Coral reef limestone (CRL) constitutes a distinctive marine carbonate formation with complex mechanical properties. This study investigates the multiscale damage and fracture mechanisms of CRL through integrated experimental testing, digital core technology, and theoretical modelling. Two CRL types with contrasting mesostructures were characterized across three scales. Macroscopically, CRL-I and CRL-II exhibited mean compressive strengths of 8.46 and 5.17 MPa, respectively. Mesoscopically, CRL-I featured small-scale highly interconnected pores, whilst CRL-II developed larger stratified pores with diminished connectivity. Microscopically, both CRL matrices demonstrated remarkable similarity in mineral composition and mechanical properties. A novel voxel average-based digital core scaling methodology was developed to facilitate numerical simulation of cross-scale damage processes, revealing network-progressive failure in CRL-I versus directional-brittle failure in CRL-II. Furthermore, a damage statistical constitutive model based on digital core technology and mesoscopic homogenisation theory established quantitative relationships between microelement strength distribution and macroscopic mechanical behavior. These findings illuminate the fundamental mechanisms through which mesoscopic structure governs the macroscopic mechanical properties of CRL.
Obtaining high-quality 10000-meter-deep seafloor sediment samples is the prerequisite and foundation for conducting deep-sea geological and environmental scientific research. The bottom structure of the deep seafloor is complex, and the physical and mechanical properties and disturbance resistance of sediments of different lithologies vary greatly, so the sediment sampler inevitably disturbs the sediments during the sampling process and affects the quality of the sediment samples. A new type of deep-sea sediment pressure retaining sampler is introduced, the force state and elastic-plastic state of the sampler destroying sediments are analyzed, the radial disturbance model of sediment coring based on the spherical cavity expansion theory is established, and the radius of sediments undergoing plastic deformation around the spherical holes is used as an index for evaluating the radial disturbance of sediments. The distribution of stress and strain fields in the sediments during the expansion of the spherical cavity and the influencing factors of the radius of the radially disturbed region (plastic region) are analyzed using an arithmetic example, and the influence law is analyzed. A sediment disturbance experimental platform was built indoors to simulate the sediment coring process. The radial stress field and pore water pressure of the sediment during the coring process were monitored by sensors arranged inside the sediment, and the results of indoor tests verified the correctness of the perturbation theory model. The sampler was carried aboard the deep-sea manned submersible FENDOUZHE and conducted on-site tests at depths of 9298.4 and 9142.8 m in the Kuril-Kamchatka Trench. Pressure-preserved sediment samples were retrieved, with preservation rates of 94.21% and 92.02%, respectively, which are much higher than the current technical indicator of 80% of pressure-holding ratio for deep-sea sediments. The retrieved sediments have obvious stratification characteristics and little disturbance.
Deep-sea mineral resource transportation predominantly utilizes hydraulic pipeline methodology. Environmental factors induce vibrations in flexible pipelines, thereby affecting the internal flow characteristics. Therefore, real-time monitoring of solid-liquid two-phase flow in pipelines is crucial for system maintenance. This study develops an autoencoder-based deep learning framework to reconstruct three-dimensional solid-liquid two-phase flow within flexible vibrating pipelines utilizing sparse wall information from sensors. Within this framework, separate X-model and F-model with distinct hidden-layer structures are established to reconstruct the coordinates and flow field information on the computational domain grid of the pipeline under traveling wave vibration. Following hyperparameter optimization, the models achieved high reconstruction accuracy, demonstrating R2 values of 0.990 and 0.945, respectively. The models’ robustness is evaluated across three aspects: vibration parameters, physical fields, and vibration modes, demonstrating good reconstruction performance. Results concerning sensors show that 20 sensors (0.06% of total grids) achieve a balance between accuracy and cost, with superior accuracy obtained when arranged along the full length of the pipe compared to a dense arrangement at the front end. The models exhibited a signal-to-noise ratio tolerance of approximately 27 dB, with reconstruction accuracy being more affected by sensor failures at both ends of the pipeline.
As the main geomaterials for coral reefs oil or gas extraction and underground infrastructure construction, coral reef limestone demonstrates significantly distinct mechanical responses compared to terrigenous rocks. To investigate the mechanical behaviour of coral reef limestone under the coupling impact of size and strain rate, the uniaxial compression tests were conducted on reef limestone samples with length-to-diameter (L/D) ratio ranging from 0.5 to 2.0 at strain rate ranging from 10-5 ·s-1 to 10-2 ·s-1. It is revealed that the uniaxial compressive strength (UCS) and residual compressive strength (RCS) of coral reef limestone exhibits a decreasing trend with L/D ratio increasing. The dynamic increase factor (DIF) of UCS is linearly correlated with the logarithm of strain rate, while increasing the L/D ratio further enhances the DIF. The elastic modulus increases with strain rate or L/D ratio increasing, whereas the Poisson’s ratio approximates to a constant value of 0.24. The failure strain increases with strain rate increasing or L/D ratio decreasing, while the increase in L/D ratio will inhibit the enhancing effect of the strain rate. The high porosity and low mineral strength are the primary factors contributing to a high RCS of 16.7%-64.9% of UCS, a lower brittleness index and multiple irregular fracture planes. The failure pattern of coral reef limestone transits from the shear-dominated to the splitting-dominated failure with strain rate increasing or L/D ratio decreasing, which is mainly governed by the constrained zones induced by end friction and the strain rate-dependent crack propagation. Moreover, a predictive formula incorporating coupling effect of size and strain rate for the UCS of reef limestone was established and verified to effectively capture the trend of UCS.
This study proposes and systematically evaluates an optimized integration of warm surface seawater injection with depressurization for the long-term exploitation of marine natural gas hydrates. By employing comprehensive multiphysics simulations guided by field data from hydrate production tests in the South China Sea, we pinpoint key operational parameters-such as injection rates, depths, and timings-that notably enhance production efficiency. The results indicate that a 3-phase hydrate reservoir transitions from a free-gas-dominated production stage to a hydrate-decomposition-dominated stage. Moderate warm seawater injection supplies additional heat during the hydrate decomposition phase, thereby enhancing stable production; however, excessively high injection rates can impede the depressurization process. Only injection at an appropriate depth simultaneously balances thermal supplementation and the pressure gradient, leading to higher overall productivity. A “depressurization-driven sensible-heat supply window” is introduced, highlighting that timely seawater injection following initial depressurization prolongs reservoir dissociation dynamics. In this study area, commencing seawater injection at 170 d of depressurization proved optimal. This optimized integration leverages clean and renewable thermal energy, providing essential insights into thermal supplementation strategies with significant implications for sustainable, economically feasible, and efficient commercial-scale hydrate production.
Natural gas hydrate (NGH) has a bright future as a clean energy source with huge reserves. Coring is one of the most direct methods for NGH exploration and research. Preserving the in-situ properties of the core as much as possible during the coring process is crucial for the assessment of NGH resources. However, most existing NGH coring techniques cannot preserve the in-situ temperature of NGH, leading to distortion of the physical properties of the obtained core, which makes it difficult to effectively guide NGH exploration and development. To overcome this limitation, this study introduces an innovative active temperature-preserved coring method for NGH utilizing phase change materials (PCM). An active temperature-preserved corer (ATPC) is designed and developed, and an indoor experimental system is established to investigate the heat transfer during the coring process. Based on the experimental results under different environment temperatures, a heat transfer model for the entire ATPC coring process has been established. The indoor experimental results are consistent with the theoretical predictions of the heat transfer model, confirming its validity. This model has reconstructed the temperature changes of the NGH core during the coring process, demonstrating that compared to the traditional coring method with only passive temperature-preserved measures, ATPC can effectively reduce the core temperature by more than 5.25 °C. With ATPC, at environment temperatures of 15, 20, 25, and 30 °C, the duration of low-temperature state for the NGH core is 53.85, 32.87, 20.32, and 11.83 min, respectively. These findings provide new perspectives on temperature-preserving core sampling in NGH and provide technical support for exploration and development in NGH.
Due to complex geological structures and a narrow safe mud density window, offshore fractured formations frequently encounter severe lost circulation (LC) during drilling, significantly hindering oil and gas exploration and development. Predicting LC risks enables the targeted implementation of mitigation strategies, thereby reducing the frequency of such incidents. To address the limitations of existing 3D geomechanical modeling in predicting LC, such as arbitrary factor selection, subjective weight assignment, and the inability to achieve pre-drilling prediction along the entire well section, an improved prediction method is proposed. This method integrates multi-source data and incorporates three LC-related sensitivity factors: fracture characteristics, rock brittleness, and in-situ stress conditions. A quantitative risk assessment model for LC is developed by combining the subjective analytic hierarchy process with the objective entropy weight method (EWM) to assign weights. Subsequently, 3D geomechanical modeling is applied to identify regional risk zones, enabling digital visualization for pre-drilling risk prediction. The developed 3D LC risk prediction model was validated using actual LC incidents from drilled wells. Results were generally consistent with field-identified LC zones, with an average relative error of 19.08%, confirming its reliability. This method provides practical guidance for mitigating potential LC risks and optimizing drilling program designs in fractured formations.
The spatiotemporal characterization of plume sedimentation and microorganisms is critical for developing plume ecological monitoring model. To address the limitations of traditional methods in obtaining high-quality sediment, a novel sampling system with 6000 m operational capability and three-month endurance was developed. It is equipped with three sediment samplers, a set of formaldehyde preservation solution injection devices. The system is controlled by a low-power, timing-triggered controllers. To investigate low-disturbance rheological mechanisms, gap controlled rheological tests were conducted to optimize the structural design of the sampling and sealing assembly. Stress-controlled shear rheological tests were employed to investigate the mechanisms governing yield stress in sediments under varying temperature conditions and boundary roughness. Additionally, the coupled Eulerian-Lagrangian (CEL) method and sediment rheological constitutive models were employed to simulate tube-soil interaction dynamics and sediment disturbance. The radial heterogeneity of sediment disturbance and friction variation of the sampling tube were revealed. The tube was completely “plugged” at a penetration depth of 261 mm, providing critical data support to the penetration depth parameters. The deep-sea pressure test and South China Sea field trials demonstrated the system’s capability to collect and preserve quantitative time-series sediment samples with high fidelity.
Reef limestone is buried in the continental shelf and marine environment. Understanding the mechanisms governing filter cake formation in coral reef limestone strata is essential for various engineering activities in coastal areas, including slurry pressure balanced (SPB) shield tunneling, which are currently not well understood. This study systematically investigates the slurry infiltration characteristics of different coral reef limestone types with inherent anisotropy, identified by growth line orientations, through a series of micro-infiltration column tests. Multiple slurry concentrations and pressures were used to analyze their effects on slurry infiltration dynamics and filter cake formation. Pre- and post-infiltration CT scanning was conducted to examine skeletal morphology and reconstruct the pore network structure of coral reef limestone samples. The results show that while increased slurry concentrations and pressures generally improve filter cake formation, excessive pressure can compromise filter cake integrity. By employing Dijkstra’s algorithm in a pore network model, the study identified primary seepage pathways, highlighting the significant role of near-vertical throat clusters in the infiltration process. A comprehensive analysis of pore structure and connectivity indices before and after infiltration revealed that the orientation of growth lines in coral reef limestone is the primary factor influencing macroscopic slurry infiltration behavior. These findings offer valuable insights for the design and execution of tunneling projects through coral reef limestone formations, especially in coastal regions.
Retrogressive landslides in sensitive clays pose significant risks to nearby infrastructure, as natural toe erosion or localized disturbances can trigger progressive block failures. While prior studies have largely relied on two-dimensional (2D) large-deformation analyses, such models overlook key three-dimensional (3D) failure mechanisms and variability effects. This study develops a 3D probabilistic framework by integrating the Coupled Eulerian-Lagrangian (CEL) method with random field theory to simulate retrogressive landslides in spatially variable clay. Using Monte Carlo simulations, we compare 2D and 3D random large-deformation models to evaluate failure modes, runout distances, sliding velocities, and influence zones. The 3D analyses captured more complex failure modes-such as lateral retrogression and asynchronous block mobilization across slope width. Additionally, the 3D analyses predict longer mean runout distances (13.76 vs. 11.92 m), wider mean influence distance (11.35 vs. 8.73 m), and higher mean sliding velocities (4.66 vs. 3.94 m/s) than their 2D counterparts. Moreover, 3D models exhibit lower coefficients of variation (e.g., 0.10 for runout distance) due to spatial averaging across slope width. Probabilistic hazard assessment shows that 2D models significantly underpredict near-field failure probabilities (e.g., 48.8% vs. 89.9% at 12 m from the slope toe). These findings highlight the limitations of 2D analyses and the importance of multi-directional spatial variability for robust geohazard assessments. The proposed 3D framework enables more realistic prediction of landslide mobility and supports the design of safer, risk-informed infrastructure.
Low-angle submarine landslides pose a greater threat to offshore infrastructure compared to those with steep sliding angles. Understanding the preparation and triggering mechanism of these low-angle submarine landslides remains a significant challenge. This study focuses on a deformed low-angle submarine landslide in the shelf-slope break of the Pearl River Mouth Basin, South China Sea, integrating sedimentology, geophysics, and geotechnology to investigate potential failure mechanisms. The architecture and deformation characteristics of the submarine landslide were elucidated by analyzing multibeam and seismic data. Within the context of the regional geological history and tectonic framework, this study focuses on the factors (e.g., rapid sedimentation, fluid activity, and earthquakes) that potentially contributed to the submarine slope failure. Furthermore, a series of stability evaluations considering the effects of rapid sedimentation and earthquakes was conducted. Our findings indicate that the most probable triggering mechanism involves the combined effects of sedimentation controlled by sea-level fluctuations, high-pressure gas activity, and seismic events. The high-pressure gas, which acts as a long-term preconditioning factor by elevating pore pressures and reducing shear resistance within the sediment, accumulated beneath the upper and middle sections of the low-permeability stratum that was formed during sea-level rise and ultimately evolved into the sliding mass. The overpressure generated by gas accumulation predisposed the submarine slope to instability, and a frequent or moderate earthquake ultimately initiated local failure. This study enhances the mechanistic understanding of low-angle slope failures in the shelf-slope break zone and provides critical insights for assessing marine hazard risks.
Deep-sea mining has emerged as a critical solution to address global resource shortages; however, the mechanical interaction between tracked mining vehicles (TMVs) and soft seabed sediments presents fundamental engineering challenges. This study establishes a multiscale modelling framework coupling the discrete element method (DEM) with multi-body dynamics (MBD) to investigate track-seabed dynamic interactions across three operational modes: flat terrain, slope climbing, and ditch surmounting. The simulation framework, validated against laboratory experiments, systematically evaluates the influence of grouser geometry (involute, triangular, and pin-type) and traveling speed (0.2-1.0 m/s) on traction performance, slip rate, and ground pressure distribution. Results reveal rate-dependent traction mechanisms governed by soil microstructural responses: higher speeds enhance peak traction but exacerbate slip instability on complex terrain. Critical operational thresholds are established-0.7 m/s for flat terrain, ≤0.5 m/s for slopes and ditches-with distinct grouser optimization strategies: involute grousers achieve 35%-40% slip reduction on slopes through progressive soil engagement, while triangular grousers provide optimal impact resistance during ditch crossing with 30%-35% performance improvement. These findings provide quantitative design criteria and operational guidelines for optimizing TMV structural parameters and control strategies, offering a robust theoretical foundation for enhancing the performance, safety, and reliability of deep-sea mining equipment in complex submarine environments.