Hot Oil Pipeline Research Articles

Thermo-mechanical Behaviour of Soft Clay between Operation and Shutdown of Submarine Hot Oil Pipeline

The interaction between the pipeline and the surrounding soil is a critical factor in the buckling analysis of the pipeline operating at high temperatures and pressures. The heating pipeline alters the temperature and induces excess pore pressure in the surrounding soil, which in turn affects the mechanical properties of the soil. Therefore, a comprehensive understanding of the thermo-mechanical response of the soil surrounding the pipeline during operation and shutdown is essential. To address this issue, this study developed a temperature-controlled pipe model test device that simulates the heating and cooling cycle of the pipe and the temperature field and pore pressure response of the surrounding kaolin clay. The test results showed that the soil temperature at the closest location to the pipe (0.1 D) reached a peak value of approximately 48.7°C when the pipe was heated from 12.5°C to 55°C. The peak temperature decreased with increasing distance, reaching 22.5°C at 0.875D. Heat transfer analysis using ABAQUS software revealed that the influence range of the pipeline on the soil temperature field during the heating cycle was about 2D. Additionally, a parametric study focused on the thermal conductivity (λsoil) of kaolin, and the best-fit value was determined to be 0.6 W/(m·°C. The excess pore pressures were generated in the kaolin surrounding the pipes during the heating cycle, and negative pore pressures were observed after cooling cycle. Furthermore, a theoretical solution for calculating the thermally induced pore pressure was derived, and preliminary verification showed good agreement between the theoretically computed and experimentally measured results. These findings provide valuable insights into the thermo-mechanical response of the soil surrounding pipelines during operation and shutdown, which can improve the design and safety of oil and gas pipelines.

Investigation into the heat transfer models for the hot crude oil transportation in a long‐buried pipeline

AbstractBased on reasonable simplifications of a two‐dimensional (2D) energy equation, a basic heat transfer equation in 1D form is obtained for the hot crude oil transportation in a long‐buried pipeline. To enclose the heat transfer equation, the overall heat transfer coefficient and the numerical simulation of temperature fields of pipe wall and soil are introduced, and they together with the basic heat transfer equation constitute 1D and cross‐dimensional heat transfer models of crude oil transportation, respectively. Some numerical procedures, including grid generation, partial differential equation discretization, and algebraic equation solution, are combined to solve the two heat transfer models. Based on the designed cases with different pipeline parameters, the relative deviation between the numerical results from the two heat transfer models does not exceed 1%, and the two heat transfer models agree well. Furthermore, the nonuniform natural soil temperature field is designed, and its influence on the oil temperature and the deviation of the 1D heat transfer model is investigated. Under the condition of the nonuniform natural soil temperature field, the atmospheric temperature as the ambient temperature in the 1D heat transfer model causes an apparent deviation, whereas the soil temperature at the buried depth of the pipeline in the natural soil temperature field as the ambient temperature does not. This study can provide a reference for the reasonable selection and use of the heat transfer models of the buried hot oil pipeline.

GPU-accelerated transient thermo-hydraulic simulation of weakly compressible restart flow of a non-Newtonian fluid in a long-buried hot oil pipeline

For the complex transient thermo-hydraulic restart process of a long-buried hot oil pipeline, it takes a long time to solve the restart simulation model, which cannot meet the requirements of rapid restart prediction and assessment. The modern GPU is specialized for compute-intensive and massively parallel computation, and it has great potential to accelerate transient thermo-hydraulic restart simulation. According to the computational characteristics of restart model solutions, four types of GPU parallel strategies, from the mapping of grids to the convergence judgment of the discrete equation solution, are combined to accelerate the whole process of a transient thermo-hydraulic restart simulation. The results indicate that the total computation time of the transient thermo-hydraulic simulation is reduced from 185.4 min to 8.8 min, corresponding to an acceleration rate of 21.1, which exhibits a good acceleration effect for a single GPU and is beneficial to the rapid assessment of restart schemes. Based on the GPU-accelerated numerical results, the complex transient thermo-hydraulic restart process with the changing soil temperature field, the non-Newtonian thixotropic behavior of crude oil and flow pattern evolution, is investigated. The analysis reveals that the dominant influencing factors of the restart process vary with the restart time, and the flow pattern evolution plays an important role in the thermo-hydraulic characteristics of the restart process.

Quantitative Effects of Different Factors on the Thermal Characteristics of New Submarine Hot Oil Pipeline During the Preheating Process

Quantitative Effects of Different Factors on the Thermal Characteristics of New Submarine Hot Oil Pipeline During the Preheating Process

Analysis of the thermal process of hot oil pipeline shutdown and restart

For a long transport pipeline in service in the east, the thermodynamic change rules along the pipeline during shutdown and restart were simulated and analyzed respectively, and the characteristics of soil temperature field during restart were clarified. The change of the dangerous section position was analyzed emphatically, and the reasonable shutdown time was determined according to the temperature of the dangerous section position. The research of this paper can provide theoretical support for the determination of shutdown time in the process of shutdown.

Development of mathematical models and optimization of operation modes of the oil heating station of main oil pipelines under conditions of fuzzy initial information

The relevance of the study is substantiated by the fact that when managing the processes of oil transportation through main pipelines, it becomes necessary to determine and select the optimal operating modes of the oil pipeline units, taking into account the fuzziness of some part of the initial information. In this regard, solving the problem of multi-criteria selection of effective operating modes for an oil heating station for a hot oil pipeline system, which is often described in a fuzzy environment, based on the apparatus of fuzzy set theories, is an urgent scientific and practical problem. A method for the synthesis of models in the conditions of fuzzy output parameters of the object has been developed, with the help of which fuzzy models of the investigated oil heating station of the main oil pipeline have been built. Based on the modification and combination of various optimality principles, mathematical formulations of the problem of multi-criteria selection of effective operating modes for an oil heating station in a fuzzy environment are obtained. By modifying and adapting the principles of guaranteed results and equality in a fuzzy environment, a heuristic method has been developed for solving the formulated problem of selecting object's operation modes using the initial fuzzy information. The proposed heuristic method for multi-criteria selection in a fuzzy environment is based on the use of the experience and knowledge of the decision-maker. The proposed approach is implemented in the formulation and solution of the problem of multi-criteria selection of operating modes of the oil heating station in Atyrau of the Uzen-Atyrau-Samara main oil pipeline. As a result of the application of the proposed method, an improvement in the degree of fulfillment of a fuzzy restriction on environmental impact was achieved by 2 %, as well as the optimal values of the operating parameters of the object were improved: the temperature was reduced by 1.85 % (5.67 K), pressure – by 0.04 % (kPa) and fuel consumption – by 2.9 % (0.0002 kg/s). The obtained results have confirmed the effectiveness of the proposed approach to solving the assigned tasks.

Heat Transfer Simulation Study of Hot Oil Tubes Based on COMSOL

Heat insulation board is added on the adjacent side of hot oil pipeline and other pipelines to increase thermal resistance and reduce the influence of hot oil pipeline on the temperature of the pipeline. Because the heat conduction problem of the soil around the hot oil pipeline after the addition of eat insulation board becomes very complex, this paper studies and analyzes the heat transfer process of the hot oil pipeline through constructing an equivalent simplified mathematical calculation model and heat transfer simulation based on COMSOL.

Study on the Operation Safety and Reliability of a Waxy Hot Oil Pipeline with Low Throughput Using the Probabilistic Method.

When the hot oil pipeline is running at a low throughput, it easily enters into an unstable condition, which seriously threatens the safety of the hot oil pipeline operation. In this study, the unsteady heat transfer and flow mathematical models for the hot oil pipeline system were established first by comprehensively considering the uncertainty of parameters during pipeline operation, such as the operating parameters (throughput and oil temperature), physical properties of crude oil (freezing point, viscosity, and thixotropic parameters), and environmental parameters (buried deep soil temperature and soil thermal conductivity). Then, the efficient Latin hypercube sampling (LHS) stochastic numerical algorithm was applied and further developed to quantitatively describe the operation safety of hot oil pipelines with low throughput in the form of probability. On the basis of the abovementioned research, the qualitative relationship between pipeline flowrate and friction loss is obtained. Finally, taking an actual crude oil pipeline as an example, the failure probabilities of the pipeline under different operating conditions were analyzed in detail. Combined with the target safety level of pipeline operation, the minimum allowable throughput of pipelines was determined. This study revealed the flow and heat transfer law of hot oil pipelines with low throughput and determined its operation safety and reliability under different operating conditions.

A new model of overall heat transfer coefficient of hot wax oil pipeline based on dimensionless experimental analysis

Aimed at biggish prediction error of the overall heat transfer coefficient caused by running parameters fluctuations, analyze the correlations between various elements including outlet oil temperature, oil physical properties, flow rate, environmental temperature, wax deposition rate as well as running time after pigging and the overall heat transfer coefficient. And four dimensionless number determining the laws of overall heat transfer coefficient was put forward. Simplified model came in the use of collinearity diagnosis theory and dimensionless experimental analysis, followed by determining the optimal functional form of prediction model and physical meaning for each dimensionless numbers, then establish a new prediction model. Prediction of the overall heat transfer coefficient for several pipelines came after applying Ten-fold Cross Validation to solve the model. The results showed that, compared with back calculation, the error of overall heat transfer coefficient reduces by 3.62%, and average relative error of end-point temperature lowers by 2.88%. The model is of good steadiness and generalization, and better characterization on heat transfer mechanism of overall heat transfer coefficient, to some degree, solving the biggish prediction error led by running parameters fluctuations.

Standard friction prediction model of long-distance hot oil pipelines

We developed a predictive model for the pipeline friction in the 520–730 m3/h transmission range using the multi-layer-perceptron–back-propagation (MLP–BP) method and analyzing the unit friction data after the pigging of a hot oil pipeline. In view of the shortcomings of the MLP–BP model, two optimization methods, the genetic algorithm (GA) and mind evolutionary algorithm (MEA), were used to optimize the MLP–BP model. The research results were applied to the standard friction prediction of three sections of a hot oil pipeline. After the GA and MEA optimizations, the average errors of the three sections were 0.0041 MPa for the GA and 0.0012 MPa for the MEA, and the mean-square errors were 0.083 and 0.067, respectively. The MEA-BP model prediction results were characterized by high precision and small dispersion. The MEA-BP prediction model was applied to the analysis of the wax formation 60 and 90 days after pigging. The analysis results showed that the model can effectively guide pipe pigging and optimization. There was little sample data for the individual transmission and oil temperature steps because the model was based on actual production data modeling and analysis, which may have affected the accuracy and adaptability of the model.

Study on the Economic Insulation Thickness of the Buried Hot Oil Pipelines Based on Environment Factors

It is important to determine the insulation thickness in the design of the buried hot oil pipelines. The economic thickness of the insulation layer not only meets the needs of the project but also maximizes the investment and environmental benefits. However, as a significant evaluation, the environmental factors haven’t been considered in the previous study. Considering this factor, the mathematical model of economic insulation thickness of the buried hot oil pipelines is built in this paper, which is solved by the golden section method while considering the costs of investment, operation, environment, the time value of money. The environmental cost is determined according to the pollutant discharge calculated through relating heat loss of the pipelines to the air emission while building the model. The results primarily showed that the most saving fuel is natural gas, followed by LPG, fuel oil, and coal. The fuel consumption for identical insulation thickness is in the order: coal, fuel oil, LPG, and natural gas. When taking the environmental costs into account, the thicker the economic insulation layer is, the higher cost it will be. Meanwhile, the more pollutant discharge, the thicker the economic insulation layer will be.

Analysis on influencing factors of buried hot oil pipeline

For oil and gas gathering and transportation field, shutdown is inevitable. After the pipeline of buried geothermal oil is shutdown, the crude oil will dissipate quickly and solidify easily. The phase change process of crude oil in the pipeline is complicated, which is accompanied by latent heat release, liquid-solid interface location tracking, crude oil rheological characteristics, natural convection treatment and other key issues. An Enthalpy-Porous medium method was used to establish the thermal model of crude oil in shutdown. Fluent software was used for numerical simulation and analysis. The influence factors such as soil depth, atmospheric temperature, initial temperature of crude oil and thermal insulation materials on the total freezing time of crude oil in the pipeline were analyzed. The results of this study can provide theoretical basis for the determination of the allowable shutdown time of pipeline.

Sensitivity analysis of heat dissipation factors in a hot oil pipeline based on orthogonal experiments.

The study of phase-change heat-transfer characteristics of crude oil has been one of the hot issues in the field of gathering and transportation. The process of phase-change heat transfer of crude oil involves many complicated problems such as natural convection treatment, latent heat treatment, phase-change interface determination and fluid characteristic change. A mathematical model based on the additional capacity heat method is proposed in this article, and the momentum equations of crude oil liquid phase are presented for Newtonian and non-Newtonian fluids. The aim of this study was to investigate the influence of different factors on the heat transfer performance during the shutdown process of an overhead pipe. Experiments were conducted to verify the model and the solution method; the experimental and model results showed good agreement with a maximum relative error of 4.57%. The temperature fields and solidification conditions of crude oil in pipelines under different shutdown conditions were determined, and the sensitivity of the main effect factors was determined through an orthogonal experiment. The results show that the order of influence was oil initial temperature >thickness of insulating layer >air temperature >thickness of wax layer. The results of the study have important guiding significance on the control of shutdown time and the determination of restarting schemes.

Analysis of ground temperature variation in differential settlement of China-Russia crude oil pipeline: field monitoring and numerical simulation

In vast areas of rich frozen soil, the intense heat exchange between the hot oil pipeline and surrounding soil results in the continuous melting of permafrost under the pipeline and the continuous sinking of the pipeline. Moreover, the settlement rate and depth of the pipeline are also different due to the influence of ice content and the thickness of rich frozen soil layer. Therefore, in this paper, based on the field measured data, by adopting the method of numerical simulation, the temperature rise process of the soil around the pipeline under the condition of different subsidence rate and different thickness of rich frozen soil layer is studied. The results show that with the increase of the thickness of the frozen soil layer, the degradation depth of the upper limit of the artificial frozen soil in the lower part of the pipeline increases gradually. Under the same thickness of the frozen soil layer, the influence of the settlement rate of the pipeline on the upper limit of the artificial frozen soil in the lower part of the pipeline mainly concentrates on the initial operation stage of the pipeline; with the increase of the thickness of rich frozen soil layer, the upper limit difference of artificial frozen soil under different settlement rates of pipelines at the same time increases gradually. Of different sedimentation rate and the rich soil layer under the condition of ice -6 m depth of geothermal heating process monitoring, the geothermal heating process can be divided into three stages: melting stage (I), rapid heating stage (II), slow heating stage(III). With the increase of the settling depth, the duration of the phase I and II is significantly reduced, the heating rate is significantly promoted, the duration of phase III is increased significantly and the heating rate is gradually reduced.

A Hybrid Meta-Heuristic Solution to Operation Optimization of Hot Oil Pipeline

In the process of pipeline transportation, the temperature and pressure of waxy and high viscosity crude oil are decreasing. In order to complete the pipeline transportation task, stations are usually set at intervals to overcome the pressure loss caused by friction and impact in the flow and the temperature loss caused by the radial temperature difference. The main equipment of the station is pumps and heaters. The operating fee of pumps and heaters is the core costs of hot oil pipelines, so it is necessary to reasonably optimize the pump and heater operating schemes. Because the hydraulic and thermodynamic of the hot oil pipeline are coupled with each other, and there are a lot of nonlinear constraints and integer variables, this optimization problem is a kind of mixed integer nonlinear programming, which is difficult to solve by using the traditional optimization method. In this paper, a hybrid meta-heuristic algorithm based on particle swarm and pattern search is proposed, which can deal with the problem effectively. The operational cost of the optimized scheme is much lower than that of the actual operating scheme. So this method can provide guidance for the cost reduction and efficiency improvement of the hot oil pipeline.

Numerical Simulation for Preheating New Submarine Hot Oil Pipelines

For new submarine hot oil pipelines, accurate simulation of preheating is difficult owing to complex transient flow and coupled heat transfer happening. Using quasi-steady equations to simulate preheating is inadequate as the hydraulic transient phenomenon is neglected. Considering this fact, this paper constructs an unsteady flow and heat transfer coupled mathematical model for the preheating process. By combining the double method of characteristics (DMOC) and finite element method (FEM), a numerical methodology is proposed, namely, DMOC-FEM. Its accuracy is validated by field data collected from the Bohai sea, China, showing the mean absolute percentage error (MAPE) of 4.27%. Simulation results demonstrate that the preheating medium mainly warms submarine pipe walls rather than the surrounding subsea mud. Furthermore, during the preheating process, the equivalent overall heat transfer coefficients deduced performs more applicably than the inverse-calculation method in presenting the unsteady propagation of fluid temperature with time and distance. Finally, according to the comparison results of 11 preheating plans, subject to a rated heat power and maximum flow, the preheating parameter at a lower fluid temperature combined with a higher flow rate will produce a better preheating effect.

Study on the restart algorithm for a buried hot oil pipeline based on wavelet collocation method

The influence of soil temperature field on the restart process of crude oil pipeline is not considered in previous studies. However, it cannot be ignored for the long-time restart of buried hot oil pipeline. Considering the long computation time for the solution of soil temperature field, the study on the hydraulic-thermal coupled acceleration algorithm for the restart process of buried hot oil pipeline is conducted. Firstly, the mathematical models considering the influence of soil temperature field are introduced in detail for the restart problem. Then based on wavelet collocation method, the adaptive gird is generated to reduce the computational cost of soil temperature field. Furthermore, the hydraulic-thermal coupled acceleration algorithm is proposed, which can realize the fast coupled solution of the restart process. Finally, the optimal values of threshold and coarsest level of resolution are investigated for the restart problem, and the time-adaptive strategy and no-time-adaptive strategy are compared. And the conclusions are drawn from the comparison and analysis of numerical results, which can provide beneficial guidance to the choices of threshold, coarsest level of resolution and adaptive strategy in future studies.

Study on Low Flow Rate Operating Characteristics of Hot Waxy Crude Oil Pipelines Based on an Unsteady-State Calculation Method

Hot crude oil pipeline operated at low flow rate might work in an unstable state, where the friction loss continuously increases with the decreasing flow rate. This will lead to harmful flow stagnation and needs proper management. In this paper, the unsteady-state thermal-hydraulic behavior of the coupled pipeline-pump system is investigated by adopting an unsteady-state numerical model instead of the traditional steady-state model. The new model considers the impacts of the heating strategies, initial operating conditions, and pump characteristics on the pipeline characteristics at low flow rate. Through obtaining the transient characteristics of the flow rate and friction loss, the stable and unstable working ranges are distinguished and critical flow rate is calculated. It is shown that the transient characteristics at low flow rate can be classified into three categories. (1) The flow rate keeps constant for the initial period and then decreases continuously while the friction loss increases monotonously. (2) The flow rate is constant while the friction loss increases firstly and then stays stable. (3) The flow rate maintains constancy while the friction loss increases firstly and then decreases to a final stable level. Besides, it can be concluded that the critical flow rate calculated by the unsteady-state method is recommended with preheating before the flow rate drop.

Study on the water-heat coupled phenomena in thawing frozen soil around a buried oil pipeline

Buried pipelines are one of the most economical and efficient methods for long distance transportation of crude oils, but when a pipeline goes through the cold regions, it may suffer serious threats caused by the freezing and thawing problems of frozen soil. In this study, the water-heat coupled problem of frozen soil around buried hot oil pipeline in cold regions is systematically studied by means of numerical simulation. The temperature field, water content field and unfrozen water migration of frozen soil under different pipeline operation periods are obtained by pure heat conduction model and water-heat coupled model respectively to illustrate the influence of water content field on temperature distribution of frozen soil and oil temperature along the pipeline. It can be concluded from the simulation results that it is the coupled interactions of temperature distribution and water migration that determine the ultimate status of transport phenomena in frozen soil. The most obvious water migration occurs in the soil area close to the buried oil pipeline where there is the largest gradient of temperature. And the change of thermal properties caused by the water migration turns out to be one of the most important factors affecting the temperature field of frozen soil.

Shear tests on deep-ocean clay crust from the Gulf of Guinea

Natural cores from the crust that is found over deep-ocean sediments off the west coast of Africa have been tested in direct shear following one-dimensional consolidation under normal stresses relevant to pipelines. Shearing tests were conducted at different speeds of 0·5, 0·05, 0·005 and 0·0005 mm/s, both within the soil and against interfaces of different roughness. The behaviour in these tests is observed to be strongly influenced by the structure of the natural soil, which contains abundant faecal pellets. The data of shear tests, especially when conducted at faster speeds, display significant randomness, which is attributed to the random location of pellets and their associated macro-voids. Nevertheless, certain trends are evident. Two key observations are made. The first is that the interface friction factor when shearing against a smooth interface can be greater than that of a rough interface. The second is that the rate of shearing has a strong influence on the residual interface-friction value when shearing against a rough interface. When shearing at 0·5 mm/s, the apparent friction factor on a rough interface was observed to drop to zero, whereas the apparent friction factor of a smooth interface tended to be larger. This is attributed to excess pore pressures created by damage to the natural structure of the soil. Slow shearing at 0·005 mm/s produced higher apparent friction factors in the range 0·15–0·30, although excess pore pressures may still be present. It is concluded that the testing of natural cores is essential for a complete understanding of the behaviour of hot-oil pipelines that are subject to pipe walking and buckling. If designers rely instead on tests that have been conducted on reconstituted soils, pipelines may misbehave.