Dynamic Characteristics of Deep-Water Risers Carrying Multiphase Flows

2018 ◽  
Author(s):  
Bowen Ma ◽  
Narakorn Srinil
Keyword(s):  
Steady State ◽  
Free Vibration ◽  
Multiphase Flow ◽  
Deep Water ◽  
Slug Flow ◽  
Internal Dynamic ◽  
Gas Flows ◽  
Time Space ◽  
Slug Flows ◽  
Oscillation Frequencies

Deep-water flexible risers conveying hydrocarbon oil and gas flows may be subject to internal dynamic fluctuations associated with the spatial variations of phase densities, velocities and pressure drops. Many studies have focused on single-phase flows in pipes whereas understanding of multiphase flow effects is lacking. This study aims to investigate the planar free-vibration characteristics of a long flexible catenary riser carrying the steady-state, multiphase slug oil-gas flows in order to understand how the inclination-dependent internal slug flows affect riser natural frequencies and modal shapes. The influence of slug characteristics such as phase velocities on the riser vibration is also studied. The catenary riser planar motions are mathematically described by a two-dimensional continuum model capturing coupled horizontal and vertical responses. Based on the selected two-phase flow rates at the wellhead, riser geometric configurations and specified slug unit lengths, a steady-state slug flow model is considered by taking into account several empirical closure correlations and riser mechanical properties, solving for the multiphase flow aspects including pressure, velocities, liquid holdup and gas fraction. By assigning an undamped free-vibration shape of an empty catenary riser as initial displacement conditions, the space-time numerical simulations are performed using a finite difference approach. Comparisons of oscillation frequencies, time histories, phase planes, time-space varying responses and dynamic stresses of catenary risers with and without slug flows are presented, identifying the dynamic modifications arising from the internal slug-induced mass momentum change and pressure loss. To understand the influence of slug flow properties, parametric studies are carried out with different gas velocities. Numerical results highlight the reduced riser tensions, decreased oscillation frequencies, multiple oscillation modes, amplified amplitudes and stresses. These key observations will be useful for the forced vibration analysis of catenary risers subject to combined internal (multiphase) and external (vortex-shedding) flow excitations.

scholarly journals Influence of Combined Empirical Functions on Slug Flow Predictions of Pipelines With Variable Inclinations

2020 ◽  
Author(s):  
Hossein Zanganeh ◽  
Victoria Kurushina ◽  
Narakorn Srinil ◽  
Omar K. Matar
Keyword(s):  
Steady State ◽  
Slug Flow ◽  
Gas Flow ◽  
Mechanistic Model ◽  
Gas Production ◽  
High Mass ◽  
Momentum Balance ◽  
Liquid Holdup ◽  
Wide Range ◽  
Slug Flows

Abstract Prediction of internal multiphase flows in subsea pipelines is an integral part of the oil and gas production system design. High mass and pressure fluctuations are often encountered during the operation with a liquid-gas slug flow regime exhibiting a sequence of long gas bubbles and aerated liquid slugs. It is important for industry to realistically identify the slug flow occurrence and predict slug flow characteristics, depending on several multiphase flow-pipe parameters. These may be achieved using a one-dimensional, steady-state, mechanistic model accounting for a mass and momentum balance of the two liquid-gas fluids within a controlled volume often referred to as a slug unit. By reducing a 3-D flow problem to a 1-D one, several empirical or closure correlations and associated empirical coefficients have been introduced in the literature and used in commercial software predicting slug flows in subsea jumpers, pipelines and risers with variable inclinations. This study aims to investigate the influence of combined 25 closure functions on the predictions of slug flows in horizontal and inclined pipes based on a steady-state mechanistic model for a wide range of superficial liquid and gas velocities. The model with studied closures is implemented by the authors of this study as the numerical tool iSLUG. The model performance is verified with respect to the estimated film liquid holdup, film length and pressure drop per length of a slug unit for an empirically specified translational velocity, slug liquid holdup, slug liquid length and pipe wall wettability. Closure combinations are analyzed using the relative performance factors and compared against available experimental data in order to identify a set of functions suitable for upward, downward and horizontal flows, and the effect of diameter and inclination on the model prediction is considered. The present method and analysis outcomes may further contribute to the improvement of transient liquid-gas flow models to predict more practical cases.

Slug Flow-Induced Oscillation in Subsea Catenary Riser Experiencing VIV

2018 ◽  
Author(s):  
S. Safrendyo ◽  
Narakorn Srinil
Keyword(s):  
Steady State ◽  
Slug Flow ◽  
Internal Flow ◽  
Dynamic Responses ◽  
Internal Diameter ◽  
Fundamental Vibration ◽  
Analysis And Design ◽  
Gas Phases ◽  
Riser Pipe ◽  
Slug Flows

Slug flow appearance in a multiphase-carrying riser with a long tie-back distance and deeper water is inevitable, depending on the operational and environmental conditions. Several state-of-the-art technologies in mitigating the effects of internal slug flows might not be completely effective or cost-efficient. In addition to the slug excitation, the external current flows can also affect the riser structural behavior and integrity by the presence of vortex-induced vibration (VIV). This study aims to investigate and understand the behavior of slug-conveying catenary riser under uniform and random slug excitations, in combination with VIV. The steady-state slugs are considered and modelled by a series of liquid and gas phases flowing at certain rates inside the riser pipe. Each slug unit consists of a slug liquid (oil, water or their mixture) and gas pocket. In the uniform slug flow cases, all slug units have their equal slug liquid lengths. Time-domain simulations are conducted for different slug units of 20D, 30D, 40D and 50D, where D is the pipe internal diameter, and for different internal flow rates. The non-uniform slug flow case is considered by randomly generating the time-varying slug liquid and unit lengths. Multi-frequency oscillations of the catenary riser are observed, triggered by the transient slug excitations rendering the fundamental vibration mode which is sustained over the ensuing steady-state slugging period. The random slug-induced vibration (SIV) entails larger response amplitudes which are critical from the fatigue life viewpoint, especially when VIV is also accounted for. For riser SIV analysis, only in-plane response is observed; nevertheless, the interaction of riser SIV and VIV generates both in-plane and out-of-plane responses with larger 3-D dynamic responses, deformations and stresses. Such combined SIV and VIV should be specially considered during the riser analysis and design by also taking into consideration the travelling random-like or intermittent slug flows.

3-D Numerical Simulations of Subsea Jumper Transporting Intermittent Slug Flows

2018 ◽  
Author(s):  
Jihyeon Kim ◽  
Narakorn Srinil
Keyword(s):  
Slug Flow ◽  
Oil And Gas ◽  
Process Condition ◽  
Structural Response ◽  
Dynamic Responses ◽  
Operating Pressure ◽  
Computational Performance ◽  
Fluid Properties ◽  
Slug Flows ◽  
Interaction Approach

Subsea jumper is the steel pipe structure to connect wellhead and subsea facilities such as manifolds or processing units in order to transport the produced multiphase flows. Generally, the jumper consists of a goalpost with two loop structures and a straight pipe between them, carrying the multiphase oil and gas from the producing well. Due to the jumper pipe characteristic geometry and multi-fluid properties, slug flows may take place, creating problematic fluctuating forces causing the jumper oscillations. Severe dynamic fluctuations cause the risk of pipe deformations and resonances resulting from the hydrodynamic momentum/pressure forces which can lead to unstable operating pressure and decreased production rate. Despite the necessity to design subsea jumper with precise prediction on the process condition and the awareness of slug flow risks, it is challenging to experimentally evaluate, identify and improve the modified design in terms of the facility scale, time and cost efficiency. With increasing high computational performance, numerical analysis provides an alternative approach to simulate multiphase flow-induced force effects on the jumper. The present paper discusses the modelling of 3-D flow simulations in a subsea jumper for understanding the development process of internal slug flows causing hydrodynamic forces acting on the pipe walls and bends. Based on the fluctuating pressure calculated by the fluid solver, dynamic responses of the jumper pipe are assessed by a one-way interaction approach to evaluate deformation and stress. A potential resonance is discussed with the jumper modal analysis. Results from the structural response analyses show dominant multi-modal frequencies due to intermittent slug flow frequencies. Numerical results and observed behaviors may be useful for a comparison with other simulation and experiment.

Investigations on Slug Flow in a Horizontal Pipe Using Stereoscopic Particle Image Velocimetry and CFD Simulation With Volume of Fluid Method

2012 ◽  
Author(s):  
Marek Czapp ◽  
Matthias Utschick ◽  
Johannes Rutzmoser ◽  
Thomas Sattelmayer
Keyword(s):  
Particle Image Velocimetry ◽  
Slug Flow ◽  
Particle Image ◽  
Volume Of Fluid ◽  
Stereoscopic Particle Image Velocimetry ◽  
Horizontal Pipe ◽  
Pressurized Water ◽  
Cooling Circuit ◽  
Image Velocimetry ◽  
Slug Flows

Investigations on gas-liquid flows in horizontal pipes are of immanent importance for Reactor Safety Research. In case of a breakage of the main cooling circuit of a Pressurized Water Reactor (PWR), the pressure losses of the gas-liquid flow significantly govern the loss of coolant rate. The flow regime is largely determined by liquid and gas superficial velocities and contains slug flow that causes high-pressure pulsations to the infrastructure of the main cooling circuit. Experimental and numerical investigations on adiabatic slug flow of a water-air system were carried out in a horizontal pipe of about 10 m length and 54 mm diameter at atmospheric pressure and room temperature. Stereoscopic high-speed Particle Image Velocimetry in combination with Laser Induced Fluorescence was successfully applied on round pipe geometry to determine instantaneous three-dimensional water velocity fields of slug flows. After grid independence studies, numerical simulations were run with the open-source CFD program OpenFOAM. The solver uses the VOF method (Volume of Fluid) with phase-fraction interface capturing approach based on interface compression. It provides mesh refinement at the interfacial area to improve resolution of the interface between the two phases. Furthermore, standard k-ε turbulence model was applied in an unsteady Reynolds averaged Navier Stokes (URANS) model to resolve self-induced slug formation. The aim of this work is to present the feasibility of both relatively novel possibilities of determining two-phase slug flows in pipes. Experimental and numerical results allow the comparison of the slug initiation and expansion process with respect to their axial velocities and cross-sectional void fractions.

scholarly journals Multiphase flow behavior in deep water drilling: The influence of gas hydrate

2020 ◽  
Vol 8 (4) ◽  
pp. 1386-1403 ◽  
Author(s):  
Jianhong Fu ◽  
Yu Su ◽  
Wei Jiang ◽  
Xingyun Xiang ◽  
Bin Li
Keyword(s):  
Multiphase Flow ◽  
Deep Water ◽  
Gas Hydrate ◽  
Flow Behavior

Non-linear free transverse vibration of thin rotating discs

2007 ◽  
Vol 221 (3) ◽  
pp. 467-473 ◽  
Author(s):  
H R Hamidzadeh
Keyword(s):  
Steady State ◽  
Free Vibration ◽  
Natural Frequencies ◽  
Mode Shapes ◽  
Stress Distributions ◽  
Rotating Speed ◽  
Rotating Discs ◽  
Non Linear ◽  
Modes Of Vibration ◽  
Free Transverse Vibration

An analytical method is adopted to determine modal characteristics of non-linear spinning discs. The disc is assumed to be isotropic and rotating under steady-state conditions. The effects of amplitude and rotating speed on natural frequencies are determined. The developed procedure is also capable of analysing natural frequencies of linear free vibration, which is independent of amplitude. Attention is confined to determine natural frequencies, mode shapes, stress distributions, and critical speeds for different numbers of nodal diameters. The developed procedure does not consider modes of vibration corresponding to nodal circles. Validity of this procedure is verified by comparing some of the computed results with those established for certain cases.

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