Interactions between adjacent underwater vehicles determine both the attached cavity flow regime and the state of their multi-degree-of-freedom motion. Based on model tests, this paper studies the formation of continuous underwater tail cavities in twin-engine vehicles and their multi-degree-of-freedom motion characteristics, it systematically explores how parameters including time interval, ejection pressure, and transport velocity influence these characteristics. The results demonstrate that secondary flow field disturbance deflects the leading vehicle's tail cavity laterally toward the trailing vehicle, inducing necking and pinch-off, while both vehicles' motion attitudes deflect in opposite directions. The shed hairpin vortex ring from the leading vehicle migrates toward the trailing vehicle, significantly accelerating the transition of the latter's rear cavity from the transparent phase to a foam cavity. The lateral pitch angle of the leading vehicle initially increased then decreased as the vertical displacement varied from 0 to L. At intervals <= 1/3L, significant vehicle deflection occurred with partial fusion of tail cavities. Hydrodynamic coupling subsequently weakened, and vehicles approached nearindependent motion at intervals >= 2/3L. Under increased ejection pressure, the trailing vehicle's tail cavity absorbs and axially stretches fragmented gas masses from the leading vehicle. This coiled cavity entrains additional gas, generating auxiliary thrust that sustains its velocity advantage. When the transport velocity is less than 0.11 m/s, transverse pressure difference delays tail cavity shedding, above 0.33 m/s, shedding accelerates with directional shift outward.

The subsea pipeline packer can quickly plug the damaged pipeline, and the sealing rubber cylinder is the packer's core. This study investigates the fretting wear behavior between the pipeline and the cylinder, which influences the sealing performance. The fretting wear tests and the surface wear morphology of nitrile rubber (NBR) and fluororubber (FKM) are conducted. The curves of friction force, the running condition fretting maps, and the friction coefficient are obtained. The fretting behavior and mechanisms with displacement amplitude (D) and normal force (F-n) are discussed. The results indicate that the fretting wear shows different operating states under different conditions. Compared to FKM, NBR transitions more easily from the mixed slip region to the fully sliding region. FKM demonstrates better resistance to fretting wear under oil lubrication.

Tail cavities are separate bubbles of initially non-condensable gas attached to the bottom of the vehicle, affecting the dynamics characteristics of the vehicle. The evolution of the tail cavity under various launch pressures is experimentally simulated, and the pulsation shedding characteristics of the tail cavity are investigated with the U-net method. Additionally, the influence of the tail shape on the bubble-carrying capability of tail cavity is discussed. Research shows that gas break-off and environmental pressure reduction induce a quasi-periodic pulsation process in the tail cavity, maintaining an approximately fixed length. Due to the asymmetric flow disturbances and the formation of an end closure by bubble cluster, the tail cavity shedding is transformed from regular vortex ring to hairpin vortex. The cavity volume pulsation frequency is consistent with cavity length. The continuous expansion of the cavity induced by environmental depressurization compensates for the loss of cavity volume due to hairpin vortex shedding. Increasing the launch pressure leads to a change in the tail vortex morphology, but does not significantly increase the size of tail cavity. The bottom extension can improve the bubble-carrying capacity and effectively solve the exit angle and air column caused by the increase of launch pressure.

The attached shoulder-tail cavities of an underwater vehicle are accompanied by complex flow phenomena and determine its motion and loading characteristics. The evolution of underwater vehicle-attached cavities is studied. The ability of the solid wall to carry the air mass near the tube is focused on. The evolutionary properties of cavity collapse and tailing surges are studied. The effects caused by the cavitation number and the Froude number are discussed. The study showed that the air mass near the tube-carrying-type shoulder cavity is prone to form disintegration before leaving the water when Fr-l <= 6.42. During vehicle traversing the free surface, the shattered shoulder cavity forms a splash jet impacting the tail cavity. As the Fr increased, the position of the disintegration point is progressively closer to the free surface, and the convex fluctuation point occurs near the leading edge of the cavity. The umbilical tailing surges gradually converge. Until Fr-l = 10.41, no significant disintegration of the cavity is observed. At low cavitation numbers, the ability to carry the air mass near the tube out of the tube is better than at high Fr. When Fr-l = 11.88, the maximum value of L(1) is 0.5L. When P-difference = 16.8, the maximum value of L(1) is 0.72L. The high pressure ratio reduces the splashing effect of the jet. When the vehicle is completely above the free surface, L(J)-D8 = 6.28, L(J)-D9 = 4.75, and L(J)-D10 = 2.80. The tail cavity is pulled from the jet to form a tailing surge.

The deep-sea pipe plug is critical equipment in emergency maintenance operations. Existing deep-sea pipeline plugs have issues such as excessively large axial dimensions and being limited to use with pipelines of a single wall thickness. To address this issue, an external anchoring - internal plugging deep-sea pipe plug is designed. Considering the tangential fretting behavior of the rubber barrel during its sealing operation, fretting wear tests are conducted to study the fretting characteristics. This research provides a theoretical basis for predicting the remaining service life of the sealing rubber barrel and concurrently enriches and advances the fretting tribology theory of sealing materials.

For the sealing structure of subsea pipeline connectors, it is crucial to understand the maximum equivalent stress of the sealing ring, the normal force generated by the pressure effect of the internal fluid acting on the contact area of the flanges and the sealing ring, and the displacement of the flange in the pre-tightening state and the working state. This paper proposes an equivalent calculation method (ECM) to quickly compute the values of these three variables. A theoretical model for the parallel contact of the axially symmetric object and an elastic foundation is proposed based on the dimensionality reduction method and Hertzian theory. Considering the contact and structural characteristics of the flanges and the sealing ring of the sealing structure, an equivalent hollow cylinder model is introduced. By combining these two models, along with the long hollow cylinder theory, theoretical formulas for the maximum equivalent stress, the normal force, and the flange displacement in the pre-tightening state and the working state of the sealing structure are derived. In addition, finite element simulations were conducted on the sealing structure of different sizes and working states. A comparative analysis was performed between the simulation results and the theoretical formulas results. The results show that, within the yield strength of the sealing ring, the maximum relative deviation of the three theoretical formulas is less than 10%. The theoretical formulas can be used for the design and engineering application of the sealing structure of subsea pipeline connectors.

The air mass near the launch tube, the launched high-pressure gas, and the ventilated cavity determine the kinematic characteristics during the process of traversing the water and the free surface. The evolutionary mechanism of the underwater vehicle's ventilated cavity has been experimentally studied by using a cone head vehicle. The ventilated cavity, air mass near the launch tube, and free surface interaction relations are obtained, and the influence brought about by changes in cavitation number and Froude number is investigated. It is shown that the initial expanded volume of the air mass near the launch tube provided an appropriate environment for the growth of the ventilated shoulder cavity. The low-pressure region in the shoulder increases the velocity of the reentrant jet toward the leading edge of the cavity, which leads to the unstable cavity boundary and collapse. The tail cavity is pinched off by a jet at the end of the ventilated shoulder cavity. In addition, the reduction of cavitation number significantly improves the bubble-carrying capacity of the ventilated cavity and reduces the percentage of reentrant jet. The reduction of environmental pressure led to an increase in the shoulder cavity's ability to traverse the free surface and the ability to entrain the tail cavity, and the shoulder and tail cavities acted in an earlier location. Increasing the Froude number has a similar effect as decreasing the cavitation number. It also resulted in the earlier collapse of the jet produced by the pinched tail cavity.

The diamond wire cutter (DWC) is a crucial tool for subsea pipeline repairs. However, in the event of a saw wire failure, replacing and connecting the diamond wire on the seabed is unfeasible due to constraints in underwater construction conditions. To extend the service life of diamond wires, this study examines the micro-wear morphology of diamond beads through low-temperature grinding experiments. The wear mechanisms of diamond grains are revealed, and the wear process of the diamond bead is deduced. Utilizing wear experimental data and a BP neural network, the wear rate prediction model of electroplated diamond wire during subsea pipe continuously cutting is constructed. Additionally, the effects of cutting parameters, wire lengths, and pipe diameters on bead wear rates are analyzed. Findings indicate that the wear behavior of diamond grains includes fracture, thermal wear, and grain detachment. Compared to the (111) crystal surface, the (100) crystal surface exhibits superior wear resistance. During electroplated diamond wire continuously cutting subsea pipes, the saw wire achieves optimal service life at a cutting speed of 22 m/s and a feed speed of 1.1 mm/min. This study offers theoretical guidance for diamond wire underwater construction programs.

The surging ocean waves contain enormous energy and are one of the most promising renewable energy sources. In order to enhance the motion response of the triboelectric nanogenerator in a wave environment, the paper proposes a triboelectric nanogenerator based on a pile-type oscillating float structure for collecting wave energy, so that it can be installed on steel piles on offshore platforms to provide a steady power supply. Due to the movement of the float and the waves, the contact of the independent layer power generation structure of the grid electrode is insufficient. To solve the problem, the study adopts the electrode form with a spring elastic support structure, which significantly improves the wave energy collection capacity. The OF-TENG experiment provides a peak rectified short-circuit current of 35.88 mu A, a peak power of 5.02 mW, and an average power of 1.724 mW. At the same time, in the water flume wave simulation experiment, the 1 mF capacitor was charged to 5V within 546.64s, successfully realizing the data transmission of the temperature and humidity sensor and powering the marine navigation indicator light, verifying the feasibility and practicality of the device as a power source. The study provides a practical approach for efficient wave energy harvesting and multifunctional applications.

The diamond wire saw is an essential cutting tool in the decommissioning process of deep-water jacket platforms. This study addresses the saw-binding phenomenon observed during the flat-cutting of jacket legs by designing a specialized diamond wire cutting robot to optimize kerf geometry. Additionally, to refine underwater cutting parameters, an experiment was conducted on the low-temperature grinding of X65 steel using electroplated diamond beads. The bead surface wear morphology was examined by using scanning electron microscopy. Experimental data were used to analyze the effects of various cutting parameters on material removal rate, bead wear, and grinding ratio. The findings reveal that the wear behavior of diamond grits on the bead surface includes grit integrity, micro-wear, fracture, and pull-off. Based on the grinding ratio data, the recommended cutting parameters are a feed speed of 1.2 mm/min and a cutting speed of 22 m/s. This research provides theoretical guidance for optimizing underwater diamond wire cutting operations.

Diamond wire saws are the preferred underwater cutting tools for the disintegration of X-series steel structures. This paper first analyzes the material removal behavior of abrasive grains during the grinding of X65 steel. Based on contact mechanics theory, a mechanical model is established for the diamond abrasive grain's griding process, encompassing the phases of rubbing, plowing, and cutting to elucidate their interactions. In accordance with the four typical postures of hexoctahedral abrasive grains, a theoretical model for the material removal rate (MRR) of the electroplated diamond bead is proposed, based on the shoelace theorem and the trajectory of the abrasive grains. Finally, low-temperature grinding experiments were conducted to reveal the chip formation mechanism of X65 steel under different cutting parameters and verify the MRR model. Observations under a scanning electron microscope (SEM) showed that the griding chips changed from the flowing chip to the knife chip as the feed speed increased. With an increase in cutting speed, the proportion of lump debris also increased. The error rate between the theoretical model of MRR and the experimental data is within 10 %, indicating the model's capability to predict the removal rate of the beads and wire saw.

The submarine pipeline stopper is an emergency device that can quickly seal damaged pipelines. Investigating the stopper's dependability and safety is vital to ensure that subsequent maintenance activities go smoothly. However, in production environments, it is challenging to get failure data for complex systems due to high experimental costs. This work proposes a fuzzy comprehensive dynamic Bayesian network (FCDBN) based on fault tree, fuzzy evaluation, and dynamic Bayesian network. Using this method, the failure rate of the stopper can be obtained, thus solving the problem of difficult data acquisition. The time slice-based qualities are taken into account while evaluating the reliability and safety. By controlling variables, each failure rate's effect is quantified. Finally, the process of failure prediction is completed. The sealing device is least reliable and most possible to fail, according to the results. The most significant influence on reliability comes from rubber barrel shoulder upwarping. The failure rate of the stopper is highest if the sealing device fails. Based on the aforementioned findings, appropriate control measures are suggested, which can greatly lower the stopper failure risk.

It's important to improve the sealing performance of the inner packer in deepwater pipelines, which is an emergency sealing and oil control equipment. The deformation characteristics and sealing stress conditions of the packer's sealant cylinder are analyzed, and the hyperelastic constitutive model is determined based on uniaxial tensile experiments. The effective contact stress and effective contact range between the rubber cylinder and the pipe wall are the main parameters for evaluating the sealing performance. Through simulation, it was found that the rubber cylinder on the force application side plays a major sealing role, and severe stress can lead to shoulder protrusions. The location of fatigue damage on the rubber cylinder's shoulder is consistent with the sea trial. Therefore, special-shaped rubber cylinders with different structures are designed to address the shoulder protrusion. The simulation shows that the sealing performance of the right-angled trapezoidal double-layer rubber cylinder with force applied on the slope side(S-RARC) is significantly better than the original rubber cylinder(ORC), and it does not produce shoulder protrusions.

The submarine pipe stopper is prone to failure at large pressure and high temperature conditions. This work focuses on the submarine pipe stopper’s sealing failure caused by excessive shear stress and high temperature. First, the working principle of the sealing device in the stopper was analyzed and its performance was verified through land and sea trials. Then a comprehensive evaluation model fit for the large working pressure was established by analyzing the rubber cylinder’s sealing performance and strength. Next, the model was modified by simulation using the nitrile rubber (NBR) cylinder and the fluororubber (FKM) cylinder were used as examples to revise the comprehensive evaluation model while taking temperature into account. The maximum contact stress and the maximum shear stress of the NBR rubber cylinder at 100°C increased by 28% and 30.62% compared to 25°C, and that of the FKM rubber cylinder increased by 24% and 26.12%. In comparison to 25°C, the maximum contact stress and maximum shear stress of the NBR rubber cylinder increased by 28% and 30.62% at 100°C, while the FKM rubber cylinder had an increase of 24% and 26.12%. At 100°C, the NBR rubber cylinder exhibits a maximum contact stress of 4.71 MPa and a maximum shear stress of 1.18 MPa, whereas the FKM rubber cylinder displays maximum contact and shear stresses of 5.09 and 2.44 MPa. Finally, the rubber cylinder’s maximum working pressure at different temperatures was derived, and the influence of its parameters was discussed. The maximum working pressure at 100°C is only 21% of what the rubber cylinder can bear at 25°C. This work is of great significance for accurately evaluating the rubber cylinder’s sealing performance in the submarine pipe stopper and provides new ideas for the rubber cylinder’s design at high-temperature conditions.

Diamond rope saws are one of the important tools for cutting large structures in deep water, and exploring their ability to remove steel materials is a prerequisite for improving cutting efficiency. In this paper, based on the observation results of an optical profilometer on the surface morphology of diamond beads, using statistical theory to analyse the data on the spatial parameters of abrasive grains, a method of three-dimensional digital characterisation of diamond beads based on the spatial description of the coordinate system of the column is proposed. Based on the above research, the theoretical models for material removal prediction and the finite element simulation model of diamond bead cutting steel material are established to analyse the removal ability of steel material under different cutting conditions with the synergistic effect of multiple abrasive grains in the process of diamond beads cutting steel material. Experiments were used to perform error analysis on the theoretical model, optimise the cutting parameters and correct the theoretical model for material removal. The results show that the cutting speed of 21 m/s and the feed speed of 1 mm/min resulted in a low theoretical and real removal error and less bead wear, which is favourable for continuous cutting. This current work provides a theoretical basis for the intelligent monitoring of the cutting process state of underwater diamond rope saws.

With the exploitation of marine resources, the development of maintenance operation technology for subsea pipelines transporting oil and gas has become the focus worldwide. Diamond bead wire serves as important engineering equipment in subsea pipeline maintenance, and the study of the wear mechanism of diamond grains on the beads is key to improving the cutting efficiency and lifetime of diamond bead wire. In this work, a theoretical model of the mechanical wear rate of the diamond grain was established, and a simulation model of the diamond grain cutting X65 pipeline steel was developed by applying smoothed particle hydrodynamics method. By studying the effects of the cutting factors of the diamond grain on mechanical wear, the mechanisms of adhesive wear, abrasive wear, and fatigue wear were clarified. The results indicate that the diamond grain in line contact or large surface contact with X65 steel contributes more to wear rate reduction than point contact; The mechanical wear rate predominated by adhesive wear and abrasive wear of the diamond grain increases with the increase of cutting speed and depth. And when the cutting speed is 22–26 m/s and the depth is 20–30 μm, the growing trend of the mechanical wear rate turns gentle, which is more suitable to balance improving cutting efficiency and reducing mechanical wear.

The real contact area of rough surfaces has significant importance in many engineering applications, such as tribology, wear, lubrication and seals. A continuous observation length-dependent mechanic model of rough contact without adhesion is proposed, which assumes that the rough surface is divided into ideal subplanes. However, the model ignores the elastoplastic deformation of asperities, and the standard deviation of ideal subplanes’ heights is assumed to vary linearly with continuous observation length, which is not precise for all the surface fractal dimensions. In this work, a revised continuous observation length model is proposed with elastic, elastoplastic and fully plastic stages. The expressions of force and real contact areas are derived. For surfaces with different fractal dimensions, the quadratic polynomial, cubic polynomial and power relationships between standard deviation and observation length are proposed, respectively. In addition, the influences of the dimensionless observation length, fractal dimension and equivalent elastic modulus on the real contact areas in different contact stages are also analyzed. It can be concluded that the quadrate real contact area decreases as the dimensionless observation length decreases, which can be applied to the percolation theory for leak seal problems.

The water-based cutting fluid plays an important role in cooling and lubricating during cutting process. In order to analyze the role of water in the cutting process from the microscopic view, this paper used molecular dynamics simulation to establish the cutting model with water lubrication by covering a water layer on the surface of iron workpiece. By comparing the cutting heat and friction coefficient under dry cutting and wet cutting, it is found that: water molecules will enter the gap between the tool and the workpiec, preventing the direct contact between the carbon atoms and the iron atoms, thereby reducing the friction coefficient. At the same time, wet cutting can reduce the surface temperature of the workpiece and play a role in cooling and lubricating.

In the field of underwater emergency maintenance, submarine pipeline cutting is generally performed by a diamond wire saw. The process, in essence, involves diamond grits distributed on the surface of the beads cutting X56 pipeline steel bit by bit at high speed. To find the effect of the different parameters (cutting speed, coefficient of friction and depth of cut) on cutting force, the finite element (FEA) method and response surface method (RSM) were adopted to obtain cutting force prediction models. The former was based on 64 simulations; the latter was designed according to DoE (Design of Experiments). Confirmation experiments were executed to validate the regression models. The results indicate that most of the prediction errors were within 10%, which were acceptable in engineering. Based on variance analyses of the RSM models, it could be concluded that the depth of the cut played the most important role in determining the cutting force and coefficient the of friction was less influential. Despite making little direct contribution to the cutting force, the cutting speed is not supposed to be high for reducing the coefficient of friction. The cutting force models are instructive in manufacturing the diamond beads by determining the protrusion height of the diamond grits and the future planning of the cutting parameters.

In this paper, the molecular dynamics simulation method was used to establish the MD model for diamond cutting iron. The cutting process was simulated, and the change rules of temperature, cutting stress and radial distribution function with timestep were analysed. The results show that: As the cutting progresses, the temperature will increase gradually, the cutting stress fluctuates around 50,000 bars, and the wear mechanism of tool is analysed form a microscopic perspective; By adjusting the posture of the tool, the rake angle and the flank angle are changed, and the size of the thermostat layer and boundary layer are adjusted at the same time, which reduces the impact of non-Newton atoms on the workpiece; The change rules of temperature and cutting stress under different rake angle and flank angle conditions are analysed, and the minimum cutting stress is obtained when the rake angle is 15 degrees; By prefabricating a crack on the flank face, the change rule of the crack edge stress is studied. It is found that due to the occurrence of crack, the contact area between the tool and the workpiece is reduced, and there is obvious stress concentration at the edge of the crack.