Research on flow assurance of deepwater submarine natural gas pipelines: Hydrate prediction and prevention

The formation of hydrate will lead to serious flow assurance problems in deepwater submarine natural gas transmission pipelines. However, the accurate evaluation model of the hydrate blocking risk for submarine natural gas transportation is still lacking. In this work, a novel model is established for evaluating the hydrate risk in deepwater submarine gas pipelines. Based on hydrate growth-deposition mechanism, the mathematical model mainly consists of mass, momentum and energy conservation equations. Meantime, the model results are obtained by finite difference method and iterative technique. Finally, the model has been applied in the production of deepwater gas field (L Gas Field) in China, and the sensitivity analysis of relevant parameters has been carried out. The results show that: (a). The mathematical model can well predict the hydrate blockage risk in deepwater natural gas pipelines after verification. (b). Hydrate is easily formed at the intersection of horizontal pipeline and vertical riser, and the maximum blocking position often occurs in middle of the riser. (c). The hydrate blockage degree and length of hydrate formation region (HFR) decrease with the increase of gas transport rate. (d). The hydrate blockage degree and length of HFR decrease with the increase of gas transport temperature. (e). The hydrate blockage degree and length of HFR increase with the extension of horizontal pipeline. (f). Injecting inhibitors can effectively inhibit hydrate formation and blockage, but the improvement of transmission measures can significantly reduce the dosage of inhibitor. It is concluded that measures such as increasing gas transportation rate and temperature, shortening horizontal pipeline length, optimizing inhibitor injection point and injection rate can play a safe, economic and efficient role in hydrate preventing and controlling.     

湿天然气管道螺旋流水合物浆液安全输送研究

研究了天然气水合物浆液在气液两相螺旋管流中流动特性,分析了以气相为连续相、水合物颗粒为离散相的气固两相螺旋流的流动机理,通过对水合物颗粒受力分析和运动分析,结合螺旋流旋涡结构演化规律,推导出水合物颗粒平动、转动的判断条件,给出了颗粒各种受力的关联式,建立螺旋管流水合物颗粒运动模型,探讨了水合物颗粒的动力学行为。分析水合物浆液流动特性得到临界速度1即水合物浆液从固定床流动向悬浮流转化速度以及临界速度2即水合物浆液从移动床流动向固定床转化速度,为水合物浆液稳定流动提供了理论判据。     

Rigorous Calculation of Critical Flow Conditions for Pressure Safety Devices

Sizing and verification of pressure relieving systems is an important topic in the design of process plants in order to assure equipments and people protection against malfunctions and hazards. The calculation of the critical flow (choke) condition is analysed with respect to existing standard calculations procedures (API and Omega methods) that implement approximate procedures and may not be extended to temperature/pressure regions near the thermodynamic critical point. These procedures may be replaced by a more rigorous calculation based on the evaluation of the local sonic velocity with equations of state. The method applies to systems composed of pure chemical components as well as to multi-component mixtures existing in the single phase and multi-phase regions. As a consequence of an exact calculation of the critical flow conditions, more accurate values of the discharged flowrate may be obtained. Comparisons with calculations performed using the standard API RP 520 procedure and Omega method are presented.     

深水测试地面流程节流油嘴段温压场研究*

为对深水高压气井测试地面流程中的节流油嘴段的温压场及水合物生成情况进行研究,采用数值模拟的方式对整个节流管路进行气体流动特性分析。并利用P-T图回归公式法,得出不同温压场条件下的水合物生成情况,进行水合物生成范围对比。结果表明:针阀出口后端气体速度随着管路入口压力和针阀直径的增大而增大,随管路出口压力的增大而减小,而气体温度随管路入口压力、管路出口压力、针阀直径的变化趋势与之相反。管路入口压力的递增、管路出口压力的递减以及针阀直径的递增,都会导致针阀突变径处的速度场、温压场波动更加剧烈,并使得水合物生成范围扩大。     

Numerical study of choked two-phase flow of hydrocarbons fluids through orifices

Valves and orifices are the most widely devices of flow control used in oil and gas industry. In particular, they are installed in relief piping system in order to control the discharge flow during potential plant overpressuring scenarios, thus ensuring plant safety. It is a common practice to flow liquid and gas mixtures through such restriction devices.Rigorous models are available to precisely size pressure relief devices operating in single phase flow; however for two-phase flow, no models are considered sufficiently reliable for predicting the relevant flow conditions.In the present paper, two-phase flow of hydrocarbons fluids through an orifice under critical conditions has been numerically investigated.The existing literature has been analyzed and data on two-phase flow of highly volatile mixtures of hydrocarbons through openings have been collected. A comparison has been carried out with numerical simulations carried out by the multiphase flow simulation tool OLGA by SPT.The Henry–Fauske model has been used as orifice choke model and the orifice discharge flow coefficient, required as input by OLGA, has been calculated by Chisholm's model.Comparison between OLGA's results and experimental data shows that Henry–Fauske model markedly underestimates the mass flow rate through the orifice, if Chisholm's model is used to calculate discharge coefficient. It was found that the error of the model could be minimized using different values of orifice discharge coefficient (C_d).A new discharge flow coefficient model, suitable for choked two-phase flow across orifices, is proposed in this study and it has been determined to match the above mentioned experimental measurements.     

New tools predict monoethylene glycol injection rate for natural gas hydrate inhibition

In the oil and gas production operations, hydrates deposition leads to serious problems including over pressuring, irreparable damages to production equipment, pipeline blockage, and finally resulting in production facilities shut down and even human life and the environment dangers. Hence, it is of great importance to forecast the hydrate formation conditions in order to overcome problems associated with deposition of hydrate. In this article, an effective, mathematical and predictive strategy, known as the least squares support vector machine, is employed to determine the hydrate forming conditions of sweet natural gases as well as the monoethylene glycol (MEG) flow-rate and desired depression of the gas hydrate formation temperature (DHFT). The outcome of this study reveals that the developed technique offers high predictive potential in precise estimation of this important characteristic in the gas industry. Beside the accuracy and reliability, the proposed model includes lower number of coefficients in contrast with conventional correlations/methods, implying an interesting feature to be added to the modeling simulation software packages in gas engineering.     

低浓度瓦斯安全输送新技术

阐述了瓦斯水合物合成的可行性,分析了水合物储存的稳定性,讨论了水合物固化储运的经济性.结果表明,应用固态水合物方式储运瓦斯具有安全、经济的特点.     

A viscosity correction factor for shear-thinning liquid flows in safety valves

At the moment there is lack in the existing standards of a sizing procedure for media of more complex rheological behavior than Newtonian. A formulation for the generalized Reynolds number to calculate the viscosity correction factor and size safety valves for the flows of shear-thinning media is proposed in this work. The various formulations in the literature are validated in their accuracy of estimation of the mass flow rates of aqueous solutions of polyvinylpyrrolidone. Among them the generalized Reynolds number defined using the wall viscosity of the annulus between the seat and the disk gives the highest accuracy without overestimating the measured mass flow rates. On the base of these results this generalized Reynolds number is recommended for introduction in the next review of both ISO 4126-1 Part 1 and API 520 for the sizing of safety valves for shear-thinning media.     

A New Homogeneous Non-Equilibrium Model to Compute Vapor-Liquid Two-Phase Critical Pressure Ratios of Multicomponent Hydrocarbon Mixtures

The critical pressure ratio (η_c) is an essential parameter for computing the vapor-liquid two-phase critical pressure and mass flow rate of multicomponent hydrocarbon mixtures flowing through valves and leakage orifices. The Homogeneous Non-Equilibrium Diener-Schmidt (HNE-DS) model widely used to calculate η_c assumes that the fluid's volume linearly changes with the pressure (using the Clausius-Clapeyron equation), which is not suitable for multicomponent gas mixtures. In this paper, a new Homogeneous Non-Equilibrium (new-HNE) model is proposed to calculate η_c of gas mixtures. Firstly, a new critical flow compressibility factor (ω_c) is developed from its thermodynamic definition and the Peng-Robinson equation of state (EOS), overcoming the inherent limitations of the Clausius-Clapeyron equation. Then, η_c is correlated to the newly derived ω_c by fitting experimental data at various pressures and gas mass fractions of both single-component and multicomponent gas mixtures, yielding the new HNE-DS model. Results show that, for the water-steam and air-water two-phase flow, the average relative deviations (ARD) between the calculated critical pressure ratios and experimental values are equal to 2.8% and 4.93%, respectively, which represents a significant improvement in comparison with the original HNE-DS model. Moreover, this new model is extended to the applications of Liquefied natural gas (LNG)/liquefied petroleum gas (LPG) fluids, and will further contribute to the calculation of the leakage mass flow rate of fluid flowing through the orifices/valves.     

Study of dry ice formation during blowdown of CO₂-CH₄ from cryogenic distillation column

Cryogenic distillation columns are generally subjected to high-pressure loadings during the natural gas purification process. The high-pressure conditions inside the column cause safety risk e.g. rupture. When an emergency arises, blowdown is a typical way of minimizing the failure hazard. However, blowdown at the cryogenic conditions involves dry ice formation due to the rapid decrease in temperature driven by the Joule-Thomson effect. The dry ice formation intensifies the failure hazard due to the orifice blockage. Therefore, optimization of blowdown parameters is necessary to avoid the dry ice formation. So far, very limited studies are available in the literature for the blowdown of CO₂-CH₄ mixture, especially at the cryogenic conditions. In this study, a computational investigation followed by the experimental validation is accomplished to analyze the dry ice formation during blowdown of CO₂-CH₄ binary mixture from the cryogenic distillation column. The composition of mixture, orifice size, and initial conditions inside vessel have a high impact on blowdown path. A 3.00 mm orifice is the most suitable size for the blowdown at cryogenic conditions as it doesn't promote solidification and discharges the inventory quickly. Based on the experimental observation, an empirical correlation is also developed to instantly find out the optimum blowdown parameters.     

对天然气水合物预防管理保障安全生产的研究

为了保障陆上气田冬季天然气安全生产,防止因天然气水合物导致安全事故,采用鱼骨图法对陆上气田天然气水合物产生的原因进行系统分析。分析显示,导致天然气水合物产生的根本原因,出现在天然气产能建设和日常生产管理各环节,以及单井、输气管道和集气站各生产场所,为防止天然气水合物产生,针对原因采用5W1H法进行管理,从而达到对冬季天然气生产主要安全风险进行有效控制的目标。     

Prediction of methanol loss in vapor phase during gas hydrate inhibition using Arrhenius-type functions

Gas hydrate formation in natural gas and NGL systems can block pipelines, equipment, and instruments, restricting or interrupting flow leading to safety hazards to production/transportation systems and to substantial economic risks. The amount of hydrate inhibitor to be injected not only must be sufficient to prevent freezing of the inhibitor water phase, but also must be sufficient to provide for the equilibrium vapor phase content of the inhibitor. The vapor pressure of methanol must be high enough so that significant quantities will vaporize. Therefore to estimate methanol vaporization losses, it is necessary to develop a new predictive tool. In this work, a simple correlation, which is a mathematically compact and reasonably accurate equation containing few tuned coefficients, is presented here for the prediction of methanol vaporization loss and vapor pressures of aqueous methanol solutions as a function of temperature and methanol mass fraction in aqueous solutions using a novel and theoretically meaningful Arrhenius-type asymptotic exponential function and Vandermonde matrix. The proposed correlation predicts the vapor pressures of aqueous methanol solutions for temperatures up to 100 °C and methanol vaporization loss for temperature between ?16 and 16 °C. Estimations are found to be in excellent agreement with the reliable data in the literature where the average absolute deviation from data is less than 1.5%. The tool developed in this study can be of immense practical value for the engineers and scientists to have a quick check on the methanol vaporization loss and vapor pressures of aqueous methanol solutions at various conditions without opting for any experimental measurements. In particular, chemical and process engineers would find the approach to be user-friendly with transparent calculations involving no complex expressions.     

体积法压井现场试验研究

为了更加精细地描述体积法压井过程中井内气体的分布和运移规律,针对体积控制法压井开展了试验研究,通过在试验井井底注气的方式,模拟气侵并实施体积法压井;测量并分析压井过程中套压、立压、出口流量及注气速率等参数的变化规律。研究结果表明:体积法压井控制过程中,由于井筒内气侵气体以多个短气柱和弥散型气泡分布为主,在钻井液泄流过程中套压很难实现理想状态的稳定控制;井口出现气体并不代表气侵气体以气柱形式全部到达井口,通过气液混合泄流,可有效控制井底压力实现井底准恒压;当部分气体在井口形成气柱后,可提前启动置换流程,这种体积控制流程和置换流程结合的方式可有效降低地层破裂风险;置换流程中压井液的注入会产生井底附加压力,针对薄弱地层,需考虑此参数以降低风险。     

Numerical analysis of phase behavior during rapid decompression of rich natural gases

The effect of the condensation process on the gas and liquid phase behavior during rapid decompression of rich natural gases is studied in the paper numerically. A one-dimensional mathematical model of transient thermal two-phase flow of compressible multi-component natural gas mixture and liquid phase in a shock tube is developed. The set of mass, momentum and enthalpy conservation equations are solved for the gas and liquid phases. The approach to model a liquid condensation process during rapid decompression of rich natural gas mixture is proposed. The mass transfer between the gas and the liquid is taken into account by introducing the appropriate terms into the governing equations. Thermo-physical properties of multi-component natural gas mixture are calculated by solving the Equation of State (EOS) in the form of the Soave–Redlich–Kwong (SRK-EOS) model. The proposed liquid condensation model is integrated into the GDP model. A simple case of GDP model, where the liquid was not considered, was extensively validated on base and dry natural gases. The proposed two-phase model is validated against the experiments where the decompression wave speed was measured in rich natural gases at low temperature. It shows a good agreement with the experimental data.     

建筑物通道内火灾烟气运动的数值模拟

发生在各种长通道内的建筑火灾受到人们越来越多的关注.研究此类火灾烟气运动的特点,对于发展火灾烟气控制技术,减少烟气对人员的伤害具有重要的意义.应用场模拟方法,对有自然通风的建筑物通道内的火灾烟气运动进行了数值模拟.模拟得到的通道内烟气温度的分层分布与实验数据基本相符合,烟气对环境空气卷吸量的计算值也与已有经验关系式的结果相一致.利用数值模拟还研究了释热率对通道内火灾烟气运动的影响.结果表明,在有自然通风的条件下,释热率的增大对通道上层热烟气的温度和速度有较明显的影响,导致火灾的危害性变大.     

Prediction of methanol loss in liquid hydrocarbon phase during natural gas hydrate inhibition using rigorous models

Methanol is the most widely used natural gas hydrate inhibitor and it is only effective as a hydrate inhibitor in the aqueous phase. Methanol is not regenerated in natural gas inhibition process due to its intermittent application in most cases. However, a significant cost is associated with the process because of methanol loss while utilizing this inhibitor. In this work, several intelligent models along with a new mathematical correlation are presented in terms of methanol concentration in aqueous phase and temperature to precisely forecast the methanol loss in the saturated hydrocarbons phase. An excellent match was noticed between the calculated results and literature data.     

湿法脱硫后烟气中SO3/硫酸雾测试方法的试验

在自行设计和搭建的模拟湿法脱硫烟气系统中,研究了基于控制冷凝法原理的SO_3/硫酸雾采集装置对湿法脱硫后SO_3/硫酸雾的捕集性能,重点考察了螺旋管与石英棉冷凝温度、采样枪温度、采样流量、采样时间和SO_3/硫酸雾质量浓度对SO_3/硫酸雾捕集性能的影响。结果表明,在研究范围内,湿法脱硫后SO_3/硫酸雾采集装置优化的操作参数为:螺旋管及石英棉冷凝温度95℃,采样枪温度280℃,采样流量17 L/min,采样时间30 min。SO_3/硫酸雾质量浓度较高时,螺旋冷凝管对SO_3/硫酸雾的捕集起主要作用,其捕集的SO_3/硫酸雾比例在75.8%~80.8%,远高于螺旋冷凝管与石英棉捕集SO_3/硫酸雾的比例;SO_3/硫酸雾质量浓度较低(约1mg/m3)时,采样枪对SO_3/硫酸雾的捕集起主要作用,其捕集SO_3/硫酸雾的比例大于50%。     

建筑火灾区域模拟竖孔流动的计算

本文根据建筑火灾双层区域模拟思想，给出其常微分控制方程组，并分析了其压力求解方法，然后，运用伯努力利方程推导了相邻房间通过矩形竖孔（门或窗）的质量流率计算公式，还讨论了竖孔流动中性面产生条件。在此基础上，结合一两房间、两孔建筑中烟气运动实例，运用Ｃ．Ｗ．Ｇｅａｒ刚性稳定算法对火灾发展及烟气流动过程进行了数值模拟；给出各竖孔中性面位置、数量和各主环境之间通过竖孔的质量流率；还给出各房间气体平均温升、     

以Cu-ZSM-5为催化剂乙烯选择性催化还原烟气脱硝的试验研究

介绍了采用乙烯作为还原气体、Cu-ZSM-5为催化剂的烟气脱硝工艺流程.首先,研究了催化剂的制备方法,Na-ZSM-5分子筛作为催化剂载体,用浸渍法制成了不同负载量的Cu-ZSM-5分子筛催化剂.其次,研究了培烧条件对催化剂活性的影响,包括反应温度、气体流速以及反应气中O2的浓度等催化还原反应工艺参数.最后,研究了掺杂Ce4+离子对Cu-ZSM-5分子筛的活性影响.     

Small scale experiments on boiling liquid expanding vapor explosions: Vessel over-pressure

The boiling liquid expanding vapor explosion (BLEVE) is a type of physical explosion that has caused massive damage in the petrochemical industry. In this paper, a study has been made of the conditions that could lead to a BLEVE. A device was built to simulate the occurrence of suddenly initiated release through a top orifice. As there is some danger in using liquefied petroleum gas (LPG) in the experiments, water was used as the test fluid. The change of pressure and temperature was measured during the experiment. It was determined that two pressure peaks result after the pressure is released: the first pressure peak seems to occur because of the vapor pressure caused by the swelled two-phase layer after the initial venting, the second pressure peak is possibly due to a dynamic impact or ‘liquid hammer’ and is maintained by bubbles collapse or something like cavitation at the surface of the inner wall of the head space that occurs with the ejection of two-phase flow.Liquid heights, orifice size, and the degree of liquid superheating all have differing influence on the magnitude of the measured over-pressure; the greater the degree of liquid superheat, the stronger the over-pressure; smaller opening areas delay and reduce the magnitude of the first over-pressure; at fill levels between 60% and 80%, the impact pressure appears more violent than with other fills.