During the high-rate， high-frequency injection-production of salt-cavern compressed air energy storage， gas carries solid particles upward， causing severe erosion to core surface equipment. To minimize the erosion caused by large particles， it is essential to accurately predict the maximum cuttings-carrying diameter during gas production in salt-cavern compressed air energy storage. Based on the dynamics of gas-borne cuttings carrying， a model integrating the incipient-motion theory and the suspension theory was established to predict the maximum cuttings-carrying diameter during gas production of salt-cavern energy storage wells， and it was verified with field data. Through theoretical derivation and multi-factor coupling analysis， the effects of the cuttings density and shape， storage medium， gas production rate， and the distance between the lower end of the injection-production tubing and the residue surface at the cavern bottom （the lower-end distance in brief） on the cuttings-carrying diameter were systematically investigated， and a shape-coefficient table for irregular particles together with a quantitative predictive fitting formula was established. Field verification shows that the model is stable and accurate， with an accuracy of 92.56%. It is indicated that the cuttings density is negatively correlated with the maximum cuttings-carrying diameter. The gas production rate is approximately exponentially and positively correlated with the cuttings-carrying diameter. The lower-end distance， by affecting the gas velocity at the tubing outlet， is positively correlated with the cuttings-carrying diameter. The relative molecular mass and viscosity of the storage medium alter the cuttings-carrying capacity through the fluid drag force. The shape coefficient of irregular cuttings significantly affects the cuttings-carrying diameter. The proposed model can accurately predict the size of produced solid particles and provides theoretical support for the design of injection-production schemes， the selection of surface equipment， and the optimization of operating plans for salt-cavern compressed air energy storage.
Underground gas storage is a major component of compressed air energy storage power plants， and studying the factors affecting the stability of the surrounding rock of caverns in faulted rock masses is of great significance for the stability evaluation of underground structures and for cavern design and operation. However， existing studies mostly focus on a single surrounding rock type， a single operating condition or a single fault parameter， and rely mainly on finite element or empirical models. Based on the finite difference method and combined with structural-plane surveys and in-situ stress measurements， a realistic numerical model was established by incorporating different lateral pressure coefficients， surrounding rock classes and fault locations. The effects of the fault location and distance on the displacement， plastic zone and failure mode of the surrounding rock under different in-situ stress coefficients and rock classes were analyzed， and the effects of joints and faults on cavern stability under operating and seismic conditions were further investigated. The results show that the fault location has the most pronounced effect on the adjacent cavern wall， with the amplification effect being greatest for vertical faults—the maximum displacement around the cavern reaches 2.0864 cm—whereas inclined faults have a relatively small effect and the deformation is not very sensitive to the dip angle. The maximum stress concentration factor varies nonlinearly with the gas storage pressure and is significantly affected by the fault type. The vertical fault model shows the largest reduction in the maximum stress concentration factor， reflecting its strongest disturbance to the stress distribution. Under seismic action， the heterogeneity and slip characteristics of faults significantly amplify the dynamic response of the surrounding rock. The maximum z-direction displacement of the vertical fault model reaches 4.96 cm， and the responses of the inclined fault models are relatively moderate but still 3.4%-5.1% higher than those of intact rock. The research findings are referential for the engineering design of underground gas storage.
After solution mining of salt cavern， the sediment space， formed by the settling of interlayers and insolubles， will cause serious economic losses and resource waste if it cannot be used for energy storage. Although twin-well salt-cavern storage can increase the gas storage capacity， utilization of the sediment space is still at the experimental stage， and its mechanism， injection-production behavior and key influencing factors remain unclear. Accordingly， based on the formation characteristics of the A twin-well salt-cavern storage in China， the structural features of the rock-salt beds and the properties and volume fraction of the sediment space were analyzed. On this basis， a multiphase-flow model of gas injection and brine drainage in twin-well salt-cavern storage was built with computational fluid dynamics software， and the effects of the gas injection rate， permeability and connecting-channel angle on gas injection and brine drainage were investigated. The results show that the sediment space accounts for 55.52%-71.77% of the A twin-well salt cavern and that the gas-storable space in the sediment can reach 10.25%-26.50%， indicating a prominent storage value. Porosity is positively correlated with permeability， and the volume-fraction equation can characterize the geological features. During gas injection and brine drainage， gas floats upward to displace brine； the brine-displacement rate drops sharply after reaching its peak， and the cumulative displaced volume increases rapidly at first and then levels off. A higher gas injection rate corresponds to a higher brine-displacement rate， but the injection rate affects the cumulative displaced volume by only 3.56%. Compared with a 0° channel， a 20° connecting channel prolongs the gas-production time by 550 s and increases the displaced brine volume by 43.91%， so an inclined connecting channel is recommended. Verification by a 12.36×104 m3 compressed air energy storage project in Hubei Province shows that a high-injection， low-displacement mode of 100 m3/h has achieved a displaced brine volume of 40×104 m3， confirming that the sediment space can be reused. The research findings provide a theoretical basis for optimizing injection-production engineering in twin-well salt-cavern storage.
The power generation of a well-site photovoltaic （PV） power station is closely related to its revenue， and its generating capacity depends on many factors， such as the station design， module performance and system condition. A scientific evaluation of PV station efficiency is therefore essential for station development. For the first batch of PV power stations built in the Changqing Oilfield， a distributed PV performance test scheme was designed. The generation efficiency of the distributed PV stations was comprehensively evaluated from the series-parallel mismatch loss， AC/DC line losses， inverter efficiency， system efficiency and infrared thermal imaging scans of the arrays， and optimization suggestions were proposed accordingly. The results show that the series-parallel mismatch loss of the target stations is small， and the strings with relatively large mismatch loss exhibit clear differences in electrical performance. The AC and DC line losses are both within the standard limits and have little influence on the system efficiency. The PV system efficiency first increases and then decreases over time； inverter over-sizing loss and shading are the main causes of system-efficiency loss， and higher system efficiency is observed at low irradiance. The well-site environment increases the number of hot spots in PV generation， and shortening the grid-connection distance effectively reduces the AC/DC line losses. The research findings provide a reference for the large-scale deployment of intelligent intermittent distributed PV generation at well sites.
By switching the wellhead drilling fluid circulation path， a continuous circulation drilling system can effectively eliminate the circulation interruptions and bottomhole pressure fluctuations caused by frequent pump starts and stops when making or breaking connections， thereby significantly improving drilling safety. The pressure loss characteristics of the main and side circulation lines of the control manifold directly determine the amplitude and duration of the pressure fluctuations during path switching， as well as the total circulation pressure loss. To address the shortcomings of existing continuous circulation system manifolds， including too many elbows and transition sections， undersized drift diameter， and excessive footprint， three principles for structural optimization were proposed： fewer abrupt changes in the flow path， better valve selection， and optimized spatial layout. Accordingly， a novel three-dimensional continuous circulation manifold suitable for offshore platforms was designed. A flow-field pressure loss analysis model of the novel manifold was built and used， together with laboratory tests， to reveal the pressure loss control performance and influencing factors of the novel manifold. The flow-field and transient analyses of the main and side circulation paths of the novel manifold at a flow rate of 2 500 L/min indicate that the large-radius bends， optimized throttle valve and enlarged drift diameter greatly reduce the local vortex intensity， effectively suppress fluid disturbance and pressure fluctuation in the manifold， and reduce the local flow resistance， thereby lowering the flow pressure loss. Compared with the manifold of the existing CCDS-2 continuous circulation system， the main and side circulation paths of the novel manifold demonstrate pressure losses reduced by 64.7% and 55.8% at 2 000 L/min and by 75.5% and 66.4% at 3 500 L/min， respectively. These results meet the dual requirements of offshore continuous circulation operations for compact spatial layout and low frictional pressure loss. The study provides theoretical support for the miniaturized integrated design and pressure loss control of continuous circulation equipment.
Nitrogen， featuring low cost， high safety， and green， pollution-free properties， has been widely used as a dissolution inhibitor in salt-cavern solution mining. However， the calculated nitrogen injection volumes and pressures at stages of dissolution deviate considerably from field values， and the supplementary nitrogen injection volume strongly affects the stability of the gas-liquid interface， which in turn causes marked changes in dissolution time， performance and cost. To address this problem， an accurate prediction model for nitrogen injection volume and pressure throughout the entire nitrogen-blanket solution mining process in salt caverns was established by considering the gas-liquid interface control method and the dissolution characteristics of nitrogen. The model was verified against field data from the nitrogen-blanket solution mining of two wells （A and B） at the Pingdingshan salt-cavern gas storage in Henan， China. The results show that the model accounts for nitrogen re-injection and retention in the gas-supplementing， gas-withdrawing and dissolving stages. A nitrogen injection volume model was built by incorporating the casing program， openhole structure， direct and reverse circulation conditions， and brine dissolution. A nitrogen injection pressure model was constructed by combining the U-tube theory， the wellbore flow-friction equation， and the brine-discharge wellhead pressure. Both models yield an accuracy exceeding 90%. The research findings provide a theoretical basis for calculating the nitrogen injection volume and pressure throughout nitrogen-blanket solution mining in salt caverns and are of great significance for promoting the low-carbon application of clean energy storage technologies.
Guided by the goals of “carbon peaking and carbon neutrality”， green and low-carbon upgrading of the oil and gas industry has become an inevitable trend， and the traditional single diesel-generator power-supply mode of drilling rigs can no longer meet the technical and environmental requirements under this transformation. Currently， the core power-supply equipment of land drilling rigs is dominated by diesel generator sets. To accommodate the drastic load fluctuations of rigs， the sets must be sized for the maximum load， so the generators operate at low load for long periods with low fuel efficiency. Moreover， the slow dynamic response of diesel generators cannot follow load changes in time， readily causing delayed power supply and aggravated equipment wear and thus limiting drilling efficiency. As a core technical route for power-supply optimization， drilling-rig energy storage systems have been preliminarily explored， but existing schemes have obvious limitations. For instance， conventional PQ and VF control modes cannot coexist， making it difficult to meet both peak-shaving and high-frequency fluctuation-smoothing demands； the hybrid “supercapacitor + battery” architecture increases cost and maintenance difficulty； and a supercapacitor-only scheme has insufficient energy density. Accordingly， a multi-energy coordinated energy-storage control strategy was developed to achieve energy saving， consumption reduction and stable operation through coordinated operation of the energy storage system and the diesel generator sets. A multi-energy coordinated architecture centered on a lithium-battery energy storage system was proposed， a closed-loop energy management system was constructed， and a deeply coupled multi-objective optimization strategy of “high-speed load identification-PQ dual-function coordinated control-adaptive generator start-stop” was developed. Relying on a high-frequency load-power sampling module， the total load power is decoupled into low-and high-frequency components， with the energy storage system compensating for the insufficient dynamic response of the diesel generators. A closed-loop control model based on the load rate kept the generator load rate within the high-efficiency range of 60%-70%， and a multi-condition-triggered adaptive generator start-stop strategy balanced operating efficiency and power-supply reliability. Application results show that， by configuring the energy storage system， the specific fuel consumption of the generators drops to 272 g/（kW·h）， the average fuel-saving rate reaches 21%， and the total daily operating time of the generator sets is reduced to 24 h， effectively mitigating equipment wear. By breaking through the limitations of conventional technologies and simplifying the hardware architecture while balancing response speed and economy， the system can effectively smooth load fluctuations and optimize generator operating conditions， providing an effective technical solution for the green and digital upgrading of drilling-rig energy systems， with good prospects for engineering application.
Well type and borehole size of a salt-cavern gas storage directly affect its construction and operation efficiency， safety and economy. However， their applicability has not been analyzed systematically， insufficiently supporting future storage construction under complex conditions. In this study， the technical characteristics of three well types （vertical， directional and horizontal wells） were analyzed， and depending on the requirements for well trajectory and cavity-pillar designs， the feasible parameter limits of cluster wells were investigated. For conventional and large boreholes， the effects of storage depths and cavity volumes on drilling， dissolution and injection-production were examined. Finally， recommended applicability ranges of well types and borehole sizes for salt-cavern storage were proposed through multi-constraint demonstration. The results show that cluster wells can be considered when the bottomhole displacement of a directional well exceeds 120 m. The number of wells that can be placed on the same pad can be determined from the maximum bottomhole displacement at different storage depths： up to seven wells with conventional boreholes at a depth of 1，500 m， and seven wells with large boreholes at 2，000 m. At pump rates of 50， 100， 150 and 200 m3/h， the energy consumption of large boreholes is only 59.0%， 30.9%， 20.5% and 15.8% of that of conventional boreholes， respectively， and large boreholes also offer comprehensive advantages in broadening downhole-tool selection， shortening the construction period and increasing the injection-production capacity. Vertical wells， with mature technology and low cost， should follow the principle of “using vertical wells wherever possible” in salt-cavern storage construction. Directional wells can overcome surface-site constraints and support the intensive deployment of cluster wells. Horizontal wells are suitable for storage construction under complex conditions such as thin salt beds and low-grade salt； and large boreholes show broad prospects in medium-deep salt storage， although their adaptability requires further demonstration. The research findings provide theoretical basis and engineering reference for selecting well types and borehole sizes under different geological conditions.
Wedge throttle valve is a key component of choke manifold in modern drilling engineering. Its vibration characteristics are affected by the solid structure itself and also by the coupling of flow-field factors such as fluid pressure and flow rate. To investigate the vibration mechanism of wedge throttle valve systems in ultra-deep wells and identify potentially hazardous operating conditions， the modal characteristics and vibration stresses of the valve system under flow-field action were analyzed by fluid-solid coupling simulation. Three-dimensional simulation models of the fluid domain and the components of the throttle valve system were established and verified. The modal frequencies， mode shapes and flow-induced vibration stresses of the system under different openings were obtained， together with their variation with pressure and flow rate. The simulation results show that the operating conditions and valve opening have little influence on the natural frequencies of the throttle valve， theoretically ruling out the possibility of resonance. However， flow-induced vibration analysis indicates that the vibration stress of the valve stem at a small opening （10 mm） varies significantly with operating conditions and reaches high levels under some conditions， showing that a risk of vibration-induced fracture still exists under specific operating conditions. Although the overall resonance risk of the structure is low， local regions remain sensitive to vibration-induced fracture， and the identification and control of these high-risk conditions should be strengthened in design and operation. These results provide a theoretical basis for understanding the vibration characteristics of wedge throttle valves and an important reference for the early warning of vibration faults and for structural optimization in engineering practice.
In 10，000-m ultra-deep wells， fatigue cracks may readily initiate and cause fractures at drillpipe threads owing to stress concentration under the extreme alternating tensile-compressive-bending-torsional loads. To address this severe problem， a metal magnetic memory （MMM） testing device capable of adaptive scanning of the thread conical surface was developed. The device enables rapid and accurate damage inspection and early stress concentration assessment for internal and external threads of various drillpipes commonly used in oilfields. Its key technology lies in a self-centering， diameter-adjustable mechanical structure. Specifically， the external-thread unit is equipped with an expansion mechanism and a double-taper diameter-adjusting turntable. It is inserted into the drillpipe bore and expanded to realize rapid coaxial centering， and allows the sensor array to fit the thread conical surface precisely by rotating the turntable to adjust the inspection radius in a stepless manner. The internal-thread unit adopts a taper-matched positioning cone structure and achieves centering through its natural taper fit with the thread. The software integrates a Gaussian filtering module （for denoising） and a gradient analysis module， converting raw magnetic-field fluctuations into intuitive， interpretable defect-indication information. To verify the device's performance and reliability， systematic tests were performed on defective external-thread specimens and defect-free internal-thread specimens under controlled laboratory conditions. The results show that the device can effectively and stably identify abnormal magnetic memory signals corresponding to typical defects such as cracks and galling. On the gradient-analysis map， defect locations appear as distinct local peak jumps with clear features， and the localization agrees well with the actual defect positions. In defect-free threaded sections， the magnetic memory signal exhibits regular， smooth periodic fluctuations corresponding to the thread geometry， without abnormal jumps. The research findings are of great engineering value and significance for the timely identification of early damage， the effective prevention of catastrophic fracture accidents， and the assurance of safe and efficient drilling operations.
Alternating loads induced by cyclic injection and production in gas-storage wells may cause bonding failure at the cement-sheath interface， which promotes the extension of the free deformation section of the casing and in turn leads to wellhead uplift. To overcome the limitation that existing models do not systematically quantify multi-factor coupling failure， a method was proposed for calculating the length of the casing free deformation section by integrating four factors： cement top， cementing quality， and axial and radial bonding failures at the cement-casing interface caused by changes in ambient temperature and pressure. Combined with a wellbore temperature-field prediction model and an elasto-plastic mechanical model of the casing-cement sheath-formation system， real-time dynamic prediction of wellhead uplift was achieved. Through full-scale interface bonding tests， a shear bonding strength of 0.97 MPa and a radial bonding strength of 0.09 MPa at the casing-cement interface were obtained， and the critical conditions under which the axial stress difference dominates interface shear failure during temperature loading while radial tensile stress causes interface debonding during temperature unloading were clarified. Field verification shows that the predicted wellhead uplift agrees with the measured value by 92%， indicating a significant improvement in the prediction accuracy under complex conditions. According to quantitative analysis， when the production fluctuation exceeds 500，000 m3/d， the risk of radial interface bonding failure increases by 60%； after more than 30 temperature fluctuations， the growth of the casing free deformation section slows down. This study provides a multi-physics coupling framework for wellbore-integrity evaluation of gas-storage wells， which is helpful for wellbore-integrity management and production scheme regulation.
Installing a baffle in a gate valve can effectively prevent fluids and solid particles （e.g. mud and sand） from entering the valve cavity. However， no methods are available to calculate the baffle thickness and clearance for existing baffled gate valves， resulting in extensive damage and scrapping in practical applications. Field statistics show that abnormal opening/closing torques account for 71.0% of all gate-valve failures， and intrusion of solid particles from the drilling fluid into the valve cavity is one of the main causes of such abnormalities. Accordingly， a fluid-solid coupling numerical model of a baffled gate valve was established to analyze the effects of the baffle clearance and gate opening on the sand-blocking performance. It is found that the maximum flow velocity in the flow passage increases with the baffle clearance-when the clearance is less than 3 mm， the growth rate of the maximum flow velocity is 4.5%； when the clearance exceeds 3 mm， the growth rate reaches 19.05% at most. A one-way coupling method was then used to analyze the effect of the baffle thickness on the maximum stress. It is indicated that the maximum stress is positively correlated with the clearance. When the clearance is no greater than 3 mm and the thickness no less than 9 mm， the maximum stress is 423 MPa， below the allowable stress of the baffle material （491.18 MPa）. With the clearance set to 1-2 mm and the thickness to about 10 mm， the baffle stress ranges from 280.12 MPa to 322.86 MPa and the deformation from 0.69 mm to 0.96 mm. Comparative tests between conventional and extended valve seats show that the extended seat achieves a shorter and more stable full-opening time and better sand-control performance， verifying the rationality of the structural design. The research findings provide theoretical support for the structural optimization of low-torque gate valves and the development of high-grade gate valves for ultra-deep and extra-deep wells.
Coiled tubing is a rigid body. Its winding on a reel is far more complex than that of flexible bodies such as steel cables， and the winding force depends on the reel radius， the dimensions of the coiled tubing itself and the geometry of the transition zone from the straight state to the final bent state. Field empirical calculations of the winding force are inaccurate， which leads to an unreasonable selection of the reel-motor output torque and to fluctuations of the winding force. This in turn causes mechanical scratches on the coiled-tubing surface during winding， shortens its service life and compromises operational safety. To address this problem， the energy method was used to establish mechanical models of the winding force of coiled tubing on the reel under high and low tensions， yielding the relationship between the winding force and the reel radius and tubing dimensions. Assuming a linear variation of the bending curvature in the winding transition zone， a mechanical model of the winding force as a function of the transition-zone length was established， fully accounting for the plastic behavior of the coiled tubing during winding and for the effect of the transition section from the straight segment to the reel. The relationship between the winding force and the reel-motor output torque required to drive the reel was also derived by the energy method. The results show that the actual field winding force is 20.8% greater than the minimum winding force， and the actual reel output torque is 63.4% greater than the minimum required torque. The maximum relative error between the theoretical and simulated bending moments in the transition zone occurs 0.8 m from the tangent point of the straight segment， with a maximum relative error of 12.1%. These results provide a theoretical basis for the selection of reel motors in the field and for the winding force required in numerical simulation and optimal design of coiled-tubing reels.
In China， bedded salt rocks contain numerous thin interlayers. Horizontal-well solution mining can significantly improve the construction efficiency of salt-cavern gas storage in such rock strata. To investigate the effect of the injection-production spacing—a key solution mining factor—on cavity formation， a test platform based on physical similarity was built. Cavity shape， cavity volume， dissolving rate and brine mass concentration were selected as evaluation indices to test the influence of different injection-production spacings on the morphology of horizontal cavities and to analyze the development of salt caverns under different spacings. The results show that， based on the geometric similarity ratio Kl = 2，000 （1 cm in the model corresponding to 20 m in the prototype）， the cavity volume increases and then decreases with increasing spacing， reaching a maximum at a spacing of 15 cm （300 m in the prototype）. The brine mass concentration increases with the spacing； however， if the spacing is too small the brine can hardly reach saturation， whereas if it is too large the brine readily becomes saturated and further dissolution becomes difficult. The cavity dissolution rate reaches its maximum at this spacing， and the final cavity is relatively regular and cylindrical at a spacing of 12.5 cm （250 m in the prototype）. A comprehensive analysis of the cavity volume， shape and brine mass concentration under multiple spacings indicates that the optimal spacing is 250-300 m. These results provide a basis for optimizing the cavity-shape design and process parameters of horizontal-well solution mining for salt-cavern gas storage.
In oil and gas drilling and production operations， flat gate valves frequently fail to open normally because of increased torque. This problem becomes more severe under the abnormally high pressures encountered when drilling 10，000-m ultra-deep wells. However， the mechanism underlying this torque increase remains unclear. Considering the effects of the unilateral ultra-high pressure and high/low-temperature environments faced by well-control manifolds on the wear of the plates and seats of flat gate valves， the friction and wear behaviors of the metal sealing surfaces of the plate and seat were investigated through experiments and simulations. The variation of the friction coefficient of the contact surface under different contact pressures and high/low-temperature conditions was examined. A quantitative relationship between the torque variation of a flat gate valve and its service life was established， and the mechanism of the increase in opening/closing torque under ultra-high pressure was revealed. The results show that， under three temperature conditions， the friction coefficient exhibits distinct staged characteristics as the number of friction cycles increases from 0 to 9，000. The friction coefficient is significantly negatively correlated with the contact pressure， that is， the friction coefficient tends to decrease gradually as the contact pressure increases. Over 0-9，000 reciprocating friction cycles， the friction coefficient increases to 2.16， 2.53 and 2.65 times its initial value under low-， normal- and high-temperature conditions， respectively， leading to a marked increase in the valve torque， which confirms that the change in the friction coefficient is a major cause of the torque variation. At a valve-cavity pressure of 105 MPa， excessive torque caused by high friction occurs after 500 friction cycles （250 opening-closing operations） under low-temperature conditions， 20 friction cycles （10 operations） under normal-temperature conditions， and 10 friction cycles （5 operations） under high-temperature conditions. The research findings provide guidance for the safe use of flat gate valves under ultra-high pressure conditions.
Check valve is one of the key sealing components in a surface manifold control system. The high differential pressure generated at the instant of its opening aggravates erosion and wear of the sealing surface， thereby leading to seal failure and channeling accidents. Based on a coupled computational method combining the k-ε turbulence model with a general erosion model， a three-dimensional fluid simulation model of the check valve was established to investigate its erosion resistance at an ultra-high pressure （175 MPa）. The flow-field characteristics of the check valve at different flow rates and the influence of the sealing-surface cone angle on the valve erosion rate were analyzed. The results show that as the sealing-surface cone angle increases， the erosion rate of the valve-core contact surface increases， the erosion rate of the valve-seat contact surface decreases and remains consistently much higher than that of the valve-core contact surface， and the overall erosion rate generally decreases. The minimum contact stress of the sealing surface decreases （to 158.39 MPa at 120°） with increasing cone angle， and both the average and minimum contact stresses decrease as power functions of the cone angle. Considering both the erosion rate and the sealing requirements， a cone angle of 87° is recommended for the best overall performance， which satisfies the sealing requirements while effectively reducing the erosion rate. The research findings provide a theoretical basis for the optimal design of the sealing-surface cone angle of check valves under ultra-high pressure conditions.
As a key connecting component in drilling engineering， drillpipe threaded connectors are subjected to complex stress for long periods and thus highly susceptible to fatigue damage caused by stress concentration， which seriously affects their service life and safety. Considering the complex structural characteristics of drillpipe threads， a damage evaluation method based on the metal magnetic memory （MMM） testing was proposed. A three-dimensional model of the in-service drillpipe threads for Well SDTK-1 was established， and the stress-concentration distribution was simulated by the finite element method. Through experimental testing， a damage evaluation workflow suitable for drillpipe threads was developed. This workflow， comprising key steps such as signal denoising， feature extraction， stress concentration identification， and crack determination， can effectively identify stress concentration and cracks in the threaded zone， and the deviation degree is introduced as a parameter to quantify the severity of the stress concentration. The results show that the proposed damage evaluation method based on MMM testing is suitable for engineering applications at drilling sites and provides a scientific and effective technical support for the reliability assessment of drillpipe threads.
With the development of offshore petroleum and new energy in China， large-diameter steel cylinder structures are increasingly applied in offshore engineering. In this context， it is necessary to effectively evaluate the stability of foundation trench slope before and after the vibratory penetration of steel cylinders. In this study， taking an actual engineering project in the Bohai Sea as an example， the strength reduction finite element method （SRFEM） and the limit equilibrium method （LEM） were employed to comparatively analyze the stability of foundation trench slope. The analysis was conducted by simulating two conditions （before and after the vibratory penetration of the steel cylinder） and considering wave loads with different return periods. Moreover， the wave-induced pressure acting on the soil was equivalently transformed and applied to the slope surface to reflect the influence of wave action on slope stability. The results show that， under the condition before the vibratory penetration of steel cylinder， the safety factors and potential slip surface locations obtained by the two methods are in good agreement， with differences below 4%， indicating that the foundation trench excavation scheme has sufficient safety margins. Under the condition after the vibratory penetration of steel cylinder， the safety factors obtained by SRFEM are slightly higher than those by LEM， with differences below 11%. LEM yields relatively conservative results， since it simplifies the operation process after the vibratory penetration. The wave loads with different return periods have generally limited impacts on slope stability. However， considering the dynamic nature of wave action， the spatial distribution of wave-induced pressure at different time points significantly affects the location and extent of potential slip surface. The research findings provide useful references for the design of foundation trench excavation in similar offshore steel cylinder protection structures.
Hydrothermal resources are often extracted using the coaxial heat exchange technology. Composite pipe is universally preferred as the core component of the coaxial heat exchanger. Thus， this pipe performance has a direct impact on the extraction efficiency. Currently， the thermal insulation performance of composite pipes for coaxial heat exchange in geothermal recovery is mostly investigated by field tests. This method can realistically represent the performance of composite pipes， but it is heavily energy-consuming in pumping for maintaining fluid circulation， in addition to several limitations such as high operation cost， high safety risk， and probability of medium leakage. In this study， a fiber-reinforced aerogel-filled composite pipe was designed for coaxial heat exchange in hydrothermal extraction. Moreover， to effectively evaluate its heat transfer performance and avoid uncontrollable risks in field tests， a low-cost， boundary-condition-controllable test system was established. By changing the working conditions such as heat-source fluid temperature （45-85 ℃） and circulation flow rate （9.5-15.5 m³/h）， key thermodynamic parameters （e.g. along-pipe temperature drop， heat loss rate， and inner/outer pipe wall temperature difference） of the composite pipe were determined， and the insulation performance of the composite pipe was evaluated indoor quantitatively. This study reveals the local thermal bridge effect at the metal joint of pipe string. It is found that the composite pipe， after equivalent correction for heat loss， achieves an average temperature drop of 0.41-2.03 ℃ and an average heat loss rate of 3.38%-14.69% under given conditions. Moreover， the heat loss along the pipe increases nonlinearly with the rising heat-source temperature and reduces with the increasing circulation flow rate. Contrast test proves that the composite pipe is much superior to the PERT-Ⅱ thermal pipe in comprehensive thermal insulation performance， and its temperature drop reaches 65.8% under typical conditions. This study provides experimental evidence for the structural design and practical application of composite pipes for coaxial heat exchange in geothermal extraction.
Due to multiple factors such as the complex and changing environment， uneven light conditions， and diverse drill-string shapes at rig sites， existing drill-string status recognition methods are insufficient in accuracy. Moreover， it is difficult to dynamically delineate hazardous areas during rig operations. To address these challenges and improve the safety control of rig operations， this study proposes a drill-string status recognition and hazardous area localization method based on an improved YOLOv8 model. For drill-string status recognition， a module of super token attention （STA） mechanism was introduced into the backbone network， making the model more capable of extracting key features of drill string in complex rig-site scenarios while effectively suppressing background interferences. In addition， the feature pyramid network architecture was optimized by improving the multi-scale feature fusion strategy， so that the model can more adequately capture the shallow texture details and deep semantic information to enable accurate recognition of various drill-string statuses （e.g. horizontal， inclined， and vertical）. For hazardous area localization， the typical processes of rig operations were considered， and the position and orientation features and motion trend of the drill sting in each operation stage were differentiated. Then， the current operation stage was determined from the recognition results of the improved YOLOv8 model， and a model of dynamic mapping for drill-string status vs. operation stage was constructed. The research and test results show that the improved YOLOv8 model achieves a recognition accuracy of 95.7% for drill-string status， with the mAP@0.5 increasing to 92.3% compared to the baseline model， and the accuracy of operation stage auto-classification exceeding 90%. This method enables high-precision real-time localization and dynamic delineation of hazardous areas， and can synchronously monitor the operational process while outputting a continuous risk assessment curve. The results provide intuitive and quantitative evidences for the site safety managers in decision-making.
