Annulus Conditions Research Articles

Numerical Permeation Models to Predict the Annulus Composition of Flexible Pipes

The migration of acid gases through the pressure sheath of flexible pipes may induce a corrosive environment that can lead to steel armors’ failure by SCC (stress corrosion cracking). This permeation process depends on temperature, partial pressures, materials, and the pipe’s geometry. However, there are few works related to permeation modeling in flexible pipes, and these works usually contain significant simplification in pipes’ geometry. Hence, this work proposes two finite element (FE) permeation models and discusses the effects of the pipe’s characteristics. The models were developed in Ansys®, considering two- (2DFE) and three-dimensional (3DFE) approaches, and rely on gas fugacities instead of concentrations to describe the mass transport phenomenon. A radial temperature gradient is also considered, and the heat transfer is uncoupled from the mass transfer. Dry and flooded annulus analyses were conducted with the proposed models. In dry conditions, the results obtained with the 2DFE and the 3DFE approaches showed no significant differences, demonstrating that 3D effects are irrelevant. Hence, the permeation phenomenon is ruled by the permeation properties of the polymeric layers (pressure and outer sheaths) and possible shield effects promoted by the metallic armors. In contrast, the flooded annulus analyses resulted in a non-uniform fugacity distribution in the annulus with significant differences between the 2DFE and the 3DFE approaches, showing the importance of modeling the helical geometries of the metallic armors in this condition. Finally, a conservative 2DFE approach, which neglects the contribution of the pressure sheath, is proposed to analyze the flooded annulus condition, aiming to overcome the high computational cost demanded by the 3DFE approach.

Multiannuli Solution Uses Intelligent Pipe for Real-Time Monitoring

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 211546, “Intelligent Pipe: A Multiannuli Monitoring Solution,” by Francois-Xavier Bulard, Vallourec; Emmanuel Tavernier, OpenField Technology; and Lucas Kling, Vallourec. The paper has not been peer reviewed. _ The authors describe a multiannular monitoring solution that measures pressure and temperature for each well annulus. These key parameters are captured by sensors based on microelectromechanical-system technology. A transmission methodology is used that allows power supply and data retrieval even for annuli beyond the B annulus. All electronic elements of the solution will be qualified through an international recommendation protocol [Advanced Well Equipment Standards (AWES)] that consists of a series of mechanical and long-term tests to ensure that the system will survive for 20 years at 125°C. The solution could be used in all storage applications, especially those related to the energy transition. Introduction The initial goal of the intelligent-pipe solution was to add functionality to tubulars by embedding miniaturized sensors into the pipe body. The secondary objective was to monitor the pipe’s environment to properly characterize well loads and compare them to its strength resistance. The main application of the solution was then to monitor annulus conditions in real time during the whole life of the well and to anticipate any leakage or annular-pressure-buildup issue that could not be monitored currently on subsea wells. A tubular design was developed to incorporate pressure and temperature sensors without affecting the integrity of the pipe. The design was verified by a testing protocol performed by an in-house testing laboratory. This protocol consisted, first, of finite-element analysis (FEA) performed on different tubular designs with the objective of optimizing the different shapes of the grooves created to incorporate the sensors. The second steps included physical tests performed on a specimen inside test frames that confirmed that the tubular integrity was not affected after several pressure and axial load cycles, even when applying maximum pipe-resistance loads. This mechanical test qualification demonstrated that the monitoring mandrel was able to withstand the constraints observed in a well. The qualification level achieved was the same as that achieved on the premium pipe connection. Much of the complete paper is dedicated to the development of the permanent solution; this synopsis focuses on the system’s pilot deployment and its global system qualification. First Deployment in a Deepwater Subsea Well The operator’s objective was to monitor several annuli of a subsea well to determine the effect of completion operations. This characterization was completed before the well-production phase. The main outputs of the pilot were annuli trapped pressure and temperature. The pilot was defined initially in two stages. The first was to monitor the B and C annuli conditions (pressure and temperature) immediately after casing deployment. The second was to do so again after running completions and various stimulation operations. A secondary objective was to retrieve historical sensor data, pressure, and temperature records from the time of sensor deployment to data-retrieval operation.

Semi-empirical equations to predict the bisymmetric dry and wet hydrostatic collapse strengths of flexible pipes

This work proposes closed-form equations to calculate the bisymmetric dry and wet hydrostatic collapse strengths of flexible pipes with imperfections. Initially, the bisymmetric hydrostatic collapse is discussed, relying on the classic Timoshenko's equation. Then, the main parameters involved in the collapse mechanism are indicated. Particularly, approaches to account for the effect of the carcass’ cold work and the pressure layers’ residual manufacturing stresses are suggested. After that, an equivalent three-dimensional shell finite element (FE) model is revisited and employed to predict the flexible pipes' wet and dry collapse strengths. Several FE collapse analyses were performed addressing different material properties, initial ovalities, and interlayer gaps. The obtained responses showed that the hypotheses assumed by the modified Timoshenko's equation only hold for the dry collapse condition with no interlayer gaps. However, the calculated collapse pressures with this equation deviate from the FE values as higher ovalities are considered. Hence, an alternative approach was employed to overcome these limitations. In this approach, the FE collapse pressures and the flexible pipes’ characteristics constituted datasets analyzed with a symbolic regression (SR) tool. These analyses led to the closed-form equations, named semi-empirical equations, that best fitted the datasets. Sensitivity analyses of these equations showed that the bisymmetric collapse is significantly affected by the yield strength of the pressure layers’ materials and imperfections. Furthermore, under wet annulus conditions, the confinement provided by the pressure armor also has a substantial impact. Finally, comparisons with experimental tests verified the applicability of the proposed equations.

Development of an analytical model for predicting the wet collapse pressure of curved flexible risers

Flexible risers are designed with strong anti-collapse capacities which enable them to operate in deep-water reservoirs. However, this anti-collapse capacity is susceptible to the pipe curvature in the flooded annulus condition. For the curved riser sections within the touch-down zone, significant reduction of collapse capacity can occur once their external sheaths are worn out by the seabed, resulting in the so-called “wet collapse”. Mostly, wet collapse studies of curved flexible risers are performed through costly numerical simulations since there are no alternative analytical approaches. In view of it, this work presents an analytical model for predicting the wet collapse pressure of curved flexible risers. The analytical model is developed based on a spring-supported arch model that from our previous work, which is able to take the curvature-induced factors into account. With the stability theories of arched structures, the wet collapse pressure of curved flexible risers can be solved. To verify this analytical model, 3D full FE models are employed. The critical collapse pressures predicted by these two kinds of approach are in good agreement, indicating this proposed analytical model can be an useful tool to facilitate the collapse analysis in pipe design stage.

Analytical estimation on the number of bending cycles to initiate armour wires lateral buckling in flexible pipes

The flexible pipe industry has been confronted with the potential threat of armour wires lateral buckling for over two decades. Cyclic bending is recognized to play a significant role in the lateral buckling mechanism since it facilitates the armour wire sliding against the frictional resistance. If a flexible pipe is exposed to axial compression larger than the critical load for a certain period, the risk of lateral buckling may be controlled by limiting the number of bending cycles. Thus, the estimation of the bending cycles number to initiate buckling is favorable to prevent buckling failure and enlarge the potential application scope of flexible pipes. The present paper presents an analytical model for an approximate estimation of the number of bending cycles required to initiate the lateral buckling in wet annulus conditions. The analytical results are compared with the available test data in the literature and relatively good correlations are observed on the cycle numbers to buckle. Based on this analytical model, parameter studies are performed and the effects of axial compression, bending curvatures, and friction coefficient on the number of bending cycles to initiate buckling, as well as the stress states at the onset of buckling, are discussed.

Numerical investigation of gas permeation and condensation behavior of flexible risers

Flexible risers are widely employed in deep-sea oil and gas development. Methane, carbon dioxide, water vapor, and other small molecular gases can permeate into the sheaths and accumulate with water condensation in the annulus, which may cause overpressure and corrosion accidents. A mechanistic model was proposed to describe the permeation of gas phase in the multiphase flow though the flexible riser inner surface and its condensation in the internal annular space. The governing equations were solved efficiently by a combination of the control volume method and the finite difference method. After validation of the models, the thermo-hydraulic characteristics, concentration fields, annulus conditions, flow patterns, and some key parameters were comprehensively investigated. The results showed that with increasing the inlet pressure and temperature, the initiation time of venting and its duration decreased exponentially. The venting frequency can be adjusted by selecting an appropriate shielding factor and venting pressure. When the shielding factor is larger than 0.75 or the venting pressure is greater than 500 kPa, the venting cycle will be significantly prolonged. The gas permeation rates varied in a wide range of 2 times with flow patterns.

Frictional effect on circular armour wire lateral buckling in umbilicals

The present paper addresses the frictional effect on the circular armour wire lateral buckling in an umbilical for wet annulus condition. A nonlinear static finite element model is developed where only one single circular armour wire is simulated and the contact interface with the neighboring layer is represented by a deformable rigid toroid surface. The isotropic Coulomb friction model regularized with an elastic domain is implemented to simulate the shear interaction. Case studies are carried out by applying different axial compression and cyclic bending curvatures between zero and a maximum value in one bending direction. The minimal axial compression that may cause the armour wire lateral buckling after numerous bending cycles is approximately estimated by fitting the numbers of bending cycles to initiate the buckling under different axial compression through an empirical model. The friction coefficient and the maximum elastic slip employed in the Coulomb friction model are varied for the parametric studies, and their effects on the armour wire sticking-slipping status, the end shortenings and deflections in cyclic bending, the critical lateral buckling limit, as well as the stress state at the onset of lateral buckling are extensively discussed.

Curvature effect on wet collapse behaviours of flexible risers subjected to hydro-static pressure

ABSTRACT Flexible risers that for deep-water production are designed with strong collapse capacity. However, this collapse capacity is susceptible to pipe curvature in the flooded annulus condition. For the curved riser sections within the touchdown zone, significant reduction of collapse capacity can occur once the external sheaths are worn out, resulting in the so-called ‘wet collapse’. Therefore, the collapse capacity of flexible risers should be well-determined during design stage. Mostly, curved collapse analyses are performed through costly numerical simulations since there is no well-established analytical models . This study focuses on the wet collapse mechanism of curved flexible risers, aiming to pave the way for the development of analytical solutions. 3D full FE models were adopted to gain insight into the wet collapse mechanism of curved risers, which had been verified by the data from public literature. The investigation indicates that the deformed cross-sectional shape of the carcass is the dominant factor of the curvature effect.

Utilizing a novel fiber optic technology to capture the axial responses of fully grouted rock bolts

Rock bolts are one of the primary support systems utilized in underground excavations within the civil and mining engineering industries. Rock bolts support the weakened rock mass adjacent to the opening of an excavation by fastening to the more stable, undisturbed formations further from the excavation. The overall response of such a support element has been determined under varying loading conditions in the laboratory and in situ experiments in the past four decades; however, due to the limitations with conventional monitoring methods of capturing strain, there still exists a gap in knowledge associated with an understanding of the geomechanical responses of rock bolts at the microscale. In this paper, we try to address this current gap in scientific knowledge by utilizing a newly developed distributed optical strain sensing (DOS) technology that provides an exceptional spatial resolution of 0.65 mm to capture the strain along the rock bolt. This DOS technology utilizes Rayleigh optical frequency domain reflectometry (ROFDR) which provides unprecedented insight into various mechanisms associated with axially loaded rebar specimens of different embedment lengths, grouting materials, borehole annulus conditions, and borehole diameters. The embedment length of the specimens was found to be the factor that significantly affected the loading of the rebar. The critical embedment length for the fully grouted rock bolts (FGRBs) was systematically determined to be 430 mm. The results herein highlight the effects of the variation of these individual parameters on the geomechanical responses FGRBs.

Exploration of Interocular Suppression Using Perceptual Reverse Correlation and Computational Modeling

Binocular rivalry is often studied with participants making perceptual reports about which of two overlapping incongruent images presented to different eyes, such as rivalrous gratings, are visible at any particular moment in time. However, the rivalrous gratings being judged are also directly activating the neural mechanisms involved in the perceptual decision process, i.e. interocular suppression. Taking advantage of the spatially extended nature of interocular suppression with a continuation of a combined psychophysical and computational modeling study (Nichols & Wilson, 2009), the current study temporally modulated the presence of surround annuli while participants made judgments on the visibility of static circular gratings that were presented to either one eye (monocular) or both eyes with different orientations (rivalrous). A perceptual reverse correlation technique was used with moving gratings in the surround annuli changing direction every second while also randomly changing with regard to which eye or eyes they were presented to. Key qualitative findings were replicated, e.g. the presence of an annulus in the eye contralateral to a monocular center grating is necessary for the suppression of the grating but removal of the annulus is not necessary for the reappearance of the grating. New qualitative findings included how the presence of an ipsilateral annulus is similarly influential for the return to perceptual dominance for both monocular and rivalrous center gratings. New quantitative measurements included the relative necessity and sufficiency of different annulus conditions for interocular suppression. Implementation of the model with fluctuations in the input strength of the surround components that inhibit the center components to match the experimental design revealed many qualitative similarities to the data but also some qualitative and quantitative differences. Future tuning of the model to establish greater consistency with the psychophysical data can shed light on the role of contralateral and ipsilateral surrounds in interocular suppression. Meeting abstract presented at VSS 2017

Analysis of Water Cooling of CPV Cells Mounted on Absorber Tube of a Parabolic Trough Collector

Abstract In the present paper, an analytical approach has been developed and presented to estimate the thermal performance of multi-junction solar panel under high concentration with liquid cooling on both sides of the panel. For such system, the receiver of parabolic collector is modified to incorporate solar cells. A long panel of flexible solar cells is mounted on the outer side of circular receiver. The water is allowed to flow from inside as well as outside of receiver, thus reducing the temperature of the panel significantly. Analytical model is developed and thermal analysis has been carried out using MATLAB (vR2012a). The thermal model predicts the temperature variation of the cell along its length, which given the improved efficiency of the panel. To validate the proposed model, the simulation is performed in COMSOL (v5.1). The results show that temperature of the liquid can be maintained up-to 85C, thus reducing the temperature of panel from initial temperature (without liquid cooling) of above 134C, under stagnant air in the annulus conditions. The results obtained are within the close approximation. The future scope would include the experimental validation of the proposed system.

Forward and Converse Lyapunov Theorems for Discrete Dynamical Systems

This technical note addresses the stability analysis of nonlinear dynamic systems. Three main contributions are made. First, we show that the standard assumption of a continuous Lyapunov function can be (and in some cases must be) relaxed. We introduce the concept of the `weak' Lyapunov function, which requires that an annulus condition be satisfied. We believe that this annulus condition is a more natural construct, because it is precisely what is needed to make the forward Lyapunov theorem true. Second, we provide an example of a nonlinear system with stable equilibrium point that cannot be shown to be stable with a continuous Lyapunov function. Finally, we demonstrate a simpler and less restrictive proof of the converse Lyapunov theorem.

The Effect of Annulus Performance Parameters on Rotor-Stator Cavity Sealing Flow

The amount of cooling air assigned to seal high pressure (HP) turbine rim cavities is critical for performance as well as component life. Insufficient air leads to excessive hot annulus gas ingestion and its penetration deep into the cavity compromising disk or cover plate life. Excessive purge air, on the other hand, adversely affects performance. This paper is a continuation of the authors' work on ingestion reported by Mirzamoghadam et al. (2008) (“3D CFD Ingestion Evaluation of a High Pressure Turbine Rim Seal Disk Cavity,” ASME Paper No. GT2008-50531), where the main focus of that investigation was to qualitatively describe ingestion driven by annulus circumferential pressure asymmetry under constant annulus conditions and rotational speed. In this paper, the research team investigated the variation of annulus circumferential pressure fluctuation and rotational speed on the double overlap platform rim seal cavity reported in part-1. The outcome from this study was to map out the resulting nondimensional minimum sealing flow (minimum value of Cw or Cw,min) as it relates to entrained ingestion in the absence of cavity cooling flow (Cw,ent). As was done in part-1, the runs were made with 3D computational fluid dynamics (CFD) in setup/run mode option using Fine/Turbo. At two rotational speeds, annulus conditions were varied by reducing turbine inlet pressure (i.e., mass flow) from the baseline operating condition, and the resulting pressure fluctuation was quantified. In addition, an investigation to assess the selected aft-located mixing plane steady state solution for this study as compared to the forward-located steady run was performed using unsteady (nonlinear Harmonics) CFD as the referee. The results yielded the linear decrease in Cw,ent at fixed rotational Reynolds number as annulus Reynolds number was decreased. Moreover, the rate of change in entrained flow sharply increases with increase in rotational Reynolds number. As annulus mass flow is reduced to a critical value defined by annulus-to-rotational Reynolds number ratio, the CFD prediction for Cw,ent converges to the turbulent boundary layer entrainment solution for the rotor, and Cw,min reverts to the rotational Reynolds number dominating region. The results from this study were compared to what has been observed by a previous study for a single overlap platform geometry. The resulting design curve allows insight in relating cavity purge flow requirements versus turbine cycle parameters which could lead to better efficiency.

A Theoretical Approach to Predict the Fatigue Life of Flexible Pipes

This paper focuses on a theoretical approach to access the fatigue life of flexible pipes. This methodology employs functions that convert forces and moments obtained in time‐domain global analyses into stresses in their tensile armors. The stresses are then processed by well‐known cycle counting methods, and S-N curves are used to evaluate the fatigue damage at several points in the pipe’s cross‐section. Finally, Palmgren‐Miner linear damage hypothesis is assumed in order to calculate the accumulated fatigue damage. A study on the fatigue life of a flexible pipe employing this methodology is presented. The main points addressed in the study are the influence of friction between layers, the effect of the annulus conditions, the importance of evaluating the fatigue life in various points of the pipe’s cross‐section, and the effect of mean stresses. The results obtained suggest that the friction between layers and the annulus conditions strongly influences the fatigue life of flexible pipes. Moreover, mean stress effects are also significant, and at least half of the wires in each analyzed section of the pipe must be considered in a typical fatigue analysis.

LDA Measurements of Feed Annulus Effects on Combustor Liner Port Flows1

A detailed LDA experimental study is reported on the effect of variations in feed annulus conditions on port flow jet characteristics. The data are relevant to primary and dilution jet flows as found in gas-turbine combustor liners. Alteration of jet velocity ratios and associated bleed flows past the jet ports for a typical annulus height/port diameter configuration was observed to produce significant variations in separation regions in the annulus, distortions in downstream annulus profiles, and jet exit conditions. Profiles of jet exit velocity, flow angle, and turbulent kinetic energy distribution have been provided which should prove invaluable as boundary conditions for related CFD studies. For the first time the influence of swirl in the approach annulus was examined, at a level consistent with residual swirl passing down the annulus from compressor exit conditions. Noticeable deviations in jet characteristics were again observed, reducing jet entry angles by some 20 deg over the rear half of the port.

Detection and identification of monochromatic stimuli under chromatic contrast

Detection thresholds for monochromatic stimuli superimposed on an achromatic background were determined under conditions of simultaneous color and brightness contrast. Chromatic annuli selectively raised the thresholds for stimuli of similar wavelength composition relative to that of stimuli of different wavelength composition. The stimuli were then equated for detectability at near-threshold intensity levels under all annulus conditions and their discriminability from one another was evaluated. All the stimuli were readily discriminated at threshold intensities under all annulus conditions, indicating that the chromatic mechanisms were active at threshold levels. The different annulus conditions did not differentially affect stimulus discriminability beyond the selective effect on detection thresholds. The selective effects on the detection thresholds may therefore have occurred within the chromatic visual mechanisms rather than the achromatic mechanism.