Showing papers on "Overpressure published in 2010"

Effect of Ignition Location, Vent Size, and Obstacles on Vented Explosion Overpressures in Propane-Air Mixtures

Abstract: The authors present results of vented explosion tests using stoichiometric propane-air mixtures in a room-size enclosure 63.7 m3 in volume. The tests were focused on the effect of ignition location, vent size, and obstacles on explosion development and pressure buildup. The dependence of the maximum pressure generated on the experimental parameters was analyzed. It was found that the pressure maxima may be caused by pressure transients defined by the interplay of several factors, such as the maximum flame area and burning velocity in the chamber, and the overpressure generated by the external explosion. A simple model was proposed that allowed for the estimation of the maximum pressure for each of the main pressure transients. The model was found to agree with the experimental data within the experimental uncertainty.

Oil generation as the dominant overpressure mechanism in the Cenozoic Dongying depression, Bohai Bay Basin, China

Abstract: The Dongying depression in the Bohai Bay Basin is a young, prolific oil-producing basin in China. The gray to black mudstones, calcareous mudstones, and oil shales of the Eocene Shahejie Formation (Es3 and Es4) are the major source rocks that are primarily dominated by type I kerogens with a high total organic carbon of up to 18.6%. The Es3 interval is characterized by a sedimentation rate of up to 500 m/m.y. Widespread overpressures are present in the Eocene Es3 and Es4 intervals in the depression, with pressure coefficients up to 1.99 from drillstem tests. Among the sonic, resistivity, and density logs, only the sonic-log displays an obvious response to the overpressure from which the top of the overpressure can be clearly identified. Acoustic traveltime versus effective vertical stress analysis of more than 300 wells in the Dongying depression suggests that the acoustic traveltime of the normally pressured and overpressured mudstones is reduced with increasing vertical effective stress. Pore pressures are accurately predicted in the Dongying depression using an Eaton (1972) exponent of 2.0 by comparing the predicted pressure coefficients with measured pressure coefficients. Disequilibrium compaction has been previously proposed as the sole cause of the high-magnitude overpressures in the Eocene strata of the Dongying depression because of rapid deposition of fine-grained sediments. However, our data indicate that the overpressures are caused by oil generation from the source rocks within the Es3 and Es4 intervals. The overpressured sediments display a normal compaction as evidenced from the overpressured mudstones exhibiting no anomalously low density, the apparent lack of correlation between mudstone densities and effective vertical stress, and the overpressured reservoir sandstones showing no anomalous high-matrix porosities or anomalous geothermal gradient. The depths to the top of the overpressure intervals range from 2000 to 3000 m (6562–9843 ft) following closely source rock depths. All the overpressured reservoirs and source rocks have a minimum temperature of approximately 87C, and overpressured source rocks generally have vitrinite reflectance (Ro) values of 0.6% or higher. Overpressures are not found in the strata within which the Ro values are less than 0.5%. The overpressured reservoirs in the Es3 and Es4 intervals are predominantly oil saturated or oil bearing. Organic-rich source rocks with overpressures are capable of generating hydrocarbons and thus can maintain an abnormally high pressure. The calcite precipitation in the calcareous mudstones may have caused a significant reduction in porosity and permeability to form an effective pressure seal. The presence of widespread microfractures in the source rocks may relate to episodic expulsion of hydrocarbons or overpressure dissipation. Overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks through active faulting and fracturing.

Influence of tectonic overpressure onP-Tpaths of HP-UHP rocks in continental collision zones: thermomechanical modelling

Abstract: The principle of lithostatic pressure is habitually used in metamorphic geology to calculate burial/exhumation depth from pressure given by geobarometry. However, pressure deviation from lithostatic, i.e. tectonic overpressure/underpressure due to deviatoric stress and deformation, is an intrinsic property of flow and fracture in all materials, including rocks under geological conditions. In order to investigate the influences of tectonic overpressure on metamorphic P-T paths, 2D numerical simulations of continental subduction/collision zones were conducted with variable brittle and ductile rheologies of the crust and mantle. The experiments suggest that several regions of significant tectonic overpressure and underpressure may develop inside the slab, in the subduction channel and within the overriding plate during continental collision. The main overpressure region that may influence the P-T paths of HP-UHP rocks is located in the bottom corner of the wedge-like confined channel with the characteristic magnitude of pressure deviation on the order of similar to 0.3 GPa and 10-20% from the lithostatic values. The degree of confinement of the subduction channel is the key factor controlling this magnitude. Our models also suggest that subducted crustal rocks, which may not necessarily be exhumed, can be classified into three different groups: (i) UHP-rocks subjected to significant (>= 0.3 GPa) overpressure at intermediate subduction depth (50-70 km, P = 1.5-2.5 GPa) then underpressured at depth >= 100 km (P >= 3 GPa); (ii) HP-rocks subjected to >= 0.3 GPa overpressure at peak P-T conditions reached at 50-70 km depth in the bottom corner of the wedge-like confined subduction channel (P = 1.5-2.5 GPa); (iii) lower-pressure rocks formed at shallower depths (< 40 km depth, P < 1 GPa), which are not subjected to significant overpressure and/or underpressure.

Overpressure and earthquake initiated slope failure in the Ursa region, northern Gulf of Mexico

Abstract: [1] We use fluid flow and slope stability models to study the evolution of overpressure and slope stability in the Ursa region, northern Gulf of Mexico. Our predictions match measured overpressures (pressures in excess of hydrostatic) from Integrated Ocean Drilling Program Expedition 308 Site U1324 above 200 m below seafloor (mbsf) but overpredict deeper (200–610 mbsf) overpressures by 0.4–1.1 MPa. Modeled overpressure at Site U1322 matches measurements for the entire section (0–240 mbsf) with exception of the measurement at 240 mbsf. Slope stability models that integrate modeled overpressure, vertical stress, and effective stress during deposition predict slope failure at 61 ka on the eastern end of the region. This failure corresponds to the base of a mass transport deposit that has been interpreted as a retrogressive failure initiated by high overpressure. Overpressure alone could not drive failure of a second mass transport deposit (MTD2) that has its base along the 27 ka horizon. With an earthquake acceleration model coupled with our slope stability model, we predict that horizontal acceleration from a magnitude 5 earthquake within 140 km of the Ursa region at 27 ka would initiate the failure that created MTD2; the same earthquake at 20 ka would have to be within 40 km for failure. This magnitude and maximum rupture distance are consistent with seismicity near the Ursa region. We therefore propose that in some cases, overpressure drives failure on low-angle slopes; however, earthquakes, even on passive margins, may play a critical role in initiating slope failure in sediments weakened by overpressure.

Subharmonic scattering of phospholipid-shell microbubbles at low acoustic pressure amplitudes

Abstract: Subharmonic scattering of phospholipid-shell microbubbles excited at relatively low acoustic pressure amplitudes (<;30 kPa) has been associated with echo responses from compression-only bubbles having initial surface tension values close to zero. In this work, the relation between subharmonics and compression-only behavior of phospholipid-shell microbubbles was investigated, experimentally and by simulation, as a function of the initial surface tension by applying ambient overpressures of 0 and 180 mmHg. The microbubbles were excited using a 64-cycle transmit burst with a center frequency of 4 MHz and peak-negative pressure amplitudes ranging from 20 of 150 kPa. In these conditions, an increase in subharmonic response of 28.9 dB (P <; 0.05) was measured at 50 kPa after applying an overpressure of 180 mmHg. Simulations using the Marmottant model, taking into account the effect of ambient overpressure on bubble size and initial surface tension, confirmed the relation between subharmonics observed in the pressure-time curves and compression-only behavior observed in the radius-time curves. The trend of an increase in subharmonic response as a function of ambient overpressure, i.e., as a function of the initial surface tension, was predicted by the model. Subharmonics present in the echo responses of phospholipid-shell microbubbles excited at low acoustic pressure amplitudes are indeed related to the echo responses from compression-only bubbles. The increase in subharmonics as a function of ambient overpressure may be exploited for improving methods for noninvasive pressure measurement in heart cavities or big vessels in the human body.

Investigation of Air-Blast Effects from Spherical-and Cylindrical-Shaped Charges:

Abstract: Although the distributions of peak incident overpressure and impulse generated from spherical charges and cylindrical charges of the same weight can differ greatly close to the point of detonation, spherical charges are assumed for nearly all explosive-effects computations per modern standards for blast-resistant design such as UFC-3-340-02 and the soon-to-be published ASCE Standard for the Blast Protection of Buildings. A blast-testing program was performed using a reinforced concrete slab as the target to investigate the reflected peak overpressure and impulse distributions as a function of charge shape, orientation, and scaled distance. The charge shapes were cylindrical and spherical, and the charge mass varied from 0.24 to 8.0 kg. Nine pressure transducers were installed on the surface of the slab to record the distribution of pressure histories over the face of the target. A finite element model of the explosive and the target was validated using the experimental data. The validated model was then used to undertake a parametric analysis to more broadly study the effects of detonation point, ratio of charge length to charge diameter, charge orientation and standoff distance on the distribution of reflected overpressure. Numerical results are compared with predictions of UFC-3-340-02. For cylindrical charges, the ratio of charge length (L) to diameter (D), the orientation of the longitudinal axis of the charge, and detonation point within the charge affected the distributions of reflected peak overpressure and impulse in the immediate vicinity of the explosive. The UFC-3-340-02 underpredicts substantially the reflected peak overpressure and impulse on a target aligned with the vertical axis of a cylindrical charge with an aspect ratio of 1.0.

Pore fluid pressure and internal kinematics of gravitational laboratory air‐particle flows: Insights into the emplacement dynamics of pyroclastic flows

Abstract: [1] The emplacement dynamics of pyroclastic flows were investigated through noninvasive measurements of the pore fluid pressure in laboratory air-particle flows generated from the release of fluidized and nonfluidized granular columns. Analyses of high-speed videos allowed for correlation of the pressure signal with the flow structure. The flows consisted of a sliding head that caused underpressure relative to the ambient, followed by a body that generated overpressure and at the base of which a deposit aggraded. For initially fluidized flows, overpressure in the body derived from advection of the pore pressure generated in the initial column and decreased by diffusion during propagation. Relatively slow diffusion caused the pore pressure in the thinner flow to be larger than lithostatic at early stages. Furthermore, partial auto-fluidization, revealed in initially nonfluidized flows, also occurred and contributed to maintain high pore pressure, whereas dilation or contraction of the air-particle mixture with associated drag and/or pore volume variation transiently led the pressure to decrease or increase, respectively. The combination of all these processes resulted in long-lived high pore fluid pressure in the body of the flows during most of their emplacement. In the case of the initially fluidized and slightly expanded (∼3–4%) flows, (at least) ∼70%–100% of the weight of the particles was supported by pore pressure, which is consistent with their inertial fluid-like behavior. Dense pyroclastic flows on subhorizontal slopes are expected to propagate as inertial fluidized gas-particle mixtures consisting of a sliding head, possibly entraining basement-derived clasts, and of a gradually depositing body.

Numerical study of blast characteristics from detonation of homogeneous explosives

Abstract: A new robust numerical methodology is used to investigate the propagation of blast waves from homogeneous explosives. The gas-phase governing equations are solved using a hybrid solver that combines a higher-order shock capturing scheme with a low-dissipation central scheme. Explosives of interest include Nitromethane, Trinitrotoluene, and High-Melting Explosive. The shock overpressure and total impulse are estimated at different radial locations and compared for the different explosives. An empirical scaling correlation is presented for the shock overpressure, incident positive phase pressure impulse, and total impulse. The role of hydrodynamic instabilities to the blast effects of explosives is also investigated in three dimensions, and significant mixing between the detonation products and air is observed. This mixing results in afterburn, which is found to augment the impulse characteristics of explosives. Furthermore, the impulse characteristics are also observed to be three-dimensional in the region of the mixing layer. This paper highlights that while some blast features can be successfully predicted from simple one-dimensional studies, the growth of hydrodynamic instabilities and the impulsive loading of homogeneous explosives require robust three-dimensional investigation.

Basin scale" versus "localized" pore pressure stress coupling - implications for trap integrity evaluation

Abstract: In petroleum industry, the difference between pore pressure (Pp) and minimum horizontal stress Sh (termed the seal or retention capacity) is of major consideration because it is often assumed to represent how close a system is to hydraulic failure and thus the maximum hydrocarbon column height that can be maintained. While Sh and Pp are often considered to be independent parameters, several studies in the last decade have demonstrated that Sh and Pp are in fact coupled. However, the nature of this coupling relationship remains poorly understood. In this paper, we explore the influences of the spatial pore pressure distribution on Sh/Pp coupling and then on failure pressure predictions and trap integrity evaluation. With analytical models, we predict the fluid pressure sustainable within a reservoir before failure of its overpressured shale cover. We verify our analytical predictions with experiments involving analogue materials and fluids. We show that hydraulic fracturing and seal breach occur for fluid pressure greater than it would be expected from conventional retention capacity. This can be explained by the impact of the fluid overpressure field in the overburden and the pressure diffusion around the reservoir on the principal stresses. We calculate that supralithostatic pressure could locally be reached in overpressured covers. We also define the retention capacity of a cover (RC) surrounding a fluid source or reservoir as the difference between the failure pressure and the fluid overpressure prevailing in shale at the same depth. In response to a localized fluid pressure rise, we show that the retention capacity does not only depend on the pore fluid overpressure of the overburden but also on the tensile strength of the cover, its Poisson’s ratio, and the depth and width of the fluid source.

Overpressure-generating mechanisms in the Peciko Field, Lower Kutai Basin, Indonesia

Abstract: The Peciko Field contains gas in multiple stacked reservoirs within a Miocene deltaic sequence. In the deeper reservoirs, gas is trapped hydrodynamically by high lateral overpressure gradients. We have analysed overpressure and compaction in this field by using wireline log, pressure, temperature and vitrinite reflectance data. The top of the overpressure is located below 3 km burial depth, below the depth range for transformation of discrete smectite to mixed-layer illite/smectite. Density-sonic and density-resistivity crossplots for mudrocks show reversals within the transition zone into hard overpressure below 3.5 km depth. Vitrinite reflectance measurements indicate that the start of unloading coincides with the onset of gas generation. Moreover, mudrock density continues to increase with depth in the overpressured section to values above 2.6 g cm –3 . We conclude that gas generation and chemical compaction are responsible for overpressure generation, contradicting previous interpretations that disequilibrium compaction is the principal mechanism for generating overpressure in the Lower Kutai Basin. The particular circumstances which make our radical interpretation plausible are that it is a warm basin with lateral reservoir drainage, so the overpressured mudrocks are probably overcompacted as a result of diagenesis.

On the efficiency of semi-closed blast inhibitors

Abstract: Numerical efficiency analysis of those blast inhibitors (BI) consisting of a vertical cylinder open at the top is presented and compared with the existing data from other authors. It was proven computationally that such low-height “semi-closed” BI do not provide blast wave suppression according to the minimum practical requirements (overpressure and pressure impulse reduction at several times) and, if one wishes to compare, other existing criteria (Bowen injury diagrams and similar guidelines adopted in different countries).

Role of the Chalk in development of deep overpressure in the Central North Sea

Abstract: A high magnitude of overpressure is a characteristic of the deep, sub-Chalk reservoirs of the Central North Sea. The Upper Cretaceous chalk there comprises both reservoir and non-reservoir intervals, the former volumetrically minor but most commonly identified near the top of the Tor Formation. The majority of non-reservoir chalk has been extensively cemented with average fractional gross porosity of 0.08, and permeability in the nano- to microDarcy range (10 −18 –10 −21 m 2 ), and sealing properties comparable to shale. Hence deeply buried chalk is comparable to shale in preventing dewatering and allowing overpressure to develop. Direct pressure measurements in the Chalk are restricted to the reservoir intervals, plus in rare fractured chalk, but reveal that Chalk pressures lie on a pressure gradient which links to the Lower Cenozoic reservoir above the Chalk and the Jurassic/Triassic reservoir pressures below. Hence a pore pressure profile of constantly increasing overpressure with increasing depth is indicated. Mud weight profiles through the Chalk, by contrast, show many borehole pressures lower than those indicated by these direct measurements, implying wells are routinely drilled underbalanced. The Chalk is therefore considered the main pressure transition zone to high pressures in sub-Chalk reservoirs. In addition to its role as a regional seal for overpressure, the Base Chalk can be shown to be highly significant to trap integrity. Analysis of dry holes and hydrocarbon discoveries relative to their aquifer seal capacity (the difference between water pressure and minimum stress) shows that the best empirical relationship exists at Base Chalk, rather than Base Seal/Top Reservoir, where the relationship is traditionally examined. Using a database of 65 wells from the HP/HT area of the Central North Sea, and extending the known aquifer gradients from the Fulmar reservoirs via Base Cretaceous to Base Chalk, leads to a risking threshold at 5.2 MPa (750 psi) aquifer seal capacity. Discoveries constitute 88% of the wells above the threshold and 36% below, with 100% dry holes where the aquifer seal capacity is zero (i.e. predicted breached trap). This relationship at Base Chalk can be used to identify leak points which control vertical hydrocarbon migration as well as assessing the risk associated with drilling high-pressure prospects in the Central North Sea.

Estimating Pore Pressure Using Compressional and Shear Wave Data from Multicomponent Seismic Nodes in Atlantis Field, Deepwater Gulf of Mexico

Abstract: Summary Pore pressures for the shallow sub-seafloor sediments at the Atlantis field are estimated by analysis of P and S wave velocities observed seismic data recorded by a patch of ocean-bottom multicomponent nodes. A modified Eaton’s algorithm for pressure prediction is applied, and estimated overpressure locations are compared to known hazardous shallow water flow zones evaluated from a batch-set drilling project.

Dynamics of soap bubble bursting and its implications to volcano acoustics

Abstract: [1] In order to assess the physical mechanisms at stake when giant gas bubbles burst at the top of a magma conduit, laboratory experiments have been performed. An overpressurized gas cavity is initially closed by a thin liquid film, which suddenly bursts. The acoustic signal produced by the bursting is investigated. The key result is that the amplitude and energy of the acoustic signal strongly depend on the film rupture time. As the rupture time is uncontrolled in the experiments and in the field, the measurement of the acoustic excess pressure in the atmosphere, alone, cannot provide any information on the overpressure inside the bubble before explosion. This could explain the low energy partitioning between infrasound, seismic and explosive dynamics often observed on volcanoes.

Explosion Phenomena and Effects of Explosions on Structures. I: Phenomena and Effects

Abstract: This is Part I of a three-part paper on the phenomena of explosions and effects of explosions on structures. The overall paper is intended to provide the reader with a practical overview of the various types of explosions and methods available to predict explosion effects with emphasis on air blast. The response of structures to air blast is discussed and methods of analysis are presented. Since the subject area is broad, reference sources for more in-depth information on many of the topics presented are provided. In this paper, Part I, the background is provided on explosion phenomena and the types of effects produced by explosions of various general types. Physical, chemical, electrical, and nuclear explosions are discussed. Explosion effects for external (outdoor) explosions presented include air blast characteristics of peak overpressure, positive phase impulse, time of arrival, and positive phase duration. The ranges of peak overpressure and event duration from different types of explosions are compa...

Modulations of a weak shock wave through a turbulent slit jet

Abstract: The pressure modulation of a weak shock wave induced by a Nd:YAG laser pulse when passing across a turbulent slit jet was experimentally investigated. With the slit jet the peak overpressure became smaller by an average of 12%, with a standard deviation of 27%. Clear relationships were obtained between the overpressure history and the experimentally observed shock front deformation, which was visualized as differential schlieren images. The peak overpressure was increased when the originally spherical blast wave front was locally flattened, whereas it was decreased when a hump in the shock wave front was formed.

Effect of variation in gas distribution on explosion propagation characteristics in coal mines

Abstract: In order to investigate the effect of variation in the distribution of gas on explosion propagation characteristics in coal mines, experiments were carried out in two different channels with variation in gas concentration and geometry. Flame and pressure transducers were used to track the explosion front velocity. The flame speed (Sf) showed a slight downward trend while the methane concentration varied from 10% to 3% in the experimental channel. The peak overpressure (Pmax) dropped dramatically when compared with normal conditions. As well, the values of Pmax and Sf decreased when the methane concentration dropped from 8% to 6%. The flame speed in the channel, connected to a cylinder with a length varying from 0.5 to 2 m, was greater than that in the normal channel. The peak overpressure was also higher than that under normal conditions because of a higher flame speed and stronger pressure piling up. The values of Pmax and Sf increased with an increase in cylinder length. The research results indicate that damage caused by explosions can be reduced by decreasing the gas concentration, which should be immediately detected in roadways with large cross-sections because of the possibility of greater destruction caused by more serious explosions.

Development of a Methodology to Assess Mechanical Impulse Effects Resulting from Lightning Attachment to Lightweight Aircraft Structures

Abstract: The effect of lightning attachment to structures and vehicles is a cause of major concern to a number of different industries, in particular the aerospace industry, where the consequences of such an event can be catastrophic In 1963, a Boeing 707 was brought down in Maryland killing 81 people on board, triggering the improvement of lightning protection standards However, commercial jets are still struck on average once every 10,000 hours of flight time and between 1963 and 1989 forty lightning related accidents were recorded within the USA alone The rapid increase in the use of composite materials in aircraft design and the consequent increase in complexity when determining the effects of a lightning strike, has led to new challenges in aircraft protection and the requirement for improved understanding and standardisation The attachment of lightning to a structure causes damage through three mechanisms Primarily a supersonic acoustic shock wave, caused by rapid heating of the arc channel during initial attachment, resulting in a large and rapid overpressure Secondly a magnetic force generated by the fields developed in the high current areas around the lightning attachment point Finally a mechanism specifically related to composite materials, where the rapid vaporisation of an expanded copper foil layer (designed to quickly transmit current across a structure, thus reducing its focus) trapped between the composite material and the protective paint layers causes an additional overpressure, which is exacerbated by additional paint layers acting to contain the explosion and direct it inwards The work described in this paper looks to develop a technique to measure these forces in order to better understand and assess their effects A novel methodology has been developed to allow the estimation of peak overpressure forces produced by the acoustic shock wave resulting from the attachment The methodology utilises Digital Image Correlation and ultra-high speed photography to acquire full-field displacement measurements of panel deflections at frame rates of up to 1,000,000 frames per second The experimental results are used in the optimisation of a finite element model, outputting the parameters of an acoustic shock that produces representative displacements, velocities and accelerations The method is currently being validated by performing a series of tests on aluminium panels subject to instrumented impact testing The next stage will be to use the technique developed to aid a program of investigation into the effects of artificial lightning strike events on aluminium and composite panels

Method of detecting at least one malfunctioning high-pressure gas tank

Abstract: A method of detecting a malfunctioning high-pressure gas tank (4) comprised within a vehicle (1), which method comprises retrieving (202, 204) first and second system gas pressure values (p1, p2) at first and second points of time (t1, t2); retrieving (206) data related to temperature and pressure of gas supplied to an engine (12) between the first and second points of time; determining (208) an expected gas pressure drop (D expected ) within the system (2) based on the data; determining (210) an actual system gas pressure drop (D actual ) by comparing the first and second pressure values (p1, p2); determining (212) a pressure drop delta (Δ) by comparing the actual pressure drop (D actual ) and the expected pressure drop (D expected ); and triggering an indication if the delta (Δ) exceeds a predetermined threshold (Δ threshold ), thereby providing a warning of occurrence of at least one malfunctioning tank (4). With the present invention, a potential rupturing of the tank (4) or uncontrolled release of gaseous fuel through a manual shut off valve (9) resulting from overpressure in a tank (4), may be avoided.