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As the exploration of oil and gas moves into deeper water, suction embedded plate anchors (SEPLAs) are developed to answer the growing demand for anchor-mooring systems. The keying process of SEPLAs has been extensively reported assuming a spatially homogeneous soil which is a departure from reality. This study conducts a comprehensive investigation of the keying process of SEPLA in spatially varying clay through a large deformation random finite element method coupled with the Monte Carlo simulations. The numerical model is validated with the results from the existing centrifuge test and numerical analysis. The spatial randomness of soil strength is assumed to be a three-dimensional (3D) random field to explore the failure mechanism and statistical characteristics of rotational response and pullout behaviour. The normalised factor of the upper 5% fractile of embedment loss is presented as an algebraic expression of the coefficient of variation of soil strength. The relationship between safety factor and failure probability is determined based on the requirement of pullout capacity. Finally, the applicability of the present results to different soil properties is verified. [ABSTRACT FROM AUTHOR]

Skirted spudcan foundations can effectively provide a considerable bearing capacity and moment fixity to withstand the complex combined vertical, horizontal, and moment loadings encountered in offshore engineering practices. The failure envelope is a general solution that characterizes the bearing capacities of offshore foundations under combined loading conditions. In this study, a series of probe tests were carried out using finite element analysis to investigate the undrained failure envelope of skirted spudcan foundations in soft clay. Particular attention was paid to exploring the influences of the soil strength heterogeneity, the footing–soil interface roughness, and the embedment depth ratio. The analysis results indicate that the envelope's size is influenced by these conditions to varying degrees, while the envelope's shape is only slightly influenced. The failure envelope associated with smooth interface conditions provides conservative results, particularly for the deeply embedded scenarios. A closed-form algebraic expression was derived to describe the failure envelope of skirted spudcan foundations, which takes proper account of the soil strength heterogeneity, footing–soil interface roughness, and embedment depth ratio. [ABSTRACT FROM AUTHOR]

The mobile jack-up drilling rigs supported by individual foundations are gradually extended to deep water for drilling activities in the offshore industry. These individual foundations are expected to provide considerable bearing capacity and moment fixity to ensure the entire system safety under the combined vertical, horizontal and moment loadings. Spudcan fitted with skirt at deep embedment may provide additional capacity to meet these demands. This paper carries out a series of probe tests using finite element analysis to investigate the potential benefits of deeply embedded skirted spudcan foundations under combined loading in soft clay. Four discrete skirt aspect ratios are considered to discuss the effects of the skirt length on the various bearing capacities. The failure envelopes for each of them are presented to identify the potential benefits. The results show that both the size and the shape of the failure envelopes changed greatly with increasing skirt aspect ratio. A modified analytical expression related to the skirt aspect ratio is proposed to interpret and predict the behavior of deeply embedded skirted spudcan foundations. • The potential benefits of deeply embedded skirted spudcan foundations under combined loading in soft clay were investigated. • The effects of the skirt length on the various bearing capacities were discussed. • The size and the shape of the failure envelopes changed greatly with increasing skirt aspect ratio. • A modified analytical expression related to the skirt aspect ratio is proposed. [ABSTRACT FROM AUTHOR]

Abstract Strike-slip fault zones are an important type of structure formed during continental orogenic events. Exploring the influence of strike-slip faults on the distribution of natural gas by using the 3D finite element method (FEM) is a frontier in the field of geoscience. In this paper, the effects of strike-slip faults on the heterogeneity of in situ stress and reservoir quality were systematically studied in a shallow commercial coalbed methane reservoir in the Shanxi Formation in the southern Qinshui Basin. Systematic 3D FEM modeling and in situ stress evaluation of a highly developed strike-slip fault zone were conducted. The results indicate that the simulated in situ stress distributions of σ H , σ h and σ v in the target layer present ranges of 10–55 MPa, 3–23 MPa and 5–30 MPa, respectively, which are consistent with acoustic emission and hydraulic fracturing measurements. Along the strike-slip faults, the distribution of horizontal in situ stress is mainly affected by the fault length and vertical throw. The stress magnitude differs greatly between the two sides of a given strike-slip fault, affecting the compaction degree and petrophysical properties of the gas-bearing reservoir on both sides of the fault. Along the strike of a given strike-slip fault, the minimum horizontal in situ stress exhibits distinct segmentation characteristics, which affect the distribution of wells with a higher and lower yields. For the 2 s-order faults (the Sitou and Houchengyao faults), wells with a higher gas production capacity are typically located on the northwestern side of the faults. In the middle and southern sections of the Sitou-Houchengyao strike-slip fault zone, both highly concentrated faults and large-scale opening faults (basement and surface faults) produce greater stress concentrations, and the gas production capacity of the gas wells in these areas is poor. When the main strike direction of the faults is consistent with the loading direction of the boundary stress, the stress concentration is relatively low; whereas, the stress concentration increases as the angle between the main fault strike and the loading direction increases. The shear stress of the target layer is characterized by clockwise rotation, which is an important control on the stress heterogeneity of the strike-slip faults in the study area. Highlights • 3D FEM modeling of a highly developed strike-slip fault zone. • Effects of strike-slip faults on stress heterogeneity. • Influence of strike-slip faults on the distribution of natural gas. [ABSTRACT FROM AUTHOR]

Abstract The tight gas sandstone reservoirs in the Paleozoic of the Sulige gas field in China are highly heterogeneous, and fractures are key factors for stable reservoir production. Low-amplitude folds or nose-like structures are widely developed in the Upper Paleozoic strata in this area. To effectively predict gas well productivity, in this paper, a 3D FEM numerical simulation based on the deformation and energy variation of the rock mass was used to predict the "sweet spots" of gas well productivity in a tight gas sandstone reservoir using the He8 segment of the Middle Permian Xiashihezi Formation in the Central Sulige block as an example. The paleotectonic stress field of the study area during the maximum episode of compression in the Yanshanian movement was restored, and the two rupture parameters of the integrated rupture rate (I F) and strain energy density (U) were constructed. The strain energy density distribution has a high correlation with gas well productivity, indicating that it can better predict the rock rupture degree in low-amplitude tectonic zones. A complex relationship exists between the strain energy density distribution and low-amplitude folds. The high strain energy density zones are mainly distributed among the high positions and wing areas of the low-amplitude fold zone, but the top area of the low-amplitude fold does not necessarily have a high strain energy density. Portions of the high strain energy density zones are located in the gentle tectonic zone, located near but outside the low-amplitude fold zone. The strain energy in these gentle tectonic zones with a high strain energy density value is relatively high, and the rock mass is prone to rupture. This study is of great value in enriching the prediction of "sweet spots" in tight gas sandstone reservoirs in low-amplitude tectonic zones worldwide. Highlights • Strain energy density distribution in low-amplitude tectonic zone. • 3D FEM simulation study. • Effect of strain energy distribution on gas well productivity. [ABSTRACT FROM AUTHOR]

Skirted spudcan foundations can provide a considerable bearing capacity and moment fixity for resisting the complex combined vertical, horizontal, and moment loadings arising from the harsh environment in the offshore oil and gas industry. These potential benefits are conducive to the extension of mobile jack-up drilling rigs into deep water. In this study, a series of probe tests were carried out using finite element analysis to investigate the effects of the skirt length and embedment depth on the bearing capacity of skirted spudcan foundations in soft clay under combined loading. The results indicate that both the skirt length and the embedment depth can significantly affect the behavior of a skirted spudcan and apparently improve the various bearing capacities under combined loading. As the skirt length and embedment depth increase, the failure envelope's size gradually expands and the failure envelope's shape changes accordingly. A spudcan with a skirt length of 0.25 D embedded in about 2 D of sediments is suggested to obtain an optimal application in practice. A closed-form algebraic expression was derived to interpret and predict the behavior of skirted spudcan foundations, which properly considers the effects of the skirt length and embedment depth. • The undrained bearing capacities of embedded skirted spudcan foundations under combined loading in soft clay were investigated. • The effects of the skirt length and the embedment depth on the various bearing capacities were discussed. • A spudcan with skirt length of 0.25 D embedded at a depth of about 2 D was suggested to obtain optimal application in practice. • A closed-form algebraic analytical expression was proposed, which can describe the capacity envelope under combined loading and it is applicable to each skirt length and embedded depth. [ABSTRACT FROM AUTHOR]

• The elastic stiffness coefficients for a skirted spudcan geometry footing in soft clay were investigated. • The skirted spudcan footing can provide higher elastic stiffness coefficients than the circular plate or spudcan footings. • The effects of the soil shear strength, embedment depth, soil backflow, and skirt length conditions on the elastic stiffness coefficients were discussed. • An approximation expression with three special variables is proposed for predicting the corresponding elastic stiffness coefficients. Skirted spudcan foundations can potentially provide considerable bearing capacity and moment fixity that can withstand the complex combined vertical, horizontal, and moment loadings arising from the harsh environments encountered in the offshore industry. The analysis of a skirted spudcan foundation's stiffness should be given special consideration during the preliminary stage of the design process. This technical note carries out a series of probe tests using finite element analysis to investigate the elastic stiffness coefficients of a skirted spudcan foundation in soft clay. The elastic stiffness coefficients of skirted spudcan footings with different soil shear strengths, embedment depths, and skirt lengths are presented. The results indicate that skirted spudcans can provide higher elastic stiffness coefficients, and the elastic stiffness coefficients are significantly influenced by the soil shear strength, embedment depth, soil backflow, and skirt length. An approximation expression with three special variables is developed for predicting the elastic stiffness coefficients of a foundation, except for the footing on the surface. This may potentially complement the elastic stiffness coefficients in the existing jack-up assessment guidelines, which do not account for skirted spudcan shaped footings embedded at different depths. [ABSTRACT FROM AUTHOR]

The core factors in the study of fractures within tight sandstone reservoirs are tectonics, geomechanics and fluid mobility, which are not independent but are coupled to each other. In this paper, taking the upper Paleozoic gas reservoir within the low-amplitude tectonic zone in the Sulige gas field of the Ordos Basin as an example, the developmental characteristics and geomechanical model of fractures and their relationships with the productivity of tight gas sandstone reservoirs were systematically studied. The results show that this low-amplitude structural zone develops mainly low-order ruptures. The paleotectonic stress field of the target layer in the Yanshanian period was restored using a 3D finite element method. The horizontal principal stresses exhibited stress diffusion and were affected by local low-amplitude structures. There was a positive correlation between σ H and σ h , and σ H / σ h was distributed primarily between 1.35 and 1.95 with an average of 1.62. The strain energy density (U) was calculated to quantitatively characterize the microfracture distribution of the target layer based on the principle of energy conservation. There are certain positive correlations between U and the horizontal principal stresses. The geostresses of high- U regions underwent a certain degree of stress torsion, which manifested as a stress deflection reaching up to 30° during stress propagation. Finally, we proposed the concept of U -value geomechanical sweet spots in a tight gas sandstone. Based on this criterion, commercial gas wells can be effectively identified. • Geomechanical simulation of low-order ruptures. • Fracture development characteristics in a low-amplitude structural region. • Concept of U -value geomechanical sweet spots of tight gas sandstone. [ABSTRACT FROM AUTHOR]

The spatial variability of soil has long been recognised, yet there is a dearth of research on the response of piles under lateral loading that considers this variability, especially for the lateral load transfer (P-y) curve in spatially variable clay. To address this gap, the study presents a three-dimensional random finite element analysis utilising a non-stationary random field to model soil variability with a linearly increasing trend. The results reveal that the spatial variability significantly affects the lateral capacity of the pile, emphasising the importance of accounting for this variability in pile design. Additionally, the study finds that while the spatial variability affects the failure mechanism, the mechanism remains comparable to that observed in deterministic analyses. The spatial variability has a substantial effect on ultimate soil resistance pressure, which can be fitted using a normal probability distribution function. An adjust factor is suggested to calculate the characteristic value, and from this characteristic value, the fractiles of ultimate soil resistance pressure can be calculated. This study provides an evaluation method for the spatial variability effect of soil, facilitating a cost-effective design of pile foundation. • The study explores the net soil pressure, failure mechanism and lateral load transfer (P-y) curves at varing depths in spatially variable soil. • The research discusses the impact of inherent variation and autocorrelation of soil on the response of lateral loaded piles. • A framework is proposed to integrate the effects of spatial variability into the current engineering guidelines. • An illustrative example is provided to demonstrate the practical significance of the research outcomes. [ABSTRACT FROM AUTHOR]

Reservoirs where tectonic fractures significantly impact fluid flow are widespread. Industrial-level shale gas production has been established from the Lower Cambrian Niutitang Formation in the Cen'gong block, South China; the practice of exploration and development of shale gas in the Cen'gong block shows that the abundance of gas in different layers and wells is closely related to the degree of development of fractures. In this study, the data obtained from outcrop, cores, and logs were used to determine the developmental characteristics of such tectonic fractures. By doing an analysis of structural evolution, acoustic emission, burial history, logging evaluation, seismic inversion, and rock mechanics tests, 3-D heterogeneous geomechanical models were established by using a finite element method (FEM) stress analysis approach to simulate paleotectonic stress fields during the Late Hercynian—Early Indo-Chinese and Middle-Late Yanshanian periods. The effects of faulting, folding, and variations of mechanical parameters on the development of fractures could then be identified. A fracture density calculation model was established to determine the quantitative development of fractures in different stages and layers. Favorable areas for shale gas exploration were determined by examining the relationship between fracture density and gas content of three wells. The simulation results indicate the magnitude of minimum principal stress during the Late Hercynian — Early Indo-Chinese period within the Cen'gong block is −100 ∼ −110 MPa with a direction of SE-NW (140°–320°), and the magnitude of the maximum principal stress during the Middle-Late Yanshanian period within the Cen'gong block is 150–170 MPa with a direction of NNW-SSE (345°–165°). During the Late Hercynian — Early Indo-Chinese period, the mechanical parameters and faults play an important role in the development of fractures, and fractures at the downthrown side of the fault are more developed than those at the uplifted side; folding plays an important role in the development of fractures in the Middle-Late Yanshanian period, and faulting is a secondary control. This 3-D heterogeneous geomechanical modelling method and fracture density calculation modelling are not only significant for prediction of shale fractures in complex structural areas, but also have a practical significance for the prediction of other reservoir fractures. [ABSTRACT FROM AUTHOR]

Understanding the rock mass deformation and strain state is critical to a range of endeavors. Geomechanical modeling is an excellent approach simulating the formation processes of faulting and folding, which can reproduce the geometry of fold structures and track strain and stress through the whole deformation process. In this study, five series of two-dimensional (2D) elastic-plastic finite element models were built to investigate the influences of shortening distance, ramp cutoff angle, interlayer slip, fault friction, fault number and fault slice width on structural styles and strain distribution patterns during fault-propagation folding in the Kuqa Depression of Tarim Basin. The results indicate that, i) All above mentioned factors can influence the structural style and strain distribution of fault-propagation fold (FPF) to varying degrees. Fault friction, interlayer slip and fault slice width are the most important parameters influencing the deformation geometry and strain patterns of FPF; ii) There are critical values for fault friction coefficient and shortening distance to create a secondary fold besides the FPF; iii) A "step pattern" occurs both in the forelimb and backlimb of the FPF when fault slice width reaches a certain high value; iv) Structural position in the FPF controls volumetric changes. The evolution of FPF can be divided into two stages: layer parallel shortening and volume loss, and folding; v) The steep forelimb region of FPF develops more natural fractures, which may serve as a favorable location for potential reservoirs in the Kuqa Depression. The results are also expected to provide clues for the exploration and development of oil and gas in regions with similar geological conditions. [ABSTRACT FROM AUTHOR]