Bare Riser Research Articles

Reconstructing flexible body vortex-induced vibrations using machine-vision and predicting the motions using semi-empirical models informed with transfer learned hydrodynamic coefficients

This work assesses the validity of transfer learning the hydrodynamic coefficient database, consisting of the added mass and lift coefficients, applicable to flexible bodies undergoing vortex-induced vibrations. Specifically, the hydrodynamic coefficient database learned on data collected by Braaten and Lie (2005) are used to predict the motions observed during in house bare riser model experiments at the MIT Towing Tank. A fully immersed vertical flexible riser model with a length-to-diameter ratio of 145 is towed at different flow speeds and top tensions. Motion is tracked using underwater cameras and the motions are reconstructed using a machine-vision framework eliminating the need for expensive sensing hardware. The vibration amplitude, frequency, and mode shape are determined and the results are compared with those in the literature. Finally, blind predictions of the in-house observed experiments are made using the software VIVA informed with transfer learned hydrodynamic coefficients learned on the experiments by Braaten and Lie (2005).

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.

A numerical study on the vortex-induced vibration of flexible cylinders covered with differently placed buoyancy modules

Buoyancy modules are essential parts in the offshore oil and gas drilling and production practices. They provide weight compensation for the riser and help avoid excessive top tension exerted by the floating platform. However, the deployment of buoyancy modules with larger diameter than the bare riser may alter the geometric shape of the riser system and greatly influence the hydrodynamic performance. Engineering designs with buoyancy modules are mostly based on experience and rigid cylinder forced vibration experimental data. Since it is difficult if not impossible to perform real sea experiments, numerical simulation becomes important in aiding the buoyancy module design and setup. In the present work, a fluid–structure interaction code based on the computational fluid dynamic model is firstly validated with the riser experimental data from MARINTEK (the Norwegian Marine Technology Research Institute) and then used to simulate the riser responses with different continuous buoyancy module setups. The Reynolds number based on the incoming velocity and the bluff body diameter ranges from 4×103 to 1.68×104. The simulation results suggest that the shape alteration from the buoyancy deployment will change the vortex-induced vibration(VIV) response mode along with the vibration frequency. The cylinder fatigue damage, travelling wave response and the fluid–structure energy transfer characters are discussed under different buoyancy module setups.

Viñedos en terrazas en la región vitivinícola del Duero, Portugal: una perspectiva de la gestión del suelo y el agua

Los viñedos del Duero son un ejemplo sorprendente de protección del suelo, materializado, en un paisaje fuertemente humanizado, donde las terrazas cubren una gran parte de la región. Este trabajo tiene como objetivo presentar una perspectiva sobre las mejoras en la gestión del suelo y el agua en los viñedos en terrazas del Duero, como respuesta a las responsabilidades reales determinadas por el estatuto del Patrimonio Mundial de la UNESCO, para preservar un paisaje cultural, vivo y evolutivo. Después de enfatizar sobre la importancia de los viñedos en terrazas del Duero en el territorio continental portugués y las limitaciones naturales para la producción de cultivos que caracterizan el valle del Duero, se describen los tipos de terrazas presentes en el paisaje del Duero, junto con los cambios en el uso del suelo debido a las operaciones de las terrazas agrícolas. Además de la rehabilitación de las estructuras de piedra seca como parte de las intervenciones de preservación del patrimonio cultural de la región, las áreas de riesgo crítico en las laderas aterrazadas identificadas recientemente se identifican como una prioridad para las intervenciones de protección del suelo y gestión del agua. Se trata de las áreas de viñedos más expuestas al impacto directo de las lluvias erosivas y comprenden los pasillos entre líneas de la vid, especialmente en viñedos no aterrazados, los taludes desnudos de tierra en terrazas de origen reciente, y las redes de caminos y drenaje agrícolas, espacialmente coincidentes, en laderas empinadas plantadas extensivamente. Prácticas innovadoras de gestión del suelo y agua deben desarrollarse y probarse localmente en un diálogo cercano con los actores regionales.

Numerical Investigation of Vortex-Induced Vibrations of a Flexible Riser with Staggered Buoyancy Elements

This paper presents a numerical algorithm used together with a Finite-Element based Vortex-Induced Vibration (VIV) prediction software VIVANA to obtain a hydrodynamic force coefficient database based on response measurements of flexible riser VIV model test of staggered buoyancy elements. The excitation coefficient database is parameterized; and the representative parameters are systematically varied in VIVANA simulations until the predicted responses in terms of mode number, response frequency, root-mean-square (RMS) response amplitude, and RMS curvature agree well with the experimental data under same flow and boundary conditions. An optimal set of force coefficient database for both the buoyancy element and the bare riser section is obtained and a significant improvement in VIV prediction by VIVANA is achieved. The obtained hydrodynamic force coefficient database can be also applied to other empirical VIV prediction programs. The sensitivity of using the same database to predict the VIV of different configurations has also been investigated.

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.

Numerical simulation on the forced oscillation of rigid riser with helical strakes in different section shapes

In order to study the helical strakes' suppression effects on the vibration riser, triangle and circular sections helical strakes are proposed. With the technology of dynamic mesh, forced oscillation in steady flow of the riser with helical strakes in different cross-section shapes at low Reynolds number are numerically simulated. The analysis is focused on the lock-in range, as well as the force status and trailing vortex structure under different values of amplitude (A) and oscillation frequency (fe), and draw comparisons to the bare riser's results. Research shows the lock-in range will increase with the increase of amplitude ratio (A/D), the riser with all three different shaped helical strakes have larger lock-in range than bare riser. Before entering into the lock-in region, riser with different shaped helical strakes all have good effects on suppressing the vibrations of riser. Within and after lock-in region, riser with triangle and circular sections helical strakes have smaller lift force than that with rectangular section, which reduced lift coefficient by 65.83% and 83.53%. The trailing vortex structures of the riser with helical strakes are in strength three-dimensional character. Inside the lock-in region, riser with triangle and circular sections have better suppression effect than that with rectangular section. Outside the lock-in region, riser with triangle section helical strakes has the best vortex suppression effect, of which the influence on the flow field is greatest and the vortex-induced force is least.

Suppression of vortex-induced vibrations of a flexible riser by adding helical strakes

3-D computational fluid dynamics/ computational structure dynamics (CFD/CSD) numerical two-way coupling simulations are conducted for a flexible rise in order to study the dynamic response performance of the riser with and without helical strakes exposed to the vortex-induced vibration (VIV). The VIV responses of a PVC riser without helical strakes are computed and compared with experimental data, to verify the accuracy of the present two-way coupling method. Subsequently, the dynamic behaviors of a short PVC riser with different kinds of helical strakes are studied. The vibration amplitudes along the riser, the trajectories of the riser’s monitor point and the vortex shedding contours are obtained in a series of simulations. The helical strakes’ VIV suppression mechanisms are found involving the breaking of the vortex structures and the reduction of the vortex shedding frequency of the bare riser. Moreover, a good suppression effect can be achieved by attaching the helical strake structure with a reasonable geometrical configuration (such as the appropriate strake height, strake pitch, the number of starts and strake coverages) to the flexible riser. The effect is also diverse at different reduced velocity (Ur). The remarkable effect is found at Ur = 7 for the short riser, with about 97% reduction in the transverse vibration response.

VIV response of a long flexible riser fitted with different helical strake coverages in uniform and linearly sheared currents

ABSTRACTAn experimental investigation was conducted on a flexible riser with and without various helical strake coverages, with the aim of further improving the understanding of the response performance of the vortex-induced vibration (VIV) of a riser with helical strakes. Two current profiles, including uniform and linearly sheared currents, were simulated. The VIV displacements, VIV strains and hydrodynamic forces were directly measured from the test. Based on the fatigue damage method, the fatigue damage was obtained from the measured strain. Strakes with different coverages were analysed, and response parameters such as the displacement response and fatigue damage were studied. The results of the straked model test indicate that the response characteristics of a bare riser model can be quite distinct from those of a riser with helical strakes, and the response performance closely depends on the coverage of the helical strakes. The results also show that the flexible riser with 75% and 50% strake coverages are the best configurations to suppress VIV fatigue damage in uniform and linearly sheared currents, respectively.

Experimental study on response performance of VIV of a flexible riser with helical strakes

Laboratory tests were conducted on a flexible riser with and without helical strakes. The aim of the present work is to further understand the response performance of the vortex induced vibration (VIV) for a riser with helical strakes. The experiment was accomplished in the towing tank and the relative current was simulated by towing a flexible riser in one direction. Based on the modal analysis method, the displacement responses can be obtained by the measured strain. The strakes with different heights are analyzed here, and the response parameters like strain response and displacement response are studied. The experimental results show that the in-line (IL) response is as important as the cross-flow (CF) response, however, many industrial analysis methods usually ignore the IL response due to VIV. The results also indicate that the response characteristics of a bare riser can be quite distinct from that of a riser with helical strakes, and the response performance depends on the geometry on the helical strakes closely. The fatigue damage is further discussed and the results show that the fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction.

VIV response of a long flexible riser fitted with strakes in uniform and linearly sheared currents

An experimental investigation was conducted on a flexible riser with and without various strake arrangements. The aim of the present work was to further improve the understanding of the response performance of the vortex-induced vibration (VIV) of a riser with helical strakes. Two current profiles, including uniform and linearly sheared flows, were simulated. The uniform current was simulated by towing the riser model in one direction using the towing carriage, and the linearly sheared current was simulated by fixing one end of the riser and using a driven cantilever to traverse a circular arc. Based on the modal superposition method, the displacement responses were obtained from the measured strain. Strakes with different heights and pitches were analysed, and response parameters such as the displacement response and fatigue damage were studied. The results of the bare model test show that the lock-in phenomenon displays multi-order characteristics, and the VIV displacement decreases with an increased order of the lock-in regime. The results of the straked model test indicate that the response characteristics of a bare riser can be quite distinct from those of a riser with helical strakes, and the response performance depends closely on the geometry of the strake configuration.

A new method for strake configuration design of Steel Catenary Risers

A number of studies of vortex-induced vibration (VIV) having important effect on the fatigue performance of riser have performed analytically as well as experimentally in the last decade. However, there are few research results on the configuration design of VIV suppression devices such as fairings, strakes, and etc. Therefore, a systematic design procedure of strake configuration (i.e., location and length) using the modal approach for steel catenary riser (SCR) is proposed in this study. The proposed method considers the dominant mode for VIV suppression through modal analysis of bare riser to find appropriate location and various design parameters are introduced to calculate appropriate length of VIV suppression devices. From the numerical example of SCR, it is shown that the required strake length from the optimal location for VIV suppression of SCR is much shorter than that of the conventional method. This is achieved by staggered strake installation at the effective location. Furthermore, no additional calculations or engineering cost are required for the proposed method, because the proposed method follows exact same analysis procedure of the conventional one except using the modal information of bare SCR obtained from initial analysis of bare SCR.

Experimental investigation of the response performance of VIV on a flexible riser with helical strakes

Experimental investigations were conducted on a flexible riser with and without helical strakes. A uniform current was obtained by towing a riser model in a tank, and the vortex-induced vibration (VIV) suppression of strakes with different heights and pitches was studied. The experimental results indicate that the response characteristics of a bare riser can be quite distinct from those of a riser with helical strakes, and the suppression performance depends on the geometry of the helical strakes. The VIV responses in the cross-flow (CF) and in-line (IL) directions can be coupled via variations in the tensile force. The fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser fitted with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction. The experimental results also confirmed that the strake height has a greater influence on the VIV response than the strake pitch, and the drag exerted on the riser increases with the strake pitch and height.

Investigation of a new vortex-induced vibration suppression device in laboratory experiments

In order to mitigate vortex-induced vibration (VIV) of marine risers, especially to eliminate the phenomenon of frequency slock-in’, a new suppression device of crescent-shaped flow spoiler was designed with seven different layout schemes. VIV model tests with six flow levels were conducted in a large wind-wave-current flume. In all cases, vibration responses in both in-line and cross-flow cases were measured. With the installation of suppression devices vibration frequency evolution of a riser was analyzed by Morlet wavelet transform. The principle of VIV suppression was interpreted through vibration characteristics. Fatigue life of the riser was calculated by the Palmgren-Miner rule. Compared with a bare riser, vibration of an outfitted riser with suppression devices disturbed the steady flow, the vibration amplitudes in the two flow directions were reduced, and the riser fatigue life was improved.

Experimental Research on Interference of Marine Risers with Suppression Device

An experimental research on interference of two risers with suppression device was conducted. The arrangement of the two risers is side-by-side, one is bare and another is a riser with triple strakes. Centre-to-centre distances from 3 to 10 diameters of the risers were studied. The riser model was a 18mm diameter organic glass with wall thickness of 2 mm. The current velocity ranges from 0.3 to 0.8 m/s, with approximate increments of 0.1 m/s. Dynamic strain records were obtained through the dynamic strain gauges which were sticked on the surface of the risers. The experimental data were contrastive studied by dynamic response, amplitude and frequency, respectively. The experimental results indicate that interference have little effect on the oscillatory characteristic of the riser with triple strakes while have important effect on the oscillatory characteristic of the bare riser with different distances.

Experimental investigation of vortex-induced vibration of long marine risers

There is considerable disparity between predictions of marine riser vortex-induced vibration (VIV) fatigue damage, and often the agreement between computer models and observed VIV-related damage is inaccurate by orders of magnitude. Resulting problems for deepwater riser design are the need for large safety factors on fatigue damage predictions and the use of expensive vortex-suppression devices (e.g. helical strakes). Understanding is especially limited for long risers, which may be excited in multiple and higher modes. Norwegian Deepwater Programme (NDP)—a group of oilfield licensees in Norway—has commissioned experiments on riser models over a range of scales and current conditions in order to improve the ability to predict VIV. Recently, testing of a model with an L/ D (length-to-diameter ratio) of 1400 was conducted at Marintek's Ocean Basin in Trondheim. The riser was tested without VIV suppression and with various strake arrangements. Testing was performed on an innovative test rig that could simulate uniform and linearly sheared currents, with a composite fibre model that featured a dense array of high-quality instrumentation. In-line and cross-flow responses were considered important with respect to fatigue. Indeed, this study reinforces recently published results that in-line fatigue damage is as severe as cross-flow fatigue damage. However, industry analysis approaches generally ignore in-line damage due to VIV. The findings also indicate that the response character of a bare riser can be quite distinct from that of a riser partially or fully covered with helical strakes. In addition, while helical strakes of different types can be effective in mitigating VIV fatigue of long risers, their performance is dependent on their geometry. Finally, the results suggest that a key consideration in VIV fatigue design is the length of suppression coverage and the nature of the flow to which the bare section of the riser is exposed.

A Comprehensive Approach to Deepwater Marine Riser Management

Summary A system for riser management has been developed and used that comprises (1) a variable buoyancy system, (2) hardware to hang off a disconnected marine riser near the keel of a drillships (3) a heave acceleration prediction capability, (4) instrumentation for monitoring heave acceleration and riser buoyancy, tension, pressure, vibrations, and inclination, and (5) a technique for pressure, vibrations, and inclination, and (5) a technique for making hang-off and disconnect decisions based on forecasts and real-time vessel motion measurements. Introduction When conducting a deepwater drilling operation from a dynamically positioned (DP) vessel, riser disconnect can become necessary at positioned (DP) vessel, riser disconnect can become necessary at any time. It can be caused either by loss of station or by heave in excess of tensioner/slip joint stroke capability. Drilling vessels supporting disconnected long marine risers in heavy seas are vulnerable toriser buckling,overloading the attachment point to the vessel,moonpool impact/riser bending, andriser handling problems. All of these problems are minimal with a short riser because when drilling in relatively shallow water ( less than 1,000 ft [300 m]) with an anchored vessel, one can plan to disconnect and pull the riser for major storms. However, in 5,000- to 7,500-ft (1500- to 2300-m] water depths with a DP ship, this is not the case. Pulling the riser requires at least 36 hours, which approaches the range of reliable weather forecasts. One assessment of winter storms in the mid-Atlantic region (Wilmington Canyon) indicated that we would need to pull and secure the riser six or more times during the winter to prevent riser damage-if we had sufficient warning. The expected downtime for pulling and running the riser was unacceptable. Furthermore, the pulling and running the riser was unacceptable. Furthermore, the mid-Atlantic is an area of rapidly developing storms, which often can preclude pulling a long riser. Therefore, it became necessaryto develop a method of surviving storms without pulling the riser,to improve forecasting, andto develop a systematic, logical approach for operating the marine riser. The specific constraints can be summarized asdrilling in the mid-Atlantic in 5,000 to 7,500 ft [1500 to 2300 m] of water in year-round weather,use of Discoverer Seven Seas drillshipsuse of semi-conventional riser system, andstartup of the drilling operation in less than 1 year. Background A neutrally buoyant riser is desirable for minimizing top tension requirements and riser stresses. However, if it is disconnected from the blowout preventer (BOP) stack or wellhead and suspended from the vessel, neutral buoyancy is detrimental. If the riser is attached to the vessel, it must move up and down with the vessel. Newton's Second Law of Motion is where m is the total riser system mass, a is the vertical acceleration of the riser-the same as the vessel's heave acceleration, and F is the resultant vertical force on the riser. Obviously there must be a cyclic vertical force applied to the riser to make it follow the vessel. The upward force applied to the riser by the vessel is limited either by the strength of the attachment point or by the tensile strength of the riser. Buckling of long, slender tubes is well understood. Since compression in the riser can lead to failure, the vessel cannot be allowed to exert a large, downward force on it. The only practical way to achieve a net downward force is to give the riser system a net in water weight, which obviously cannot be concentrated near the top of the riser or the buckling problem will remain. Thus, it becomes apparent that heave acceleration of the vessel is the most important vessel motion. The force/mass ratio or weight/mass ratio of the riser determines whether it is capable of falling with the vessel and not being pushed or compressed. When the vessel accelerates upward, the tension at the top of the riser must include both the riser's in water weight and the force needed to accelerate it upward. Many techniques for increasing the weight and, therefore, the weight/mass ratio, have been proposed and/or developed:bare (unbuoyed) riser joints,variable buoyancy (air can) riser joints,heavy weights on the Lower Marine Riser Package (LMRP), andheavy drilling mud inside the riser. JPT P. 835