Internal Slug Flow Research Articles

Slug flow parameters applied to dynamic riser analysis

Slug flow is a typical pattern in the petroleum production system. Its main feature is intermittency. This feature influences the riser dynamic behavior due to the coupling of its geometric configuration and time and space variation of internal fluid parameters. Although the studies that illustrate slug flow intermittency and dynamic riser behaviors have been growing recently, this discussion still needs to be expanded. Most slug models proposed in the literature are simplified and do not adequately capture the intermittency to a proper riser dynamic influence determination. Previous models neglected at least one of the following slug flow mechanisms: random features, bubble shape, liquid slug aeration, wake, and pipeline inclination effects. This paper aims to cover all these phenomena and present a model that provides realistic slug flow data for any riser analysis program. The dynamic riser analysis is not the scope of this paper. This article used a new slug tracking model that captures the intermittency to study the influence of the internal flow over the dynamic riser behavior. The model considers the liquid slug hold-up and its change over the pipe, the oil and gas thermodynamic properties, and the geometrical transition from typically horizontal plane interfaces to vertical concentric bubble shapes. The slug tracking model tests use three gas and liquid ratios (0.55, 0.7, and 0.82) with a 2.05 m/s constant mixture velocity. The maximum relative pressure deviation was 24.1. Time and space analysis of the slug flow parameters demonstrates the slug tracking capacity to reproduce the intermittency of the bubble and liquid slug lengths, void fractions, velocities, and pressures. This intermittence is essential to describe the internal slug flow influence on the structure behavior.

Fatigue reliability assessment of offshore catenary risers conveying internal slug flow

AbstractThis paper proposes a fatigue reliability assessment method of offshore steel catenary risers (SCRs) caused by internal slug flow. Offshore risers subject to slug flow are modeled first by employing a finite element method scheme. Subsequently, a combination of Kriging metamodel and Monte Carlo simulation (MCS) is established to assess the fatigue reliability of SCRs conveying internal slug flow. An explanatory example of a 16‐inch SCR is presented to validate the performance of the proposed method and the results indicate that the failure rates of the SCR are significantly affected by the frequency of slug flow. Moreover, the accuracy and feasibility of the present method are validated by a comprehensive comparison with the existing experiments and numerical results. The present method contributes to the design and maintenance of offshore risers with internal slug flow.

Three-dimensional dynamic behaviour of flexible catenary risers with an internal slug flow

This work analyses the three-dimensional dynamic response of flexible catenary risers when subjected to the effects of an internal slug flow. The interaction between the flexible riser and the internal slug flow is a rather complex phenomenon, and this is because the forces due to the acceleration of the internal fluid influence the riser dynamics, which, in turn, influences the slug flow motion in its interior. In order to carry out this analysis, a new computational tool called SLUGFLEX has been developed, which is composed by two computational codes: the code that calculates the structural dynamic response of the flexible riser, and the other one that predicts the development of the internal slug flow. In the structural analysis, the spatial discretization of the riser is performed using spatial Euler beam finite elements, which suffer large displacements, although with small deformations. The tangent stiffness matrix and the internal force vector of the structure are calculated using the corotational approach. The internal slug flow, whose key feature is the intermittence of two different regions: a liquid slug and a large bubble of gas, is mathematically modelled by using a new one-dimensional lagrangian slug tracking method, which has been based on a model developed for straight pipes, and over which the effects of the curvature of the riser and its dynamics were added. The dynamic response of the riser is calculated by using the constant average acceleration method, in combination with the incremental iterative Newton–Raphson scheme for follower loads. The numerical results show good agreement when compared to the experimental ones obtained from the literature.

Coupling vibration response of a curved flexible riser under the combination of internal slug flow and external shear current

Apart from the vortex-induced vibration (VIV) caused by flow over flexible risers, internal slug flow-induced vibration (FIV) is extensively encountered in practice as risers are conveying oil–gas two-phase flow. This paper presents an experimental study of the coupling response of a flexible catenary riser under the combination of internal slug flow and external shear current. The vibration displacement and the internal flow regime are synchronously captured using the non-intrusive technique of high speed imaging method. The experimental results illustrate that the contribution of internal FIV and external VIV depends on the phase difference and the water depth. For the in-plane response, the contribution of internal slug flow is greater than the external flow at the upper part, while the out-of-plane response is dominated by the external current. Unlike the standing wave behavior in the internal FIV case, a traveling wave occurs with the participation of external flow, associated with the multimodal response. Additionally, the response frequencies vary over time, possessing oscillating characteristics. Thus unstable response and mode switching are observed in the deflected shapes of the flexible riser. Due to the vigorous out-of-plane vibration, the longer liquid slugs are split into shorter segments. As the gas–liquid ratio increases, the liquid slug length is decreased gradually, resulting in the reduction of system mass. Both the in-plane and out-of-plane responses are enhanced accordingly.

A parametric analysis of the influence of the internal slug flow on the dynamic response of flexible marine risers

This paper presents a two-dimensional parametric analysis of the influence of an internal slug flow on the structural dynamic response of flexible risers. The parametric analysis allow us to study the behaviour of the parameters involved in the dynamic response of flexible risers with an internal slug flow, such as displacements, top tension, and bending moment, when any of the parameters characterizing the slug flow is modified. To accomplish this goal, a computational tool is developed. The spatial discretization of the riser is performed using beam finite elements and the corotational approach is used for the two-dimensional formulation of the beam element. The beam elements have a linear elastic behaviour and they are submitted to large displacements, although with small deformations. The incremental-iterative Newton-Raphson scheme for follower loads is used to solve the equilibrium equations in dynamic analysis. Time domain numerical integration is performed using the constant average acceleration method. The slug flow is viewed as a single-phase fluid with variable density, which is modelled by a periodic function. The numerical results show good coherence when compared to the experimental ones, qualifying the computational tool to conduct parametric studies about dynamic of flexible marine risers with an internal slug flow.

Experimental investigation on the flow-induced vibration of a free-hanging flexible riser by internal unstable hydrodynamic slug flow

Unstable hydrodynamic slug flow is extensively encountered in marine risers, resulting in internal flow-induced vibration, which is quite different with the vortex-induced vibration caused by a cross flow. Experiments were performed in an air-water test loop to study the internal slug flow-induced vibration of a free-hanging flexible riser with aspect ratio (the ratio of the length to the diameter of a riser) of 158. A non-intrusive technique of high speed imaging method was employed to record the vibration displacements of the riser model and the flow regime of two-phase mixture in the riser simultaneously. The results show that an in-plane vibration is created by the internal slug flow, and the responses along the two directions in the plane are interdependent to each other. In the considered gas-liquid ratio (the ratio of air flow rate to water flow rate), the second-order mode dominates the response. The hydrodynamic behaviors of slug flow, including the gas-liquid ratio, liquid slug length and superficial velocity, affect the local pressure fluctuation and hence influence the vibration amplitude. The fluid-structure interaction is reflected from the same frequencies presenting in the vibration and pressure fluctuation.

Slug Flow and Waves Induced Motions in Flexible Riser

Flexible risers provide optimum solutions for deep water offshore fields. Reliable dynamic analysis of this kind of slender structure is crucial to ensure safety against long time fatigue failure. Beyond the effects from wave loads, the influence from transient internal slug flow on the slender structure dynamics should also be taken into account. In this study, two coupled in-house codes were used in order to identify and quantify the effects of an internal slug flow and wave loads on the flexible riser dynamics. One code carries out a global dynamic analysis of the slender structure displacements using a finite element formulation. The other program simulates the behavior of the internal slug flow using a finite volume method. The slug flow is influenced by the dynamic shape of the riser, while the time varying forces from internal slug flow plus external waves will influence the shape. Hence, a fully coupled analysis is needed in order to solve the coupled problem. By means of the distributed simulation, these two programs run synchronously and exchange information during the time integration process. A test case using hydrodynamic forces according to the linear Airy wave theory coupled with an internal unstable slug flow was analyzed and the results shown amplification of the dynamic response due to the interaction between the two load types, effects on the effective tension caused by the internal two-phase flow, and influence on the internal slug flow caused by the wave-induced response.

Hydroelastic response of marine risers subjected to internal slug-flow

It is the purpose of this study to investigate the dynamic behaviour of catenary pipelines for marine applications, assuming the combined effect of harmonic motions imposed at the top, and the internal slug-flow. The analysis is based on the assumption of a steady slug-flow inside the pipe that results in a relatively simplified model for the formulation of the internal flow. The slug-flow model is described using several assumptions and empirical correlations which attempt to reveal the ill-understood and concealed properties of the slug-flow. The pipeline dynamics are investigated in the two dimensional space omitting the out-of-plane vibrations. The system of differential equations is generic and accounts for the steady effect of the internal liquid as is conveyed through the structure.The two models, those of the internal slug-flow and the pipeline’s dynamical model, are properly combined through the internal flow terms of the dynamic equilibrium system. The solution provided is achieved using a frequency domain technique which is applied to the linearized governing set. The effect of the slug-flow is assessed through comparative computations with and without internal flow effects. The conclusions are drawn having the structure excited under axial and normal motions paying particular attention to the variation of the dynamic components along the complete length of the pipeline.

Finite Element Analysis of Fatigue in Pipelines due to Slug Flow

Transportation of Oil and Gas from the production site to the terminal or refinery requires the use of long pipelines which procure to develop slug flow regime. This slug may cause cyclic stresses leading to fatigue and results in damaging the pipelines. This paper aims to utilize that design guide and calculate the total fatigue life for the pipeline due to internal slug flow by performing evaluation of the stress range and fatigue life span of the pipeBeam 3element with moving loads in ANSYS commercial software have been used to simulate the slug flow across the pipeline and assess the dynamic response. The total fatigue life for different selected slug parametrs was calculated. The model was validated by comparing with analytical solution in a previous published research and reasonable agreement was revealed. The results show that increasing the slug to pipe weight ratio resulting in higher dynamic stresses and reduction in the fatigue life of the pipe. This model can be utilized for the offshore pipeline if the external environmental effects are considered.