Net cage group nonlinear motion response analysis method and model

Abstract

The invention relates to a net cage group non-linear motion response analysis method and model. The method comprises the following steps: S1, obtaining non-linear motion response and mooring force data of a net cage group under the action of waves through marine actual observation or a wave flume test; s2, obtaining an intrinsic mode function of the nonlinear motion response and an intrinsic mode function of the mooring force of the net cage group through an empirical mode decomposition method; s3, performing Hilbert-Huang transformation on the IMF in the step S2 to obtain a frequency-frequency energy spectrum; and S4, converting the nonlinear motion response data of the net cage group from a relationship among amplitude, time and frequency to a relationship between amplitude and frequency. According to the method, signals, such as pulse interference and noise, which cannot be processed by fast Fourier transform, appearing in strong nonlinear waves or structural motion responses can be captured, quantitative analysis can be carried out on the strong nonlinear action between data, and the method can be widely popularized in the fields of offshore cage culture, floating structures and the like.

Description

Technical Field

[0001] The present disclosure relates to the technical field of marine cage aquaculture, and particularly to a method and model for analyzing the non-linear motion response of a cage group. Background Art

[0002] The research on deep-sea cages in China began in 1998. Hainan Province took the lead in introducing HDPE (High Density Polyethylene) gravity deep-sea cages from Norway. In 2001, Zhejiang Province introduced disc-shaped cages from the United States. In the following year, lifting cages were introduced from Japan. With the rapid development and application of anti-wave cage technology, large-scale industrial applications of deep-sea cages represented by HDPE gravity cages have emerged in China. By the end of 2020, the aquaculture water volume of deep-sea cages in China was 38.2139 million cubic meters, a year-on-year increase of 97.40% compared with 2019, and the aquaculture fish output reached 293,100 tons. With the continuous expansion of the scale of scientific research investment and industrial support funds from the central and local governments to support the transformation and upgrading of the fishery, the importance of developing deep-sea and far-sea aquaculture has become increasingly prominent.

[0003] Deep-sea cages have good anti-wave performance, can resist the complex wave and current field environment in the deep sea and far sea, and have the advantages of large aquaculture capacity, high efficiency and good quality. According to the working environment, operation form and structural characteristics of deep-sea cages, cages can be divided into various different forms. According to the operation type, they can be divided into four types: fixed cages, floating cages, semi-submersible cages and submerged cages. According to the deformation of the cage structure, they can be divided into flexible cages and rigid cages. According to the sealing method of the aquaculture cage, it can be divided into open cages or closed cages. After decades of development, deep-sea cages have been widely used in the United States and Norway. Various countries around the world have also developed deep-sea cages with different structures for different aquaculture environments, and still mainly gravity cages.

[0004] In order to maximize the development and utilization of the ocean and improve aquaculture efficiency, gravity cages are mostly arranged in groups. In the past thirty years, China has made breakthrough progress in the research on the anti-wave performance of the single structure of gravity cages. However, the research on the anti-wave performance of gravity cage groups is still relatively weak. Summary of the Invention

[0005] Based on this, the purpose of the present disclosure is to provide a research method and system for the anti-wave performance of cage groups, so as to provide theoretical support for the design and layout of cage groups.

[0006] To achieve the above purpose, the present disclosure adopts the following technical solutions:

[0007] A method for analyzing the non-linear motion response of a cage group, comprising the following steps:

[0008] S1. Obtain the non - linear motion responses and mooring force data of the cage group under wave action through actual marine observations or wave flume tests;

[0009] S2. Obtain the intrinsic mode functions (IMFs) of the non - linear motion responses of the cage group and the intrinsic mode functions of the mooring forces through the empirical mode decomposition (EMD) method;

[0010] S3. Perform the Hilbert - Huang transform on the IMFs in S2 to obtain the frequency - frequency energy spectrum;

[0011] S4. Transform the non - linear motion response data of the cage group from the relationship among amplitude - time - frequency to the relationship between amplitude - frequency.

[0012] Preferably, in step S2, perform EMD decomposition on the time series of the non - linear motion responses and the mooring forces of the cage group.

[0013] 3. The method for analyzing the non - linear motion response of the cage group according to claim 2, wherein step S2 is carried out as follows:

[0014] S21. Fit two envelope lines corresponding to the extreme points of the original signal x(t) through a cubic spline function, and take the mean m of the envelope lines; 1 ;

[0015] S22. Subtract the average envelope m from the original signal x(t); 1 , to obtain the data h with the low - frequency trend filtered out; 1 ;

[0016] S23. Repeat steps S21 - S22 for the data h; 1 until the constraint conditions of the IMF are satisfied, and obtain IMF1, where IMF1 = h; 1 = c; 1 ;

[0017] S24. Subtract the IMF1 from the original signal x(t), and denote r; 1 = x(t)-h; 1 ;

[0018] S25. Take the r; 1 as the original data and repeat steps S21 - S22 until IMF2 = h; 2 = c; 2 ;

[0019] S26. Repeat steps S21 - S25 until all IMFs are obtained, and the final data is decomposed into;

[0020] Preferably, the non-linear motion response is caused by low frequency and wave frequency.

[0021] To achieve the above object, the present disclosure also provides the following technical solutions:

[0022] A non-linear motion response analysis model for a cage group, which is used to perform the non-linear motion response analysis method of the cage group as described above; the model is arranged in a water tank and includes a cage group, a mooring unit, a wave-making unit, a data acquisition unit and a data analysis unit;

[0023] The cage group is fixed by the mooring unit and suspended in the water tank. The wave-making unit is used to generate waves in the water tank. The data acquisition module is used to collect the height of the waves and the non-linear motion response and mooring data of the cage group under the action of the waves;

[0024] The data analysis unit performs EMD and Hilbert-Huang transforms on the non-linear motion response and mooring data collected by the data acquisition module.

[0025] Preferably, the cage group is provided with at least 3 gravity cages, and the gravity cages are also provided with a floating ring unit and a netting unit.

[0026] Preferably, the wave-making unit includes a wave maker.

[0027] Preferably, the mooring unit includes an anchor and a rope connecting the anchor and the gravity cage.

[0028] Preferably, the data acquisition unit includes a tension sensor, a wave height meter and a high-speed camera for capturing the non-linear motion response of the cage group. The tension sensor is arranged on the rope.

[0029] Preferably, the data analysis unit includes a computer.

[0030] The technical solutions claimed by the present disclosure have achieved the following beneficial effects:

[0031] 1) The method in the present disclosure can capture signals such as pulse interference and noise that cannot be processed by fast Fourier transform when strong non-linear waves or structural motion responses appear, and can perform quantitative analysis on the strong non-linear interaction between data.

[0032] 2) The method in the present disclosure can handle strong non-linear waves, especially the non-linear motion response of the cage group caused by deformed waves such as focused waves, filling the gap in the existing calculation theory for predicting the non-linear motion response of structures caused by deformed waves.

[0033] 3) It can be widely promoted in the fields of offshore cage aquaculture and non-linear dynamic response analysis of floating structures. Brief Description of the Drawings

[0034] To more clearly illustrate the technical solutions in the embodiments of the present disclosure, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only the embodiments of the present disclosure. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on the provided drawings.

[0035] Figure 1 It is a schematic diagram of the non - linear motion response analysis model of the cage group.

[0036] Figure 2 It is the surge diagram of the cage obtained by the non - linear motion response analysis method of the cage group.

[0037] Figure 3 It is the different modes and Hilbert - Huang energy spectra of the cage obtained by the non - linear motion response analysis method of the cage group. Detailed Embodiments

[0038] To make the objectives, technical solutions and beneficial effects of the embodiments in the present disclosure clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present disclosure with reference to the drawings in the embodiments of the present disclosure. Obviously, the described embodiments are part of the embodiments of the present disclosure, rather than all of them. Based on the embodiments in the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present disclosure.

[0039] Embodiment 1

[0040] This embodiment provides a non - linear motion response analysis method for a cage group, including the following steps:

[0041] S1. Obtain the non - linear motion response and mooring force data of the cage group under wave action through on - sea actual observations or wave flume / tank tests;

[0042] S2. Obtain the intrinsic mode functions (IMFs) of the non - linear motion response of the cage group and the intrinsic mode functions of the mooring force through the Empirical Mode Decomposition (EMD) method;

[0043] S3. Perform Hilbert - Huang transform on the IMFs in S2 to obtain the frequency - frequency energy spectrum;

[0044] S4. Convert the non - linear motion response data of the cage group from the relationship among amplitude - time - frequency to the relationship between amplitude - frequency.

[0045] Among them, in step S2, perform EMD decomposition on the time series of the non - linear motion response and the mooring force of the cage group.

[0046] In a preferred solution, step S2 is carried out as follows:

[0047] S21. Fit two envelope lines corresponding to the extreme points of the original signal x(t) through a cubic spline function, and take the mean m of the envelope lines 1 ;

[0048] S22. Subtract the average envelope m from the original signal x(t) 1 , and obtain the data h with the low - frequency trend filtered out 1 ;

[0049] S23. Repeat steps S21 - S22 for the data h 1 until the constraint conditions of the IMF are met, and denote IMF1 = h 1 = c 1 ;

[0050] S24. Subtract the IMF1 from the original signal x(t), and denote r 1 = x(t) - h 1 ;

[0051] S25. Take the r 1 as the original data and repeat steps S21 - S22 until obtaining IMF2 = h 2 = c 2 ;

[0052] S26. Repeat steps S21 - S25 until all IMFs are obtained, and the final data is decomposed into

[0053] In a preferred solution, the non - linear motion response is caused by low - frequency and wave - frequency.

[0054] Specifically, the non - linear motion response analysis of the cage group in this embodiment is carried out in a water tank. The size of the water tank is 30m (length) × 2m (width) × 1.2m (height). A single cage is scaled down at a ratio of 1:50. After scaling down, the diameter of the cage is 0.8m, the height of the cage is 0.5m, and the stiffness of the floating ring is 27.59 Nm 2, the netting density is 0.36, and the three cages are connected by anchor ropes. In this embodiment, the selected focusing wave period is 1.2 s, the effective wave height is 0.06 m, the focusing position is 10 m away from the wave-making board, and the focusing time is 20 s. During the test, a preset focusing wave is first generated by the wave-making machine, and a focus is formed 10 m away from the wave-making board, which just acts on the frontmost cage, thus causing the nonlinear motion of the cage.

[0055] The nonlinear motion response of the cage, such as the nonlinear surge response, is captured by a high-speed camera. When the wave acts on the cage, the high-speed camera can capture the time history data of the cage's motion in real time. These time history data are transmitted to the computer acquisition system through the camera to form the original data, and then processed by a program. After removing the noise, subsequent empirical mode decomposition (EMD) and Hilbert-Huang transform can be carried out.

[0056] Appendix Figure 2 shows the surge diagram of the cage obtained by the method for analyzing the nonlinear motion response of the cage group in this embodiment.

[0057] Figure 3 are the different modes and Hilbert-Huang energy spectra of the cage obtained by the method for analyzing the nonlinear motion response of the cage group.

[0058] From Figure 2 it can be seen that at the moment when the wave propagates to 29 s, the surge motion of the frontmost cage reaches a peak value of 6.4 cm. In the Hilbert-Huang energy spectrum, the surge motion of the cage is jointly composed of the wave-frequency response (0.5 Hz - 1.0 Hz) represented by modes 1 - 2 and the low-frequency response (0.05 Hz - 0.2 Hz) represented by modes 3 - 4, where the frequency range of the incident wave is 0.5 Hz - 2.0 Hz. By comparing the motion responses and frequency ranges of the wave frequency band and the low-frequency band, it can be known that the wave-frequency motion represented by modes 1 - 2 is excited by the first-order wave load, and the motion amplitude is small. While the low-frequency motion represented by modes 3 - 4 is excited by the second-order wave load caused by the difference-frequency wave, and its motion response is much larger than that of the wave frequency band.

[0059] Embodiment 2

[0060] This embodiment provides a cage group nonlinear motion response analysis model for carrying out the cage group nonlinear motion response analysis method in Embodiment 1.

[0061] The model is set in a wave flume / tank and includes a cage group, a mooring unit, a wave-making unit, a data acquisition unit, and a data analysis unit.

[0062] The cage group is fixed by the mooring unit and suspended in the water tank. The wave-making unit is used to generate waves in the water tank. The data acquisition module is used to collect the wave height and the nonlinear motion response of the cage group under the action of waves and the mooring data. The data analysis unit performs EMD and Hilbert-Huang transform on the nonlinear motion response and mooring data collected by the data acquisition module.

[0063] Refer to the appendix Figure 1 , the size of the water tank is 30m (length) × 2m (width) × 1.2m (height). Considering the actual sizes of the water tank and the cages, in order to minimize the side wall effect of the water tank as much as possible, the model scale used in the experiment is 1:50. Therefore, the diameter of a single cage in the full-scale platform is 40m, and the diameter of the scaled-down cage is 0.8m, and the height of the cage is 0.5m. The model includes a wave height gauge for measuring the wave height and at least 3 gravity cages. The gravity cages are also respectively provided with a floating ring unit and a netting unit. The lower part of the netting can be provided with sinkers. The stiffness of the floating ring is 27.59 Nm 2 , and the density of the netting is 0.36.

[0064] Among them, the gravity cage is fixed by a fixed anchor and suspended in the water tank. A tension sensor is provided on the rope between the fixed anchor and the gravity cage. Among them, the tension sensor and the wave height gauge respectively transmit the tension mooring data and the wave height data to the computer acquisition system.

[0065] In this embodiment, the selected focusing wave period is 1.2s, the effective wave height is 0.06m, the focusing position is 10m away from the wave-making board, and the focusing time is 20s. During the experiment, first, the preset focusing wave is generated by the wave-making machine, and a focus is formed 10m away from the wave-making board, which just acts on the frontmost cage, thus causing the nonlinear motion of the cage.

[0066] The model is also provided with a high-speed camera, which is used to capture the nonlinear motion response of the cage, such as the nonlinear surge response. When the wave acts on the cage, the high-speed camera can capture the motion time history data of the cage in real time. These time history data are transmitted to the computer acquisition system through the camera to form the original data, and then processed by a program. After removing the noise, the subsequent empirical mode decomposition (EMD) and Hilbert-Huang transform can be carried out.

[0067] The embodiments described above are only exemplary descriptions of the present disclosure, and do not limit the scope of the present disclosure. Without departing from the design spirit of the present disclosure, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present disclosure should fall within the protection scope determined by the present disclosure.

Claims

1. A method for analyzing the non - linear motion response of a cage group, characterized in that, it comprises the following steps: S1. Obtain the non - linear motion response and mooring force data of the cage group under wave action through actual offshore observations or wave flume tests; S2. Obtain the intrinsic mode functions (IMFs) of the non - linear motion response of the cage group and the intrinsic mode functions of the mooring force through the empirical mode decomposition (EMD) method; S3. Perform Hilbert - Huang transform on the IMFs in S2 to obtain the frequency - frequency energy spectrum; S4. Transform the non - linear motion response data of the cage group from the relationship among amplitude - time - frequency to the relationship between amplitude - frequency.

2. The method for analyzing the non - linear motion response of a cage group according to claim 1, characterized in that, in step S2, perform EMD decomposition on the time series of the non - linear motion response and the mooring force of the cage group.

3. The method for analyzing the non - linear motion response of a cage group according to claim 2, characterized in that, step S2 is carried out as follows: S21. Fit two envelope lines corresponding to the extreme points of the original signal x(t) through a cubic spline function, and take the mean value m of the envelope lines 1 ; S22. Subtract the average envelope m from the original signal x(t) 1 to obtain the data h with the low-frequency trend removed 1 ; S23. For the data h 1 Repeat steps S21 - S22 until the constraint conditions of the IMF are satisfied, and obtain IMF1, where IMF1 = h 1 = c 1 ; S24. Subtract the IMF1 from the original signal x(t), and denote it as r 1 = x(t) - h 1 ; S25. Repeat steps S21 - S22 with the said r 1 as the original data until IMF2 = h 2 = c 2 ; S26. Repeat steps S21 to S25 until all IMFs are obtained. The final data is decomposed into 4. The method for analyzing the non - linear motion response of a cage group according to any one of claims 1 to 3, characterized in that, the non - linear motion response is caused by low - frequency and wave - frequency.

5. A non - linear motion response analysis model of a cage group, characterized in that, the model is used to perform the method for analyzing the non - linear motion response of a cage group according to any one of claims 1 to 4; the model is set in a flume and includes a cage group, a mooring unit, a wave - making unit, a data acquisition unit and a data analysis unit; the cage group is fixed by the mooring unit and suspended in the flume, the wave - making unit is used to generate waves in the flume, and the data acquisition module is used to collect the wave height and the non - linear motion response and mooring data of the cage group under wave action; the data analysis unit performs EMD and Hilbert - Huang transform on the non - linear motion response and mooring data collected by the data acquisition module.

6. The non - linear motion response analysis model of a cage group according to claim 5, characterized in that, the cage group is provided with at least 3 gravity cages, and the gravity cages are also provided with a floating ring unit and a netting unit.

7. The non - linear motion response analysis model of a cage group according to claim 6, characterized in that, the wave - making unit includes a wave - maker.

8. The non - linear motion response analysis model of a cage group according to claim 6, characterized in that, the mooring unit includes an anchor and a rope connecting the anchor and the gravity cage.

9. The non - linear motion response analysis model of a cage group according to claim 8, characterized in that, the data acquisition unit includes a tension sensor, a wave height gauge and a high - speed camera for capturing the non - linear motion response of the cage group, and the tension sensor is arranged on the rope.

10. The non - linear motion response analysis model of a cage group according to claim 5, characterized in that, the data analysis unit includes a computer.

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