynamic characteristics of VIV of the flexible riser transporting straight flow and spiral flow are investigated and the effects of the spiral flow incidence velocity and angle controlled by the jet device on VIV responses are evaluated. Also, the VIV dynamics considering different damping ratios are examined. The results show that the rotational angular velocity of the spiral flow would amplify the mean deflection of the in-line (IL) direction, and the incidence velocity has a significant nonlinear effect on the dominant frequency of the riser, which would induce the high-order dominant vibration modes when the incidence velocity is large sufficiently. However, the incidence angle has relatively less effects on the VIV responses of flexible riser. Moreover, the locked interval of VIV is less influenced by damping ratio below 0.1.

Numerical prediction of vortex-induced vibrations of a long flexible riser with an axially varying tension based on a wake oscillator model

A numerical study based on the wake oscillator model has been conducted to determine the vortex-induced vibration (VIV) responses of a flexible riser with an axial time-varying tension. Three different types of flows, viz., linear shear, exponential shear, and real stepped flows, have been considered. The coupling equations of a structural oscillator and a wake oscillator have been solved using a standard central finite difference method of the second order. The VIV response characteristics including the structural displacement, structural frequency, displacement envelope, and displacement evolution for three different flow profiles have been systematically compared. Subsequently, for each flow profile, the effect of the different tension models on the VIV characteristics has been investigated. The numerical results indicated that, both the VIV displacement and VIV frequency in real stepped flow had diverse characteristics from those in linear shear flow. Compared with the corresponding VIV characteristics in real stepped flow, the VIV displacement in exponential shear flow was similar, however, the VIV frequency in exponential shear flow was disparate. The VIV displacement with the constant tension model had larger values than that with the real tension model.

Optimized parametric hydrodynamic databases provide accurate response predictions and describe the physics of vortex-induced vibrations

We address the problem of obtaining accurate predictions of the vortex-induced vibrations (VIV) of flexible structures placed in cross-flow by reconstructing fluid force databases in parametric form. The forces are expressed in terms of hydrodynamic coefficients, which depend on several variables and parameters, including the structural properties and geometry, frequency and amplitude of vibration, and the flow conditions, so that a comprehensive database obtained with systematic experiments would require an intractably large number of tests. Therefore, we develop a method for determining optimal parametric hydrodynamic databases directly from a variety of free- and forced-vibration experiments on both rigid and flexible cylinders, by minimizing the error between the predicted responses using a software driven by the force database, and the measured data. We first describe the process of selecting an appropriate parametrization of the fluid forces as function of reduced frequency and non-dimensional amplitude of vibration, informed by Gaussian-Process-Regression-guided automated experiments. Several examples are selected to demonstrate the method’s ability to acquire optimal hydrodynamic databases, including data sets highlighting the effect of Reynolds number and the addition of in-line motion to cross-flow rigid cylinder VIV. The proposed methodology is further demonstrated to be effective when used with the semi-empirical program VIVA for flexible cylinder VIV, yielding significant improvements in prediction accuracy compared to baseline results. It therefore serves as a potential solution for the development of a digital twin for marine risers, an in-situ, real-time riser response prediction and structural health monitoring system.