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A pressure relief valve (PSV) cannot depressurize a vessel or system; it can only limit its maximum pressure during upset or emergency conditions. A blowdown system is usually specified to accomplish the removal of the pressurized, volatile, hazardous, and flammable compounds such as hydrocarbon or ammonia vapor in vessels or systems. The pressure is removed from the vessel using a blowdown system; however, some hydrocarbon liquid will remain in the vessel. This paper explains different criteria for designing a blowdown system. Examples are presented to show how the blowdown time is affected by several factors. The Joule–Thomson effect and its effect on the vessel's inner wall temperature are illustrated using charts and tables. Several applications of the blowdown system are provided that relate to actual offshore and refining operations. Different mechanisms by which the blowdown system is activated are discussed, and a simplified design procedure to design a blowdown system is also provided. [ABSTRACT FROM AUTHOR]

The carbon capture, utilization and storage (CCUS) technology can effectively improve global climate change. Extensive pipelines are used in CCUS to transport large amounts of pure and impure supercritical CO2(S‐CO2) from captured sites to storage, owing to pipelines being an economical and efficient means of transportation. In the event of an accidental rupture of a supercritical CO2 pipeline, causing substantial economic losses and catastrophic consequences to surroundings. Hence, it is necessary that the study of the pressure variation and temperature distribution of CO2 during pipeline blowdown contributes to understanding fracture initiation and propagation, as well as designing reliable pipelines. A laboratory‐scale setup is used to investigate the evolution of leakage behaviours and analyze heat transfer of pure and impure supercritical CO2 with different release orifices. The depressurization process, variation of inner and wall of pipeline temperature were measured. Results show that the larger is the leakage orifice, the lower is the minimum temperature of the inner pipeline. The larger is the concentrations of nitrogen (N2) in supercritical CO2 mixture, the lower is the minimum temperature of the inner pipeline, and the higher is the risk of pipeline brittle fracture. During the entire release process, the Nusselt number (Nu) was determined based on measurements to discuss the process of heat transfer between the CO2 mixture and pipeline wall. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Class 1 hydrate reservoirs are regarded as the most promising targets for gas production. Previous investigations in the northern South China Sea have proven that the low‐permeability hydrate reservoir at site GMGS3‐W19 is a Class 1 hydrate deposit. However, the dynamic production behaviors of this type of reservoir are not yet fully understood. Here, we report gas recovery from hydrate reservoirs containing underlying free gas layers with different initial gas saturations and investigate the effects of absolute permeability and depressurization pressure on gas production performance. The results show that high initial gas saturation of the underlying free gas layer and low bottom‐hole pressure are advantageous for enhancing the gas production performance. The effects of absolute anisotropic permeability and isotropic permeability on gas recovery from Class 1 hydrate reservoirs are different. For long‐term gas production from this hydrate reservoir with any initial gas saturation, a higher horizontal permeability (permeability anisotropy more than 10) in the hydrate layer is remarkably beneficial to stimulate gas extraction and decrease the amount of produced water replenished by inflows from permeable overlying and underlying sediments. Moreover, the gas production rate is highly consistent with the hydrate dissociation ratio, one of the main factors affecting the gas production performance, in Class 1 hydrate reservoirs with different permeabilities. However, for this type of hydrate reservoir with permeability anisotropy larger than 5, the larger the horizontal permeability in the hydrate layer can form more secondary hydrates. The increment in secondary hydrate formation negatively influences the hydrate dissociation ratio and gas recovery. Thus, it is also significant to weigh the permeability anisotropy of the hydrate layer and the secondary hydrate formation amount during the production progress. These findings can contribute to estimating the gas recovery efficiency in Class 1 hydrate reservoirs and optimizing the production process in similar areas. [ABSTRACT FROM AUTHOR]

With the implementation of the production tests in permafrost and offshore regions in Canada, US, Japan, and China, the study of natural gas hydrate has progressed into the stage of technology development for industrial exploitation. The depressurization method is considered as a better strategy to produce gas from hydrate reservoirs based on production tests and laboratory experiments. Multi‐well production is proposed to improve gas production efficiency, to meet the requirement for industrial production. For evaluating the applicability of multi‐well production to hydrate exploitation, a 2D model is established, with numerical simulations of the performance of the multi‐well pattern carried out. To understand the dissociation behavior of gas hydrate, the pressure and temperature distributions in the hydrate reservoir are specified, and the change in permeability of reservoir sediments is investigated. The results obtained indicate that multi‐well production can improve the well connectivity, accelerate hydrate dissociation, enhance gas production rate and reduce water production as compared with single‐well production. [ABSTRACT FROM AUTHOR]

The permeability anisotropy (ie, horizontal, kr, to vertical, kz, permeability ratio) of gas hydrate reservoirs may be a significant factor that affects hydrate dissociation and gas production. Here, site SH2, a candidate for field testing comprising a clayey silt gas hydrate reservoir in the Shenhu area in the South China Sea, was chosen to investigate the effect of permeability anisotropy on gas production behavior through numerical simulations. The spatial distribution of physical fields (ie, pressure, temperature, and saturation) and the evolution of gas and water recovery are comparatively analyzed. The simulation results show that permeability anisotropy has a significant impact on gas extraction, especially when the horizontal permeability is higher than the vertical one. The sensitivity analyses indicate that permeability anisotropy in both medium‐permeability (10‐100 mD) and low‐permeability (1‐10 mD) sediments are conducive to hydrate dissociation and that a higher horizontal permeability is more favorable for gas production. Comparatively, permeability anisotropy in low‐permeability sediments is not beneficial for improving production efficiency. In addition, our work also suggests that heterogeneous hydrate saturation in a reservoir‐scale model should be employed in future predictions because the gas production potential will be overestimated in a homogeneous reservoir under the same conditions. These findings contribute to a clear understanding of the influence of reservoir properties on gas hydrate dissociation processes and provide a reference for future field trials and commercial production. [ABSTRACT FROM AUTHOR]

Summary: There are four primary classes of natural gas hydrate deposits in natural world. The differences between them are in the distribution of the methane hydrate, free gas, and free water layers. A reactor which the height was 120 mm and the inner diameter was 103 mm was used in hydrate formation and dissociation, and 17 thermocouples measured the distribution of temperature during the mining process. Different experimental means were applied to simulate three classes of methane hydrate deposits, in which water and glass beads filled in different orders. The hydrate samples were dissociated by depressurization, and the results showed no need to test a backpressure greater than 94% of the equilibrium pressure of methane hydrate. Class 1 sample's methane hydrate decomposition rate was slower than that of class 3 sample at the beginning of depressurization when backpressure was 2.3 and 2.6 MPa, but then, the opposite happened. The average dissociation rate decreased nearly linearly with the increase in backpressure for the class 1 samples. [ABSTRACT FROM AUTHOR]

Summary: The cargo compartment of an airplane in flight is a complex environment with dynamic pressure (pressurization and depressurization), nonconservative oxygen, and unidirectional ventilation. In this study, n‐heptane pool fires were performed under static pressure, pressurization, and depressurization in a full‐scale airplane cargo compartment. The static pressure included 30 and 90 kPa, the pressurization was from 30 to 90 kPa at rates of 6, 12, 19, and 25 kPa/min, while the depressurization was from 90 to 30 kPa at rates of 6, 12, 17, and 20 kPa/min. The effects of pressure, oxygen concentration, and ventilation on pool fire characteristics including fuel mass loss rate (MLR), flame centerline temperature, and flame shape under each condition were concluded. The results show that the predominant factor of MLR was different in three conditions. The flame is divided into four regimes, in which the fuel vapor regime is used to emphasize the influence of fuel vapor on flame temperature above the fuel surface. The concept of average flame shape is put forward to reflect the flame occurrence probability. And its bottom, which named average flame root, presents the negative correlation with compartment pressure. [ABSTRACT FROM AUTHOR]

The depressurization method is one of the most promising methods for the exploitation of hydrate reservoirs and has been conducted in several field tests. In this work, the production behaviors of methane hydrate in sand sediments by different depressurization strategies were comparatively investigated using a cubic hydrate simulator (CHS) with a capacity of 5.832 L. The experimental conditions are based on the hydrate reservoirs parameters of the South China Sea. The results indicate that the hydrate dissociation rate is related to production pressure and heat conduction between the sediments and the surroundings. Some hydrate form again at the beginning of depressurization and the progress of hydrate dissociation during stable depressurization stage is basically along with the P−T curve. The cumulative amounts of gas production are almost the same, affirming it depends on the final depressurization amplitude, and less water produced when a short shut‐in period is conducted before hydrate dissociation. Compared to single‐step depressurization strategy, the multistep depressurization possesses a more well‐distributed gas production, and the sensible heat of sediments contributes less to hydrate dissociation. Despite the production period is longer, its temperature over the whole production process is higher, which has the potential to reduce the secondary hydrate formation and the damage of wellbore and sediments effectively in actual hydrate production. Two different depressurization strategies were carried out to produce gas from the hydrate reservoir, respectively. The experimental conditions are based on the hydrate reservoirs parameters of the South China Sea. For the multistep depressurization strategy, the sensible heat of sediments contributes less to hydrate dissociation and the gas production is well‐distributed over the production process compared to single‐stage depressurization. [ABSTRACT FROM AUTHOR]

Abstract: Depressurization of hydrate‐bearing sediments (HBS) can cause the movement of fine particles, and in turn, such fines migration affects fluid flow and mechanical behavior of sediments, ultimately affecting long‐term hydrocarbon production and wellbore stability. This study investigated how and to what extent depressurization of HBS causes fines migration using X‐ray computed tomography (CT) imaging. Methane hydrate was synthesized in sediments with 10% fines content (FC), composed of sands with silt and/or clay, and the hydrate‐bearing samples were stepwisely depressurized while acquiring CT images. The CT images were analyzed to quantify the spatial changes in FC in the host sediment and thus to capture the fines migration during depressurization. It was found that the FC changes began occurring from the hydrate dissociation regions. This confirms that the multiphase flow caused by depressurization accompanies fines migration. Depressurization of HBS with a hydrate saturation of ~20–40% caused FC reduction from ~10% to ~6–9%, and the extent of fines migration differed with the particle sizes of the host sands and the types of fines. It was found that fines migration was more pronounced with coarse sands and with silty fines. Such observed level of FC reduction is estimated to increase sediment permeability by several factors based on the Kozeny‐type permeability model. Our results support the notion that the extent of fines migration and its effect on fluid flow behavior need to be assessed in consideration of physical properties of host sediment and fine particles to identify optimum depressurization strategies. [ABSTRACT FROM AUTHOR]

In the present study, we have developed a numerical method which can simulate the dynamic behaviour of a seabed ground during gas production from methane hydrate-bearing sediments. The proposed method can describe the chemo-thermo-mechanical-seismic coupled behaviours, such as phase changes from hydrates to water and gas, temperature changes and ground deformation related to the flow of pore fluids during earthquakes. In the first part of the present study, the governing equations for the proposed method and its discretization are presented. Then, numerical analyses are performed for hydrate-bearing sediments in order to investigate the dynamic behaviour during gas production. The geological conditions and the material parameters are determined using the data of the seabed ground at Daini-Atsumi knoll, Eastern Nankai Trough, Japan, where the first offshore production test of methane hydrates was conducted. A predicted earthquake at the site is used in the analyses. Regarding the seismic response to the earthquake which occur during gas production process, the wave profiles of horizontal acceleration and horizontal velocity were not extensively affected by the gas production. Hydrate dissociation behaviour is sensitive to changes in the pore pressure during earthquakes. Methane hydrate dissociation temporarily became active in some areas because of the main motion of the earthquake, then methane hydrate dissociation brought about an increase in the average pressure of the fluids during the earthquake. And, it was this increase in average pore pressure that finally caused the methane hydrate dissociation to cease during the earthquake. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The correct design of a safety valve or rupture disk can only be done if predictions on the venting behavior can be made. To date, those predictions have limitations with regard to foaming or highly viscous systems. An experimental study is presented that describes the influence of hydrophilic particles on the venting of strongly and weakly foaming systems. It is shown that hydrophilic particles increase the mass discharge over time and reduce the pressure decrease over time by increasing the foaming of aqueous systems. These effects are not significantly influenced by the reactor characteristics. At higher additive concentrations, the addition of particles is not as influential anymore. Thus, models for foamy two-phase systems are applicable for the design of the described foamy three-phase systems as well. © 2013 American Institute of Chemical Engineers Process Saf Prog 32: 239-243, 2013 [ABSTRACT FROM AUTHOR]

This article discusses how pipeline networks that experience sudden pressure differences, even in the absence of initial system flow, can create severe transient pressures. If a pipe is under static pressure and a valve is suddenly opened, a depressurization wave propagates through the system. As with any transient, reflection is based on the pipe material, and at a dead end the original and reflected waves superimpose. This superposition is almost invariably large enough for local pressures to drop below the gauge value and reach vacuum pressures, inducing cavitation. For the experiments reported in this article, negative pressures were measured for several seconds and a low-pressure event was followed by rapidly increasing pressures and associated impressive acoustic effects. This investigation highlights that depressurization events are potentially destructive and must be treated with appropriate caution because such shock loadings can cause structural failures. [ABSTRACT FROM AUTHOR]

Rate equations proposed in Part I were extended to dissociation processes induced by different methods such as depressurization, raised temperature, or simultaneous changes in temperature and pressure. For all cases, the dissociation rate of hydrate into the two-phase coexisting region (V-Lw) can be expressed by a general form of the product of the rate constant and the logarithm of the solubility ratio of methane in the aqueous solution, XR to XS: XR is the solubility of methane in the aqueous solution that is hypothetically in equilibrium with the hydrate phase under the dissociation pressure and temperature, and Xs is the equilibrium solubility of methane in water under the same dissociation condition. The virtual solubility of methane, XR, can be estimated by extrapolating the solubility curve of methane under the conditions in equilibrium with the stable hydrate to the region at which the dissociation occurs by a simple thermodynamic treatment. The dissociation equation was applied to experimental results of dissociation processes induced by depressurization with methane bubble formation. The experimentally observed dissociation rates were well correlated by the rate equation with the dissociation constants determined in Part I. The results suggest that the dissociation equation can be applied generally to dissociation processes that occur by different methods, and the rate constant is a universal parameter within the experimental range examined in this work. [ABSTRACT FROM AUTHOR]

The development of a rigorous mathematical model for simulating the processes leading to the transition of an initially stable through-wall defect into an elongated running fracture in pressurized pipelines is described. The application of the model to the puncture of a gas pipeline followed by its isolation indicates significant and rapid localized cooling of the pipe wall to temperatures well below its ductile to brittle transition temperature. The resulting order of magnitude drop in the fracture toughness coupled with the pressure stresses at the defect plane are shown to lead to catastrophic pipeline failure in the form of a running fracture some 2000 s after its puncture. Significantly, delay in pipeline isolation after its puncture is shown to have a profound effect in circumventing such failures. The above study for the first time quantitatively highlights the importance of taking into account the expansion-induced cooling effects as a credible failure scenario when undertaking safety assessment of pressurized pipelines. [ABSTRACT FROM AUTHOR]