Due to the unique constraints of the offshore environment, there exist many challenges, including limited construction space, high investment costs, and demanding production requirements, which pose significant obstacles to large-scale offshore fracturing. Large fracturing vessels and supporting technologies can effectively enhance the productivity of low-quality reservoirs, and standardized and batch-operating models enable intensive industrial advantages, which can reduce operational costs, achieve cost-efficient offshore fracturing development characterized by "fewer wells with higher production". However, the corresponding fracturing operation modes and associated technical systems still need further research. Based on the fact that the large-scale fracturing vessel can meet the demands of large-scale and batch offshore fracturing operations in the future, four development directions of fracturing vessels are prospected. By systematically sorting out the main offshore operation facilities, a solution for the connection of fracturing tubing strings and corresponding wellheads based on four modes is proposed, namely "oil production platform + modular drilling rig or drilling ship, unmanned platform + drilling ship or workover ship, exploration well fracturing by using drilling ship and fracturing during jacket period". These solutions provide technical reference for the large-scale continuous operation of fracturing ships.

Multi stage limited-entry fracturing is one of the most effective methods to improve the production capacity of low-permeability reservoirs in the Bozhong 25-1 Oilfield. However, when reservoir characteristics and flow limiting fracturing process parameters jointly affect each other, the mechanism of multi cluster fracture propagation is still unclear. Based on a large-scale true triaxial fracturing simulation experimental platform, we conduct experimental research on the multi cluster fracture propagation law of artificial rocks with similar physical properties in the target layer. The influence of factors such as perforation cluster spacing, cluster number, and horizontal stress difference on the multi cluster fracture propagation state is analyzed. The results indicate that increasing the spacing between clusters has little effect on the initiation pressure of rocks, but it weakens the interference between cracks and promotes the uniform expansion of multiple clusters of cracks; the increase in the number of perforation clusters will weaken the resistance of rock plastic deformation to crack propagation and increase the scale of crack propagation; when the horizontal stress difference is less than 6 MPa, multiple clusters of cracks exhibit a staggered expansion of transverse and longitudinal cracks; the increase in fracturing fluid displacement and viscosity can increase the net pressure inside the fracture, and the morphology of multiple clusters of fractures becomes more complex. This study provides a reference for optimizing the parameters of flow limiting fracturing technology in low-permeability.

The Bohai Sea oilfield has abundant low-permeability reserves, which are important replacement resources for increasing reserves and production. Among them, the C Oilfield in the Bohai Sea has well-developed natural fractures in the buried hill reservoirs. Volume fracturing to create complex fracture networks is a key technical means for development. To address the issue that the influence law of single-layer proppant placement of secondary fractures on conductivity is still unclear during network fracturing, a prediction model for the conductivity of secondary fractures with multi-size distribution was established based on the plane contact theory. The correctness of the model was verified through model degradation and experimental analysis, and the influencing factors were analyzed. The research results show that increasing the proportion of large-sized proppants is beneficial to maintaining the width of secondary fractures; the permeability of secondary fractures is approximately linearly and positively correlated with the proportion of large-sized proppants; in the case of multi-size proppant combinations, as the proportion of large-sized proppants increases, the conductivity of secondary fractures improves, and large-sized proppants play a key role in conductivity. The research conclusions provide theoretical guidance for the selection of proppant sizes in the fracture network fracturing design process in the Bohai Sea.

The development of low-permeability reservoirs offshore faces significant challenges such as high costs, high risks, and high requirements. Large-scale fracturing technology based on fracturing vessels enables continuous and efficient reservoir stimulation, representing a key pathway for the economical and effective exploitation of offshore low-permeability oil and gas resources. Taking the "Haiyang Shiyou 696" fracturing vessel as an example, which enables continuous stimulation at a rate of 12 m$^{3}$/min and can increase fracture half-length by 1.5 times compared to conventional offshore fracturing, the recovery factor increased by approximately 5.0 percentage points. Through a comparative analysis of hydraulic jet fracturing, pumping bridge plug-perforation combined fracturing, and cemented sliding sleeve fracturing: it is concluded that the cemented sliding sleeve fracturing, with a non-operating time accounting for only 10%~15%, offers superior operational continuity and is best suited for the high-efficiency mode of fracturing vessels. Furthermore, key supporting technologies have been developed, including a seawater-based integrated variable-viscosity fracturing fluid system, continuous proppant supply, casing tie-back and downhole safety control. A multi-method fracture monitoring approach is also recommended to enhance post-fracturing evaluation accuracy. The research findings provide systematic technical support for the large-scale development of China$'$s offshore low-permeability oil and gas resources and the achievement of the "fewer wells and higher production" objective.

To meet the requirements for hydraulic fracturing of deep and ultra-deep offshore oil and gas wells while ensuring the thermal stability and proppant-carrying capacity of fracturing fluids, a high-temperature and high-salinity hydrophobic associative polymer, TH-200, was synthesized using acrylamide, acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and the hydrophobic monomer DM. The structure of polymer TH-200 was characterized by infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and thermogravimetric analysis, and its main performance as a fracturing fluid thickener was further evaluated. The results show that polymer TH-200 is a quaternary copolymer with excellent thermal stability. It exhibits a viscosity increase rate of 92 % in seawater, indicating good viscosity enhancement and salt resistance, and is therefore suitable for high-mineralization environments such as seawater. The critical intermolecular association mass fraction is approximately 0.25%. Molecular dynamics simulations revel that as the AMPS monomer concentration increases, the gyration radius of polymer segments significantly improves, with minimal influence from temperature and mineralization. 0.1% polymer solution prepared in seawater achieves a drag reduction rate of up to 78%. The seawater-based fracturing fluid formulated with 1.0 % polymer TH-200, 0.5% chelating regulator, and 1.0% organic zirconium cross linking agent maintains a viscosity of 110 mPa$\cdot$s after shearing at 200 ℃ and 100 s$^{-1}$ for 90 minutes. These results demonstrate favorable delayed cross linking performance, excellent thermal and shear resistance and outstanding proppant-carrying capabilities, meeting the demands for fracturing operations in offshore reservoirs at 200 ℃.

The thin interbedded reservoir with low permeability in Bohai Oilfield is characteristic of long span, developed bottom water and highly interbedded sand-mud layer. Large displacement cross-layer fracturing is an important means to develop long-span thin interbeds, but there is no related offshore practice and research. For Shahejie reservoir with low permeability and thin interbeded sandstone in Bohai Oilfield, the effect of perforation location and stage limit on fracture propagation at 5~12 m$^3$/min fracturing displacement rate is studied and unperforated height of thin bottom-water interbeded reservoir is determined by establishing a thin interbeded fracture propagation model. The results show that there is an optimal value of perforation cluster number for different displacement rates. When the perforation cluster number is less than this value, the fracture can penetrate the layer longitudinally, but the layer number of perforation transformation is limited. When the perforation cluster number is more than this value, the fracture is difficult to penetrate the layer and is restricted within the perforation layer. The 12 m$^3$/min single-stage fracturing pump rate combined with multi-cluster perforation can be used to fully stimulate the 50 m thin interbedded reservoir. For the long-span thin interbedded reservoir of more than 50 m, the multi-stage fracturing combined with large displacement rate is an effective means to fully stimulate the thin interbedded reservoir. With the increase of displacement rate, the unperforated height in the bottom water reservoir should increase. Under the fracturing displacement rate of 12 m$^3$/min, the unperforated height of the bottom water reservoir in Shahejie should exceed 9.5 m. The research results can provide a theoretical basis for the large displacement fracturing construction design of large-span thin bottom water interbeded reservoir in the Bohai Oilfield.

The Bohai Oilfield possesses abundant low-permeability reserves. However, their economic development has been constrained by offshore operational limitations that hinder the application of large-scale network fracturing techniques. With the recent deployment of large fracturing vessels, large-scale network fracturing is poised to become a key trend for developing offshore low-permeability reservoirs. The study focuses on a low-permeability reservoir in the Bohai Sea. We characterizes rough fracture surfaces by splitting and scanning core samples from the target layer, establish a complex fracture model based on the operational capacity of fracturing vessels, and carry out physical experiments and numerical simulation analysis of proppant transport. Key findings include: rough fracture walls increase transport resistance, making it necessary to use slug flow to polish fracture surfaces; the transport mechanism of sand-carrying slickwater follows a dynamic equilibrium under the influence of gravity settling and fluid drag. Therefore, to improve placement efficiency, the strategy should focus on enhancing the proppant$'$s ability to migrate into the deeper sections of the fracture; through optimized parameter design, slickwater injection at 12 m$^3$/min with a staged proppant blend 40/70 + 30/50 + 20/40 mesh can significantly enhance intra-fracture proppant placement. These results provide scientific guidance for fracturing parameter design and hold critical implications for the efficient development of offshore low-permeability oilfields.

In response to the challenges of achieving balanced stimulation in the thin-interbedded, low-permeability reservoirs with large span in the Bohai Sea, this study establishes mechanical models for the migration and sealing of diverting balls to investigate the influence of different diverting parameters on their migration and sealing behavior. Based on discrete element theory, the method for controlling fluid flow distribution at fracture openings during diverting fracturing was developed. A three-dimensional model for fracture propagation during diverting fracturing was established to study the effects of different ball injection coefficients, diverting timing, and diverting frequency on multiple fracture propagation. The research indicates that: 1) The optimal pump rate for efficient seating of diverting balls is 4 m$^3$/min. 2) Diverting balls with low density and fracturing fluids with high viscosity enhance the plugging effect; when the ratio of diverting ball diameter to perforation diameter exceeds the critical plugging value of 1.6, the plugging effectiveness deteriorates. 3) Under the model parameters, the optimal ball injection coefficient ranges from 1.8 to 2.0, with 1 to 2 diverting operations, and employing mid or early-stage diverting operations significantly improves the uniformity of fracture propagation. Based on theoretical and numerical simulation studies, the post-fracturing production of Well H-2 was increased threefold, achieving a productivity of 100 m$^3$/d. This research provides theoretical guidance and technical reference for optimizing the parameters of diverting processes in offshore thin-interbedded sandstone reservoirs.

Accurately evaluating the vibration characteristics of fracturing pump units plays a crucial role in enhancing the safety of offshore fracturing operations. This study established a refined numerical model of the fracturing cabin coupling system based on a vibration characteristic test on a single fracturing skid, and focused on simulating and analysing combined vibrations under multiple working conditions. Parameters considered include the number of activated fracturing pump, spatial layout, and load frequency, et. al. The research results indicate that the static response accounts for more than 80.0% of the total response, while the dynamic response contributes less than 20.0%. The vibration of the fracturing pump attenuates along the deck in a sinusoidal half-wave pattern, with a peak amplification effect of 2.0~2.5 times observed at the activated pump. As the number of activated fracturing pumps increases from 1 to 5, the maximum dynamic response generally shows an upward trend, with peak displacement increasing by 57.5%, peak acceleration by 6.5%, and peak stress by 5.1%. A five-level startup strategy for fracturing pumps was proposed, specifically starting from the edge toward the centre. When the bidirectional inclination angle increases from the baseline state (0°) to the critical threshold, the peak displacement increases by 190.0%, the peak acceleration increases by 141.0%, and the peak stresses on the three core components, i.e., deck, deck beam, and connection base, increase by 20.0%, 22.0%, and 21.0%, respectively. The failure mode of the coupling system manifests as the overall strength failure of the deck, with the failure morphology highly consistent with the modal shape.

The sand-mixing tank serves as the core equipment in fracturing operations, with its mixing efficiency determining the performance of fracturing fluids and thereby influencing the entire fracturing process. To address the unclear impact of ocean sloshing loads on the mixing process within the sand-mixing tank of fracturing ship, a solid-liquid two-phase mixing model for the fracturing ship's sand-mixing tank considering rolling excitation was established based on the CFD method, investigating the effects of different rolling amplitudes and periods on its mixing characteristics. The results reveal that rolling excitation primarily affects the velocity and distribution of the particle phase while having minimal impact on the overall flow field of the mixed phase in the tank, with the particle phase exhibiting a movement trend consistent with the rolling direction. The distribution of the particle phase is influenced by the tank's position: in the left-leaning state, the volume fraction of the particle phase on the left side is significantly higher than that without shaking, while the right side shows a significant decrease, and the opposite occurs in the right-leaning state. Roll excitation can accelerate the mixing process. As the roll amplitude increases or the period decreases, the difference in the volume fraction of the particle phase between the left and right sides of the tank increases, and the tendency of the particle phase to settle becomes more obvious. Consequently, the average density of the mixed phase at the outlet also increases, while the density uniformity index at the outlet shows a decreasing trend. The uniformity index is lower than that under the non-sloshing condition when $T$=8 s and $A$>6°, or when $A$=6° and $T$<8 s. The research results can lay a theoretical foundation for exploring the influence of sloshing loads on mixing tanks.

The high-pressure manifold of the offshore fracturing vessel provides a high-pressure passage for the fracturing fluid. Currently, the research focuses on structural optimization, erosion and wear, vibration, non-destructive testing and can$'$t quantitatively evaluate the blasting energy and impact characteristics generated after blasting failure. This paper quantitatively characterizes the blasting energy of high-pressure pipelines, and finds a calculation method for the burst energy and equivalent TNT equivalent of high-pressure manifolds; the main hazards of blasting are shock waves, and the blasting area is divided into four areas: extremely severe damage area according to the degree of human injury and object damage, severe damage area, moderate damage area, and minor damage area according to the degree of human injury and object damage. The simulation analysis revealed the distribution patterns of maximum deformation, maximum stress amplitude, and maximum pressure rise rate in the energy isolation plate following a manifold rupture. This study can be used for energy calculation and consequence prediction of pipeline blasting for offshore fracturing vessels. It can also provide a basis and reference for safe construction, protection, and evaluation of high-pressure construction.

Based on the three-dimensional potential flow theory, the hydrodynamic performance of China$'$s first offshore fracturing ship is investigated, focusing on its motion response under different wave directions, wave spectral peak periods, and wave heights. The frequency-domain response of the moored fracturing ship is analyzed using AQWA, through which the motion response amplitude operators, added mass, and radiation damping are obtained. The numerical results are validated through comparison with seakeeping experimental data conducted by the research group to evaluate the validity and accuracy of the model. Subsequently, the time-domain mooring response of the fracturing ship under various wave parameters is analyzed based on the frequency-domain results. The study indicates that the wave period has little influence on the added mass in the sway direction, while the added mass in the surge direction decreases first, then increases, and subsequently decreases. In contrast, an opposite trend is observed for the added mass in the heave direction. The added mass in the pitch and yaw directions also decreases initially, followed by an increase and a subsequent decrease as the wave period increases, whereas variations in the roll direction is relatively small. Moreover, under different wave periods, the radiation damping in all degrees of freedom is found to first strengthen and then weaken. When wave direction angle from 0° to 180°, the significant amplitude of the roll motion first increases and then decreases, reaching its maximum value at 90°. In contrast, the significant pitch angle first decreases and then increases, with a maximum at 180°. With an increase in the wave spectral peak period, the significant amplitudes of roll and heave show an increasing trend, whereas the significant pitch response is observed to first rise and then decline. Additionally, increasing wave height leads to increases in the significant amplitudes of roll, pitch, and heave. The research findings provide a theoretical basis and reference for the analysis and practical application of motion response of large vessels under wave conditions.

In response to the issue of material tank strength failure induced by ocean loads on the first offshore fracturing ship, this study systematically conducts a fluid-structure interaction analysis of this specialized marine equipment under real sea conditions. A three-dimensional simulation model of typical material tanks and their supporting structural components is established, enabling the integration of internal flow field with fluid-structure interaction calculation. Based on indoor large-scale pool experiments, ship acceleration data under different sea conditions are determined to simulate the effects of ocean loads. The study found that as the wave spectral peak periods increases, both deformation and stress within the tank gradually rise. Additionally, with greater wave height, the stress and deformation show an upward trend. The maximum deformation and stress occur when the wave direction angle is at 90°. Moreover, greater filling ratio and increased liquid density also leads to higher deformation and stress levels. The analysis identified that the point of highest frequent maximum stress during the swing of the fracturing ship is located at the bend in the middle area of the tank wall bottom. The study found that the most critical extreme working conditions for the material tank structure are a liquid density of 1.35 g/cm$^3$, a wave direction angle of 90°, height of 4 m wave spectral peak periods of 13.00 s, and filling ratio of 95%. By accurately identifying the bend at the tank wall bottom as a critical hazard zone and outlining the most dangerous combination of extreme conditions, the research provides significant theoretical insights and data support for the design of material tank structure parameters during the swaying of the first offshore fracturing ship.

As a critical component of offshore fracturing vessel diesel engines, the crankshaft is prone to vibration due to long-term high-speed operation and mass imbalance caused by manufacturing tolerances, both of which can compromise engine reliability. This study established a finite element model of the crankshaft, taking the firing condition of the second cylinder as the working scenario, to systematically investigate its static and dynamic characteristics. Modal analysis was conducted to determine the first six natural frequencies and their associated mode shapes. This revealed that low-order modes could cause significant deformation under resonance conditions. Furthermore, harmonic response and random vibration analyses were performed to identify regions of dynamic response and stress concentration under external loads. The results of the topology-based lightweight design indicate that the mass of the crankshaft was reduced from 37.7 kg to 19.3 kg (a weight reduction of approximately 48.8%), while the first natural frequency increased from 545.36 Hz to 560.32 Hz (a rise of 10.3%). This effectively avoids resonance and mitigates the risk of failure. This study provides theoretical guidance and an engineering reference for optimising the structure, lightweight design, and enhancing its vibration reliability in the context of crankshafts in offshore fracturing vessel diesel engines.

Aiming at the problems of high energy consumption, high noise, and environmental pollution existing in the independent diesel engine drive scheme adopted by traditional fracturing vessels, relying on the "Haiyang Shiyou 623" large-scale fracturing vessel, this study carried out research on the key technologies of integrated drive between the fracturing system and the ship$'$s electric propulsion system. In the research, an integrated drive architecture of the common power plant was adopted, the AC 6 600 V power distribution system was designed as a closed-ring system configuration, the common power plant was divided into 5 redundant groups, a dual fault isolation mechanism of main protection and backup protection was configured, and the Advanced Generator Protection (AGP) system and voting mechanism were integrated. The fracturing pump adopted a 24-pulse diode rectifier inverter (DFE) frequency drive scheme, three-winding transformers were used to isolate the fracturing system from the ship$'$s 400 V system, and active power filters were added to address the problem of excessive harmonics. The research achieved flexible operation of the power plant and optimization of fuel consumption, significantly reduced the impact range of the maximum single failure, effectively controlled the potential hazards of hidden failures, greatly improved the stability and reliability of the power plant. This integrated solution breaks through the technical bottlenecks of the traditional drive mode, significantly reduces energy consumption and pollutant emissions, promotes the development of offshore fracturing operations towards energy conservation, high efficiency and environmental protection.

The exploitation of marine low-permeability oil and gas resources has driven the development of advanced fracturing technologies, among which all-electric fracturing ships have emerged as a highly integrated solution. These ships offer signi-ficant advantages, including enhanced operational efficiency, reduced environmental impact, lower noise levels, and precise control, making them a pivotal direction for future large-scale marine volumetric fracturing operations. This study focuses on the transition of fracturing vessels from a mounted to an integrated design, conducting research on the key technologies and equipment essential for the fracturing operation system of a 25 000 hp all-electric fracturing ship. By analyzing the overall structural design of the vessel, 15 specialized subsystems for marine fracturing operations were systematically established. A three-tiered layout structure was further developed to optimize functional integration and spatial efficiency. The research emphasizes eight critical technological areas: fracturing power supply and distribution, fracturing pump injection, sand supply and mixing, mixing and proportioning, high-pressure manifold systems, hose reel mechanisms, acid mixing systems, and centralized control centers. Building on these advancements, a series of marine fracturing equipment was successfully developed, rigorously tested, and certified by the China Classification Society (CCS). A fracturing operation system has been developed that is suitable for various sea conditions, has the ability to operate continuously, and can achieve the automation of the entire fracturing process. This achievement not only lays a solid foundation for the development of integrated fracturing ships but also provides robust technical support for the efficient, environmentally sustainable, and safe exploitation of marine low-permeability oil and gas resources.

In response to the protection requirements of high redundancy and fault tolerance for closed bustie medium voltage power systems in DP-2 or DP-3 class ships, a selective protection scheme that balances reliability and rapidity is proposed to solve the problem of fault isolation under single-point fault superimposed with hidden faults, ensuring that the power loss area does not exceed the maximum allowable failure range of the dynamic positioning system-typically one section of the medium-voltage main busbar. Based on the operational characteristics of closed bustie power systems, each section of the medium-voltage system is divided into four protection zones: generator, busbar, load feeder, and bustie cable. A two-tier protection scheme consisting of primary and backup protection is constructed through the coordination of differential protection and directional pilot protection. Based on the 6.6 kV closed bustie power system architecture of a typical DP-2 class ship, a hardware-in-the-loop (HIL) test platform including various types of integrated relay protection devices in one section of the main busbar and digital real-time simulation systems was built. Simulation tests were conducted in four types of areas with eight operating conditions including single point faults and superimposed hidden faults, testing the effectiveness of both primary and backup protections in each zone. The results showed that the protection system could operate correctly under various operating conditions with dynamic changes in fault current levels, verifying the coordination, reliability and rapidity of the proposed protection scheme.