Downstream Riser Research Articles

An improved void fraction prediction model for gas-liquid two-phase flows in pipeline-riser systems

This paper introduces the application of void fraction prediction models (25 models total) from a simple horizontal pipe to a complex, real pipeline. The flow mechanism in the pipeline-riser system is explained based on the upstream phase interface and differential pressure in the downstream riser. The transition boundaries of “stratified to intermittent flow” and “stratified smooth to stratified wavy flow” in the horizontal section were predicted well by the models based on simple pipes. Electrical capacitance volume tomography was used to obtain the upstream void fraction just before the downstream gas-liquid eruption, α1, or during multiple cycles, α2. The difference between α1 and α2 are ranging between −10 % and 5 %. Finally, an improved correlation is proposed based on the model found to be best (Lockhart and Martinelli, with error values (root mean square error (RMSE) and mean absolute percentage error (MAPE)) of about 12 % and 18 %); the corresponding error levels (RMSE and MAPE) amount roughly to 5 % and 10 %, respectively. The improved model also delivers a good prediction for α1 when separating in two flow regimes. Overall, the finally proposed correlation reads and is found superior in terms of accuracy to all previously published correlations.

Experimental study of the hydraulic jump phenomenon induced by the downstream riser structure in a pipeline–riser system

• The interface wave in the horizontal section before the riser was studied by an ECVT. • Flow regime transition boundaries shift to lower velocities due to the riser. • The period of hydraulic jump is consistent with that of severe slugging flow. • The hydraulic jump upstream is mainly caused by pressure fluctuations of the riser. This paper describes experiments conducted on the gas–liquid two-phase flow characteristics in a pipeline-riser system under 1 MPa. The hydraulic jump phenomenon (HJP) in the upstream horizontal section was quantitatively studied using electrical capacitance volume tomography (ECVT). Flow regime transition boundaries were shifted to lower gas and liquid velocities due to the impact of the riser at the end of the horizontal pipe. The HJP’s period is consistent with that of severe slugging flow; additionally, the liquid slug production and the gas–liquid eruption stages always occur before the HJP in the upstream horizontal section. The Pearson correlation coefficient between the HJP upstream and the pressure fluctuations in the three main structures (horizontal, downward, and riser sections), the period consistency, and the sequential relationship between them were analyzed to verify that the pressure fluctuations in the downstream riser were the main cause of the HJP upstream.

Experimental Study on Vortex-Induced Vibration Coupling Wake Interference of Multi-Riser Groups with Sensitive Spacing

The “riser group—fluid between risers” is taken as the carrier, and the experiment on vortex-induced vibration of tandem riser groups coupling interference effect under sensitive spacing is performed. The least-square method is used to linearly fit the reduced velocity and main frequency, and the rule of Strouhal numbers is analyzed. Each mode is separated based on the mode decomposition theory, and the mode conversion mechanism is also explored. The concept of “interference efficiency” is introduced to study the dynamic characteristics and response evolutions of different riser groups. The results show that the wake shielding effect widely exists in tandem riser groups, and the interference effect of midstream and downstream risers on their upstream risers is significantly lower than that of upstream risers on midstream and downstream risers. The trajectories of midstream and downstream risers lag behind their upstream risers due to multiple shadowing effects, the vibration frequency range of downstream riser is widened and the dominant frequency is extremely unstable. Compared with the isolated riser, wake interference suppresses the vibration displacement of the midstream and downstream risers in the in-line direction, and enhances the displacement of upstream and midstream risers in the cross-flow direction. The interference effect of the fluid between risers at low velocities is stronger than that at higher velocities, and the cross-flow displacements of upstream risers are always in the interference enhancement region. It is urgent to pay attention to the cross-flow displacement of upstream and midstream risers in tandem riser groups considering the safety design.

Static behaviours and collision onset criterion of two adjacent vertical risers

ABSTRACT This study aims to investigate the static behaviours of two risers and establish the generalised dual-riser collision onset criterion (COC). The dual-riser model is developed based on the vector form intrinsic finite element method where the Blevins wake model is used to consider the wake effects. The governing equations including a virtual damping force are solved by the central difference method to obtain the static behaviours of risers. Then, the effects of initial spacing, top tension factor and current velocity on the static behaviours of two vertical risers arranged tandem are investigated. Further, the dual-riser COC is established with three non-dimensional parameters by linear-regression analysis. The results show that a larger top tension factor of the upstream riser and a smaller that of the downstream riser is a more effective combination to avoid collisions. The dual-riser COC can be described by a linear function of non-dimensional parameters.

Study on the collision probability of vertical risers considering the wake interference effect

This paper develops a model to predict the collision probability of risers. Considering the wake interference effect, a dual-riser model is built based on the vector form intrinsic finite element. The central difference method is employed to obtain the dynamic responses of risers. Then, an analytical geometry-based procedure is established to identify a minimum distance from the displacement response in a duration. A random sample of the minimum distance is obtained based on a series of repeated such processes. Further, the collision probability is predicted from this random sample utilizing the probability paper method and the least-squares fitting method. Finally, the effects of critical parameters (i.e., initial spacing, top tension factor (TTF), reduced velocity) on the collision probability of two vertical risers arranged tandem are investigated. Results show that the collision probability is considerably underestimated without considering the wake interference effect. Moreover, an increase in TTF of the upstream riser (URTTF) or a decrease in that of the downstream riser (DRTTF) is more feasible to reduce the collision probability. For every 0.1 increases in URTTF, the collision probability drops by 2 orders of magnitude. For every 0.1 decreases in DRTTF, the collision probability drops by 1-2 orders of magnitude.

Assessment of methods for short-term analysis of riser collision probability

Reliability- and performance-based design of a riser cluster requires an accurate estimation of the extreme values of the stochastic relative distance between the risers during a given time duration. This study presents a comprehensive assessment of six methods for estimating the collision probability between two risers with limited simulation length. A pair of steep-wave risers in tandem arrangement subjected to combined current and wave loads is modeled. The wake effect generated by the upstream riser acting on the downstream riser is considered.Firstly, the critical locations at which collisions during a short duration are likely to occur are identified. After obtaining nonlinear riser responses, 3-hour short-term extreme relative motions are calculated, which are used for the collision probability estimation. Several methods for the extreme value analysis, including the Gumbel probability paper method, the general extreme value method and the average conditional exceedance rate method are presented. This paper also proposes two methods which offer reliable and satisfactory results for the extreme value analysis of highly skewed processes.

Experimental study on dynamic response of smooth and straked risers in tandem arrangement coupling interference effect

An experiment was conducted in a combined wave–current water flume on two tandem risers subjected to uniform flow. The riser model has an effective length of 2.0 m. The aspect ratio is 111.11. The upstream riser is smooth and the downstream riser fitted with three-strand helical strakes with pitch 17.5D and height 0.25D. By varying the external flow velocities and spacing ratios, through comparisons with the dynamic response of isolated smooth and isolated straked riser, the paper observe how interference effect impacts the dynamic characteristics and dynamic response of two risers in tandem arrangement, reveal how the suppression efficiency of the three-strand helical strakes responds to spacing ratio and external flow velocity, and explore the wake excitation effect of inter-riser fluids on the downstream riser and their dynamic feedback to the upstream riser. The results show that the dominant frequency of the upstream smooth riser is sensitivity to the change of the spacing ratio is low, and the displacement response is offset or enhanced in different degree due to the difference of the interference efficiency. The downstream straked riser dominates frequency and displacement higher than the isolated straked riser. The wake vortex of the upstream smooth riser acts on the downstream riser, occupying a dominant position in the vibration of the downstream riser. It degrades the vibration suppression efficiency of the three-strand helical strakes: the suppression efficiency is the highest at spacing ratio of 8, being a merely 70.57%. With the increase of the spacing ratio, the CF displacement of the riser gradually decreases, and the IL displacement gradually increases. The interference efficiency partition and suppression efficiency at different spacing ratios reflects the dynamic feedback of the upstream smooth riser is much smaller than the interference effect of the downstream suppression riser.

Static analysis of interaction between two adjacent top tensioned risers with consideration of wake effects

Collision between adjacent risers has become an important issue as the oil and gas industry moves to deeper waters. In order to estimate the clearance between two marine risers, a static analysis which is mainly concerned with the wake effects is performed in this paper. A new wake model, which is used to predict the wake flow around the downstream riser, is developed based on Prandtl's shearing stress hypothesis. The wake effects with respect to riser interference in uniform current flow are then investigated. In this work, the two risers are both simplified as circular cylinders top-tensioned and pinned at the bottom. A procedure for iteratively predicting the wake velocity distribution and estimating the clearance between two risers is introduced to find the final convergent result by combining the new wake model with the global riser analysis software Riflex. The effects of different factors like riser spacing, top tension force and current velocity are also studied. The results indicate that these factors significantly influence the riser interference. Since the current velocity cannot in general be controlled for a specific site, the riser spacing and the top tension force become the primary design parameters that can be chosen by the operator.

Study on Vortex-induced Vibration Characteristics of Large Slenderness Ratio Tandem Risers

The fluid mechanics software-Fluent was used to simulate vortex-induced vibration (VIV) of flexible tandem risers with large slenderness ratio. In this paper, VIV of a single riser and the flexible tandem risers with large slenderness ratio at a spacing of 2-16 times were numerically simulated, which was based on slicing principle for 2-DOF vortex-induced fluid-structure coupled vibration. The ratio of vibration frequency between upstream and downstream risers, dimensionless amplitude and the effects of reduced speed of the risers were discussed under different spacing conditions. The preliminary results show that the VIV characteristics of the upstream riser is similar to the single riser, “lock-in” occurs obviously, the frequency ratio of lift force and drag force is 1:2 over the entire reduced velocity range, and there is no obvious “lock-in” in the downstream riser due to the influence of the wake of upstream riser. The maximum cross-flow amplitude reaches 0.9 times the diameter of the riser, and complex coupling phenomena such as the 1:1.5 times frequency ratio and double “8” track were observed.

Experimental study on vortex induced vibration of risers with fairing considering wake interference

Vortex Induced Vibration (VIV) is a typical flow-structure interference phenomenon which causes an unsteady flow pattern due to vortex shedding at or near the structure's natural frequency leading to resonant vibrations. VIV may cause premature fatigue failure of marine risers and pipelines. A test model was carried out to investigate the role of a stationary fairing by varying the caudal horn angle to suppress riser VIV taking into account the effect of wake interference. The test results show significant reduction of VIV for risers disposed in tandem and side-by-side. In general, fairing with a caudal horn of 45° and 60° are efficient in quelling VIV in risers. The results also reveal fairing can reduce the drag load of risers arranged side-by-side. For the tandem configuration, a fairing can reduce the drag load of an upstream riser, but will enlarge the drag force of the downstream riser.

Laboratory measurements of vortex- and wake-induced vibrations of a tandem arrangement of two flexible risers

The dynamic response of two flexible model risers in tandem arrangement immersed in a stepped current was analyzed. The risers, with an external diameter of 20 mm and a total length of 6200 mm, had an aspect ratio of 310. They were hinged to the support structure at the center-to-center distances away 3-12 times the external diameter. The top 1200 mm was exposed to a uniform current at a speed which was up to 0.9 m/s (the Reynolds number was 18000) and the rest in still water. The dynamic responses, which were obtained through the Fiber Bragg Grating strain gauges mounted on the surface, were analyzed by studying the cross-flow amplitudes and modal weights. The cross-flow vibration were observed up to the third mode, and the modal transformation from the second mode to the third mode was clearly observed. The experiment confirmed that the typical vortex-induced vibration (VIV) had occurred on the up-stream riser. But for the down-stream riser, the main excitation mechanism was wake-induced vibration (WIV). The modal transformation of WIV was more complex than that of VIV, which might be helpful for other researchers to study the interference effect.

Instability of a vertical riser in the wake of an upstream vertical riser

The static solution and its associated dynamic stability of a vertical riser situated in the wake of an upstream riser is investigated in the paper. Both the upstream and downstream risers are top-tensioned and pinned at the bottom ends on the seabed. Due to the wake effects, the downstream riser is subject to mean lift force as well as drag along its length. It is shown that under certain current conditions there exist multiple stable and unstable static solutions of the downstream riser. There also exists a critical current velocity and once this velocity is exceeded no static solutions of the downstream riser can be found. The disappearance of the static solutions is closely related to the loss of stability which is believed to be the main cause of ensuing clashing between the two risers. A further dimensional analysis reveals that the instability onset criteria can be described by three nondimensional parameters.

Riser diachroneity, lateral erosion, and uncertainty in rates of strike‐slip faulting: A case study from Tuzidun along the Altyn Tagh Fault, NW China

At late Quaternary timescales, offset fluvial terrace risers are among the most common landforms used to determine rates of strike‐slip faulting. Although diachroneity in the age of riser segments on opposite sides of the fault has been noted previously, an unexplored source of uncertainty associated with deriving slip rates from these markers centers on quantifying the size of the displacement uncertainty such diachroneity introduces. To evaluate the impact of riser diachroneity, we investigated the Tuzidun site (37.73°N, 86.72°E) along the Cherchen He reach of the active central Altyn Tagh Fault. The east bank of the channel is flanked by a left‐laterally offset terrace riser. While the measured offset is 54 ± 3 m, geochronologic measurements and analysis of riser topography indicate that the downstream riser segment formed between 6.0 ± 0.8 ka and 5.7 ± 0.4 ka, while the upstream riser segment may have been laterally refreshed as recently as 0.5 ± 0.2 ka. A valley wall on the west bank of the channel places a maximum limit of 38 ± 6 m on the amount of possible lateral erosion of the upstream riser. This bound, in turn, limits the total offset since formation of the downstream riser to range from 54 ± 3 to 89 ± 7 m. Together, these observations bracket the millennial Altyn Tagh Fault slip rate to range from 9.0 ± 1.3 to 15.5 ± 1.7 mm a−1. More generally, this investigation shows that the observed riser displacement does not necessarily correlate with the age of either riser segment (downstream or upstream of the fault) in cases where one segment is displaced while the other is subjected to lateral erosion. If this diachroneity goes undetected, erroneous slip rate measurements are likely to result.