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Fatigue failure and stress damage are the main damage forms of marine risers.To study the variation rule of surface magnetic field signals of X80 steel during tensile fatigue process under geomagnetic excitation, the fatigue tensile test of X80 steel specimens under typical working conditions was carried out with an MTS810 250 fatigue tensile testing machine. During the experiment, the variation of magnetic field on the surface of the tensile specimen was monitored by a magnetic memory sensor, the fatigue failure of the specimen was observed by a handheld microscope, and the morphology of the fracture was analyzed with the help of scanning electron microscope(SEM).The results show that the magnetic memory is sensitive to the tensile fatigue damage. Thus, the characteristic parameters of magnetic memory detection for tensile fatigue damage of X80 steel were proposed, which took the extreme value of magnetic memory signal with background removed HE(X) and the line slope of the extreme value of tangential gradient signal SK(X) as the eigenvalues. As a result,zero crossing phenomena occur to HE(X) and SK(X) in the stage of crack initiation. The acceleration of HE(X) and SK(X) speeds up with the increase of the fatigue cycle, which is consistent with the crack growth law during the fatigue process. Generally, HE(X) and SK(X)can reflect the fatigue failure process of the specimen, and they can be used to monitor the fatigue damage of marine risers. This research results could provide a reference for the field application of the magnetic memory detection method in the fatigue damage detection of marine risers.

Vortex-induced vibration (VIV) of marine risers under external currents may cause serious fatigue damage, which is a challenging issue especially when the internal flow is included simultaneously. The temperature gradient between the conveying fluid and the surrounding seawater makes the internal flow effect more complicated. In this paper, the effect of internal flow with a non-uniform temperature environment is added to the riser's dynamic governing equation, and the semi-empirical model is adopted to simulate VIV. A large number of cases are designed to analyse the dynamic response and fatigue damage of the riser when the internal flow and non-uniform temperature environment are individually considered. Then, the combined effect of the internal flow and temperature environment is compared with the simple superposition of their individual effects. The increase in the internal flow temperature could trigger mode jump and enlarge the fatigue damage. [ABSTRACT FROM AUTHOR]

Due to the influence of the geometric complexity and the interlayer contact of composite materials, the numerical calculation of axial compression failure of marine risers is complex and time-consuming.Therefore, a simplified model of axial compression failure of marine risers, in which the 8-layer pipe section is simplified to a 5-layer pipe section, was proposed assuming the equivalent stiffness.The rationality and the practicability of the simplified model was verified through stiffness checking and experimental comparison.The failure response of riser under axial compression was studied based on the simplified model, which improved the convergence of nonlinear analysis and computational efficiency.The results show that, when the wear-resistant layer and the outer protective layer reach the yield strength of the material, the inner and outer tensile armor layers will expand rapidly in radial direction to form a separation gap.The wear-resistant layer and the outer protective layer will be destroyed if they reach the ultimate strength of the material, leading to a maximum separation gap in the inner and outer tensile armor layers and depriving the riser's resistance to axial compression.The simplified model could simulate the axial compression failure of marine risers, providing technical support for the design and installation of marine risers.

The target of this paper is designing a boundary controller for vibration suppression of marine risers with coupling mechanisms under environmental loads. Based on energy approach and the equations of axial and transverse motions of the risers are derived. The Lyapunov direct method is employed to formulated the control placed at the riser top-end. Stability analysis of the closed-loop system is also included. [ABSTRACT FROM AUTHOR]

This study investigates the effects of multiphase internal flows that consider hydrate phase transitions on the parametric stability of marine risers. A numerical model of the multiphase internal flow that considers a hydrate phase transition is established. The model first solves the flow parameters and subsequently obtains the natural frequencies of risers with different gas intake ratios. The stability charts of marine risers with different gas intake ratios are plotted by applying Floquet theory, and the effects of the gas intake ratio on the instability and vibration response of the risers are identified. The natural frequency increases with an increase in the gas intake ratio; thus, instability zones move to higher frequency ranges in the stability charts. As the increasing gas intake ratio reduces the damping effect of the Coriolis force, the critical amplitude of the heave in the unstable region decreases, especially when hydrodynamic damping is not considered. As a result, higher-order unstable regions are excited. When in an unstable region, the vibration response curve of a riser with a high gas intake ratio excited by parametric resonance diverges quickly due to parametric resonance. [ABSTRACT FROM AUTHOR]

The marine risers are often subjected to parametric excitations from the fluctuation top tension. The top tension on the riser may fluctuate with multiple frequencies caused by irregular waves. In this paper, the influence between different frequency components in the top tension on the riser system is theoretically simulated and analyzed. With the Euler-Bernoulli beam theory, a dynamic model for the vibrations of the riser is established. The top tension is set as fluctuating with time and it has two different frequencies. The influences from the fluctuation amplitudes, circular frequencies and phase angles of these frequency components on the riser system are analyzed in detail. When these two frequencies are fluctuating in the stable regions, the riser system may become unstable because ?1+?22On. The fluctuation amplitudes of these frequencies have little effect on the components of the vibration frequencies of the riser. For different phase angles, the stability and dynamic behaviors of the riser would be different. [ABSTRACT FROM AUTHOR]

This paper formulates and solves an inverse optimal gain assignment control problem for the evolution systems perturbed by unknown (bounded) disturbances in Hilbert spaces. The control design ensures global well-posedness and global (practical) K ∞ -exponential (respectively, strong or weak asymptotic) stability of the closed-loop system, minimizes a cost functional that appropriately penalizes state, control, and disturbances in the sense that a cost functional, which is positive definite in state and control, and is negative definite in disturbances, is minimized. Thus, the proposed control law minimizes both state and control while attenuates the disturbances. Moreover, it is not required to solve a Hamilton–Jaccobi–Isaacs equation (HJIE) but the Lyapunov functional used in the control design is exactly the solution of a family of HJIEs. The developed results are illustrated via an application to design new inverse optimal gain assignment boundary control laws for mitigating vibration of extensible marine risers under either a positive or zero or even negative pretension subject to sea loads. This optimal control problem for marine risers has not been addressed in the literature. [ABSTRACT FROM AUTHOR]

• State-of-the-art and historical development of composite risers for deep waters. • Research findings on composite risers from detailed bibliographic review. • Overview on composite riser assessment based on structural and mechanical behaviour. • End-fitting designs, and stress profile from load responses on composite risers and end-fitting. • Marine industry application of marine riser configurations, like SLHR for composite risers. The need for the utilisation of marine risers in deep seas has increased in recent years and this is due to increased drilling explorations with the shift in trend from shallower to deep and deeper waters. Also, there have been increased applications in sea-crossing infrastructure resulting in the need for longer risers, leading to a significant weight increase of marine risers used. Composite materials can thus be utilised in marine riser engineering to provide lightweight, fatigue-resistant, corrosion-resistant, low-bending stiffness and high-strength characteristics. In this paper, the history and potential of composite marine risers, including the first successful deployment of a composite riser joint offshore on the Heidrun Platform in 1995, are reviewed. The paper also discusses the advances achieved on composite marine risers for deep waters and presents some recommendations on their use, in light of their current significance and growth. [ABSTRACT FROM AUTHOR]

This paper considers a co-rotational beam formulation for beams, which is used for the finite element analysis of flexible risers and pipelines made of functionally graded materials. The influence of material gradation is addressed using an exponential variation of properties throughout the thickness of the pipe. Space discretization of the equilibrium equations is derived based on the Euler–Bernoulli assumptions considering two-node Hermitian beam elements which are referred to a co-rotation coordinate system attached to the element local frame of coordinates. The geometric non-linear effects of the beam are considered under large displacement and rotations, but under small-strain conditions. The deflections of the riser result from forces caused by self-weight, buoyancy, sea currents, waves, the action of floaters, seabed-structure interactions, and ship’s motion. We provide numerical examples and compare our results with the ones available in the literature. In addition, applications related to practical offshore engineering situations are considered to highlight the behavior of functionally graded materials (FGMs) as compared to homogeneous risers. [ABSTRACT FROM AUTHOR]

To analyze the stress wave propagation associated with the vortex-induced vibration (VIV) of a marine riser, this paper employed a multi-signal complex exponential method. This method is an extension of the classical Prony's method which decomposes a complicated signal into a number of complex exponential components. Because the proposed method processes multiple signals simultaneously, it can estimate the 'global' dominating frequencies (poles) shared by those signals. The complex amplitude (residues) corresponding to the estimated frequencies for those signals is also obtained in the process. As the signals were collected at different locations along the axial direction of a marine riser, the phenomena of the stress wave propagation could be analyzed through the obtained residues of those signals. The Norwegian Deepwater Program (NDP) high mode test data were utilized in the numerical studies, including data sets in both the in-line (IL) and cross-flow (CF) directions. It was found that the most dominant component in the IL direction has its stress wave propagation along the riser being dominated by a standing wave, while that in the CF direction dominated by a traveling wave. [ABSTRACT FROM AUTHOR]

Experiments with a flexible cylinder of circular cross-section towed behind a stationary circular cylinder at various distances are described in this paper. The dynamics of the model immersed in the wake of the leading one are fully characterised after data processing and computation of amplitude and frequency responses, as well as modal content as a function of reduced velocity. Reduced velocity was controlled by changing the towing speed of the system and by adjusting different pre-tensions on the model. The amplitude response of the model has been found to be dominated by resonance or lock-in regions in the reduced velocity axis at the different structural modes of the cylinder model. Separation distance does not produce significant qualitative changes in the response amplitudes and frequencies observed when compared to the response of the same cylinder without wake interference. However, there is a clear effect of the incoming wake, because amplitude responses are considerably higher than those expected in isolated cylinders at the same reduced velocities. Also, the response has been found to be more multi-modal with responses made of large amplitude contributions of several adjacent modes. [ABSTRACT FROM AUTHOR]

A floating production system is exposed to many sea conditions over its lifetime. Consequently, fatigue design of a riser system is challenging, owing to the need to consider the long-term wave condition, characterized by the joint statistics of the significant wave height and peak wave period. The problem becomes computationally intractable if dynamic analysis is performed in the time domain, which is often necessary due to nonlinearities. This paper outlines a new efficient approach for long-term riser fatigue analysis, in conjunction with time domain simulations. The time series of an irregular wave elevation is widely simulated as the sum of regular wave components with random amplitudes and phase angles. Importance sampling is the basis of the proposed approach; however, it is not exploited for variance reduction. Instead, this technique allows one to simulate the wave amplitudes from a distribution different from the original one. This implies that following a dynamic analysis, the results for many sea states can be obtained by simply altering the importance sampling weighting function. The proposed approach is enhanced by a succession of additional techniques to reduce the sampling variability. Since importance sampling is not an approximate method, but an alternative means of simulation, the predicted long-term fatigue damage is unbiased. The standard error is estimated via bootstrapping. [ABSTRACT FROM AUTHOR]

Due to the complexity of riser dynamic behavior in various sea conditions, the reentry of the deepwater drilling riser to the seabed wellhead is difficult to perform. To facilitate reentry operation, Ant Colony Optimization (ACO) algorithm is adopted to optimize the movement of the riser upper end. Under optimized movement, the distance between the riser lower end and the seabed wellhead is shortened, and the oscillation of the riser lower end is damped quickly. To test the efficiency of reentry motion optimization, the water-tank experiments are carried out. The experimental results show the good efficiency of the movement optimization. For a 1500 m riser in deepwater drilling, the numerical simulations of ordinary and optimized reentry motions are shown also. [ABSTRACT FROM AUTHOR]

A vertical two-dimensional numerical model is employed to study the scour beneath a vibrating pipe exposed to steady currents. The ability of the model is first validated against previous scour experiments concerning both fixed and vibrating pipes. Both the development of scour depth and the final bed profile are generally well reproduced by the model. Subsequently, a broad range of the vibration parameters, including the amplitude and frequency, are tested to reveal their influences on the equilibrium scour depth. A generic relationship between the vibration frequency, amplitude, and scour depth is established by constructing suitable nondimensional groups. The maximum scour depth is shown to increase with the vibration amplitude and frequency. At small frequencies, both the frequency and amplitude have a pronounced influence on the scour depth; with the increase of the vibration frequency, the influence of the frequency gradually diminishes and the scour depth is mainly decided by the vibration amplitude. Two empirical formulas are proposed to describe the quantitative relationship between the frequency, amplitude, and scour depth. [ABSTRACT FROM AUTHOR]

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Flexible Segment Model (FSM) based optimization method is proposed as a robust dynamics calculation method for underwater Moving Slender Bodies (MSBs). In the method, the underwater MSB is divided into a series of flexible segments, their deformations are analyzed individually, and the dynamics calculation is accomplished by the optimization of a mechanical equilibrium function in a simple form. Because the whole deformation of a MSB is decomposed into small ‘curve’ deformations of all segments, the dynamics calculation needs a relatively small number of segments for accuracy. The comparison with experimental results shows that the numerical results by FSM based optimization method have good accuracy. Moreover, the tests of sensitivity and error tolerance show the numerical calculation by FSM based optimization method has high stability. [ABSTRACT FROM AUTHOR]

Marine risers are important components operating in offshore oil and gas industry. The vortex-induced vibration design of marine risers requires accurate knowledge of natural frequencies and mode shapes. Free vibration of marine risers are re-examined in this paper by means of variational iteration method, which is relatively new technique capable of dealing with eigenvalue problems rather efficiently. Solutions from the variational iteration method are compared to approximate solutions previously proposed in literatures via a numerical example. Furthermore, validation of the technique is demonstrated by comparing experimentally measured natural frequencies of model marine riser with the predicted ones. [ABSTRACT FROM AUTHOR]

The goal of this paper is to present a nonlinear approach to identify the structural parameters of a vertical riser. To carry out this task, Kalman-like observers are proposed so that they can deal with the nonlinear behavior that is exhibited in the restoring force of a riser and the noise of the measurement signals that are used to perform the identification. The observers are designed from a spatial discretization of a nonlinear model with distributed parameters that is described by a system of quasi-linear partial differential equations. Some results from numerical simulations—showing the feasibility of the proposed approach—are presented. [ABSTRACT FROM AUTHOR]

Experimental results show how vortex-induced vibration (VIV) amplitudes of flexible cylinders can be reduced up to a 90% by covering less than half of the length of the cylinder with splitter plates elastically mounted to the surface of the cylinder. The VIV amplitude reduction takes place along with drag coefficient reductions of up to a 50% for the reduced velocities investigated. [ABSTRACT FROM AUTHOR]

Based on dynamic response signals a damage detection algorithm is developed for marine risers. Damage detection methods based on numerous modal properties have encountered issues in the researches in offshore oil community. For example, significant increase in structure mass due to marine plant/animal growth and changes in modal properties by equipment noise are not the result of damage for riser structures. In an attempt to eliminate the need to determine modal parameters, a data-based method is developed. The implementation of the method requires that vibration data are first standardized to remove the influence of different loading conditions and the autoregressive moving average (ARMA) model is used to fit vibration response signals. In addition, a damage feature factor is introduced based on the autoregressive (AR) parameters. After that, the Euclidean distance between ARMA models is subtracted as a damage indicator for damage detection and localization and a top tensioned riser simulation model with different damage scenarios is analyzed using the proposed method with dynamic acceleration responses of a marine riser as sensor data. Finally, the influence of measured noise is analyzed. According to the damage localization results, the proposed method provides accurate damage locations of risers and is robust to overcome noise effect. [ABSTRACT FROM AUTHOR]

Abstract: A series of experiments describing the dynamic response resulting from vortex shedding excitation, on isolated long flexible cylinders with two different low mass ratios (mass to displaced fluid mass), are presented. The cylinder models had aspect ratios (length to diameter) of 158 and 187 and mass ratios of 1.1 and 2.7 respectively, both with very low damping ( of critical). The models were towed to generate a stepped current in a 100m long still water tank with a cross-section of 3.8×2.5m, and curvatures were measured at 11 strain gauge stations distributed along the length of the models, resulting in an extensive data set. Reynolds numbers over 37000 were reached and responding modes up to the fifth were observed, showing maximum amplitudes of more than 3 diameters in the model with the lowest mass ratio. Drag coefficients with values well over 3 were also measured. [Copyright &y& Elsevier]

Abstract: The use of composite risers may offer important advantages over the use of conventional steel risers in deepwater oil fields. However, the design of laminated composite risers is much more complex than the design of steel risers, due to the large number of parameters that need to be chosen to define the riser layup. This work presents a methodology for optimum design of composite catenary risers, where the objective is the minimization of cross-sectional area of the riser and the design variables are the thickness and fiber orientation of each layer of the composite tube. Strength and stability constraints are included in the optimization model and multiple load cases are considered. The methodology can handle both continuous and discrete variables. Gradient-based and genetic algorithms are used in the computer implementation. The proposed methodology is applied to the optimization of composite catenary risers with different water depths, liner materials, and failure criteria. The numerical examples show that the proposed methodology is very robust. [Copyright &y& Elsevier]

This paper presents a design of boundary controllers implemented at the top end for global stabilization of a marine riser in a three dimensional space under environmental loadings. Based on the energy approach, nonlinear partial differential equations of motion, including bending-bending and longitudinal-bending couplings for the risers are derived. The couplings cause mutual effects between the three independent directions in the riser's motions, and make it difficult to minimize its vibrations. The Lyapunov direct method is employed to design the boundary controller. It is shown that the proposed boundary controllers can effectively reduce the riser's vibration. Stability analysis of the closed-loop system is performed using the Lyapunov direct method. Numerical simulations illustrate the results. [ABSTRACT FROM AUTHOR]

Abstract: A marine riser must be designed with a high reliability as failure is disastrous. It is desirable for risers to be arranged in proximity to maximize the available subsea space. However, collision between neighboring risers is an issue that needs to be addressed. This paper presents a novel and efficient procedure for estimating the probability of riser clashing, so that the designer can optimally balance economy and risk. The proposed approach is based on post-processing the nodal positions obtained from time domain analyses, which are typically performed to satisfy other design requirements. A certain process is defined to characterize the instantaneous severity of the system response with respect to a potential collision event. The crossing rate of the process is extrapolated to the failure threshold to estimate the failure probability. Case studies, involving two flexible risers, are conducted to verify various hypotheses made in the method. Amongst other things, it is demonstrated that the extrapolation strategy is robust, and will generally result in a conservative prediction for riser clashing. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Vortex-induced vibration (VIV) of long flexible cylindrical structures (e.g. risers, pipelines, tendons, mooring lines) enduring ocean currents is ubiquitous in the offshore industry. Though significant effort has gone into understanding this complicated fluid–structure interaction problem, major challenges remain in modeling and predicting the response of such structures (for example a riser). The work presented in this paper provides a systematic approach to estimate and analyze the vortex-induced motions of a marine riser. A systematic framework is developed, which allows reconstruction of the riser motion from a limited number of sensors placed along its length. A full reconstruction criterion is developed, which classifies when the measurements from the sensors contain all information pertinent to riser VIV response, and when they do not, in which case additional, analytical methods must be employed. Reconstruction methods for both scenarios are developed and applied to experimental data. Finally, a systematic study on the error during the reconstruction is also undertaken. The methods developed in this paper can be applied to: improve understanding of the vortex shedding mechanisms, including the presence of traveling waves and higher-harmonic forces; develop tools for in-situ estimation of fatigue damage on marine risers; and estimate the vortex-induced forces on marine risers. [ABSTRACT FROM AUTHOR]

The present work investigates the linear out-of-plane vibrations of catenary risers. The study outlines in detail the complete theoretical procedure for setting up the mathematical formulation that governs the non-linear vibrations of the structure in three dimensions including torsion effects. The governing set is subsequently reduced using valid assumptions, and the equivalent linearized system is eventually derived. It is shown that the linearization procedure uncouples the in-plane and the out-of-plane vibrations of the riser. The reduced linear set of the ordinary differential equations that govern the out-of-plane vibrations is solved in the frequency domain using a centred differences numerical scheme. [ABSTRACT FROM AUTHOR]

Abstract: Significant past effort has gone into understanding the complicated flow–structure interaction problem of vortex-induced vibration (VIV) of long flexible cylindrical structures (e.g., risers, mooring lines, tendons, conductors) in the ocean environment. However, major challenges persist with regard to riser VIV modeling and response prediction. The existing prediction schemes are based on a number of hypotheses, experimental facts and data like strip theory, energy balance, correlation length and, most importantly, the use of lift force coefficient databases. Recent advances in observing the VIV motions on experimental risers with high confidence shows that some of these assumptions may not be valid. One important source of the discrepancies between theoretical estimates and experimental observations arise from the use of experimentally obtained lift coefficient databases. These databases were obtained under the laboratory conditions of limited Reynolds number, and under the assumption that the cross-flow motions are not influenced by restraining the in-line motions. In this paper we develop a method to improve the modeling capability of riser VIV by extracting empirical lift coefficient databases from field riser VIV measurements. The existing laboratory-based lift coefficient databases are represented in a flexible parameterized form using a set of carefully chosen parameters. Extraction of the lift coefficient parameters is posed as an optimization problem, where the objective is to minimize the error between the prediction using a theoretical model and the experimental data. Application of the method to data from the Norwegian Deepwater Programme experiments shows that the new optimal databases significantly reduce the error in estimating the riser VIV cross-flow response. [Copyright &y& Elsevier]

Abstract: In the offshore oil and gas industry, marine risers are critical conductors connecting platforms to the wellheads at the seabed for drilling, production and transmission, the vortex-induced vibration design of which requires accurate knowledge of the natural frequencies and mode shapes. The vibration characteristics of marine risers are re-examined in this paper using the method of differential transformation, which is relatively new technique capable of dealing with eigenvalue problems rather efficiently. Natural frequencies and mode shapes of marine risers are examined for various boundary conditions, and solutions are compared with numerical and experimental results published in the literature where available. [Copyright &y& Elsevier]

Abstract: Long flexible cylinders (e.g., risers, tendons and mooring lines) exposed to the marine environment encounter ocean currents leading to vortex-induced vibration (VIV). These oscillations, often driven at high frequencies over extended periods of time, may result in structural failure of the member due to fatigue damage accumulation. Recent developments in instrumentation and installation of data acquisition systems on board marine risers have made accurate measurement of riser responses possible. This paper aims at using the data from these data acquisition devices (typically strain gages and accelerometers) in order to understand the evolution of the riser VIV, with the final aim of estimating the fatigue damage. For this purpose we employ systematic techniques to reconstruct riser VIV response using the data from the available sensors. The reconstructed riser response allows estimation of the dynamic axial stresses due to bending and consequently the estimates of the fatigue damage along the entire riser. The above methods can take into account the fatigue damage arising from complicated riser motions involving the presence of traveling waves even with the use of very few sensors. An alternate approach using a Van der Pol wake oscillator model is also explored to obtain fatigue life estimates caused by riser VIV. [Copyright &y& Elsevier]

Using recent advances in data analytics techniques and tools, this paper proposes a novel data-centric paradigm for riser fatigue analysis in which practitioners can efficiently make use of large hindcast and measured metocean datasets directly in the design process. In the conventional fatigue design method for offshore risers systems, in order to reduce the computational cost practitioners tend to condense large hindcast metocean datasets into small wave datasets, so-called wave scatter diagrams, to represent the long-term variation in significant wave height, H s , and peak period, T p, at a site of interest. These methods tend to predict rather conservative riser loading conditions and hence fatigue lives due to the information lost in the condensing method, which can limit the application of steel catenary and lazy wave risers (SCRs and SLWRs) with larger diameters (outside diameter, OD > 18") whose design is governed by fatigue. This paper proposes enhanced frameworks for more accurate prediction of the fatigue life of SCRs and SLWRs located in swell dominant regions by enabling the direct use of large measured and hindcast metocean datasets in the fatigue design process of risers. Firstly, we introduce a novel detailed framework that uses an "ANN-based technique" with a "representative (P50) year" concept to enable large metocean datasets to be directly used for accurate estimation of riser fatigue life that is suitable for detailed design stages. Secondly, we propose an alternative innovative framework based on the use of Monte Carlo methods for more efficient long-term fatigue assessment of risers. Finally, we demonstrate the usefulness of the proposed frameworks by calculating the riser fatigue lives and comparing them against those obtained via conventional wave-condensing strategies. For the case studied here, the results show that using the proposed framework can significantly increase the estimated riser fatigue life over the values estimated using conventional wave condensing methods. [ABSTRACT FROM AUTHOR]

Abstract: A finite differences (FD) solution method is proposed for the numerical treatment of the dynamic equilibrium problem of 2D catenary risers. The method is based on the so-called Box approximation, which in the scope of the present contribution is applied to the complete nonlinear model as well as to the reduced linearized formulation. The application of the Box method transforms the original governing systems into convenient sets of algebraic equations, which in turn are solved efficiently by the relaxation method. Extensive numerical calculations are presented that describe the dynamic behaviour of the structure and evaluate the amplification in loading due to the dynamic components. The effect of the geometric nonlinearities is assessed through comparative calculations that concern both mathematical formulations examined in the present, i.e. the complete nonlinear, and the reduced linearized model. Special attention is paid to the heave excitations as they amplify significantly the magnitudes of the loading components. [Copyright &y& Elsevier]

Abstract: One of the main problems in predicting the response of flexible structures subjected to vortex shedding is that a totally reliable model for the fluid loading does not exist. It is also very difficult to measure the distributed force exerted by the fluid along the length of a marine riser without disturbing the system in some way. The methodology described here uses experimental response data obtained in a test programme undertaken in the Delta Flume in Holland during May 2003, linked to a finite element method model (FEM) of the riser. The length-to-diameter ratio of the model used was around 470, the mass ratio (mass/displaced mass) was 3 and the Reynolds number varied between 2800 and 28000. By using the response data as the input to the numerical model, the instantaneous distributed in-line and transverse forces acting on a flexible cylinder can be studied. [Copyright &y& Elsevier]

Abstract: Helical strakes are known to reduce and even eliminate the oscillation amplitude of vortex-induced vibrations (VIV). This reduction will increase the fatigue life. The optimum length and position of the helical strakes for a given riser will vary with the current profile. The purpose of the present paper is to describe how data from VIV experiments with suppressing devices like fairings and strakes can be implemented into a theoretical VIV model. The computer program is based on an empirical model for calculation of VIV. Suppression devices can be accounted for by using user-defined data for hydrodynamic coefficients, i.e. lift and damping coefficients, for the selected segments. The effect of strakes on fatigue damage due to cross flow VIV is illustrated for a vertical riser exposed to sheared and uniform current. Comparison of measured and calculated fatigue life is performed for a model riser equipped with helical strakes. A systematic study of length of a section with strakes for a set of current profiles is done and the results are also presented. [Copyright &y& Elsevier]

The paper deals with the non-linear dynamic response in the transverse direction of vertical marine risers or a tensioned cable legs subjected to parametric excitation at the top of the structure. The dynamic model contains both elastic and bending effects. The analytical approach reveals that the dynamic lateral response is governed by effects originated from the coupling of modes in transverse direction. The mathematical model is being treated numerically by retaining a sufficient number of transverse modes. Numerical results are given for specific case studies and refer both to the time histories of the lateral response for all modes of motion, and to the corresponding power spectral densities obtained through FFT. The numerical predictions are suitably plotted and discussed. The calculations concern both the undamped and the damped dynamic system. The damping in the system is a non-linear Morison type term, which describes the effect of the hydrodynamic drag. Both coupled and uncoupled equations are treated and points as well as regions of coupled and uncoupled stability and instability are defined. It is shown that the impacts originated from the coupling, evaluate new instabilities for the respective undamped system. The numerical results obtained through FFT of the time histories, provide qualitative conclusions for the features of the dynamic response for the modes of motions considered. Special attention has been paid to the effect of the hydrodynamic drag for the parametric excitation frequencies that guide the dynamic system to lie within a region of coupled instability. [Copyright &y& Elsevier]

Fairing is one of the most effective devices to suppress vortex-induced vibration (VIV) of marine risers. However, when the rotational friction coefficient of the fairing device is too high to rotate, severe galloping oscillations occurs. In this paper, a two-dimensional mass-spring-damping system was constructed based on the commercial software ANSYS. The galloping mechanism and the two degree-of-freedom (DOF) galloping characteristics of the fairing were studied by numerical methods. In particular, the influence of the natural frequency ratio of the in-line (IL) direction and the cross-flow (CF) direction on the galloping oscillation was discussed. The mass ratio of the riser-fairing system for which the simulations were performed was 1.7, and the damping ratio was 0.003. The Reynolds number based on the cylinder diameter and the inflow velocity was 1000. The range of reduced velocities based on the natural frequency in the CF direction was 1.76–18.52. The results showed that the activation of the inherent modes of the structure and the reattachment of the shear layer in the wake had an important contribution to the generation of the galloping. The movement in the IL direction could promote the reattachment of the shear layer to a certain extent, thereby expanding the scope of the galloping conditions. When the natural frequency ratio was equal to 1 or 2, a "double peak" phenomenon was found in the vibration amplitude curve in the CF direction. In the 2-DOF vibration, the fairing moved in the shape of an "8″ or a drop-shaped trajectory in most cases. On the whole, the interaction between the movement of the fairing in the IL direction and the movement in the CF direction could not be neglected, and the natural frequency ratio also significantly affected the vibration characteristics. • The activation of the natural mode and the reattachment of the shear layer cause galloping. • Movement in the in-line direction promotes galloping in the cross-flow direction. • The fairing gallops in the shape of an "8″ or a drop-shaped trajectory in most cases. • When the natural frequency ratio β = 1 or 2, a "double peak" phenomenon appears in galloping. [ABSTRACT FROM AUTHOR]