A plug-in oil hose joint is used as the research object in this study. Aimed at the problem of pipeline breakage due to insufficient strength of the joint during the process of oil transportation on the sea, a three-dimensional geometric model of the joint is constructed according to the structural characteristics of the plug-in joint. The mechanical properties of the plug-in hose joint were studied by numerical simulations and field experiments. The strength of the joint was optimized by applying materials with different properties and adjusting the circular angle of the joint. The tensile strength test results of the plug-in oil hose joint are consistent with the theoretical analysis, which shows that the method proposed in this paper can effectively improve the tensile strength of the oil hose joint. It is of great significance to expand the scope of the plug-in connector and ensure the safe and stable use of the hose system.
A plug-in connector has the advantage of quick connection and is widely used in a variety of industrial and engineering activities, especially in the offshore soft oil pipeline system. Since characteristics of the soft pipeline include small bending rigidity and high tensile strength, the shape of the soft pipeline can be easily changed to transmit the external load to the hose connector position, resulting in the hose joint becoming the weak part of the entire hose system. This is particularly evident in the use of marine hose systems. If the joint is damaged, oil leakage can occur, along with possible fire and other security threats. Therefore, the study of joint strength has attracted much attention.
During the past decade, the study of joint strength has developed in a variety of directions. Some scholars have studied the fatigue strength of welded joints. Lazzarin et al. tested more than one hundred aluminum symmetric double butt joints to clarify the influence of geometrical, technological, and environmental factors on the fatigue behavior and failure modes of bolted joints [
At the same time, other researchers focused on the shear strength of solder joints during aging. Tomlinson and Fullylove tested the single overlap joints of copper and brass soldered with tin and tin-based solders containing 1% Cu, 3.5% Ag, 5% Sb, 57% Bi, 2% Ag, and 36% Pb and found that brass joints were generally slightly stronger than copper joints and tended to show more scatter [
There are also experts devoted to the prediction and optimization of lap joint strength. Hattori applied a new adhesive strength evaluation method, using two stress-singularity parameters to single-lap joints, which are generally used as adhesive strength test specimens [
It should be mentioned that much of the work so far has focused on the prediction and strength optimization of welded joints, lap joints, and solder joints. Little is known about plug-in oil hose joints. We demonstrate through an extensive literature review that the existing models are not capable of handling the specifics of the problem in this study.
This paper aims to determine a method for improving plug-in oil hose joint strength by theoretical analysis and experimental verification. Following the introduction, the remainder of this paper is organized as follows. Section
When the hose system is used on land, it is less affected by the outside world in general situations. However, for the marine pipeline, the situation is completely different. Considering the environmental factors, the marine hose system is influenced by ocean waves, ocean current, wind load, and so on. In the actual process, since the direction of ocean waves, ocean currents, and wind load action can change at any time, the shape of the soft hose changes frequently, leading to a complex force situation on the pipeline. Considering that the bending moment of the hose joint is small and is mainly subjected to the axial stretching effect, this paper mainly analyzes the mechanical properties and strength optimization of the plug-in hose joint under axial load.
To facilitate the connection and disassembly of the joint, several auxiliary structures are designed externally. However, these structures increase the difficulty of modeling the mechanical analysis of the joint and also make few contributions to improving the tensile strength of the joint. Therefore, according to the forces acting on the joint, a three-dimensional geometric joint model is established by ignoring the local difference and simplifying the complicated and less influential structure. A three-dimensional view of the joint is shown in Figure
Three-dimensional view of the joint.
Three-dimensional view of the assembly.
Different views of the joint. (a) Front view. (b) Back view. (c) Side view. (d) Axial view.
During actual usage on the sea, due to the ocean waves and currents changing at any time, the soft pipeline system is subjected to forces from different directions. Under the action of various loads, the hose is stretched, bent, rotated, and so on. However, because the bending stiffness of the hose is small, the loads are finally transmitted to the hose joint part, resulting in the tensile behavior of the joint in the axial direction. Therefore, this paper assumes that the insert-type connector is only affected by the axial load, and the joint does not bear the torque. Meanwhile, other joint behaviors, such as rotations, are not in the scope of the study. In addition, the joint material is uniform, and the joint has no initial defects due to other causes.
Based on the basic model of the plug-in connector, a three-dimensional geometric model is established using SolidWorks. To determine the connection position of the connector assembly and its effective width, the angle
Diagrammatic sketch of angles
Sectional view of the assembly hose joint.
Sketch map of mesh generation.
Assembly meshing diagram.
Mechanical properties of joint materials.
Materials | Parameters | Values | Parameters | Values |
---|---|---|---|---|
Aluminum alloy LY12M | Elastic modulus |
0.71 × 105 | Poisson’s ratio |
0.31 |
Density (kg/m3) | 2.8 × 103 | Yield strength |
300 |
Applied load to the joint.
Boundary condition definition of the joint.
Figure
Deformation maps of each part of the joint. (a) Deformation map of the fixed end. (b) Deformation map of the movable end. (c) Sectional deformation diagram of the wedge-shaped part of the joint. (d) Sectional deformation diagram of the joint slot.
In Figure
From Figure
Strain distribution diagram of the joint slot. (a) Logarithmic strain components of the joint slot. (b) Equivalent plastic strain of the joint slot.
Strain distribution diagram of the wedge-shaped part of the joint. (a) Logarithmic strain components of the wedge-shaped part of the joint. (b) Equivalent plastic strain of the wedge-shaped part of the joint.
In Figure
The stress distribution of the joint is shown in Figure
Stress distribution map of each part of the joint. (a) Stress distribution map of the fixed end. (b) Stress distribution map of the movable end. (c) Sectional stress distribution diagram of the wedge-shaped part of the joint. (d) Sectional stress distribution diagram of the joint slot.
Figure
Mises stress and S11 stress distribution curves of the joint slot. (a) Equivalent stress distribution curve. (b) X-axial stress distribution curve.
S22 stress and S33 stress distribution curves of the joint slot. (a) Y-axial stress distribution curve. (b) Z-axial stress distribution curve.
Figure
Figure
Figure
Figure
Mises stress and S11 stress distribution curves of the wedge-shaped part of the joint. (a) Equivalent stress distribution curve. (b) X-axial stress distribution curve.
S22 stress and S33 stress distribution curves of the wedge-shaped part of the joint. (a) Y-axial stress distribution curve. (b) Z-axial stress distribution curve.
Figure
Figure
Wedge slices at different positions: (a)
From Figure
Tension and displacement diagram of the joint.
From Figure
Mechanical properties of different aluminum alloys.
Materials | Parameters | Values |
---|---|---|
Aluminum alloy LY12M | Elastic modulus |
0.71 × 105 |
Density (kg/m3) | 2.8 × 103 | |
Poisson’s ratio |
0.31 | |
Yield strength |
300 | |
Aluminum alloy LY12CZ | Elastic modulus |
0.72 × 105 |
Density (kg/m3) | 2.8 × 103 | |
Poisson’s ratio |
0.33 | |
Yield strength |
380 | |
Aluminum alloy LC4-CS | Elastic modulus |
0.74 × 105 |
Density (kg/m3) | 2.8 × 103 | |
Poisson’s ratio |
0.33 | |
Yield strength |
550 |
The materials with different properties are assigned to basic hose joint models, and the corresponding simulation models are established, as shown in Figure
Joint models with different materials. (a) Aluminum alloy LY12CZ joint model. (b) Aluminum alloy LC4-CS joint model.
For finite element analysis models applied with aluminum alloy materials LY12CZ and LC4-CS, the static general analysis step is set, and the geometric nonlinearity function is opened in ABAQUS. The initial incremental step is 0.1, the minimum analysis step is 1
From Figure
Tension and displacement diagram of the joint under different material parameters.
Sectional view of different sizes of the wedge-shaped part of the joint: (a)
Hose joint with different center angles.
Sequence number | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|
Center angle |
21.5° | 22.5° | 23.5° | 24.5° | 25.5° |
After calculating the four finite element joint models corresponding to different center angles
Tension and displacement diagram of the joint corresponding to different center angles.
From Figure
To test the tensile strength of the plug-in hose connector and analyze the specific failure form of the joint during the axial drawing process, a tension experiment of the plug-in hose joint sample was conducted using a 1000 kN microcomputer-controlled servohydraulic universal testing machine and compared with the relevant results of the numerical simulation.
The experiment was performed using a 1000 kN microcomputer-controlled servohydraulic universal testing machine, which is shown in Figure
1000 kN microcomputer-controlled servohydraulic universal testing machine.
According to the basic size of the plug-in hose joint, different aluminum alloy materials have been selected for manufacturing joint samples, which are shown in Figures
Front view of the joint.
Back view of the joint.
Beam.
Connector.
Auxiliary fixture assembly.
Joint assembly.
Joint installation.
Start the test machine computer control, open the application software, and initialize all the parameters. Set the tension acting on the joint to increase gradually at a speed of 0.1 kN/s until the joint breaks. This process is presented in Figure
Software initialization.
The joint drawing process analysis is carried out using the joint with the aluminum alloy LY12M material and a 23.5° center angle as an example. At the initial state, the distance between the movable end of the plug and the fixed end is 0. As the testing machine begins to produce tension and continues to act on the movable part of the joint, the distance between the moving end of the joint and the fixed end gradually increases, and the relationship between the tension and displacement is linear. When the tension reaches approximately 70 kN, the distance between the moving end of the joint and the fixed end increases significantly. At approximately 74 kN, the moving end of the joint is separated from the fixed end, that is, the joint is broken. Since the system has automatic data collection, the software interface generates the tensile displacement curve slowly. The drawing process of the joint is shown in Figure
Joint failure process: (a)
During this experiment, the testing machine program automatically records the tension and displacement data and updates relative parameters in the software interface in real time, to analyze the joint displacement tension situation and determine if the joint breaks according to its changing trend.
For the test of the hose joint model with the aluminum alloy LY12M material and a 23.5° center angle, the tension displacement curve recorded is shown in Figure
Joint force displacement curve.
Experimental result diagram.
Tensile strength of joints with different materials.
Tensile strength of joints with different center angles.
In the previous study, the results showed that the tensile force displacement curves are nearly identical between numerical simulations and experiments. However, the theoretical value of the joint ultimate tension differs from the experimental result; in other words, the test results of the tensile strength of the joint are less than the theoretical values. Several reasons may account for this phenomenon. During the finite element analysis process, it is assumed that the joint does not have initial imperfections and that the material of the joint is isotropic. However, in the actual manufacturing process, defects are inevitable, and the sample material cannot be completely uniform. Furthermore, different manufacturing methods have a greater impact on the strength of the sample. Generally, it is reasonable that the theoretical and experimental values are different, but the difference is within a reasonable range.
It is critical to optimize the joint size and structure and improve the tensile strength of the joint based on studying the failure mode and characteristics of the joint and analyzing the stress distribution of the weak parts of the joint.
Figures
Fracture morphology of the movable end of the joint.
Fracture morphology of the fixed end of the joint.
In order to verify the feasibility of theoretical analysis, we carried out an actual oil transportation test on a floating hose system based on improved plug-in joint in Bohai, China. The whole test lasted for 2 months.
China Bohai area has been chosen as the test site since this area is wide and the sea waves and currents are representative. The tide in this area is a regular semidiurnal tide. The strong wave is in northeast direction. Its average wave height is 0.7 m, and the maximum wavelength is 3.5 m. The second strong wave is in southeastern direction. Its average wave height is 0.5 m, and the maximum wave height is 3.0 m. The hydrology of the experimental area during the test time is shown in Table
Tidal data in the experimental sea area.
Date | High tide time (low tide) | Ebb tide time (full tide) | High tide time (low tide) | Ebb tide time (full tide) |
---|---|---|---|---|
July 11 | 3:12 | 9:24 | 15:36 | 21:48 |
July 12 | 4:00 | 10:12 | 16:24 | 22:36 |
July 13 | 4:48 | 11:00 | 17:12 | 23:24 |
July 14 | 5:36 | 11:48 | 18:00 | 0:12 |
July 15 | 6:24 | 12:36 | 18:48 | 1:00 |
July 16 | 7:12 | 13:24 | 19:36 | 1:48 |
July 17 | 8:00 | 14:12 | 20:24 | 2:36 |
July 18 | 8:48 | 15:00 | 21:12 | 3:24 |
July 19 | 9:36 | 15:48 | 22:00 | 4:12 |
July 20 | 10:24 | 16:36 | 22:48 | 5:00 |
July 21 | 11:12 | 17:24 | 23:36 | 5:48 |
July 22 | 12:00 | 18:12 | 0:24 | 6:36 |
July 23 | 0:48 | 7:00 | 13:12 | 19:24 |
July 24 | 1:36 | 7:48 | 14:00 | 20:12 |
July 25 | 2:24 | 8:36 | 14:48 | 21:00 |
July 26 | 3:12 | 9:24 | 15:36 | 21:48 |
July 27 | 4:00 | 10:12 | 16:24 | 22:36 |
July 28 | 4:48 | 11:00 | 17:12 | 23:24 |
July 29 | 5:36 | 11:48 | 18:00 | 0:12 |
July 30 | 6:24 | 12:36 | 18:48 | 1:00 |
July 31 | 7:12 | 13:24 | 19:36 | 1:48 |
August 1 | 8:00 | 14:12 | 20:24 | 2:36 |
August 2 | 8:48 | 15:00 | 21:12 | 3:24 |
August 3 | 9:36 | 15:48 | 22:00 | 4:12 |
August 4 | 10:24 | 16:36 | 22:48 | 5:00 |
August 5 | 11:12 | 17:24 | 23:36 | 5:48 |
August 6 | 12:00 | 18:12 | 0:24 | 6:36 |
August 7 | 0:48 | 7:00 | 13:12 | 19:24 |
August 8 | 1:36 | 7:48 | 14:00 | 20:12 |
August 9 | 2:24 | 8:36 | 14:48 | 21:00 |
August 10 | 3:12 | 9:24 | 15:36 | 21:48 |
August 11 | 4:00 | 10:12 | 16:24 | 22:36 |
August 12 | 4:48 | 11:00 | 17:12 | 23:24 |
August 13 | 5:36 | 11:48 | 18:00 | 0:12 |
August 14 | 6:24 | 12:36 | 18:48 | 1:00 |
August 15 | 7:12 | 13:24 | 19:36 | 1:48 |
In this experiment, the improved plug-in connector is applied to the offshore oil transmission hose for the offshore oil transportation test. In order to test the ultimate tensile strength of the joint as much as possible, the hose fitted with an improved plug-in hose joint has been laid on the sea for two months so as to experience as many different extreme sea conditions as possible. If the hose system can still operate normally under long-term continuous operation, it is proved that the test result of the improved plug-in joint is consistent with the theoretical result. If the joint position of the hose system is damaged due to the load concentration, it is proved that the optimization result of the joint is not perfect enough. Figures
Oil tanker.
Floating pipeline department.
Encountering strong currents.
Plug-in joint part.
Figure
Joint morphology after the test.
In this paper, the structural characteristics of a plug-in oil hose joint are analyzed, and the mechanical properties are studied using numerical simulations and experiments. The main conclusions of the study are as follows: (1) the strain distribution of the fixed and movable ends of the joint assembly is similar, and the strain mainly occurs on the inside of the wedge-shaped part of the joint and the connecting part with the joint slot. (2) Along with the direction of slot thickness, the maximum strain occurs at the upper surface of the joint slot, and as the thickness increases, the strain gradually decreases. (3) The stress of the wedge-shaped part is mainly concentrated at its top bayonet position, and the stress concentration from the upper part to the bottom part gradually decreases. (4) The stress distribution regulation of slices at different thickness positions of the wedge part is nearly identical. The top position of the wedge has a high degree of stress concentration, and from top to bottom, the stress of the wedge cross section appears to gradually decrease. (5) Compared with aluminum alloy LY12M, the ultimate tensile force of a joint with the aluminum alloy LY12CZ material reaches 112142 N and increases by 23%. The ultimate tensile force of a joint with the aluminum alloy LC4-CS material reaches 162756 N and increases by 79%. (6) By increasing the center angle of the wedge part of the joint, that is, increasing the contact area between the wedge part and the slot of the joint, the tensile strength of the joint can be improved. (7) The tensile strength test results of the plug-in hose joint are consistent with the theoretical analysis, which verifies the conclusion of the numerical analysis.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Chang Chen conceived the project and wrote the main manuscript text, Jikun Guo prepared Figures 1–38 and put forward a unique view in representation and analysis, Shifu Zhang subsidized the subject experiment and prepared Tables 1–4, Qixin Zhang designed experiments, Dongmei Zhang assisted with Illumina sequencing, and Yongjie Niu and Hao Sun analyzed the experimental results. All authors reviewed the manuscript.
This paper is supported in part by the National Key R&D Program of China (2017YFC0806608), Education Science Fund of the Military Science Institute of Beijing, China (No. 2016JY481), and Chongqing Graduate Scientific Research Innovation Project (No. CYB17148)