A Modified CFD-based Sand Erosion Prediction Procedure For Pipe Elbows And Similarity Analyses On Erosion Tests

With resource exploitation reaching to deep and ultra-deep waters,the low-temperature and high-pressure environment makes sand particle erosion more serious and remarkable in hydrocarbon production systems.The solid particles carried with crude oil and gas from the wells usually pose considerable erosion damage on valves,chokes,blinded tees and elbows.The damage increases the risk of equipment failure and may even result in great loss to the industry and threats to the environment.The oil and gas mixture extracted from the wells and transported over long distances is often in the form of multiphase flow,which is greatly complicated especially when flowing through the fittings.The sand erosion in gas-predominant pipelines for dry gas,gas-mist and annular flows is much more serious than that for other flow patterns.The sand response behavior is also complex when entrained in the gas-liquid mixture.What's more,the monitoring work of sand erosion in deep sea is heavy and expensive.Hence,it is of great significance to develop an efficient erosion prediction procedure considering the complex pipe flow mechanism and to apply it to the engineering erosion assessment.In this dissertation,the mechanism of the complex annular flow is analyzed and described by a mechanical model,and a modified CFD erosion prediction procedure is developed by introducing the annular flow model.Then the CFD procedure is applied in the verification of a set of similarity criteria,which aims at offering engineering guidance for the oil and gas industry.Firstly,a mechanical model of film movements is developed based on the treatments on the annular flow field.The initial conditions at the inlet are determined by adopting a validated film thickness correlation for fully developed upward annular flow in vertical pipes.The overall pressure gradient is assumed to be uniform all along the axial distance within the elbow and the static pressure is also uniform on every cross section.The axial velocity normal to the cross-sectional plane is uniform respectively for the liquid film and the core region.The droplets are assumed to travel in straight lines normal to the inlet plane until colliding on and absorbed by the liquid film surface.The liquid film motion is divided into the axial direction and the radial direction.Energy conservation law and Newton's second law are respectively used in the two directions.The film motion calculation is executed by using a discrete method with an explicit solution.The average film thickness and circumferential distribution on an arbitrary cross section can be obtained for given annular flow conditions.The mechanical model is verified by comparing the predicted average film thickness and circumferential distribution with three sets of experimental data from the literature.The present work proves that the film thickness variation within the elbows can be reasonably described by the developed mechanical model.Secondly,a modified CFD-based erosion prediction procedure is proposed based on mechanistic analyses of the three flow patterns.The procedure is developed to be suitable for the erosion calculation in elbows for gas,gas-mist and annular flows.For fully developed flow field,the core region of the pipe is regarded as single-phase flow with the property of gas or the mixture of gas and droplets.The difference is the particle near-wall behavior determined by whether there is liquid film attached on the pipe wall.For gas and gas-mist flow conditions there is no velocity decay,while for annular flow the particle-wall impact velocity is calculated by using the predicted results from the mechanical film thickness model.The turbulence models are validated through investigations on the velocity field and static pressure.The numerical settings of particle number and grid density are discussed on the computation stability.Over a hundred sets of experimental data are adopted for the verification of the CFD procedure by investigating on the maximum penetration rate and its position and the erosion profile on the elbow extrados.The comparison of calculation accuracy among the CFD procedure and four empirical or semi-empirical models are also conducted.The present erosion prediction procedure proves to be efficient for gas,gas-mist and annular flow conditions.Thirdly,the corresponding relationship between engineering conditions and model experiments is described by proposing a set of similarity criteria.The criteria can help predicting the maximum penetration rate and its position in engineering conditions through the corresponding experimental design.The similarity relationship is made up respectively for gas and gas-mist flows by presenting the principal dimensionless numbers on the flow field and particle response behaviors.Typical model tests from the literature are extrapolated to a series of similar engineering cases respectively.Then two dimensionless similarity judgment numbers are calculated by the present CFD procedure and four empirical or semi-empirical models.The similarity criteria are verified by examining the equivalence trends of the two judgment numbers of the model tests and engineering cases.The present similarity criteria prove to be rational and applicable in building the corresponding relationship between model experiments and engineering.