A vibration control method for a typical cantilever structure of an offshore platform

Abstract

The invention relates to the technical field of marine platforms, and specifically discloses a vibration control method for a typical cantilever structure of a marine platform, comprising the following steps: step 1, based on a layout diagram and a structure diagram of the marine platform, using ABSQUS finite element software to preliminarily construct a geometric model of the cantilever structure, including a foundation structure attached to the cantilever structure; step 2, based on the finite element geometric model, performing vibration modal analysis to obtain the natural frequency characteristics of the cantilever structure, wherein the natural frequency includes a first-order natural frequency; the invention preliminarily constructs a finite element model of the cantilever structure of the marine platform, and obtains the natural frequency of the structure by modal analysis; statistically calculates the design period of wind and waves in the working sea area of ​​the marine platform, as well as the rotation speed and the number of blades of a propeller of the platform's natural power system, so as to obtain the excitation frequency of wind, waves and the propeller; empirically determines that the natural frequency value of the cantilever structure is not less than 4 Hz, and avoids the risk of resonance with the excitation source.

Description

Technical Field

[0001] The invention relates to the technical field of marine platforms, in particular to a vibration control method for a typical cantilever structure of a marine platform. Background Art

[0002] Cantilever structures are a common and typical structural form on offshore platforms, such as cranes, masts, external exploration platforms, and pipeline supports. As classification societies and crews have higher and higher requirements for the safety and comfort of offshore platform structures, the low-frequency vibration problem of typical cantilever structures has become an urgent problem to be solved. In the actual design and production process, the use of the Finite Element Method (FEM) for structural vibration analysis, especially structural vibration modal analysis, is currently a relatively mature analysis method. However, there are no relevant literature or publicly published articles that conduct a detailed evaluation and explanation of the control of low-frequency vibration problems of typical cantilever structures of offshore platforms.

[0003] At present, there are some public reports related to the present invention, mainly including: Document 1, a method and system for evaluating the performance of a marine mast based on virtual reality (invention patent application, 202210205494.1); Document 2, finite element analysis of structural strength and vibration of a truss-type integrated mast (Guangdong Shipbuilding, Issue 4, 2017 (Issue 155 in total)); Document 3, modal analysis of a ship's cylindrical mast (Ship Science and Technology, Vol. 28, No. 4, August 2006).

[0004] Among them: Document 1 proposes a performance evaluation method and system for marine masts based on virtual reality, conducts reliability evaluation on the performance of masts in specific application scenarios, and evaluates the displacement deformation analysis results and reliability analysis of mast modal analysis, but does not evaluate and analyze the low-frequency vibration of masts caused by propeller excitation; Document 2 describes the use of finite element analysis software to analyze and calculate the strength and vibration characteristics of truss-type integrated mast structures, but does not involve the evaluation and control criteria of modal analysis; Document 3 focuses on the processing of simulation methods such as model simplification, concentrated mass processing, and boundary conditions in the process of mast modal analysis using the finite element method, with the aim of improving the reliability of simulation analysis. It has no enlightenment for solving the low-frequency vibration problem of cantilever structures on site and needs to be improved. Summary of the invention

[0005] The object of the present invention is to provide a vibration control method for a typical cantilever structure of an offshore platform to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A vibration control method for a typical cantilever structure of an offshore platform comprises the following steps:

[0008] Step 1: Based on the layout and structure diagram of the offshore platform, use ABSQUS finite element software to preliminarily construct the geometric model of the cantilever structure, including the foundation structure attached to the cantilever structure;

[0009] Step 2: Based on the finite element geometric model, vibration modal analysis is performed to obtain the natural frequency characteristics of the cantilever structure, where the natural frequency includes the first-order natural frequency;

[0010] Step 3: Count the design period of wind and waves in the working sea area of ​​the offshore platform, as well as the speed and number of blades of the propeller of the offshore platform's inherent power system;

[0011] Step 4: Based on the calculation results of steps 2 and 3, analyze and evaluate the resonance risk of the cantilever structure.

[0012] In an optional solution: in step 1, the cantilever structure includes a mast, and the mast is installed on the top of the offshore platform.

[0013] In an optional solution: in the step 1, the offshore platform layout diagram includes the location of the cantilever structure, and the structural diagram includes the structural shape and material properties of the cantilever structure.

[0014] In an optional solution: the finite element modal analysis results of the cantilever structure in step 2 are classified according to the frequency and vibration mode, and the vibration mode is divided into bending vibration and torsional vibration, and is divided into the following according to the frequency from small to large: 1st order bending f 1b , 1st order twist f 1t , 2nd order bending f 2b , 2nd order twist f 2t .

[0015] In an optional solution: the first-order natural frequency analysis result in step 2 is 1st-order bending or 1st-order torsion.

[0016] In one optional solution: make a preliminary judgment on the results of the modal analysis in step 2, including vibration frequency judgment and vibration mode judgment. The first-order natural frequency range of the cantilever structure is between 1Hz and 5Hz. If it is lower than 1Hz, or the frequency value is greater than 5Hz, it is necessary to return to step 1 to confirm whether the simulation of material properties or point mass is correct.

[0017] In an optional solution: the step three further includes: obtaining the periodic data of wind and waves in the corresponding sea area according to the statistical database data of the sea area and converting it into the natural frequency f of wind and waves according to the frequency = 1 / period. wind 、f wave； According to the design speed of the offshore platform propeller, the excitation fundamental frequency, i.e., the shaft frequency AF = speed / 60, and the excitation blade frequency BPF = speed / 60*number of blades.

[0018] In an optional solution: the step four further includes: determining the resonance risk range of the excitation source according to the vibration of the cantilever structure, and the resonance risk range is [0.85*excitation blade frequency, 1.15*excitation blade frequency].

[0019] In an optional solution: the minimum vibration frequency limit of the cantilever structure is 4 Hz.

[0020] In one optional scheme: when the natural frequency of the cantilever structure cannot meet the requirements of the resonance risk range and the minimum vibration frequency limit, it is considered that the vibration risk is large, and the structural optimization design must be carried out, and recalculation is carried out according to steps one, two, three and four until the natural frequency of the cantilever structure meets the requirements of the resonance risk range and the minimum vibration frequency limit.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] Based on basic information such as layout drawings, structural drawings, equipment (weight, center of gravity, rotation speed), etc., the present invention preliminarily constructs a finite element model of the cantilever structure of the offshore platform, and obtains the natural frequency of the structure by modal analysis; the design period of wind and waves in the working sea area of ​​the offshore platform, as well as the rotation speed and number of blades of the propeller of the platform's inherent power system are statistically analyzed to obtain the excitation frequency of wind, waves and propellers; based on experience, it is determined that the natural frequency value of the cantilever structure is not less than 4Hz, and the risk of resonance with the excitation source is avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the process of the present invention.

[0024] Figure 2 It is a schematic diagram of a simplified model of the cantilever structure and the attached deck in the present invention.

[0025] Figure 3 Schematic diagram of boundary selection and definition for modal analysis of cantilever structure in the present invention.

[0026] Figure 4 It is a schematic diagram of the modal analysis of the initial design scheme of the cantilever structure in the present invention.

[0027] Figure 5 It is a schematic diagram of the optimized design structure of the cantilever structure in the present invention.

[0028] Figure 6 Schematic diagram of modal analysis results for the cantilever structure optimization design in the present invention. DETAILED DESCRIPTION

[0029] See also Figure 1-Figure 6 In an embodiment of the present invention, a vibration control method for a typical cantilever structure of an offshore platform includes the following steps:

[0030] Step 1: Based on the layout and structure diagram of the offshore platform, use ABSQUS finite element software to preliminarily construct the geometric model of the cantilever structure, including the foundation structure attached to the cantilever structure;

[0031] Step 2: Based on the finite element geometric model, vibration modal analysis is performed to obtain the natural frequency characteristics of the cantilever structure, including the first-order natural frequency. The simplification principle of the cantilever structure vibration analysis model is: all plane and curved panel structures are simulated with shell elements, and smaller components are ignored to achieve the purpose of rapid modeling.

[0032] Step 3: Count the design period of wind and waves in the working sea area of ​​the offshore platform, as well as the speed and number of blades of the propeller of the offshore platform's inherent power system;

[0033] Step 4: Based on the calculation results of Step 2 and Step 3, analyze and evaluate the resonance risk of the cantilever structure. The analysis model includes the cantilever structure and its foundation structure to eliminate the influence of boundary conditions on the analysis results.

[0034] The offshore platform involved in this application is about 45 meters high, 55 meters wide, with a designed draft of 9 meters, a service speed of 10 knots, a crew of 239 people, and an empty ship weight of 23,600 tons. The main deck area is about 10,000 square meters, equipped with 6 5,000-kilowatt thrusters, a 50-ton deck crane, etc.

[0035] The cantilever structure in step 1 includes a mast, which is installed on the top of the offshore platform; the mast is 4.2 meters high, the platform is 2.5 meters wide and 3 meters long, and the mast and its accessories weigh about 1.1 tons.

[0036] In order to carry out structural modeling and simulation analysis, a 3D geometric model is established. First, according to the layout diagram and structural diagram, a mast and attached deck structure model (especially the extension range of mast attachments and deck structure) is established to facilitate the simulation of attachment weight and the setting of boundary conditions. The structures are all represented by plates and shells, and other smaller auxiliary structures are ignored. The purpose of rapid modeling and improved calculation efficiency is achieved by reasonably simplifying the model scale and reducing the amount of calculation;

[0037] The boundary conditions around the extended deck are set as simply supported, which effectively reduces the impact of the boundary on the finite element modal analysis results of the mast structure;

[0038] Finally, virtual weights are added to the platforms on each layer of the mast to ensure that the overall weight distribution of the mast is consistent with the actual situation.

[0039] In the step 1, the offshore platform layout diagram includes the location of the cantilever structure, and the structural diagram includes the structural shape and material properties of the cantilever structure.

[0040] The finite element modal analysis results of the cantilever structure in step 2 are classified according to the frequency and vibration mode. The vibration mode is divided into bending vibration and torsional vibration, and is divided into the following according to the frequency from small to large: 1st order bending f 1b , 1st order twist f 1t , 2nd order bending f 2b , 2nd order twist f 2t .

[0041] The first-order natural frequency analysis result in step 2 is 1st-order bending or 1st-order torsion.

[0042] Make a preliminary judgment on the results of the modal analysis in step 2, including vibration frequency judgment and vibration mode judgment. The first-order natural frequency range of the cantilever structure is between 1Hz and 5Hz. If it is lower than 1Hz, or the frequency value is greater than 5Hz, it is necessary to return to step 1 to confirm whether the simulation of material properties or point mass is correct.

[0043] The step three further includes: obtaining the periodic data of wind and waves in the corresponding sea area according to the statistical database data of the sea area and converting it into the natural frequency f of wind and waves according to the frequency = 1 / period wind 、f wave； According to the design speed of the offshore platform propeller, the excitation fundamental frequency, i.e., the shaft frequency AF = speed / 60, and the excitation blade frequency BPF = speed / 60*number of blades.

[0044] The step 4 further includes: determining a resonance risk range of the excitation source according to the vibration of the cantilever structure, wherein the resonance risk range is [0.85*excitation blade frequency, 1.15*excitation blade frequency].

[0045] According to statistics, the excitation frequency of wind in the working area of ​​the offshore platform is 0.5Hz, the excitation frequency of waves is 0.4Hz, and the shaft frequency of the offshore platform power system (propeller) is 2Hz and the blade frequency is 8Hz;

[0046] Based on experience, it is determined that the minimum design frequency of the cantilever structure of an offshore platform is no less than 4Hz;

[0047] Therefore, the risk frequency interval is determined to be: [0,4Hz], [0.85*8Hz,1.15*8Hz].

[0048] The minimum vibration frequency limit of the cantilever structure is 4 Hz.

[0049] When the natural frequency of the cantilever structure cannot meet the requirements of the resonance risk range and the minimum vibration frequency limit, it is considered that the vibration risk is large, and structural optimization design must be carried out and recalculated according to steps one, two, three and four until the natural frequency of the cantilever structure meets the requirements of the resonance risk range and the minimum vibration frequency limit.

[0050] The initial design natural frequency of the mast structure is 1.78 Hz, which is within the risk frequency range [0,4] and does not meet the design requirements;

[0051] After structural optimization design and structural natural frequency analysis, the optimized analysis result is 4.11 Hz, which can avoid all risk frequency ranges.

[0052] The optimized design of the cantilever structure in the present invention avoids the waste of manpower, material resources and production cycle caused by discovering resonance problems and making modifications during later production and construction, greatly improving design efficiency and reducing the risk of rework.

Claims

1. A vibration control method for a typical cantilever structure of an offshore platform, characterized in that: The following steps are involved: Step 1: Based on the layout and structure diagram of the offshore platform, use ABSQUS finite element software to preliminarily construct the geometric model of the cantilever structure, including the foundation structure attached to the cantilever structure; Step 2: Based on the finite element geometric model, vibration modal analysis is performed to obtain the natural frequency characteristics of the cantilever structure, where the natural frequency includes the first-order natural frequency; Step 3: Count the design period of wind and waves in the working sea area of ​​the offshore platform, as well as the speed and number of blades of the propeller of the offshore platform's inherent power system; Step 4: Based on the calculation results of steps 2 and 3, analyze and evaluate the resonance risk of the cantilever structure; The finite element modal analysis results of the cantilever structure in step 2 are classified according to the frequency and vibration mode. The vibration mode is divided into bending vibration and torsional vibration, and is divided into the following according to the frequency from small to large: 1st order bending f 1b , 1st order twist f 1t , 2nd order bending f 2b , 2nd order twist f 2t ; The first-order natural frequency analysis result in step 2 is 1st-order bending or 1st-order torsion; Make a preliminary judgment on the results of the modal analysis in step 2, including vibration frequency judgment and vibration mode judgment. The first-order natural frequency range of the cantilever structure is between 1Hz and 5Hz. If it is lower than 1Hz, or the frequency value is greater than 5Hz, it is necessary to return to step 1 to confirm whether the simulation of material properties or point mass is correct; The step three includes: obtaining the periodic data of wind and waves in the corresponding sea area according to the statistical database data of the sea area and converting it into the natural frequency f of wind and waves according to the frequency = 1 / period wind 、f wave； According to the design speed of the propeller of the offshore platform, the excitation fundamental frequency, i.e., shaft frequency AF = speed / 60, and the excitation blade frequency BPF = speed / 60*number of blades; The step 4 includes: determining a resonance risk range of the excitation source according to the vibration of the cantilever structure, wherein the resonance risk range is [0.85*excitation blade frequency, 1.15*excitation blade frequency]; The minimum vibration frequency limit of the cantilever structure is 4 Hz; When the natural frequency of the cantilever structure cannot meet the requirements of the resonance risk range and the minimum vibration frequency limit, it is considered that the vibration risk is large, and structural optimization design must be carried out and recalculated according to steps one, two, three and four until the natural frequency of the cantilever structure meets the requirements of the resonance risk range and the minimum vibration frequency limit.

2. The vibration control method of a typical cantilever structure of an offshore platform according to claim 1, characterized in that: In the step 1, the cantilever structure includes a mast, and the mast is installed on the top of the offshore platform.

3. The vibration control method of a typical cantilever structure of an offshore platform according to claim 1, characterized in that: In the step 1, the offshore platform layout diagram includes the location of the cantilever structure, and the structural diagram includes the structural shape and material properties of the cantilever structure.

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