Under natural conditions, natural gas hydrates are found in marine environments. Therefore, it is necessary to study the formation of hydrates under salt water conditions and their basic properties. The effect of low⁃concentration MgSO4 with sodium dodecyl sulfate (SDS), alkyl glycoside (APG), dodecyl trimethyl ammonium chloride (DTAC) and other surfactants on the formation of hydrates was studied. The changes of pressure and temperature with time during the formation of hydrate were measured under the conditions of 275.15 K and 6 MPa. The results show that when a certain concentration of SDS or APG is added to the solution, the hydrate formation is significantly promoted. When a certain amount of DTAC is added, substantially no hydrate is formed in the solution. Furthermore, low concentration of MgSO4 significantly enhanced the promoting effect of surfactants. It can be seen that low concentration of MgSO4 and appropriate concentration of surfactant will promote the formation of methane hydrate, and different types of surfactants have different effects on the formation of hydrate. Compared with the promotion effect of SDS and APG on hydrate formation, the promotion effect of DTAC on hydrate formation is not obvious.

Methane hydrate can be used as a carrier to store and transport natural gas. However, they generate slowly under natural conditions,so it is necessary to improve the rate of methane hydrate formation.To this end, the effect of the amphoteric surfactant LAD⁃40 on the methane hydrate formation rate in various mass fractions (10~500 μg/g) was examined. The results show that when the pressure is constant, compared with the nucleation temperature of methane hydrate formation under pure water conditions, adding any mass fractions of LAD⁃40, the nucleation temperature of methane hydrate changes little, which fails to improve the thermodynamic conditions of methane hydrate formation; because of the existence of LAD⁃40, the gas⁃liquid contact area was increased, the nucleation time was shortened, and the hydrate form in the reactor phase. As the LAD⁃40 mass fraction increases, the methane hydrate formation rate increases linearly, and the gas consumption does not change much, however, compared with methane hydrate formation under pure water conditions, it is significantly improved.

Because of geological disasters, corrosion defects, third party damage and other factors, there are many hidden dangers in the safe transportation of oil and gas pipelines. Therefore, it is of great practical significance to study the law of leakage and diffusion of buried natural gas pipelines for the prediction and location of leakage points and the formulation of emergency plans.Numerical analysis of the leakage and diffusion process of buried natural gas pipeline is carried out, the influence of leakage velocity, wind speed and environmental temperature on CH4 the methane concentration are analyzed and summed up diffusion law.The research results can provide theoretical support for the accurate location and emergency plan of the buried gas pipeline leakage point.

Natural gas hydrate has the advantages of high gas storage rate, low pollution and large reserves, which has good prospects for development. In addition, natural gas hydrates formed in the process of natural gas processing and transportation will lead to the problem of serious such as pipeline blockage. Therefore, it is of practical significance to analyze and predict the formation of natural gas hydrate. In order to predict the formation of natural gas hydrate, aiming at the merits and demerits of previous research on the prediction methods of natural gas hydrate formation, the combination model of GM (1,1) theory and Back Propagation neural network is established by using MATLAB computer language, which is based on the artificial neural network with the ability to solve complex system problem.Considering the improvement of prediction accuracy, the difference combination method is used to combine the two methods. The GM (1,1), back propagation neural network and the combined model are used to predict and compare the pressure data obtained in the experiment. In order to further verify the accuracy of the combined model, Markov chain model is selected for the prediction test. The results show that the combined model of GM（1,1）and BP neural network has higher canprecision and this method can be widely used in many directions, which can provide a theoretical basis for the development and utilization of NGH in the future.

With the development of China's natural gas industry, the scale of natural gas pipeline transportation is also expanding, at the same time, it also brings the hidden danger of safety. The leakage accidents of urban natural gas pipeline occur frequently, which seriously affect the lives and property safety of urban residents. This paper mainly introduces the basic theory of numerical simulation and numerical simulation of the leakage of urban natural gas pipeline. Considering the influence of wind field on leakage, the variation of the wind field near the ground is analyzed, and the leakage model of the buried natural gas pipeline is established. The leakage diffusion is set in the atmospheric environment. The CFD software is selected to divide the grid and local encryption, and the steady state simulation of the wind field is carried out. After the wind field reaches the steady state, the boundary conditions of the post-processing are changed, and the leakage is simulated instantaneously. The variation law of the natural gas leakage diffusion with time is obtained, and the influence of wind speed on the leakage diffusion is analyzed quantitatively. The results show that the building has a disturbance to the wind field. During the leakage process, the gas gathers near the ground and close to the building. With the increase of wind speed, the steady-state diffusion height decreases, but the wind field has less influence on the horizontal diffusion. The greater the wind speed, the more obvious the dilution effect of the leakage gas, the smaller the dangerous area.

The objective optimization function was set by the summary of the insulation materials' annual sharing cost and the annual heat losing cost after insulation and the calculating model of buried hot oil pipelines' insulation economical thickness was given. The model included the burying depth's influences on around pipelines ambient temperature, the heat transfer coefficient of the outer wall insulation, and the overall heat transfer coefficient. With the help of C# programming language, a computer program was developed to optimize the thickness of insulation layer of buried hot oil pipeline with EXCEL to work out economic evaluation procedures, combining with examples of calculation. The results showed that: The determination of buried hot oil pipeline insulation thickness would have some impact on the economic benefits of the project, and it should take the double requirements into account determining the thickness on both technology and economy. That could provide a theoretical reference to the hot oil transportation construction's optimizing design.

Aiming at the process on leakage of buried gas pipeline in urban area, and considering gas of different constituents and concentration leakage based under multi building condition, using computational fluid dynamics (CFD) software to establish the three-dimensional physical model of buried pipeline in the leakage process, the environmental wind field and leakage rate into the boundary conditions in the form of user-defined functions were introduced, and the simulation process was divided into steady-state simulation in environmental wind field and transient simulation in leakage. And the leakage process was divided into continuous leakage and leakage after pipe valve closing, and the gas leakage rules were analyzed. The results showed that: It was necessary to simulate the steady state in environmental wind field. There existed three low velocity zones near the flow field and a large velocity gradient on the edge of the building. It showed the following characteristics that the confined leakage of oil layer diffusion, low velocity zone upstream gathering, gas cloud rising, hydrogen sulfide deposition and diffusion and so on in the gas continuous leakage stage. It showed the characteristics of the gas diffusion continuity and dissipation from top to bottom in the valve closed stage. Based on the conditions above, comparing methane, hydrogen sulfide had larger diffusion range, later dissipation time, and greater risk.

The main index factors of the soil corrosion detection， for the section of pipeline in service in Qinghai Oilfield Huatugou, included resistivity, natural corrosion potential, redox potential, pH, moisture content, salt content, chloride ion content and soil corrosion rate. Combined with three theories, gray correlation, fuzzy evaluation, and improved AHP, the appropriate comprehensive evaluation model was established, the 15point set of measured data was used in Huatugou areas of this field to clarify its application of this method, and evaluation results reached by the comparison model with the original evaluation results were contrasted. The results of this evaluation model showed that the evaluation results was closer to reality than the original evaluation results.

The numerical simulation study was performed on the sulfurous gas leakage diffusion. The effects of obstacles shape, slope size of obstacles and the distance between leakage hole and obstacles in the calculation region and different wind speeds on leakage diffusion process were analyzed. The hydrogen sulfide component and methane component safety area were simulated in the different conditions. The results showed that under the action of wind, the presence of obstacles changed the motion path of the gas leakage and led the leakage gas to store in front of its surface. The height of leakage gas diffusion was increased and its transmission in horizontal direction was prevented if the obstacle was no slope obstructions(buildings). The horizontal distance of leakage gas diffusion was increased if the obstacle was gradient obstacles(mountain), and the leakage gas started setting down in the protected area after it bypass obstacles when the wind speed was more than a certain speed, causing the safety area reduced.Reducing obstacles slope, light wind could not influence the diffusion of gas, the leakage of gas was surrounded by obstacles and diffused near the ground when the heavy wind came. Increasing obstacles slope, leakage gas diffusion regularity of slope was similar with no obstructions. The suggestion for emergency rescue and safety management of sulfurous gas leakage accident was provided.

Based on the aerial crossing pipeline structure was widely used in the construction of longdistance pipeline, due to the particularity of its structure, crossing structure extremely vulnerable to soil collapse and other geological disasters. The stress field variation law of the horizontal pipeline section of the crossing structure was studied by establishing the finite element model of the interaction between the buried pipeline and the soil. A detailed analysis of the stress changes in the pipeline was obtained when crossing pipeline structure used different horizontal pipeline section. The maximum stress value along with the variation of the settlement range of the horizontal pipeline was studied when the length of the horizontal pipe was 20 meters. The research showed that the range of horizontal section of crossing pipeline structure and soil sedimentation had greater impact on the stress field in the pipeline crossing the structure. 

Pipeline crossing structure was very easy to be affected by the settlement of soil along the river, and effective preventive measures should be adopted to effectively prevent the damage to the crossing structure. The finite element model of pipeline crossing structure and soil interaction was established, and the stress change of pipeline span structure with the fixed bracket was established. It was found that the fixed bracket could effectively reduce the stress value of the pipeline when the span structure was affected by the settlement of soil mass, and the installation position of the fixing bracket was more prominent to reduce the stress value of the pipeline span structure. The results could provide a theoretical basis for the safety construction of pipeline crossing structure.

Natural gas hydrate was becoming more and more important in Chinese energy structure, which had the advantages of high gas content, low pollution and large reserves. Therefore, it was necessary to study natural gas hydrate. Aiming at the common blockage problem in transmission pipeline for natural gas hydrate, on the background of subsea level pipelines and based on the multiphase flow model, the calculating and analyzing natural gas hydrate mathematical model was established in gassolid two phase flow based on finite volume method. And the relationship was obtained between the difference of the diameter (the diameter of the pipe and the through-flow diameter) and the distance from inlet. When the inlet flow velocity and the diameter of the pipe were fixed, the stack position of the gas hydrate in the pipeline was analyzed with the difference of the diameter. Calculation results showed that with the other conditions remaining unchanged, as the increase of the distance from the pipe inlet, the change rule of the difference of the diameter was in accord with Gauss curve. Then the change rule was fitted and Gauss function equation was obtained, which could calculate the difference of diameter under a given position of pipe. Finally, through changing the diameter size of pipeline, the simulation and analysis were carried out, whose results showed that Gauss function equation also applied in pipelines with different diameters. A theoretical basis for predicting the stack position was provided in the pipeline and improving the transport efficiency of natural gas hydrate.

The freezing-thawing circulation was the most common security problems of buried pipeline in permafrost regions, and it could make pipeline generate additional stress. For the regional with the special structure of topographic, the freezing-thawing circulation would change the original geological characteristics, and caused geological disasters under certain conditions. The stress impact of the pipeline was studied generated by the freezing-thawing landslide because of the melting permafrost. The degree of influence of axial landslide with different displacement on the pipeline was analyzed. The simulation analysis showed that different forms of landslide generated a great impact on the pipelines; furthermore, the effects of stress on the pipeline had a greater impact by changing the location of the landslide. The influence on the pipelines generated by the freezing-thawing landslide were simulated and analyzed ,which could well reflect the changes in stress and provide a theoretical basis for building construction of buried pipelines in the permafrost regions.

The earth's settlement would generate additional stress on pipeline , which was common security problems of buried pipelines. Because of the force of aerial crossing pipeline structure was more complicated, therefore a finite element mechanical model of interactions between the pipeline and soil in acrossing segments of buried pipelines was established, to analysis the changes of the stress in aerial crossing pipeline structure, when aerial crossing pipeline structure used different angles. Further, based on the model, a detailed analysis of the stress changes in the pipeline was obtained, when soil settlement occurred at different locations nearby the acrossing segments. The variation was studied on the relationships of the maximum stress within the pipeline with the position of soil settlement, when the angle between the oblique pipeline of crossing structure and the horizontal direction was 50°, provide a theoretical basis for building construction of buried pipelines.

Liquefied petroleum gas belongs to dangerous gas, once the leakage occurs, the consequences are serious. So its security problem is very important. According to the characteristics of LPG leakage, a physical model of leakage and diffusion in the limited space was established, and the diffusion process of the gas leakage was simulated. The internal flow field of the diffusion process was analyzed through simulation results, and the different of the diffusion processes were compared under the different relative humidity. The results showed that due to the moving eddy in the air, the velocity vector of the gas diffusion change from one side was higher than the other side to the other side was higher. The increase of wind speed would accelerate the generation and movement of the eddy, and the concentration distribution of C₃H₈ would change acutely. When the air relative humidity increased, the gas descended faster, and in the plane below the leakage point, the humidity increased, the concentration of C₃H₈ was higher, the alarm time of the liquefied petroleum gas alarm device would be shortened.

The occurrence of permafrost thawing settlement would lead to soil sliding to its base along slope when the pipeline was in slope segment, which lead to form a thawing landslide, and the stress distribution of the buried pipelines located in permafrost zone ramping portion had greatly affected, and might lead to pipeline stress concentration even plastic deformation phenomenon. For buried pipelines in the slopes with different angles and the length of permafrost thawing settlement, stress changes of pipeline numerically when affected by thawing landslide and thawing settlement were simulated. The study found that different positions pipeline showed different forms of stress when it was affected by thawing landslide, and stress magnitude generated by the pipeline at different locations affected slope angles. The results provided a better theoretical basis for the safe operation and construction of the building buried pipelines.

Water-driven oil in porous media fluid and solid coupled heat transfer control equations was established by the theory of percolation mechanics and the sandstone reservoir was porous medium. Different well pattern was established for the same reservoir numerical simulation, qualitative analysis of the process of oil pressure, the remaining oil distribution, and the various concentration range the proportion of oil, quantitative analysis of the volume fraction of the sum of the remaining oil and various oil recovery year. The results show that 5 to 9 years of oil remaining oil distribution changed in the inert area, during this time recovery changed smoothly; Analysis of a variety of oil recovery wells arrangement size was that five-spot well pattern was larger than nine-pot well pattern, and nine-spot well pattern was larger than Line shape well pattern. When the water intensity constant, from the perspective of recovery. For this reservoir five-spot well pattern was the best well pattern.

Because of the complex terrain in the construction of longdistance buried pipeline, the elbows are widely used in pipeline systems. Due to its specificity in terms of shape and structure, elbows not only can change the axial direction of the pipeline, but also increase the flexibility of the pipeline. However, under the special conditions of environment, bends in the piping system are often the section of concentration of stress. In the certain conditions, plastic deformation occurs at the elbow, which poses a potential threat to the safe operation of pipelines. Through the application of software of finite element analysis, stress at a particular point (elbow) of buried pipeline were numerically calculated and analyzed. By numerical calculation and analysis, when the local pipeline under pressure in the radial direction, the distribution of values of Von Mises stress were obtained at the elbow, in the crossing segment of the buried pipeline. And when the length of the applied load, the bending angle and the radius of curvature of elbow were changed, the influence of the stress of the elbow was also obtained.

Coupling simulation was used in the buried seabed pipeline, to analyze the temperature field and stress field on the different leakage points. When the oil spilled to the sea mud area, it had the different influences on temperature field of the sea of mud, and there was a regular of distribution. When the leakage point was closed to the left and right sides of the pipe affected by gravity，the isotherm of leakage point moved down，the offside of leakage point moved up，the temperature field presented asymmetric distribution. When the leakage at the bottom of the pipes pressure field distribution was solely discussed, the isobar's distribution was symmetrical on both sides of the pipe, and gradually spread to the sides and top, and diffusion velocity had certain distinction with the leakage of buried pipe.

Based on the heat loss occurred when a pipeline to transport hightemperature steam vapor overhead, it is established physical and mathematical models of steam pipeline,and compared to the three commonly used insulation materials of overhead pipeline. Research results showed that the application results of nano airgel composite reflective insulation was the best. The different steam temperatures of the temperature field of pipeline and insulation layer in, different diameter and different insulation thickness distribution were simulated. By analyzing the temperature field indicated that changing the diameter effected the heat loss of overhead vapor line greatly. When the diameter changed from 126 mm to 132 mm unit length, the heat loss increased 10.702 62 kJ/(h •m).

In reference to previous research, the simulation software to simulate overhead gas pipeline leakage diffusion in the flat areas was used and the effects of four different leakage directions and speed on leakage diffusion process were analyzed.The results showed that the risky range of upwind area was bigger than the downwind area near the ground, the higher ground was relatively safe.The risky range of upward natural gas jet was the smallest,the risky ranges of windward and downward jet were the greatest.The impact of wind speed on the risky range of windward jet was less than downward jet.Natural gas leakage in the condition of lower wind speed and calm wind spread wider.The results could be used for the emergency evacuation and rescue of overhead natural gas pipeline leakage in the future.

Due to the oilgaswater threephase flow is extremely complex, the threephase hybrid interface is weak of stability, different delivery conditions exist a variety of flow patterns. Using the VOF model simulation, the mock object was oilgaswater threephase flow in mutation tube, studying the fluid flow in pipe when the parameters(volume gas fraction, oil viscosity and velocity) was different, thus it gotten the corresponding phase distribution nephogram. Through analyzing the results showed that the gas volume fraction and velocity had great effect on mutiphase flow pattern. The effect of crude oil viscosity on flow patterns was not very obvious. The size of the pipe diameter also affected the flow pattern.

In view of the problem of different oil pipelines leakage, using the finite volume method to build the three dimensional flow and mass transfer mathematical model of the soil porous media around underground pipeline, by the CFD soft to numerical simulate the range of different oil distribution of underground and the earth’s surface when petrol, diesel fuel or crude oil pipeline leakage, the simulation results showed that when the time was the same, the volume of diesel fuel leakage was the most, and the spread was more 20% than petrol or crude oil. After 2 hours’ leakage, the diffusion rates of both underground and surface cases tended to be stable. The simulation diffusion scopes of leakage oil had a guiding significance for repair work of contaminated soil.

Based on computational fluid dynamic，a fluid solid coupling heattransfer model of submarine in two areas was built，which were respectively the area on seawater and the sea mud area，and the sea mud area was disposed of saturated silt porous. Using the software, the leakage during the process of transmission on naked placed hot oil pipeline was simulated, and the changes of distribution of crude oil in the soil and the water of the sea and the changes of the temperature field were analyzed，and different leakage points were analyzed respectively. The settlement of its own of the submarine pipeline on the naked place and the effect of the pressure in the sea water were considered in the simulation. The simulation results showed that different leakage points had a different influence on the distribution of crude oil|when the leakage in the sea mud area happened，the distribution of crude oil in the sea mud was a arcshape, after a period of time, the crude oil flowed into the sea long the sea mud and pipe insulation, and the top was a sprayshape and the flanks was helix, finally the crude oil flew onto the surface of the sea|when the leakage point was located in the sea area, oil was jet diffusion distribution form and under the action of buoyancy, the crude oil flow gradually onto the sea. As the leak position was different，the water of the sea and the sea mud there had significant differences in the distribution of temperature field. The interface of sea water and sea mud areas had a great impact on the temperature field distribution of crude oil.

When buried pipeline was running, there were many factors could cause shutdown phenomenon. This could cause serious economic losses. Oil temperature and soil temperature were the important parameters for pipeline shutdown security. So it was significant to study oil temperature field and soil temperature field for the safe operation of buried pipeline. It was simulated that the excavation area temperature during the shutdown of the pipeline. And the simulation was threedimensional. Calculation areas contained three parts: soil area, pipeline internal area and the excavation area. These temperature fields were changed after excavation. And the changes of these three areas were simulated. The safety shutdown time was obtained by pipeline internal oil temperature changes. The oil temperature was declined after excavation. The influence of excavation area was obtained by observing the oil temperature. The temperature decline reflected the influence range. The results provided a reference for the safety production.

When it is transferred the high-pour-point and viscous crude oil, heat loss was serious. And the heat loss was more serious after fault shutdown. When the shutdown time is beyond the safe shutdown time accident occurs. It is important to study the heat calculation of crude oil. Compared the winter and summer shutdown temperature drop of the crude oil. Different shutdown time was given, simulated the temperature drop of each time. Calculated the out-station temperature of shutdown crude oil. Simulated the threedimensional temperature field of soil and pipeline after the shutdown. Concluded that the change rules of crude oil and the soil temperature field at different temperature. When the outstation temperature reach a certain temperature, it can ensure shutdown and restart safety. The result provided a reference for the safety production.

Using the finite volume method, the three dimensional mathematical model of fluid-structure interaction was built in the soil as a kind of porous media around underground pipeline, by the FLUENT soft to numerically simulate the ground temperature change and the range of oil distribution in the soil and the earth's surface when oil pipeline leaked in different pressures. The simulation results show that the temperature field around pipeline was the same in different pressures. After leak, the temperature field around pipeline changed slowly, and the greater the pressure was and the more obvious the change of the temperature field was. When the pipeline pressure was 2 times of original pressure, the spread amount of oil in the soil was less than 2 times.

Buried depth of pipeline has important effects on the temperature field and thermal characteristics of soil around the pipe. Corresponding to different buried depth, the temperature field of soil around pipeline was also different. With different buried depth of pipeline, the temperature field of soil around the pipeline was simulated and calculated by using water thermal coupling model and heat transfer model. The analysis of the calculation results of two models showed that the embedded depth had certain influence on the temperature field of soil around the buried pipeline, and so do moisture migration and the water phase transition. Comparing with the temperature difference obtained by the two kinds of models in the same temperature, the results show that with different buried depth, the influence of moisture migration and the water phase change also had different influence on the soil temperature. The shallower the buried depth of pipeline was, the bigger the difference of temperature of the two models in the same place was. In order to get actual calculation results, the moisture migration and the water phase as well as water heat coupling problems in calculating should be considered.

For high temperature steam vapor transport in the buried pipeline causes heat loss and leads to energy waste problems，several factors affecting heat loss were analyzed. Steam pipeline of physical model and mathematical model was established to simulate the different steam temperatures，different buried depth and temperature distribution under different insulation thickness. And temperature distribution pipeline and the surrounding soil were calculated and analyzed under different conditions. The results show that the heat loss of buried steam lines with the steam temperature, buried depth, changed the thickness of the insulating thickness changes，and the biggest affect of the heat loss was the insulation thickness. When the insulation thickness was from 40 mm to 80 mm，the heat loss per unit length declined 110.591 W/m，and the thermal effect was smaller and smaller .

Establishing an unsteady heat transfer model for the period of pipeline shutdown combined with the thermodynamic properties of exposed pipelines, calculating physical parameters of crude oil in two conditions which changed with temperature and never changed with temperature, the result of the latter condition had great difference with the actual shutdown situation. Therefore physical parameters changing with temperature should be considered. Based on physical parameters of crude oil changing with temperature, changed the factors affecting the shutdown temperature drop such as the shutdown starting oil temperature, ambient temperature and insulation thickness and calculating the safe shutdown time under different conditions. The results show that the growth rate of the safe shutdown time was basically the same with the gradual increasing of the initial oil temperature and the thickness of the insulating layer and the rate of increase was the same and the growth rate of safety shutdown time was increasing with the gradual increasing of ambient temperature and the rate of increase was the same.