A quantitative evaluation method for the fatigue prevention and control effect of an offshore platform

Abstract

The present invention relates to a quantitative evaluation method for the fatigue prevention and control effect of an offshore platform, including: establishing a numerical model of the offshore platform based on SACS software and verifying its reliability, obtaining the fatigue stress and cycle times of the nodes by spectral fatigue analysis, and calculating the RMS of the platform fatigue stress level before fatigue prevention and control by combining the node fatigue stress, cycle times, and wave direction probability. O The spectral fatigue analysis simultaneously obtains the fatigue life T before prevention and control. O Incorporate the prevention and control method to establish a numerical model after fatigue prevention and control, and obtain the RMS of the platform fatigue stress level after prevention and control by spectral fatigue analysis under the same working conditions. R and the fatigue life T after prevention and control. R Based on RMS O and RMS R calculate the stress level reduction index D. RMS Based on T O and T R calculate the fatigue life improvement index I. T Based on the index D RMS and I T give an evaluation of the prevention and control effect and a prevention and control decision. The present invention provides a clear and easy-to-use evaluation method for the fatigue prevention and control effect of the platform, which is convenient for the personnel in this technical field to first evaluate and screen and then implement prevention and control to ensure the prevention and control effect, filling the relevant technical gaps.

Description

Technical Field

[0001] The present invention belongs to the technical field of ocean engineering, and specifically relates to a quantitative evaluation method for the fatigue prevention and control effect of an ocean platform. Background Art

[0002] As a relatively common platform form in the early stage in China, the fixed jacket platform has mostly entered the end of its service life or exceeded its service life at the present stage, and its structural integrity is generally poor. Due to the extremely harsh working environment, the platform is prone to fatigue damage under the long-term action of ocean environmental loads. Once problems occur in the local structure of the platform, it often causes the platform to tilt, break, or even collapse completely, resulting in huge economic and environmental losses. Therefore, the aging jacket platforms generally face the problem of fatigue protection and need to take certain fatigue protection measures to enable them to serve until the expiration of their service life or beyond.

[0003] For the fatigue protection work of aging platforms, the selection of prevention and control measures in the current industry is blind and lacks a scientific basis for selection. At the same time, the prevention and control effect cannot be quantitatively predicted. There is no mature practice and standard in the industry on how to quantitatively predict the fatigue prevention and control effect of the platform and scientifically screen the prevention and control measures, nor is there a relevant patent method to provide a solution. Therefore, there is an urgent need to propose a quantitative evaluation method for the fatigue prevention and control effect of an ocean platform to solve the problems of predicting the effectiveness of the platform prevention and control measures and the engineering requirements for screening the prevention and control measures. Summary of the Invention

[0004] To overcome the blank of the existing technology, a quantitative evaluation method for the fatigue prevention and control effect of an ocean platform is proposed.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] 1. A quantitative evaluation method for the fatigue prevention and control effect of an ocean platform, comprising the following steps:

[0007] 1) Based on the SACS software, establish a numerical model of the ocean platform according to the structural dimensions, material properties, section properties, center of gravity, integrity status of the in-service ocean platform, and the pile-soil constraint effect, and conduct model reliability verification. Set the main nodes and retained main degrees of freedom for platform analysis to improve the calculation efficiency;

[0008] 2) Based on the numerical model of the ocean platform established in step 1), conduct spectral fatigue analysis with the wave scatter diagram as the load to obtain the fatigue cyclic stress S of the damaged nodes of the platform under different wave directions before fatigue prevention and control of the ocean platform ij , the corresponding number of cycles C ij , and combine the wave direction probability p i Comprehensively consider the root mean square value of the fatigue cyclic stress of the node with the maximum damage under all working conditions as the platform fatigue stress level RMS O, spectral fatigue analysis can also obtain the fatigue life of the nodes. Take the life of the node with the maximum damage as the fatigue life T before platform prevention and control O ;

[0009] 3) Based on the numerical model of the offshore platform established in step 1), simplify the numerical model of the fatigue prevention and control method and incorporate the model in 1) as the numerical model of the offshore platform after fatigue prevention and control. Conduct the same-condition spectral fatigue analysis in 2) to obtain the fatigue cyclic stress S of the damaged nodes on the platform under different wave directions after the fatigue prevention and control of the offshore platform ij , and the corresponding number of cycles C ij , combined with the wave direction probability p i Comprehensively consider the root mean square value of the fatigue cyclic stress of the node with the maximum damage under all working conditions as the fatigue stress level RMS of the platform R , spectral fatigue analysis can also obtain the fatigue life of the nodes. Take the life of the node with the maximum damage as the fatigue life T after platform prevention and control R ;

[0010] 4) Use the fatigue stress level RMS of the platform before fatigue prevention and control obtained in 2) O and the fatigue stress level RMS of the platform after fatigue prevention and control obtained in 3) R Calculate the stress level reduction index D RMS . The larger the index value, the better the fatigue prevention and control effect of the platform, and the more significant the overall reduction effect of the stress level;

[0011] 5) Use the fatigue life T of the platform before fatigue prevention and control obtained in 2) O and the fatigue life T of the platform after fatigue prevention and control obtained in 3) R Calculate the fatigue life improvement index I T . The larger the index value, the better the fatigue prevention and control effect of the platform, and the more significant the improvement effect of the fatigue life;

[0012] 6) Based on the stress level reduction index D RMS , fatigue life improvement index I T Give an evaluation of the prevention and control effect. The stress level reduction index D RMS and the fatigue life improvement index I T are positively correlated, and the two can be mutually verified. If both indexes are relatively large, it indicates that the prevention and control effect of the fatigue damage prevention and control method of the offshore platform is relatively good. If both indexes are relatively small, it indicates that the prevention and control effect of the prevention and control method is not good. These two indexes can be used to guide the decision-making of selecting the fatigue damage prevention and control method of the offshore platform.

[0013] The present invention has the following beneficial effects:

[0014] (1) The quantitative evaluation method for the fatigue prevention and control effect of the offshore platform described in the present invention provides a clear and easy-to-use evaluation method for the engineering requirements of offshore platform fatigue prevention and control, filling the gaps in the related technologies in this technical field.

[0015] (2) The quantitative evaluation method for the fatigue prevention and control effect described in the present invention provides a standard practice for screening offshore platform fatigue prevention and control methods and predicting prevention and control effects in this technical field, facilitating personnel in this technical field to first evaluate and screen and then implement prevention and control to ensure the prevention and control effect.

[0016] (3) The numerical modeling method for offshore platforms based on SACS software described in the present invention proposes a refined modeling method for offshore platforms for personnel in this technical field, and details the modeling implementation steps and key points to be noted for accurate analysis.

[0017] (4) The spectral fatigue analysis process for offshore platforms based on SACS software described in the present invention proposes a fatigue analysis method for offshore platforms. The patent details the spectral fatigue analysis process based on SACS software and the key input and output files in the analysis process. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the quantitative evaluation method for the fatigue prevention and control effect of the offshore platform of the present invention;

[0019] Figure 2 It is a schematic diagram of the refined modeling method for offshore platforms based on SACS software;

[0020] Figure 3 It is a schematic diagram of the spectral fatigue process for offshore platforms based on SACS software. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be further described in detail below with reference to the accompanying drawings:

[0022] As Figure 1 shown, the present invention provides a quantitative evaluation method for the fatigue prevention and control effect of an offshore platform, which is characterized by including the following steps:

[0023] 1) As Figure 2As shown, based on the design documents and drawing materials of the offshore platform to be analyzed, a model is built using SACS software. Set the platform material, beam section, and unit group properties. Build nodes and beam elements layer by layer to establish the structure of each module. Set the offset of the tubular joint and the flush offset of the platform deck beam. Set the correction of the effective length for the buckling check of beam element compression members. Simulate the actual weight load distribution with counterweights. According to the "Classification and Construction Specification for Fixed Offshore Platforms", considering the soil type, simplify the pile-soil constraint effect as a fixed constraint at a position n times the pipe diameter below the mud surface. Set the thickness of marine organism attachment and corrosion thickness according to the platform integrity situation, and select the hydrodynamic coefficient C according to DNV RPC205 specification D , C M , and conduct the verification of the effective self-weight force transfer after considering buoyancy in the model. Verify the reliability of the model by comparing the effective self-weight after considering buoyancy with the reaction force. Verify the correctness of the wave application in the single-pile numerical model by comparing it with the single-pile wave loading test in the "Research on Wave-Current Forces on Pile Foundation Structures" published by scholar Lan Yamei. Verify the sea current loading in the single-pile numerical model through the Morrison formula. After verification, load it to the platform numerical model in the same way to ensure the loading reliability. To improve the calculation efficiency, set the main nodes and retained main degrees of freedom for platform analysis, and set the node constraint to 222000;

[0024] 2) As Figure 3 shown, based on the offshore platform numerical model established in 1) using SACS software, conduct modal analysis to obtain the mass result file and modal result file expressing the vibration mode characteristics of the platform. Use the platform model file, mass result file, modal result file, load setting file, and response setting file as inputs to conduct platform response analysis in different wave directions to obtain the platform node stress transfer function result file in different wave directions. The load setting file specifies a series of wave loads with the same wave steepness. The response setting file specifies the stress response output. Use the load setting files in all wave directions, the platform node stress transfer function result file, and the file specifying the fatigue analysis settings as inputs to conduct spectral fatigue analysis and output the fatigue cyclic stress S ij , corresponding cycle number C ij of the damaged nodes of the platform in different wave directions before fatigue prevention of the offshore platform. Combine the wave direction probability p i and comprehensively consider the root mean square value of the fatigue cyclic stress of the node with the maximum damage under all working conditions (the calculation method is as follows formula (1)) as the platform fatigue stress level RMS O , and at the same time, the fatigue life T of the platform before prevention can be obtained O, the fatigue analysis setup file specifies the platform operation years, safety factor, S-N curve, stress concentration factor, wave spectrum, wave scatter diagram, and the nodes and beam groups for the output results. The safety factor is selected according to the CCS "Technical Guide for the Fatigue Strength Assessment of Offshore Engineering Structures". The S-N curve and stress concentration factor are selected using the built-in standard methods of the software. The wave spectrum is selected from the P-M spectrum and JONSWAP spectrum, and the wave scatter diagram is selected from the measured data related to the offshore platform to be analyzed. If there is no measured data, the wave scatter diagram of the corresponding location block is selected from the "Northwest Pacific Wave Statistics Collection" edited by Fang Zhongsheng;

[0025]

[0026] In Equation (1), where p i is the probability of the i-th directional condition; RMS i is the root mean square value of the fatigue stress in the i-th direction, S ij is the j-th cyclic stress amplitude under the i-th directional condition, C ij is the number of cycles of the j-th cyclic stress amplitude under the i-th directional condition, M is the number of wave directions, N i is the number of different stress amplitudes in the i-th direction.

[0027] 3) As Figure 1 shown, based on the numerical model of the offshore platform established in step 1), the numerical model of the fatigue prevention method is simplified and incorporated into the model in 1) as the numerical model of the offshore platform after fatigue prevention, and the spectral fatigue analysis under the same conditions in 2) is carried out to obtain the fatigue cyclic stress S ij and the corresponding number of cycles C ij of the damaged nodes of the platform under different wave directions after fatigue prevention of the offshore platform. Combining with the wave direction probability p i , the root mean square value of the fatigue cyclic stress of the node with the largest damage under all conditions is comprehensively considered (the calculation method is as shown in Equation (1)) as the platform fatigue stress level RMS R , and the spectral fatigue analysis can also obtain the fatigue life T R of the platform after prevention;

[0028] 4) As Figure 1 shown, the platform fatigue stress level RMS O before fatigue prevention obtained in 2) and the platform fatigue stress level RMS R after fatigue prevention obtained in 3) are used to calculate the stress level reduction index D RMS . The calculation method is as shown in the following Equation (2). The larger the index value, the better the fatigue prevention effect of the platform and the more significant the overall reduction effect of the stress level;

[0029] D RMS =(RMS O -RMS R ) / RMSO × 100% (2)

[0030] Among them, RMS O is the fatigue stress level of the node with the maximum platform damage before prevention and control; RMS R is the fatigue stress level of the node with the maximum platform damage after prevention and control.

[0031] 5) As Figure 1 shown, use the fatigue life T O of the platform before fatigue prevention and control obtained in 2) R and the fatigue life T T of the platform after fatigue prevention and control obtained in 3) to calculate the fatigue life improvement index I

[0032] I T =(T R - T O ) / T O × 100% (3)

[0033] In the formula, T O is the life of the node with the maximum platform damage before prevention and control; T R is the life of the node with the maximum platform damage after prevention and control.

[0034] 6) Based on the stress level reduction index D RMS , fatigue life improvement index I T , give an evaluation of the prevention and control effect. The stress level reduction index D RMS and the fatigue life improvement index I T are positively correlated and can verify each other. If both indicators are relatively large, it means that the prevention and control effect of the fatigue damage prevention and control method for the offshore platform is relatively good. If both indicators are small, it means that the prevention and control effect of the prevention and control method is not good. These two indicators can be used to guide the decision-making of selecting the fatigue damage prevention and control method for the offshore platform.

[0035] The following is an illustration with specific examples:

[0036] 1) The in-service offshore platform to be analyzed is a jacket platform. According to the design drawings, it is known that the platform consists of a main platform and two wellhead platforms, A and B, with a total weight of 4,790 tons, superstructure equipment and loads of 1,870 tons, and a total steel structure weight of 2,920 tons. Among them, the jacket weighs 630 tons, the superstructure steel structure weighs 1,060 tons, the height of the platform main body is 39.2 m, the height of the first-floor superstructure is 7.5 m, the height of the second-floor superstructure is 7.9 m, the pile penetrates into the mud by 58.5 m, the length and width span of the jacket penetration into the mud is 24 m × 21.98 m, the length and width span of the working point is 24 m × 18 m, the overall dimensions of the first-floor superstructure are 28.4 m × 52.8 m, the overall dimensions of the second-floor superstructure are 32 m × 52.8 m, and the diameter of the main column of the jacket is 1,400 mm. Based on the design drawings, a model is built using SACS software, setting the platform material, beam section, and unit group attributes, building nodes and beam elements layer by layer, establishing the block structures, setting the offset of the pipe joints and the flush offset of the platform deck beams, setting the correction of the effective length for the buckling check of beam element compression members, and conducting counterweight according to the actual distribution of the structural weight. After counterweight, the total weight is 4,790 t. The platform pile foundation soil is silty clay. According to the "Classification and Construction Specification for Offshore Fixed Platforms", considering the soil as silty clay, the pile-soil constraint effect is simplified to a fixed constraint at a position 8.5D (i.e., -11.9 m) below the mud surface. According to the data in the platform integrity inspection report, the attached marine organisms are set to 25 cm, the corrosion thickness is 1.0 mm, and the pile leg is hollowed out by 2 m. The hydrodynamic coefficients C D and C M are selected according to the DNV RP C205 specification to verify the static load transfer of the model. The self-weight of the model is 4,790 t, the buoyancy is 434.5 t, and the total constraint reaction force under the effective self-weight considering buoyancy is 4,350 t. The transfer force error is only 0.12%, verifying the reliable transfer of the structure. Through the single-pile wave loading test published by scholar Lan Yamei in "Research on Wave and Current Forces on Pile Foundation Structures" as a comparison, the correctness of the single-pile numerical model wave application is verified, and the error is only 1.47%. The single-pile numerical model current loading is verified through the Morrison formula, and the error is only 2.5%. The errors of the wave and current numerical loading verifications are very small, verifying reliability, and the same loading method is used to load the platform numerical model to ensure loading reliability. To improve the calculation efficiency, the main nodes and the main degrees of freedom to be retained for the platform analysis are set. By setting the node constraints to 222,000, using the transfer force nodes of the jacket, riser, and main beams of the upper module as the main nodes, and retaining their translational degrees of freedom as the main degrees of freedom;

[0037] 2) Based on the numerical model of the offshore platform established in 1), perform modal analysis using SACS software to obtain the mass result file and modal result file representing the vibration characteristics of the platform. Using the platform model file, mass result file, modal result file, load setting file, and response setting file as inputs, conduct platform response analysis by wave direction to obtain the platform node stress transfer function result file under different wave directions. The load setting file specifies a series of wave loads with the same wave steepness, and the response setting file specifies the stress response output. Using the load setting files for all wave directions, the platform node stress transfer function result files, and the file specifying the fatigue analysis settings as inputs, conduct spectral fatigue analysis. The fatigue analysis setting file specifies that the platform operates for 15 years, selects a safety factor of 5 times according to the CCS "Technical Guide for Fatigue Strength Assessment of Offshore Engineering Structures", selects the built-in standard methods for the S-N curve and stress concentration factor, selects the P-M spectrum for the wave spectrum, and selects Block B1 as the wave scatter diagram according to "Northwest Pacific Wave Statistics Collection" edited by Fang Zhongsheng for the platform located in the Bohai Bay. The fatigue analysis outputs the fatigue cyclic stress S of the platform nodes under different wave directions before fatigue prevention and control of the offshore platform ij and the corresponding number of cycles C ij , combined with the wave direction probability p i Comprehensively consider the root mean square value of the fatigue cyclic stress of the node with the maximum damage under all working conditions as the platform fatigue stress level RMS O , calculate RMS according to formula (1) O = 17.88 MPa, obtain the fatigue life T of the platform before prevention and control O = 6 years, which does not meet the requirement of the design life of 20 years, and the platform has a fatigue risk and needs to take measures for fatigue protection.

[0038] 3) Based on the numerical model of the offshore platform established in step 1), propose two schemes for fatigue damage prevention and control: cleaning 25 cm thick marine organisms and filling the leg of the pile to empty 2 m. For scheme 1, remove the marine organism attachment setting in the model in 1) as the model after prevention and control of this scheme. For scheme 2, raise the mud surface position in the model in 1) by 2 m and reduce the water depth by 2 m correspondingly as the model after prevention and control of this scheme. Conduct spectral fatigue analysis under the same working conditions in 2) respectively to obtain the fatigue cyclic stress S of the damaged nodes of the platform under different wave directions after the two schemes ij and the corresponding number of cycles C ij , combined with the wave direction probability p i Comprehensively consider the root mean square value of the fatigue cyclic stress of the node with the maximum damage under all working conditions as the platform fatigue stress level RMS R , calculate according to formula (1), and the platform fatigue stress levels RMS R after prevention and control of scheme 1 and scheme 2 are 16.03 MPa and 13.58 MPa respectively. The spectral fatigue analysis can also obtain the platform fatigue life T after prevention and control of the two schemes RThey are 9 years and 12 years respectively.

[0039] 4) Use the RMS of the platform fatigue stress level before fatigue prevention and control obtained in 2) O and the RMS of the platform fatigue stress level after fatigue prevention and control obtained in 3) R to calculate the stress level reduction index D RMS , calculated using formula (2). The stress level reduction index for cleaning 25 cm thick marine organisms is 10%, and the stress level reduction index for backfilling the leg with a 2 m void is 50%.

[0040] 5) Use the fatigue life T of the platform before fatigue prevention and control obtained in 2) O and the fatigue life T of the platform after fatigue prevention and control obtained in 3) R to calculate the fatigue life improvement index I T , calculated using formula (3). The fatigue life improvement index for cleaning 25 cm thick marine organisms is 50%, and the fatigue life improvement index for backfilling the leg with a 2 m void is 100%

[0041] 6) The stress level reduction index of the fatigue prevention and control plan for cleaning 25 cm thick marine organisms is 10%, and the fatigue life improvement index is 50%. The stress level reduction index of the fatigue prevention and control plan for backfilling the leg with a 2 m void is 50%, and the fatigue life improvement index is 100%. It is easy to know that both plans have a certain prevention and control effect. However, relatively speaking, the fatigue prevention and control effect of backfilling the leg with a 2 m void is better. It is recommended to select this plan as the prevention and control plan.

Claims

1. A quantitative evaluation method for the fatigue prevention and control effect of an offshore platform, characterized in that, it includes the following steps: 1) Based on the SACS software, establish a numerical model of the offshore platform according to the structural dimensions, material properties, cross-sectional properties, center of gravity, integrity status, and pile-soil constraint effect of the in-service offshore platform, and conduct model reliability verification. Set the main nodes and retained main degrees of freedom for platform analysis to improve the calculation efficiency; 2) Based on the numerical model of the offshore platform established in step 1), spectral fatigue analysis is carried out with the wave scatter diagram as the load to obtain the fatigue cyclic stress S of the damaged nodes on the platform under different wave directions before fatigue prevention and control of the offshore platform ij , the corresponding number of cycles C ij , combined with the wave direction probability p i Comprehensively consider the root mean square value of the fatigue cyclic stress of the node with the largest damage under all wave direction loads as the platform fatigue stress level RMS O , spectral fatigue analysis can also obtain the node fatigue life, and take the life of the node with the largest damage as the fatigue life T of the platform before prevention and control O ; 3) Based on the numerical model of the offshore platform established in step 1), simplify the numerical model of the fatigue prevention and control method and incorporate it into the model in 1) as the numerical model of the offshore platform after fatigue prevention and control, and conduct the same working condition spectral fatigue analysis in 2) to obtain the fatigue cyclic stress S of the damaged nodes of the platform under different wave directions after the fatigue prevention and control of the offshore platform ij , the corresponding number of cycles C ij , combined with the wave direction probability p i Comprehensively consider the root mean square value of the fatigue cyclic stress of the node with the largest damage under all working conditions as the platform fatigue stress level RMS R , the spectral fatigue analysis can also obtain the node fatigue life, and take the life of the node with the largest damage as the fatigue life T of the platform after prevention and control R ; 4) Use the RMS of the platform fatigue stress level before fatigue prevention and control obtained in 2) O and the RMS of the platform fatigue stress level after fatigue prevention and control obtained in 3) R to calculate the stress level reduction index D RMS . The larger the index value, the better the fatigue prevention and control effect of the platform and the more significant the stress level reduction effect; 5) Use the fatigue life T of the platform before fatigue prevention and control obtained in 2) O and the fatigue life T of the platform after fatigue prevention and control obtained in 3) R to calculate the fatigue life improvement index I T , the larger the index value, the better the fatigue prevention and control effect of the platform and the more significant the improvement effect of the fatigue life; 6) Based on the stress level reduction index D RMS and the fatigue life improvement index I T Give the evaluation of the prevention and control effect. The stress level reduction index D RMS and the fatigue life improvement index I T are positively correlated and can verify each other. If both indicators are relatively large, it indicates that the prevention and control effect of the fatigue damage prevention and control method for the offshore platform is relatively good. If both indicators are relatively small, it indicates that the prevention and control effect of the prevention and control method is not good. These two indicators can be used to guide the decision-making on the selection of the fatigue damage prevention and control method for the offshore platform.

2. The quantitative evaluation method for the fatigue prevention and control effect of an offshore platform according to claim 1, characterized in that, The numerical model of the offshore platform is modeled based on the design documents and drawing materials of the offshore platform to be analyzed using SACS software. The platform material, beam section, and unit group properties are set. Nodes and beam elements are built layer by layer to establish the structure of each block. The offset of the pipe joint and the flush offset of the platform deck beam are set. The effective length correction of the beam element strut buckling check is set. The counterweight simulates the actual weight load distribution. According to the "Classification and Construction Specification for Offshore Fixed Platforms", considering the soil type, the pile-soil constraint effect is simplified as a fixed constraint at a position n times the pipe diameter below the mud surface. According to the integrity of the platform, the thickness of marine organism attachment and corrosion thickness are set, and the hydrodynamic coefficients C D and C M are selected. The effective self-weight force transfer verification of the model considering buoyancy is carried out. The reliability of the model is verified by comparing the effective self-weight after considering buoyancy with the same constraint reaction force. The correctness of the wave application of the single-pile numerical model is verified by comparing with the single-pile wave loading test in the publicly published "Research on Wave and Current Forces on Pile Foundation Structures" by scholar Lan Yamei. The sea current loading of the single-pile numerical model is verified by the Morrison formula. After verification, the same loading method is used to load the platform numerical model to ensure the loading reliability. To improve the calculation efficiency, the main nodes and the main degrees of freedom to be retained for platform analysis are set, and the node constraint is set to 222000.

3. The quantitative evaluation method for the fatigue prevention and control effect of an offshore platform according to claim 1, characterized in that, The spectral fatigue analysis is carried out with the wave scatter diagram as the load. The modal analysis is performed on the numerical model of the offshore platform established according to Claim 1 to obtain the mass result file and the modal result file characterizing the vibration characteristics of the platform. Taking the platform model file, the mass result file, the modal result file, the load setting file, and the response setting file as inputs, the platform response analysis is carried out in different wave directions to obtain the platform node stress transfer function result files in different wave directions. The load setting file specifies a series of wave loads with the same wave steepness, and the response setting file specifies the stress response output. Taking the load setting files in all wave directions, the platform node stress transfer function result files, and the file specifying the fatigue analysis settings as inputs, the spectral fatigue analysis is carried out to output the fatigue cyclic stress S of the damaged nodes of the platform in different wave directions before the fatigue prevention and control of the offshore platform. ij , the corresponding number of cycles C ij , and the fatigue life T of the platform before prevention O . Among them, the fatigue analysis setting file specifies the platform operation years, safety factor, S-N curve, stress concentration factor, wave spectrum, wave scatter diagram, and the nodes and beam groups of the output results. The safety factor is selected according to the CCS "Technical Guide for the Fatigue Strength Assessment of Offshore Engineering Structures". The S-N curve and stress concentration factor are selected from the built-in specification methods of the software. The wave spectrum is selected from the P-M spectrum and the JONSWAP spectrum. The wave scatter diagram is selected from the measured data related to the offshore platform to be analyzed. If there is no measured data, the wave scatter diagram of the corresponding sea area block is selected from "Northwest Pacific Wave Statistics Collection" edited by Fang Zhongsheng.

4. The quantitative evaluation method for the fatigue prevention and control effect of an offshore platform according to claim 1, characterized in that, The pre-prevention platform fatigue stress level RMS in (2) above O and the post-prevention platform fatigue stress level RMS in (3) above R , represented by the root mean square (RMS) of the fatigue cyclic stresses for all wave direction conditions obtained through spectral fatigue analysis, is calculated using the following formula: Among them, p i is the probability of the i-th directional operating condition; RMS i is the root mean square value of the fatigue stress in the i-th direction, S ij is the j-th cyclic stress amplitude under the i-th directional operating condition, C ij is the number of cycles of the j-th cyclic stress amplitude under the i-th directional operating condition, M is the number of wave directions, N i is the number of different stress amplitudes in the i-th direction.

5. The quantitative evaluation method for the fatigue prevention and control effect of an offshore platform according to claim 1, characterized in that, The stress level reduction index D in 4) above RMS , which is expressed as the percentage of the stress level reduction value after prevention and control to the stress level before prevention and control, and is calculated by the following formula D RMS = (RMS O - RMS R ) / RMS O × 100% Among them, RMS O is the fatigue stress level of the node with the maximum platform damage before prevention and control; RMS R is the fatigue stress level of the node with the maximum platform damage after prevention and control.

6. The quantitative evaluation method for the fatigue prevention and control effect of an offshore platform according to claim 1, characterized in that, The fatigue life improvement index I in 5) above T , expressed as the percentage of the life improvement amount after prevention and control to the fatigue life before prevention and control, and calculated by the following formula I T = (T R - T O ) / T O × 100% where, T O is the maximum node life of the platform before prevention and control; T R is the maximum node life of the platform after prevention and control.

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