Fatigue test device and method for simulating underwater wave flow scouring

Abstract

The invention discloses a fatigue test device and method for simulating underwater wave flow scouring. The device comprises a scouring system and a fatigue system, the scouring system comprises a box body, an upper fixing plate, a lower fixing plate, a vertical rod group, a fluid monitoring probe, a fluid data acquisition system, a medium mixing tank, a proportioning pump and a flowmeter, and the fatigue system comprises a fatigue testing machine. The application method comprises the following steps: tensile testing the yield strength of a sample through a static test, mounting the sample on a fatigue testing machine, conveying seawater with target mud and sand content to the box body at a certain flow speed, and measuring the fatigue performance of the sample by adopting a lifting method until the sample is fractured. The device can test the fatigue performance of a metal material, an electric arc additive manufacturing structural member or a welded joint in a simulated ocean platform riser system wave current scouring environment, the service environment of the ocean platform riser system is truly and accurately simulated, and the obtained fatigue performance result can evaluate the safety of the structural member.

Description

Technical Field

[0001] This invention relates to the field of metallic materials, and in particular to a fatigue testing apparatus and method for simulating underwater wave erosion. Background Technology

[0002] Offshore platforms are key equipment for offshore oil and gas development, mainly composed of a foundation, riser system, superstructure, and anchoring system. The riser system is responsible for bearing the weight of the superstructure and is affected by ocean wave and current erosion and load variations. The harsh service environment makes the riser system vulnerable to functional degradation.

[0003] The riser system of an offshore platform consists of risers, horizontal pipes, and jacket piles. The risers and horizontal pipes are supported by metal materials and are usually connected and assembled by welding. However, with the rapid development of electric arc additive manufacturing technology, which has advantages such as high material utilization efficiency, low cost, and high production efficiency, electric arc additive manufacturing technology is particularly suitable for manufacturing risers, horizontal pipes, and support structures for offshore platforms, and its application prospects are broad.

[0004] The process parameters of arc additive manufacturing, such as current, voltage, and welding speed, have a significant impact on the fatigue performance of structural components. Furthermore, internal defects, such as porosity, inclusions, and lack of fusion, may arise during the manufacturing process, adversely affecting the fatigue performance of the structural components. Therefore, the metallic materials in riser systems are prone to corrosion loss, erosion loss, and fatigue cracking during long-term service, leading to a decrease in the load-bearing capacity of the platform structure components and the overall structure, severely impacting the safety of the structure during service.

[0005] Investigations revealed that the construction of the platform's riser system altered the original ocean current patterns in the area, making the surrounding water flow extremely complex. Secondary flows formed in front of the riser system, creating vortices around it, which increased scouring intensity, leading to scouring corrosion of the riser system. Furthermore, the scouring altered the stress state at the pile tops, increasing the equivalent pile length and affecting the structure's lateral stiffness and internal forces, ultimately impacting the platform's stability.

[0006] Corrosion fatigue is the process by which metallic materials undergo brittle fracture under the combined action of periodic or aperiodic alternating loads and corrosive media. During service, platform riser systems not only face complex wave and current erosion corrosion in seawater but also endure repeated environmental loads such as waves and ocean currents, generating constantly changing stresses within the structure. Defects in structural components can easily become crack initiations for corrosion fatigue, leading to a decrease in fatigue toughness. Therefore, fatigue performance testing of offshore platform structures under service wave and current erosion is essential.

[0007] The publicly disclosed "Intelligent Accelerated Dynamic Water Erosion and Dynamic Load Fatigue Testing Instrument and Method" (Publication No. CN117451557A) belongs to the field of pavement material testing technology. It tests the performance and durability of large-void asphalt mixtures in complex environments. The dynamic water erosion involved refers to water film covering the pavement, and the fatigue involved refers to repeated rolling of vehicle loads. It does not involve the problem of seawater wave erosion or fatigue in a hydrogen environment.

[0008] The published "A test device for concrete scour under load and environmental coupling" (publication number CN216386650U) relates to the scour fatigue test of concrete engineering structures in a simulated ocean wave environment, but does not involve wave scour or fatigue performance issues.

[0009] The published "An experimental apparatus for simulating scouring fatigue" (publication number CN107202740 B) is applied to scouring fatigue tests of high-speed rotating parts such as engine compressor blades and wind turbine blades, but does not involve metal structures and wave scouring and fatigue problems in marine environments.

[0010] In summary, the aforementioned patents do not address the issue of testing the performance of offshore platform riser systems under the combined effects of wave and current scouring and fatigue, and do not construct the wave and current morphology of the offshore platform riser environment.

[0011] Therefore, establishing a test device and method for testing the fatigue performance of metallic materials during simulated wave erosion corrosion is of practical significance. It is beneficial for identifying potential safety hazards in existing systems, which is of great importance for reducing platform maintenance costs and ensuring the safety of life and property. It can also provide technical information for further optimization of material selection, thereby improving the safety and durability of marine platforms in service. Summary of the Invention

[0012] The technical problem to be solved by the present invention is to provide a fatigue testing device and method for simulating underwater wave and current scouring, to test the fatigue performance of metallic materials during underwater wave and current scouring, and to provide a basis for safety evaluation and material selection optimization of large marine structural components.

[0013] To achieve the above objectives, the present invention employs the following technical solution:

[0014] A fatigue testing device for simulating underwater wave scouring includes a scouring system and a fatigue system; the scouring system includes a housing, an upper fixed plate, a lower fixed plate, a group of upright rods, a fluid monitoring probe, a fluid data acquisition system, a medium mixing tank, a dispensing pump, and a flow meter; the fatigue system includes a fatigue testing machine.

[0015] The housing is equipped with an upper and lower fixed plate, forming a wave-current scouring space. The upper and lower fixed plates are symmetrically equipped with mounting holes I and several mounting holes II. Mounting holes I allow the specimen to pass through and be threaded onto the fatigue testing machine. The upright rod is installed between the upper and lower fixed plates through mounting holes II. The medium mixing tank, dispensing pump, flow meter, and housing are sequentially connected via pipes to form a loop. A fluid monitoring probe is horizontally mounted circumferentially on the upright rod and connected to a fluid data acquisition system. The scouring system simulates the wave-current scouring environment of the riser system, constructing horseshoe-shaped vortices and erosion vortices. Fatigue performance testing is conducted on the specimens during this wave-current scouring corrosion process. The fatigue system includes a fatigue testing machine, specifically a hydraulic servo fatigue testing machine, model MTSLandmark730. The specimen passes through mounting holes I and is threaded onto the fatigue testing machine, providing conditions for simulating the wave-current morphology of a marine platform riser system in service.

[0016] Furthermore, there should be no fewer than 14 upright rods, evenly distributed around the sample.

[0017] Furthermore, the vertical and horizontal distances between adjacent uprights are equal.

[0018] Furthermore, the upright rod is a cylinder made of polytetrafluoroethylene or plexiglass, and the diameter of the upright rod is between 1 and 3 times the diameter of the sample clamping section.

[0019] Under the impact of fluid, the group of upright rods causes horseshoe-shaped vortices and erosion vortices to form around the sample, which transforms the advection fluid into vortex fluid, thus drastically increasing the scouring depth and scouring range, thereby simulating local scouring corrosion.

[0020] Furthermore, the sample is a circular sample with a surface roughness of less than 0.8 μm and a consistent surface finish to eliminate stress concentration; the transition section has a tangential transition arc to smoothly connect the clamping section and the parallel section to eliminate stress concentration.

[0021] Furthermore, the height of the wave current scouring space in the scouring system box, i.e. the height between the upper fixed plate and the lower fixed plate, is greater than the sum of the lengths of the transition section and the parallel section of the tensile specimen, and the distance between the outer vertical rod and the box wall is ≥0.7m.

[0022] Furthermore, the fluid monitoring probe is set at the midpoint of the sample at a horizontal position, and there are ≥4 fluid monitoring probes in the circumferential direction of the upright rods; the fluid monitoring probes are evenly set at the same horizontal position as the upright rod group, and the fluid velocity and direction data are collected by the fluid data system to draw a fluid waveform diagram, and the fluid monitoring probe is set at the midpoint of the sample at a horizontal position.

[0023] Furthermore, the flushing seawater is mixed with a simulated fluid medium in a medium mixing tank, stirred evenly, and then pumped into the tank via a dispensing pump. The seawater then flows out of the tank and is sent back to the medium mixing tank for recycling. The simulated fluid medium is seawater, simulated seawater solution, or seawater containing mud and sand, with a sand and gravel concentration not exceeding 50%.

[0024] A fatigue testing method for simulating underwater wave and current scouring includes the following steps:

[0025] Step 1: Adjust the two test parameters, namely the distance between the uprights and the fluid velocity entering the tank, to simulate the target wave flow pattern. The distance between the uprights and the fluid velocity entering the tank are key parameters for constructing the wave flow pattern. The method for determining these test parameters is as follows: A uniformly mixed simulated fluid medium, including sand-laden seawater, seawater, or a simulated seawater solution, is delivered into the tank using a dispensing pump. The fluid velocity and direction are monitored using a velocity monitoring probe, with data collected every 15 minutes. Fluid waveforms are obtained through a fluid data system to determine whether horseshoe vortices and erosion vortices are formed. If not, the distance between the uprights or the fluid velocity entering the tank is adjusted until the target wave flow pattern is formed. The determined upright distance is then used to simulate fatigue test parameters for underwater wave erosion.

[0026] Step 2: Conduct a tensile test through a static test to determine the yield strength of the specimen; this provides a reference value for the loading stress in the fatigue test.

[0027] Step 3: The fluid medium is delivered into the chamber at the flow rate determined in Step 1. At this point, the test wave pattern is a horseshoe vortex and an erosion vortex. The specimen is mounted on the fatigue testing machine, and axial fatigue tests are conducted on the specimen under preset stress, stress ratio, and loading frequency. Good coaxiality between the specimen and the main shaft of the testing machine is ensured to avoid deviations and errors during the test. The fatigue performance of the specimen is determined using the lifting method. Each experiment is conducted until the specimen fractures, and the number of cycles corresponding to fracture is taken as the fatigue life of the specimen under the corresponding test conditions. The number of fatigue cycles of the specimen exceeds 10. 7 until;

[0028] Step 4: After the fatigue test, the working section of the sample is derusted with rust remover, rinsed with deionized water, and dried with alcohol for subsequent observation. The fracture fatigue performance of the metal material itself, arc additive manufacturing components, or welded joints is determined based on the fatigue life.

[0029] The fluid entering the tank has a flow rate ≥0.5m / s.

[0030] Preferably, the stress ratio R of the fatigue testing system in the fluid medium is -1 to 0.5, the loading waveform is a sine wave, the loading frequency range is 1 to 60 Hz, and the loading stress range is 0.1R. el ~0.3Rel The loading stress range is 0.1R. el ~0.3R el This method ensures that metallic materials do not fracture instantaneously due to excessive stress in corrosive environments, contributing to a sufficiently large number of corrosion fatigue cycles. This allows for the assessment of the material's corrosion fatigue life under prolonged low stress levels, ensuring the accuracy of fatigue life assessment. The service wave current, in a vortex pattern, scours the fatigue specimen. The impact force generated by the multiphase scour angle can be divided into horizontal and vertical components. It is evident that specimens subjected to axial force in fatigue testing also experience non-axial impact forces. The horizontal component generates a cutting effect on the scour surface; at small scour angles, the horizontal component is stronger, and the horizontal cutting mechanism is the main cause of material loss. The vertical component generates impact; at large scour angles, the vertical component is stronger. Solid-phase particles impact the material surface; some particles impact to form impact pits and surrounding protruding lips, while others impact the metal surface, forming microcracks. Therefore, the vortex-shaped wave current impact force has a significant effect on fatigue specimens subjected to axial force. The specimens are subjected to fatigue tests in a wave-current scouring environment, which can be used to test the fatigue performance of the metal material itself, arc additive manufacturing structural components or welded joints, and realize fatigue performance testing in a simulated wave-current scouring environment for offshore platform riser systems.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. This invention discloses a fatigue testing device for simulating underwater wave and current erosion, comprising an erosion system and a fatigue system. In the erosion system, two parameters—the spacing between the uprights and the fluid velocity entering the chamber—are determined using fluid data acquisition and fluid waveform plotting. Horseshoe-shaped vortices and erosion vortices are constructed by adjusting the number and spacing of the uprights and the fluid velocity to simulate an underwater wave and current erosion environment. The fatigue performance of metallic materials is tested in the fatigue system.

[0033] 2. This test apparatus can test the fatigue performance of samples during erosion corrosion under specific fluid conditions, enabling the testing of samples simultaneously subjected to erosion corrosion and fatigue. Its advantages include the ability to identify potential safety hazards in existing systems, providing technical information for further material optimization, and ultimately improving the safety factor of offshore platforms. Furthermore, it facilitates the observation of material failure states, enabling the identification of severely failed areas and providing valuable information for subsequent protective measures.

[0034] 3. This invention is applicable to testing the fatigue performance of metallic materials themselves, arc additive manufacturing structures, or welded joints in the service environment of offshore platform risers. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the fatigue testing device for simulating underwater wave erosion according to the present invention.

[0036] Figure 2 This is a cross-sectional view of AA.

[0037] Figure 3 This is a fluid wave flow diagram.

[0038] In the diagram: 1 is the housing; 2 is the upper fixing plate; 3 is the lower fixing plate; 4 is the upright rod; 5 is the fluid monitoring probe; 6 is the fluid data acquisition system; 7 is the fatigue testing machine; 8 is the sample; 9 is the medium mixing tank; 10 is the dispensing pump; 11 is the flow meter; 12 is mounting hole I; 13 is mounting hole II. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of an exemplary embodiment of the experimental apparatus and method is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] This invention provides a fatigue testing apparatus and method for simulating underwater wave-current erosion, specifically for testing the fatigue performance of metallic materials during erosion corrosion under specific fluid morphology. The testing apparatus of this invention can simulate variations in the morphology, velocity, sand content, solid particle size, and corrosive medium of the wave-current fluid in service with a marine platform riser system, constructing a erosion system under operating conditions for the marine platform riser system, and then measuring the fatigue performance of the metallic material itself, arc-additive manufactured structural components, or welded joints.

[0041] like Figure 1 As shown, a fatigue testing device for simulating underwater wave scouring includes a scouring system and a fatigue system.

[0042] The flushing system includes a housing 1, an upper fixed plate 2, a lower fixed plate 3, a group of upright rods 4, a fluid monitoring probe 5, a fluid data acquisition system 6, a medium mixing tank 9, a dispensing pump 10, and a flow meter 11; the fatigue system includes a fatigue testing machine 7.

[0043] The upper fixing plate 2 and the lower fixing plate 3 are symmetrically provided with mounting holes I12 and several mounting holes II13. The specimen 8 passes through the mounting hole I12 and is connected to the fatigue testing machine 7 by threads. The upright rod 4 is installed between the upper fixing plate 2 and the lower fixing plate 3 through the mounting hole II13. The space between the upper fixing plate 2 and the lower fixing plate 3 is a wave scouring space, and its height is greater than the length of the specimen transition section and the parallel section.

[0044] In a preferred embodiment, there are no fewer than 14 upright rods in group 4.

[0045] In a preferred embodiment, the vertical and horizontal distances between adjacent uprights 4 are equal.

[0046] In a preferred embodiment, the upright rod 4 is cylindrical and made of polytetrafluoroethylene or plexiglass. The diameter of the upright rod 4 is 1 to 3 times the diameter of the clamping section of the sample 8. Under the action of the wave current, the upright rod 4 causes horseshoe-shaped vortices and erosion vortices to form around the sample 8, which deforms the advection wave current, causing a sharp increase in both the scouring depth and the scouring range, thereby realizing a simulated corrosion test of the riser system of an offshore platform.

[0047] A fluid monitoring probe 5 is horizontally positioned circumferentially at the midpoint of the axial direction of the upright rod 4. The fluid monitoring probe 5 is connected to the fluid data acquisition system 6. The fluid monitoring probe 5 monitors the flow rate and direction of the fluid and collects data every 15 minutes. The fluid data acquisition system 6 collects the above data and generates a fluid waveform diagram to determine whether the fluid forms horseshoe vortices and erosion vortices.

[0048] As a preferred embodiment, sample 8 is a circular sample with a surface roughness of less than 0.8 μm.

[0049] The mixing tank 9, dispensing pump 10, flow meter 11, and housing 1 are connected in sequence via pipelines to form a loop. The mixing tank 9 provides a homogeneous single- or multi-phase fluid. The dispensing pump 10 provides the fluid at the target flow rate, which is measured by the flow meter 11. The fluid flows from the mixing tank 9 to the housing 1 via the dispensing pump 10, and the flushing fluid flows back to the mixing tank 9 for a continuous flushing process.

[0050] In a preferred embodiment, the media mixing tank 9 is equipped with a stirrer to uniformly mix seawater, simulated seawater solution, or seawater containing mud and sand. The media mixing tank 9 stores the test fluid, with a sand and gravel concentration not exceeding 50%.

[0051] A method for using a fatigue testing device for simulating underwater wave and current erosion includes:

[0052] Step 1: Adjust the spacing of the uprights 4 and the flow velocity of the fluid entering the chamber 1 to simulate the target wave flow pattern;

[0053] Step 2: Determine the yield strength of specimen 8 by conducting a static tensile test;

[0054] Step 3: The target simulated fluid medium is transported into chamber 1 at the flow rate determined in Step 1. Specimen 8 is installed on fatigue testing machine 7, and axial fatigue tests are conducted on specimen 8 under preset stress, stress ratio, and loading frequency. The fatigue performance of specimen 8 is determined using the lifting method. Each experiment continues until the specimen fractures, and the number of cycles corresponding to fracture is taken as the fatigue life of specimen 8 under the corresponding test conditions. The number of fatigue cycles of specimen 8 exceeds 10. 7 until;

[0055] Step 4: After the fatigue test, the working section of the sample 8 is derusted with rust remover, rinsed with deionized water and dried for subsequent observation.

[0056] In a preferred embodiment, the fluid entering the housing 1 has a flow rate ≥ 0.5 m / s.

[0057] In a preferred embodiment, the stress ratio R is -1 to 0.5, the loading frequency is 1 to 60 Hz, and the loading stress is 0.1R. el ~0.3R el .

[0058] As a preferred embodiment, the method for determining the test parameters of the spacing of the uprights 4 and the flow rate of the fluid entering the chamber 1 is as follows: a uniformly mixed simulated fluid medium is delivered to the chamber 1 by a dispensing pump 8, and the fluid flow rate and direction are monitored by a fluid monitoring probe 5, with data collected every 15 minutes; the fluid waveform is obtained by a fluid data acquisition system 6 to determine whether the fluid forms horseshoe vortices and erosion vortices. If not, the spacing of the uprights 4 and the flow rate of the fluid entering the chamber 1 are adjusted until the target wave flow shape is formed.

[0059] Example 1:

[0060] Taking electric arc additive manufacturing of riser structural components as an example, combined with the attached... Figure 1 , 2 Sections 3 and 4 further illustrate the present invention.

[0061] Step 1: Determine two test parameters: the spacing between the uprights and the fluid velocity entering the chamber. This fatigue testing apparatus for simulating underwater wave erosion is designed with 14 uprights. Initially, the distance between the outer uprights and the chamber wall is set to 1m. The upright spacing is 100mm, and the fluid velocity entering the chamber is 1m / s. Fluid velocity and direction are monitored using a fluid monitoring probe, and fluid waveforms are obtained through a fluid data acquisition system. (See attached image.) Figure 3 It can be seen that the fluid flow pattern is horseshoe vortex and erosion vortex. Therefore, the distance between the outer layer of the uprights and the box wall is determined to be 1m, the spacing between the uprights is 100mm, the fluid velocity entering the box is 1m / s, the uprights and the sample are fixed to the upper and lower fixed plates at a distance of 100mm, and the height of the wave scouring space is 45cm.

[0062] Step 2: Through static testing, the yield strength of the arc additive manufacturing riser structure specimen was determined to be 690 MPa, providing a reference value for the loading stress in fatigue testing.

[0063] Step 3: Seawater and silt are uniformly mixed in a media mixing tank, with a silt content of 10%. The mixture is then pumped into the test chamber at a speed of 1 m / s using a dispensing pump. At this point, the circular sample is scourned by a horseshoe-shaped vortex and an erosion vortex formed by the uprights. The circular E690 steel sample, Ф6.35 mm in diameter, with a surface roughness of 0.8 μm and consistent surface finish, is prepared using arc additive manufacturing of the riser structure. An axial tensile test is conducted. The sample is mounted on a hydraulic servo fatigue testing machine, ensuring good coaxiality between the sample and the machine spindle to avoid deviations and errors during the test. The stress ratio R of the fatigue system in the fluid medium is 0.1, the loading waveform is sinusoidal, the loading frequency is 5 Hz, and the loading stress is 138 MPa (0.2R). el The experiment was conducted for 677,863 cycles before the specimen fractured, at which point the test was stopped. The number of cycles corresponding to the specimen fracture was recorded. After the fatigue test, the working section of the specimen was treated with rust-removing fluid, rinsed with deionized water, and dried with alcohol for subsequent observation.

[0064] The fatigue life of E690 steel used in arc additive manufacturing of riser structural components is 677,863 cycles.

[0065] Example 2:

[0066] Taking alloy steel with a yield strength of 580 MPa as an example, combined with the attached... Figure 1 , 2 Sections 3 and 4 further illustrate the present invention.

[0067] Step 1: The fatigue testing apparatus for simulating underwater wave erosion is designed with 24 support rods. Initially, the distance between the outermost support rod and the chamber wall is set to 1m. The spacing between the support rods is 150mm, and the fluid velocity entering the chamber is 1.2m / s. The fluid velocity and direction are monitored using a fluid monitoring probe, and the fluid waveform is plotted using a fluid data acquisition system. Under these conditions, the fluid flow exhibits horseshoe vortices and erosion vortices. Therefore, the distance between the outermost support rod and the chamber wall is determined to be 1m, the spacing between the support rods to be 150mm, the fluid velocity entering the chamber to be 1.2m / s, and the support rods and the test specimen are fixed to the upper and lower fixing plates at a distance of 150mm. The wave erosion space height is 45cm.

[0068] Step 2: Through static testing, the yield strength of the alloy steel was determined to be 580 MPa, providing a reference value for the loading stress in fatigue testing.

[0069] Step 3: Seawater and silt are uniformly mixed in a media mixing tank, with a silt content of 20%. This mixture is then pumped into the tank at a speed of 1.2 m / s using a dispensing pump. At this point, the circular specimen is scourned by a horseshoe-shaped vortex and an erosion vortex formed by upright rods. Circular alloy steel specimens with a yield strength of 580 MPa and a diameter of Ф6.35 mm are used, with a surface roughness of 0.8 μm and consistent surface finish. An axial tensile test is employed to avoid deviations and errors during the test. The stress ratio R of the fatigue system in the fluid medium is 0.2, the loading waveform is sinusoidal, the loading frequency is 1 Hz, and the loading stress is 174 MPa (0.3R). el The experiment proceeded to 152,944 cycles before the specimen fractured, at which point the test was stopped. The number of cycles corresponding to the specimen fracture was recorded. After the fatigue test, the working section of the specimen was treated with rust-removing fluid, rinsed with deionized water, and dried with alcohol for subsequent observation.

[0070] The fatigue life of alloy steel with a yield strength of 580 MPa is 152,944 cycles.

[0071] Example 3:

[0072] Taking alloy steel with a yield strength of 490 MPa as an example, combined with the attached... Figure 1 , 2 Sections 3 and 4 further illustrate the present invention.

[0073] Step 1: The fatigue testing apparatus for simulating underwater wave erosion is designed with 34 uprights. Initially, the distance between the outer uprights and the chamber wall is set to 1m. The spacing between the uprights is 80mm, and the fluid velocity entering the chamber is 2m / s. The fluid velocity and direction are monitored using a fluid monitoring probe, and the fluid waveform is plotted using a fluid data acquisition system. Under these conditions, the fluid flow exhibits horseshoe vortices and erosion vortices. Therefore, the distance between the outer uprights and the chamber wall is determined to be 1m, the spacing between the uprights to be 80mm, and the fluid velocity entering the chamber to be 2m / s. The uprights and the test specimen are also fixed to the upper and lower fixing plates at a distance of 80mm, and the wave erosion space height is 45cm.

[0074] Step 2: Through static testing, the yield strength of the alloy steel was determined to be 490 MPa, providing a reference value for the loading stress in fatigue testing.

[0075] Step 3: Seawater and silt are uniformly mixed in a media mixing tank, with a silt content of 30%. This mixture is then pumped into the tank at a speed of 2 m / s using a dispensing pump. At this point, the circular sample is scourned by a horseshoe-shaped vortex and an erosion vortex formed by upright rods. Circular alloy steel samples with a yield strength of 490 MPa and a diameter of Ф6.35 mm are used, with a surface roughness of 0.8 μm and consistent surface finish. An axial tensile test is employed to avoid deviations and errors during the test. The stress ratio R of the fatigue system in the fluid medium is 0.3, the loading waveform is sinusoidal, the loading frequency is 1 Hz, and the loading stress is 49 MPa (0.1R). el The experiment was conducted up to 398,557 cycles when the specimen fractured, at which point the test was stopped. The number of cycles corresponding to the specimen fracture was recorded. After the fatigue test, the working section of the specimen was treated with rust-removing fluid, rinsed with deionized water, and dried with alcohol for subsequent observation.

[0076] The fatigue life of the alloy steel with a yield strength of 490 MPa is 398,557 cycles.

[0077] Based on the above results, it can be concluded that the device of the present invention can test the fatigue life of the metal material itself, the arc additive manufacturing structure or the welded joint in the simulated wave and current scouring environment of the marine platform riser system. It truly and accurately simulates the service environment of the marine platform riser system, and the obtained fatigue performance results can be used to evaluate the safety of the structure.

Claims

1. A fatigue testing device for simulating underwater wave and current erosion, characterized in that: The system includes a flushing system and a fatigue system. The flushing system simulates a wave-current scouring environment, constructs horseshoe-shaped vortices and erosion vortices, and conducts fatigue performance tests on the samples during the wave-current scouring corrosion process. The flushing system includes a housing (1), an upper fixed plate (2), a lower fixed plate (3), a group of upright rods (4), a fluid monitoring probe (5), a fluid data acquisition system (6), a medium mixing tank (9), a dispensing pump (10), and a flow meter (11). The fatigue system includes a fatigue testing machine (7). The housing (1) is equipped with an upper fixed plate (2) and a lower fixed plate (3) on its upper and lower sides, and the space formed by the upper fixed plate (2) and the lower fixed plate (3) is... For the wave scouring space, the upper fixed plate (2) and the lower fixed plate (3) are symmetrically provided with mounting holes I (12) and several mounting holes II (13). The sample (8) passes through the mounting hole I (12) and is connected to the fatigue testing machine (7) by thread. The upright rod (4) is installed between the upper fixed plate (2) and the lower fixed plate (3) through the mounting hole II (13). The upright rod (4) is circumferentially horizontally provided with a fluid monitoring probe (5). The fluid monitoring probe (5) is connected to the fluid data acquisition system (6). The medium mixing tank (9), the dispensing pump (10), the flow meter (11) and the box (1) are connected in sequence through pipes to form a loop.

2. The fatigue testing apparatus for simulating underwater wave and current erosion according to claim 1, characterized in that, There should be no fewer than 14 upright rods in group (4).

3. The fatigue testing device for simulating underwater wave and current erosion according to claim 2, characterized in that, The vertical and horizontal distances between adjacent uprights (4) are equal.

4. The fatigue testing apparatus for simulating underwater wave and current erosion according to claim 1, characterized in that, The upright rod (4) is a cylinder made of polytetrafluoroethylene or plexiglass, and the diameter of the upright rod (4) is 1 to 3 times the diameter of the clamping section of the sample (8).

5. The fatigue testing apparatus for simulating underwater wave and current erosion according to claim 1, characterized in that, The sample (8) is a circular sample with a surface roughness of less than 0.8 μm.

6. The fatigue testing apparatus for simulating underwater wave and current erosion according to claim 1, characterized in that, The height of the wave scouring space in the scouring system box (1) is greater than the sum of the lengths of the transition section and the parallel section of the sample (8), and the distance between the outer layer upright (4) and the box wall is ≥0.7m.

7. The fatigue testing apparatus for simulating underwater wave and current erosion according to claim 1, characterized in that, The fluid monitoring probe (5) is set at the midpoint of the sample (8) at a horizontal position, and the number of circumferential fluid monitoring probes (5) on the upright rod (4) is ≥4.

8. A method of using the fatigue testing apparatus for simulating underwater wave erosion as described in any one of claims 1-7, characterized in that, Step 1: Adjust the spacing of the uprights (4) and the flow velocity test parameters of the fluid entering the box (1) to simulate the target wave flow pattern; Step 2: Perform a tensile test through a static test to determine the yield strength of the specimen (8); Step 3: The fluid medium is transported to the chamber (1) at the flow rate determined in Step 1. The specimen (8) is installed on the fatigue testing machine (7). Axial fatigue tests are carried out on the specimen (8) under preset stress, stress ratio, and loading frequency. The fatigue performance of the specimen (8) is determined by the lifting method. Each experiment is carried out until the specimen (8) breaks. The number of cycles corresponding to the breakage is taken as the fatigue life of the specimen (8) under the corresponding test conditions. The number of fatigue cycles of the specimen (8) exceeds 10. 7 until; Step 4: After the fatigue test, the working section of the sample (8) was derusted with rust remover, rinsed with deionized water and dried for subsequent observation.

9. The method of using the fatigue testing apparatus for simulating underwater wave and current erosion according to claim 8, characterized in that, The method for determining the test parameters of the spacing of the uprights (4) and the flow rate of the fluid entering the chamber (1) is as follows: the uniformly mixed simulated fluid medium is delivered to the chamber (1) by the dispensing pump (8), the fluid flow rate and direction are monitored by the fluid monitoring probe (5), and data is collected every 15 minutes. The fluid waveform is obtained by the fluid data acquisition system (6) to determine whether the fluid forms a horseshoe vortex and an erosion vortex. If not, the spacing of the uprights (4) and the flow rate of the fluid entering the chamber (1) are adjusted until the target wave flow shape is formed.

10. The method of using the fatigue testing apparatus for simulating underwater wave and current erosion according to claim 9, characterized in that, The stress ratio R is -1 to 0.5, the loading frequency is 1 to 60 Hz, and the loading stress is 0.1R. el ~0.3R el The loaded waveform is a sine wave.

Images