Deep-sea Riser Research Articles

Lifetime assessment for riser systems

As marine renewable energy technologies developing, there is a growing need for energy transportation systems. During offshore operations, deep sea risers can be subjected to excessive environmental loadings, causing operational risks. In this study, hydrodynamic loads, caused by in situ sea currents, acting on a riser under real-world sea conditions were modelled and examined, with experimental data being used as a calibration tool. Major safety problems for various offshore energy systems being an accurate assessment of excessive riser external loads, under influence of local sea currents, and hence resulting vortex induced vibrations (VIV).The method outlined in this study may be applied to complex sustainable energy systems, that are exposed to environmental loads, throughout the whole period of their intended service life. Approach advocated in this study offers practical way to estimate failure risks for nonlinear multidimensional dynamic offshore riser systems in an easy yet accurate manner.With regard to defense technology, risers and umbilicals play an important role for modern submarine operations.

Test Study on Vortex-Induced Vibration of Deep-Sea Riser under Bidirectional Shear Flow

A model test was carried out to reveal the vortex-induced vibration characteristics of a deep-sea riser under bidirectional shear flow. Bandpass filtering and modal analysis were used to process the test strain data, and the amplitude and frequency response characteristics of the vortex-induced vibration of the riser in the bidirectional shear flow field were obtained. The results of the test data analysis show that the dominant frequency of the vortex-induced vibration of the riser model under bidirectional shear flow is locked in the natural frequency of the riser and does not increase with the increase in flow velocity, that the average resistance coefficient of the riser model has little change under different flow velocities because of the distribution characteristics of the “bidirectional shear” flow field, that there is an extreme value of the shear force in the middle of the riser model, and that the Strouhal number in the transverse direction of the vortex-induced vibration under bidirectional shear flow is less than the recommended value of the current vortex-induced vibration prediction software. The obtained results provide basic data for the prediction of vortex-induced vibration and research into the fatigue analysis method of a riser under an internal wave flow field.

Experimental Investigation on Vortex-Induced Vibration of Deep-Sea Risers of Different Excitation Water Depths

The vortex-induced vibration test of the deep-sea riser was carried out with different excitation water depths in the wave-current combined water flume. By dimensionally changing the multi-stage water depth and hydrodynamic parameters such as outflow velocity at various water depths, the dynamic response parameters such as dominant frequency, dimensionless displacement and vibration trajectory evolution process of the riser under different excitation water depths were explored to reveal the sensitive characteristics of the dynamic response of vortex-induced vibration of the risers under different excitation water depths. The results show that different excitation water depths will change the additional mass of the riser and the fluid damping and other parameters, which will affect the spatial correlation and stability of the vortex shedding behind the riser. In the lock-in region, the distribution range of the characteristic frequency becomes narrow and centered on the lock-in frequency. The increase of the excitation water depth gradually advances the starting point of the lock-in region of the riser, and at the same time promotes the excitation of the higher-order vibration frequency of the riser structure. Within the dimensionless excitation water depth, the dominant frequency and dimensionless displacement are highly insensitive to the excitation water depth at high flow velocity. The change of the excitation water depth will interfere with the correlation of the non-linear coupling of the riser. The “8-shaped” gradually becomes irregular, and the vibration trajectories of the riser show “O-shape”, “X-shape” and “Crescent-shape”.

Simulating High-Mode Vortex-Induced Vibration of a Riser in Linearly Sheared Current Using an Empirical Time-Domain Model

Abstract As the development of offshore oil and gas continues to move into deeper waters, the risers connecting the seabed infrastructure to the offshore platform are getting longer. This means that vortex-induced vibration (VIV) may occur at different frequencies along the riser due to current variation, making the VIV prediction challenging. To increase the VIV prediction accuracy, an empirical time-domain VIV model has been developed. The advantage of the model is that it can account for structural non-linearities such as variable tension and time-varying flow. The robustness of the time-domain VIV model has been verified with respect to the several tests, but the length/diameter (L/D) ratio of the risers used in the experiments has so far been relatively small, and the response mode number accordingly low. Therefore, additional validation is needed to understand the uncertainties for the prediction of VIV for deep-sea riser systems at high-mode order. The purpose of the present study was to evaluate the use of time-domain model for high-mode VIV response prediction of deep-sea riser systems. This was done by a comparison study applying the Hanøytangen high-mode VIV test data. The main comparison was made regarding the dominating frequency and fatigue damage estimation. For selected empirical parameters, the model demonstrated a good correlation with the experiments. However, the simplified riser model that was utilized in the simulations caused some discrepancies between numerical simulations and the experimental results, which need to be further investigated.

Experimental investigation on the interference mechanical behavior of standing deepsea riser group coupling transient collision

Interference effect coupling transient collision between adjacent marine risers has become an important issue as the oil and gas industry moves to deepsea. The vortex-induced vibration experiment on a near-spaced, standing two-riser group was conducted to analyze the evolution from interference to collision and the mechanical behavior coupling transient collision. In this paper, the risers with the spacing of 2.0 D were arranged in two forms: tandem and side-by-side. Each riser had an effective length of 2.0 m and 40% of the riser length was submerged in the uniform flow. The result shows that the collision behavior of the risers was significantly constrained by flow velocity. The interference region, collision region, and destruction region in tandem and side-by-side arrangements were different. The cross-flow displacement amplitudes in side-by-side arrangement were larger than the isolated riser, and the wake and clearance flow caused that the displacement amplitude deviated to the interference side, so that the potential collision risk in the side-by-side arrangement was intensified. During the collision, the frequency components were abruptly changed, and the vibration dominated frequency was unstable. Simultaneously, the riser vibration trajectory deviated from the standard “8”, and the vibration state appeared unstable fluctuations. The response amplitude increased or decreased drastically in a short time, which increased the fatigue damage of the structure to a certain extent.

Experimental Investigation of Disturbing the Flow Field on the Vortex-Induced Vibration of Deepwater Riser Fitted with Gas Jetting Active Vibration Suppression Device

An experimental investigation on the disturbance effect of jet-type active vibration suppression device on vortex-induced vibration of deep-sea riser was carried out in the wave-flow combined flume. The vibration suppression device was designed in which the jet pipe was horizontally fixed to the front end of the riser. By varying three different excitation spacings and multi-stage outflow velocities, the influence law of the dominant frequency, dimensionless displacement and other dynamic response parameters was studied under different excitation spacings, and the mechanism and sensitive characteristics of the disturbance suppression were explored. The results indicate that the variation of excitation spacing makes gas curtain enter the strong disturbed flow region at different velocities and angles, and the coupling relationship between excitation spacing and reduced velocity is the key factor to enter the strong disturbed flow region to achieve the optimal disturbance suppression. In the strong disturbed flow region, the influence of gas curtain on the dominant frequency is obviously affected by the flow velocity, while the vibration displacement is stable at the same amplitude and is weakly affected by the flow velocity. Gas curtain can effectively disturb the formation of vortex shedding, destroy the strong nonlinear coupled vibration of the riser, and achieve better vibration suppression effect. In the weak disturbed flow region, the vortex length of the riser tail is prolonged, the strong nonlinear coupled vibration of the riser is gradually restored, and the vibration suppression effect of the device gradually decreases.

Effect of Top Tension on Vortex-Induced Vibration of Deep-Sea Risers

With the increase of water depth, the design and use of the top-tensioned risers (TTR) are facing more and more challenges. This research presents the effect of top tension on dynamic behavior of deep-sea risers by means of numerical simulations and experiments. First, the governing equation of vortex-induced vibration (VIV) of TTR based on Euler-Bernoulli theory and Van der Pol wake-oscillator model was established, and the effect of top tension on natural vibration of TTR was discussed. Then, the dynamic response of TTR in shear current was calculated numerically by finite difference method. The displacement, bending stress and vibration frequency of TTR with the variation of top tension were investigated. Finally, a VIV experiment of a 5 m long flexible top-tensioned model was carried out at the towing tank of Tianjin University. The results show that the vibration displacement of TTR increases and the bending stress decreases as the top tension increases. The dominant frequency of VIV of TTR is controlled by the current velocity and is barely influenced by the top tension. With the increase of top tension, the natural frequency of TTR increases, the lower order modes are excited in the same current.

Investigation on the spoiler vibration suppression mechanism of discrete helical strakes of deep-sea riser undergoing vortex-induced vibration

Vortex-induced vibration (VIV) is an important factor that causes fatigue damage of high aspect ratio structures under the action of the fluid. In this paper, a type of discrete helical strakes vibration suppression device was designed. The vortex-induced vibration suppression experiment of the discrete helical strakes of the deep-sea riser under uniform flow in the wave-current combined water flume was carried out. The riser model adopted a transparent flexible plexiglass tube with a length of 2.0 m, an outer diameter of 18 mm and an aspect ratio of 111.11. The helical strakes were made of a hexahedral artificial rubber material. According to varying the equidimensional discrete spacings of the discrete helical strakes, varying the hydrodynamic and structural parameters such as the outflow velocity and the number of helixes, the physical mechanism of spoiler of the vibration suppression device was revealed. The suppression efficiency of the device on the vortex-induced vibration of the riser and the influence on the dynamic response of the riser were explored. The results show that the discrete helical strake can release the vortex behind the riser to multiple directions, destroy the spatial correlation of the vortex from multiple dimensions, and change the distribution of the original pressure field. The discrete single-helical strakes have low vibration suppression efficiency in the lock-in region and the dominant frequency of the riser doesn't change significantly compared with the counterweight bare riser. The number of diversions of the discrete double- and triple-helical strakes increases, and the disturbance to the incoming flow enhances, which makes excellent vibration suppression efficiency in the whole velocity range. And the dominant frequency is reduced, the vibration intensity is significantly reduced and the fatigue life is improved. With the increase of the discrete spacings, the vortex separation release rate increases slightly, but it doesn't affect the change of the diversion angle along the longitudinal direction of the riser. It can still significantly disturb the spatial correlation of the vortex, disperse the vortex and maintain high suppression efficiency. In the equidimensional discrete spacings, the dominant frequency, vibration suppression efficiency and fatigue damage are less sensitive to discrete spacings.

Multi-scale analysis for helically wound structures with one-dimensional periodicity

Purpose The purpose of this study is to present a multi-scale analysis methodology for calculating the effective stiffnesses and the micro stresses of helically wound structures efficiently and accurately. The helically wound structure is widely applied in ocean and civil engineering as load-bearing structures with high flexibility, such as wire ropes, umbilical cables and flexible risers. Their structures are usually composed of a number of twisted subcomponents with relatively large slender ratio and have the one-dimensional periodic characteristic in the axial direction. As the huge difference between the axial length and the cross-section size of this type of structures, the finite element modeling and theoretical analysis based on some assumption are usually unavailable leading to the reduction of computability; even the optimization design becomes infeasible. Design/methodology/approach Based on the asymptotic homogenization theory, the one-dimensional periodic helically wound structure is equivalent to the one-dimensional homogeneous beam. A novel implementation of the homogenization is derived for the analysis of the effective mechanical properties of the helically wound structure, and the tensile, bending, torsional and coupling stiffness properties of the effective beam model are obtained. On this basis, a downscaling analysis formation for the micro-component stress in the one-dimensional periodic wound structure is constructed. The stress of micro-components in the specified geometry position of the helically wound structure is obtained basing on the asymptotic homogenization theory simultaneously. Findings By comparing with the result from finite element established accurately, the established multi-scale calculation method of the one-dimensional periodic helically wound structure is verified. The influence of size effects on the macro effective performance and the micro-component stress is discussed. Originality/value This paper will provide the theoretical basis for the efficient elastoplastic analysis of the helically wound structure, even the fatigue analysis. In addition, it is necessary to point out that the axial length of the helically wound structure in the general engineering problems that such as deep-sea risers and submarine cables.

Investigation on Vortex-Induced Vibration Experiment of a Standing Variable-Tension Deepsea Riser Based on BFBG Sensor Technology.

A vortex-induced vibration (VIV) experiment on a standing variable-tension deepsea riser was conducted to investigate the applicability and sensitivity of Bare Fiber Bragg Grating (BFBG) sensor technology for testing deepsea riser vibrations. The dominant frequencies, dimensionless displacements, in-line and cross-flow couplings of the riser VIV under different top tensions were observed through wavelet transform and modal decomposition. The result indicated that, excited by the same external flow velocities, the cross-flow and in-line dominant frequencies of the riser both decreased with increasing top tension. In terms of displacement responses, increasing top tension caused the root mean square (RMS) displacement to decrease and the vibration amplitude to reduce. In terms of cross-flow and in-line coupling, the closer a location is to the ends of the riser, the smaller the trajectory is and the more standard the “8” becomes. During top tension increases, there exists a “lag” in the time when the riser’s vibration trajectory becomes an “8”. The Slalom Surround Installation approach can effectively prevent the local breakage of the optical fiber string. BFBG sensor technology can give an accurate presentation of the displacement time history, vibration amplitude and frequency of the riser, provides a clear picture of how the riser’s mode and VIV evolve as a function of flow velocity.

Analytical study of the strength of adhesive joints of riser pipes

Today, adhesive pipe joints are widely used in marine engineering equipment such as platforms and pipe lines, because of the manifest advantages, such as lower stress concentration, uniform stress distribution, lighter weight, water tightness, corrosion-resistance and better fatigue properties. However, adhesive joints in riser pipes are the most vulnerable part where most failures occur, because singular stress fields exist at the extremes of the interface between the adhesive and the adherend. In order to prevent premature failure of the joint systems, a study of adhesive pipe joints under the actual sea environment loadings, including as external pressure, internal pressure, tension, torsion, bending, and combinations of these, should be conducted. In this reported study, stress distributions were analysed in terms of the first-order laminated theory, and it was found that numerical results were in a good agreement with the obtained results obtained from the finite element analysis. In the light of these results, the failure of adhesive joints in deep sea riser pipes was discussed based on stress distributions on the interfaces as defined by theoretical analysis and Finite Element Method. Moreover, the effects of material properties and joint dimensions on the strength of adhesive joints under different load cases were studied.

Technology Focus: Production and Facilities (December 2014)

Technology Focus Two points stand out in the development and status of technology for production and facilities. The first is the amount of innovative ongoing work related to piping, pipelines, and materials for severe service. The second is the amount of work involving the use of digital simulations, including computational fluid dynamics (CFD). Pipes are the arteries and veins of production and facilities, yet too few of us are concerned about their technical status. Fortunately, others among us are concerned about this topic. As oil production moves to increasingly hostile and remote environments, the need is increasing for reliable yet affordable pipeline technology that meets the demands of severe environments. Because of the scale and the environmental severity experienced by high-pressure/high-temperature (HP/HT) and other severe service, pipelines can be prohibitively expensive to duplicate in testing facilities. Research and development increasingly uses experience-validated digital simulations and modeling, sometimes synergistically. The technical community continues to respond to severe-service challenges with research and development on, for example, how to implement corrosion-resistant-alloy (CRA) technology, how to specify and use composite materials for pipelines, and how to better install and maintain piping (including deep-sea risers and subsea lines) constructed with conventional metallurgies. Of the papers I considered for this feature, one reported on an investigation of the fatigue aspects of CRA welds for pipes in HP/HT service using finite-element analysis, while another reported on the development of an understanding of welding residual stress with the use of computational-welding-mechanics simulations. Another group of investigators provided an overview of research on the use of composite flowlines, risers, and pipework in offshore applications. An improved understanding of vortex- induced vibration was presented by another group of investigators, who used 3D dynamic CFD simulations. In a transition from the pipeline to facilities design, other investigators developed a methodology for sizing two-phase separators and slug catchers by use of a combination of dynamic pipeline modeling and CFD. But the use of digital simulations and modeling does not stop here. Operator training and process safety are being enhanced by the use of plant simulators and virtual reality. Flow assurance is taken to a new level by the development of a fully compositional wax-deposition model to help explain the location and physical character of pipeline wax deposits. Overall, it is clear that digital-simulation capabilities are quietly revolutionizing the way in which technology is developed and deployed within our industry. The fundamental understanding afforded by a variety of digital simulations is unprecedented and will continue, allowing for the rapid development and deployment of more reliable, better-performing piping and facilities technologies. JPT Recommended additional reading at OnePetro: www.onepetro.org. OTC 24797 Modeling the Shearing Effect of Flowing Fluid and Wax Aging on Wax Deposition in Pipelines by Xiaohong Zhang, Infochem-KBC, et al. OTC 25039 Welding Residual Stress and Pipeline Integrity by R. Tornqvist, DNV GL, et al. OTC 25063 Numerical Studies of Vortex-Induced Vibration of a Circular Cylinder at High Reynolds Number by M. Mobassher Tofa, Universiti Teknologi Malaysia, et al.

Tailored local design of deep sea FRP composite risers

The use of fibre reinforced polymer (FRP) composite materials in offshore engineering for deep sea riser systems has drawn considerable interest due to the potential weight savings and improvement in durability that can be achieved. The design of FRP composite risers consists of two stages: (1) preliminary local design based on critical local load cases (LCs) and (2) global analysis of the full length composite riser under global loads including platform motion, hydrostatic pressure, gravity, buoyancy, wave and current loads to determine and assess critical locations. The preliminary local design stage is necessary to obtain a first estimate of the laminate configuration – fibre orientations and layer thicknesses of the tube wall, since the deformations and hence the forces and bending moments due to the global loads, depend on the geometric configuration. This paper describes the methodology, LCs, analysis procedure and results of the first stage, the local design of the composite riser. The local design is conducted using five different LCs as prescribed by the standards. In this study, geometric configurations of eight different composite body and liner combinations were optimised to provide minimum structural weight. Previous composite riser designs have mainly focused on axial and hoop reinforcements; in the present study, fibre reinforcements at other orientations are included. The highlight of the current study is that it shows that the use of off angle fibre orientations in addition to axial and hoop reinforcements offer substantial weight savings.

Study on the Inwall Crack Extension and Fatigue Life Evaluation for Deep-Sea Steel Catenary Riser under Wave and Current Action

Aimed at deep-sea steel catenary risers(SCRs) transporting supercritical or dense CO2, this paper uses the macroscopic pipeline-transporting finite element model and the microscopic pipewall crack growth model to numerically simulate corrosive-inwall-defect-induced crack extension along radial direction for the deep-sea riser which is subjected to wave and current loads. Meanwhile, based on linear elastic fracture mechanics and fatigue crack extension theory, the effect of wave height on the fatigue life of the pipewall at touchdown zone is discussed. The above-mentioned study in this paper is expected to give a good reference to the similar study for oil&gas SCRs.

Tailoring of Composite Reinforcements for Weight Reduction of Offshore Production Risers

The use of composite materials in offshore engineering for deep sea oil production riser systems would lead to weight savings and improvement in fatigue and corrosion resistance. This paper examines the weight saving potentials offered by varying fibre orientations, stacking sequences, fibre matrix combinations and liner materials for deep sea riser tubulars. Four main load cases, recommended by design standards for the local design of composite offshore risers, are considered in the analysis using three-dimensional finite element modelling: maximum internal pressure, axial tension, top tension with external pressure and maximum external pressure.

Research on the Joint Structure of CFRP Riser Pipe

A deep-sea riser drilling vessel is now under construction in Japan for the purpose of scientific drilling and will be completed in 2005. The target water depth of the vessel is 2, 500m for the time being and will be extended to 4, 000m in future. In the field of offshore oil drilling, it is confirmed that the target water depth of 3, 000m with using steel riser pipe will be possible. However, the drilling up to the water depth of 4, 000m has not been experienced at all and will have a lot of technical issues. One of the most important issues is to make the weight of the riser pipe lighter and its axial rigidity higher.The purpose of this research is to make clear the design concept of the mechanical joint between the CFRP pipe and the steel connector and develop a CFRP riser pipe capable of drilling in 4, 000m water depth. Here, a prototype of CFRP riser pipe is designed and five experimental models are fabricated and tested. The experimental results are compared with those of FEM analysis and the findings are reported in this paper.

Influence of Inside Fluid Motion on Tension Variation of a Deep Sea Drilling Riser in Disconnected Mode

Influence of inside fluid mass which exists inside the riser pipe on tension variation of the deep sea drilling riser in disconnected mode is investigated. A simple calculation method of the tension variation is proposed taking the inside mass motion into account based on the lumped mass method. This method is verified through the comparison with the tension variation and the riser bottom displacement measured in a basin test using a riser model. The influence of the inside mass on the tension variation is numerically investigated for deep sea drilling risers with 3 different length using the calculation method. For the deep sea riser over 2, 000 m length, the mass corresponding to about 50% of the total inside mass acts on the riser top as the additional tension variation.

Existence of Fixed Point in the Frequency Response of Deepsea Riser and its Influence on the Design

The 4000 m class deepsea drilling vessel will be realized in Japan in near future. The drilling vessel is equipped with deepsea riser, and this paper discusses longitudinal dynamic response of the deepsea riser in hung-off condition. This paper shows that there exist fixed points in the frequency response of strain of hung-off deepsea risers. A fixed point is a point in a frequency response which does not change irrespective of the change of some governing parameters of the dynamic system. The fixed point is originally found in the discreet mass-spring system, and this paper shows that the fixed point exist also for a continuum system, riser & BOP system. The fixed points are shown to exist in the frequency response of axial strain with respect to the damping of riser and also the damping of BOP. Considering the position of fixed points in the frequency response, it can be concluded that the increase of damping does not always improve the dynamic response of hung-off riser. When the external exciting frequency, namely heave motion of vessel, is lower than the fixed point, strain at riser top is increased with increase of damping. On the other hand, if the riser is excited at the frequency larger than the fixed point, response is significantly improved.

Tension Variation and Limiting Wave Height of a Deep Sea Drilling Riser in Hang-Off Mode

The wave spectral analysis technique is applied to predict the limiting wave conditions for a deep sea drilling riser in the hang-off mode. In this mode, the riser is disconnected from the subsea BOP stack, hangs from the riser tensioner wires, and the top of the riser moves with the drilling vessel. The limiting wave height then, is the highest wave which does not cause compression in the top of the riser. Calculation results are presented showing the limiting wave height as a function of averaged wave period for a 2, 000 m long deep sea riser. This method is then verified through comparisons with results of calculations performed using the time domain method in response to a random sea. Additionally, the influences of internal fluid mass in the riser and the associated fluid friction on the limiting wave height are discussed. Finally, applications of the method to the analysis and design of deep sea drilling risers are presented.

Operability Improvement of Deepsea Riser by Active Control

This paper presents a research on how to keep the riser angle of iclination within the admissible extent in the deep sea drilling operation, from the viewpoint of the positioning of the drill ship and the effects of using actuators attached along the riser.In the operation of deep sea drilling where the water depth is deeper than 3000m, it is very important to keep the top and the bottom angles of the riser inclination smaller than 4 degrees to prevent the drill strings inside the riser from touching and breaking the riser. To satisfy this criteria and to reduce the payload, the riser is tensioned at the top with a tensioner, the weight in water is reduced by attaching the buoyancy module, and the ship position is controlled by using the DPS against the disturbance resulted from current or wave. However, because of the huge mass and the very low rigidity, the dynamic response of the riser is generally so weak that the riser cannot follow the ship motion well, and that it resulted in decreases of availability of the system.Based on the above circumstances, the equations of coupled motion of the ship and the 4000m riser are formulated. With these equations, the optimal control is formulated. Then simulation results by Newmark-β method are shown for a set of parameters of 750tf, 850tf top tensions and 1kt, 2kt currents. From this study it becomes clear that an actuator with about 270kgf thrust can effecitively improve the response of the riser bottom and top inclination angles by the ship motion. And the inclination angles of the riser can be manipulated by the ship positioning within the permissible range. Furthermore, it is clarified that the two actuators with maximum thrust of 500kgf can effectively control the dynamic responses of the riser, and that positioning the ship with 850tf riser top tension, the angles of inclination at the riser top and bottom can be kept in the desirable range of less than 2 degrees even when the current speed is 2kt.