A method for modifying acceleration load calculation formula for pipeline stress analysis

Abstract

The application discloses a kind of acceleration load calculation formula correction method for pipeline stress analysis, comprising the following steps: 1) based on potential flow theory, diffraction theory, establish ship motion equation;2) in AQWA software, the ship length, type width, type depth, design draft, displacement, inertia moment of general FPSO are input, time domain analysis is carried out, and its hydrodynamic coefficient is obtained, then acceleration load extreme value is predicted using SESAM software;3) according to the ship length, type width, type depth of general FPSO, design draft, structural draft, displacement under different working conditions, the acceleration of each degree of freedom is calculated according to ship motion acceleration load calculation formula;4) error analysis is carried out to the acceleration value calculated in steps 2), 3), and the acceleration formula calculation value needs to be corrected if the error is greater than d;5) introduce correction coefficient f, and the acceleration calculation value with error greater than d is corrected until the error is less than d;6) the corrected acceleration can be applied on the pipeline as acceleration load.

Description

Technical Field

[0001] This invention belongs to the field of ship structures and relates to a method for correcting the acceleration load calculation formula for pipeline stress analysis. Background Technology

[0002] FPSOs operate at sea for extended periods, facing extremely harsh sea conditions, which places higher demands on their pipelines. Besides considering wind loads, displacement loads from ship motion, and thermal expansion loads, the impact of the ship's own acceleration loads on the pipeline must also be taken into account. However, current calculations of acceleration loads acting on pipelines commonly use the acceleration at the center of gravity to approximate the actual acceleration load at the pipeline's location. This method yields overly conservative stress results in most cases, potentially compromising pipeline safety. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides an acceleration load prediction method for pipeline stress analysis. This method can solve for the ship's acceleration based on the ship's equation of motion and hydrodynamic coefficients, and can be used to analyze pipeline stress, thereby improving the safety performance of the pipeline.

[0004] The technical solution of the present invention is as follows: A method for correcting the acceleration load calculation formula for pipeline stress analysis, the specific operation steps of which are as follows:

[0005] Step (1): Based on potential flow theory and diffraction theory, establish the ship motion equations;

[0006] Step (2): Based on the length, beam, depth, design draft, displacement and moment of inertia parameters of the general-purpose FPSO, calculate its hydrodynamic coefficients using ANSYS AQWA software, and use SESAM software to make long-term predictions of acceleration.

[0007] Step (3): Based on the length, beam, depth, design draft, actual draft, and displacement of the general-purpose FPSO, calculate the acceleration values ​​of each degree of freedom according to the calculation formula for ship motion acceleration defined in the classification society's specifications.

[0008] Step (4): Perform error analysis on the acceleration values ​​calculated in steps (2) and (3) and compare them with the minimum error d;

[0009] Step (5): Correct the acceleration calculation values ​​in step (4) where the error value is greater than the minimum error, and introduce a correction coefficient f. i ;

[0010] Step (6): Compare the corrected acceleration calculation value with the acceleration prediction value in step (2), analyze the error, and continue to execute step (5) for acceleration values ​​with errors greater than the minimum error d.

[0011] Repeat this process until the error is less than the minimum error d.

[0012] Step (7): The modified acceleration calculation formula can be substituted into the general FPSO pipeline stress analysis as the acceleration load calculation formula; calculate the actual stress value of the stress concentration point on the pipeline, and provide a reference for pipeline design.

[0013] Furthermore, when establishing the equations of motion for a ship, it is necessary to first define a coordinate system to describe the ship's motion. The method for defining the ship's coordinate system is as follows:

[0014] The origin of the coordinate system is the intersection of the waterline and the stern vertical line of a general-purpose FPSO under full load conditions. The intersection of the waterline and the centerline is the x-axis, pointing towards the bow as positive; the intersection of the waterline and the midships plane is the y-axis, pointing towards the port side as positive; and the intersection of the centerline and the midships plane is the z-axis, pointing upwards as positive.

[0015] In step (1), the establishment of the ship's motion equations is specifically as follows:

[0016]

[0017] Where: M jk Let A represent the generalized mass matrix of the ship. jk B represents the added mass. jk C represents the damping coefficient. jk Indicates the static restoring force coefficient. Indicates complex amplitude;

[0018] in, η k The displacement of the ship from its equilibrium position is represented by k = 1, ..., 6, which represent pitch, sway, heave, roll, pitch and yaw respectively.

[0019] Furthermore, in step (2), the calculation of hydrodynamic coefficients using ANSYS AQWA software is as follows:

[0020] After hydrodynamic calculations, wave spectrum analysis and long-term forecasting are performed to obtain the acceleration limit values ​​for each degree of freedom, i.e., the acceleration forecast values.

[0021] In the acceleration prediction process, the following assumptions are made:

[0022] 31) The wave direction angle is 0° to 180°, the wave direction interval is 15°, there are a total of 13 wave directions, and it is assumed that the probability of each wave direction is 1 / 13;

[0023] 32) The wave frequency is 0.2–1.8 rad / s, with an interval of 0.05 rad / s;

[0024] 33) The sea state used is the North Atlantic sea state;

[0025] 34) The spectrum used is the JONSWAP spectrum; its expression is as follows:

[0026]

[0027] Wherein: T p Indicates the period of the spectral peak, s; H s ω represents the significant peak height, m; γ represents the peak enhancement factor, taken as 3.3; σ represents the peak shape parameter: when ω≤ω p σ = 0.07; when ω > ω p , σ=0.09.

[0028] Furthermore, in step (3), the acceleration motion modes of each degree of freedom of the general-purpose FPSO are as follows:

[0029] oscillation acceleration a surge : The acceleration generated by the swaying of a general-purpose FPSO along the x-axis;

[0030] sway acceleration a sway : The acceleration generated by the y-axis sway of a general-purpose FPSO;

[0031] heave acceleration a heave : The acceleration generated by the wobbling of a general-purpose FPSO along the z-axis;

[0032] Roll acceleration a roll : The acceleration generated by the oscillation along the x-axis of a general-purpose FPSO;

[0033] Pitch acceleration a pitch : The acceleration generated by the y-axis oscillation of a general-purpose FPSO;

[0034] The oscillation acceleration a surge sway acceleration a sway heave acceleration a heave Roll acceleration a roll Pitch acceleration a pitch and the longitudinal acceleration a at any position x-env lateral acceleration a y-env Vertical acceleration a z-env The calculation formula is as follows:

[0035]

[0036]

[0037]

[0038]

[0039]

[0040]

[0041]

[0042]

[0043] Where: g represents the gravitational acceleration constant, taken as 9.81 m / s². 2 θ represents the roll angle of the ship's rolling motion; T θ The roll period, which represents the roll motion, is related to the roll radius and the initial stability height. The pitch angle represents the pitch motion; The pitching period, representing the pitching motion, is related to the ship's length and the ratio of draft to structural draft under different loading conditions; B represents the beam; L represents the length; f T This indicates the ratio of actual draft to structural draft under operating conditions; a pitch_x This represents the longitudinal acceleration caused by pitching, in m / s². 2 ;a roll_y The lateral acceleration caused by the roll is expressed in m / s². 2 ;a pitch_z Represents the vertical acceleration caused by pitch, in m / s². 2 ;a roll_z This represents the vertical acceleration caused by the roll, in m / s². 2 .

[0044] Furthermore, in step (4), the error analysis process is as follows: calculate the difference 'a' between the calculated value of the motion acceleration formula and the predicted acceleration value, and compare it with the minimum error 'd', where the minimum error 'd' is defined as 10 times the order of magnitude of the calculated value 'b'. -2 times, that is:

[0045] d~10 -2 b

[0046] Furthermore, in step (5), the introduction of the correction coefficient f i The specific method for determining it is as follows:

[0047] When the difference between the calculated acceleration and the predicted acceleration is greater than the minimum error d, the formula needs to be corrected by introducing a correction coefficient f. i The correction factor fi With length L, beam B, depth D, and design draft T SC Actual draft (T) LC C W For water surface coefficient, C B The square coefficient;

[0048] Among them, f i (i = 1, 2, 3, 4, 5, 6, 7, 8) represent the sway, heave, pitch, roll accelerations, and correction coefficients for longitudinal, lateral, and vertical accelerations at any position, respectively. The mathematical model can be expressed as:

[0049] f i =f i (L,B,D,T LC ,T SC C W C B );

[0050] Specifically, the correction factor f i The specific method for determining it is as follows:

[0051] The statistical methods employed include correlation analysis and regression; correlation analysis is primarily used to determine the dependent variable f. i With independent variables L, B, D, T LC ,T SC C W C B Is there a relationship between them? The regression method compares highly correlated variables with the dependent variable f. i The relationships between them are expressed using functions;

[0052] The functional relationship can be expressed as:

[0053] f i =f(L, B, D, T) LC T SC C W C B )

[0054] The correction coefficient f after regression i Substituting the original acceleration calculation formula, we obtain the new acceleration calculation formula.

[0055] Furthermore, based on the obtained modified acceleration calculation formula, the acceleration value is recalculated by substituting the length, beam, and depth of the general-purpose FPSO, as well as the design draft, structural draft, and displacement under different working conditions.

[0056] As described in step (6), compare the acceleration with the predicted value and re-analyze its error, comparing it with the minimum error d. Repeat this process for acceleration values ​​with errors greater than d, until the difference between the two is less than the minimum error d.

[0057] If the difference between the calculated acceleration value and the predicted acceleration value is less than the minimum error d, then the two calculated values ​​are considered to be in agreement and can be used directly.

[0058] Furthermore, in step (7), the acceleration of the ship's motion obtained by the modified acceleration calculation formula is applied to the pipeline as a uniformly distributed load, and its influence on the pipeline stress is analyzed by the stress analysis software CAESARⅡ.

[0059] The beneficial effects of this invention are as follows: The features of this invention are: 1. The calculation formulas for acceleration at the center of gravity and acceleration at any position of a general-purpose FPSO are modified, making the calculation of acceleration load more convenient when calculating the pipeline stress at its deck; 2. The acceleration load borne by the pipeline at different positions of the general-purpose FPSO is calculated by using the modified acceleration calculation formula at any position, and the result is more accurate, which can improve the safety performance of the pipeline. Attached Figure Description

[0060] Figure 1 This is a flowchart of the operation of the present invention;

[0061] Figure 2 This is a schematic diagram of the six degrees of freedom motion of the ship under different wave conditions in this invention;

[0062] Figure 3 This is a time-history curve of the ship's acceleration in six degrees of freedom under different wave conditions in this invention;

[0063] Figure 4 These are the amplitude-frequency response curves of the ship at a specified position under different wave conditions in this invention, representing the six degrees of freedom.

[0064] Figure 5 These are the acceleration time-history curves of the ship at a specified position under different wave conditions in this invention, representing the acceleration time-history curves of the five degrees of freedom in different wave conditions. Detailed Implementation

[0065] To more clearly illustrate the technical solution of the present invention, the present invention will be further described below. Obviously, the following description only describes a portion of the embodiments. For those skilled in the art, the technical solution of the present invention can be applied to other similar scenarios without creative effort. To more clearly illustrate the technical solution of the present invention, the technical solution of the present invention will be further described in detail below with reference to the accompanying drawings:

[0066] Considering the diverse environmental loads faced by general-purpose FPSOs during maritime navigation, in order to simplify the calculation method for acceleration loads on FPSO piping systems, an adaptation study is conducted on the acceleration calculation formulas given by classification societies, and the formulas are then modified. Therefore, this invention provides a modified method for acceleration load calculation formulas for pipeline stress analysis.

[0067] like Figure 2 As shown, by inputting the main parameters of the FPSO into ANSYS AQWA, the amplitude response curves of the FPSO under different wave conditions and different degrees of freedom are obtained, namely the RAO curves.

[0068] like Figure 3 As shown, time-domain analysis of the FPSO yields its acceleration response, and hydrodynamic calculations provide acceleration time-history curves for each degree of freedom.

[0069] like Figure 4 As shown, in the software SESAM, the acceleration amplitude-frequency response curves of each degree of freedom of the FPSO at any position under different wave directions can be obtained.

[0070] like Figure 5 As shown, through time-domain analysis, the acceleration time-history curve at any location can be obtained.

[0071] By using SESAM software to perform long-term predictions of acceleration for each degree of freedom, long-term predicted values ​​of acceleration under different exceedance probabilities can be obtained. The calculations are based on the following assumptions:

[0072] 31) The wave direction angle is 0° to 180°, and the calculated wave direction interval is 15°;

[0073] 32) The wave period is 2 to 62 seconds, with a time interval of 1.2 seconds;

[0074] 33) The spectrum used is the JONSWAP spectrum; its expression is as follows:

[0075]

[0076] Wherein: T p For the spectral peak period, s; ​​H s ω is the significant wave height, m; γ is the peak enhancement factor, taken as 3.3; σ is the peak shape parameter: when ω≤ω p σ = 0.07; when ω > ω p , σ=0.09.

[0077] Table 1 shows the long-term values ​​of the acceleration motion components for each degree of freedom calculated by SESAM software:

[0078] Table 1. Long-term values ​​corresponding to acceleration components

[0079] <![CDATA[10 -2 Exceeding probability <![CDATA[10 -8 Exceeding probability Probability coefficient Heave(Vertical motion) 4.840E+00 1.968E+01 0.246 Pitch(Pitching) 4.044E-02 1.587E-01 0.255 Roll(Rolling) 1.814E-01 7.453E-01 0.243 Surge(Surging) 9.056E-01 6.275E+00 0.144 Sway(Swaying) 2.707E+00 1.127E+01 0.240

[0080] As shown in Table 2 are the calculated values of the acceleration formula, the predicted values of the acceleration, and the difference between the two. Among them, the calculated value of the acceleration formula is the surge acceleration a surge , sway acceleration a sway , heave acceleration a heave , roll acceleration a roll , pitch acceleration a pitch of the motion. The calculation formula is as follows:

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087]

[0088]

[0089] Where: g: gravitational acceleration constant, taken as 9.81 m / s 2 ; θ: roll angle of the ship's rolling motion; T θ : roll period of the rolling motion, related to the roll radius of gyration and the initial metacentric height; pitch angle of the pitching motion; pitch period of the pitching motion, related to the ship length and the ratio of the draft under different loading conditions to the structural draft; B: ship width; L: overall length; f T : ratio of the draft under the actual working condition to the structural draft; a pitch_x represents the longitudinal acceleration generated by pitching, m / s 2 ; a roll_y represents the lateral acceleration generated by rolling, m / s 2 ; a pitch_z represents the vertical acceleration generated by pitching, m / s 2 ; a roll_z represents the vertical acceleration generated by rolling, m / s 2 .

[0090] According to the predicted value of the acceleration at the center of gravity by the software and the calculated result of the acceleration using the acceleration calculation formula, and calculating the error between the two, as shown in Table 2:

[0091] Table 2. Simulation values ​​and calculated values ​​at the center of gravity, and their errors.

[0092]

[0093] According to Table 2, the error is the difference 'a' between the calculated value of the motion acceleration formula and the software simulation value, and it is compared with the minimum error 'd', where the minimum error 'd' is defined as 10 times the order of magnitude of the calculated value. -2 times, that is:

[0094] d~10 -2 b

[0095] When the difference between the calculated acceleration and the predicted acceleration exceeds the minimum error d, the formula needs to be corrected by introducing a correction coefficient f. i The correction factor f i With length L, beam B, depth D, and design draft T SC Actual draft (T) LC C W For waterline coefficient (referring to design draft), C B f is the square coefficient; where f i (i = 1, 2, 3, 4, 5, 6, 7, 8) represent the sway, heave, pitch, roll accelerations, and correction coefficients for longitudinal, lateral, and vertical accelerations at any position, respectively. The mathematical model can be expressed as:

[0096] f i =f i (L,B,D,T LC ,T SC C W C B )

[0097] Next, we analyze L, B, D, T. LC ,T SC C W C B The correlation under full-load conditions was investigated using statistical methods such as correlation analysis and regression. Correlation analysis was primarily used to determine the dependent variable f. i With independent variables L, B, D, T LC ,T SC C W C B Is there a relationship between them? The regression method compares highly correlated variables with the dependent variable f. i The relationships between them are expressed using functions.

[0098] In the correlation analysis, five additional ships and general-purpose FPSOs were selected as the analysis samples, covering large, medium and small vessels.

[0099] When correcting the acceleration formula at the center of gravity, a correlation analysis was performed using the ship's roll acceleration as an example. The results are shown in Table 3.

[0100] Table 3. Correlation analysis results of the roll acceleration correction factor f5

[0101]

[0102] f in the table BL f is the ratio of the ship's width B to its length L. TL For actual draft T LC The ratio to the length L.

[0103] In Table 3, L, B, and D showed the highest Pearson correlation at only 0.47, indicating that their correlation with f5 was not significant. LC With T SC The Pearson correlation reached 0.84. According to the criteria for judging the range of Pearson correlation coefficient values ​​(correlation greater than 0.8 is extremely strong), T LC T SC It has a very strong correlation with f5; f BL The correlation is weak, f TL The significance level is 0.075 > 0.05, indicating that its correlation with f5 is not significant.

[0104] Therefore, the variable with a strong correlation to the correction coefficient f5 is T. LC T SC .

[0105] Next, a regression analysis was performed, and the results are shown in Table 4.

[0106] Table 4 Regression analysis results of the roll acceleration correction factor f5

[0107] R value <![CDATA[R 2 ]]> Error of standard estimation Durbin-Waston 0.931 0.867 0.01 2.644

[0108] R: A measure of the degree of multiple correlation. The larger the multiple correlation coefficient (maximum 1), the stronger the linear correlation between elements or variables.

[0109] R 2 This value is used to determine the goodness of fit of a multiple linear regression equation; it indicates the extent (proportion) of the variance in the dependent variable explained by the independent variable. The closer its value is to 1, the better the fit.

[0110] Error of standard estimation: The smaller the error value, the smaller the approximation error between the estimator and its true value.

[0111] Durbin-Watson test value: It is generally believed that the Durbin-Watson test value is distributed between 0 and 4. The closer it is to 2, the greater the possibility that the observations are independent of each other.

[0112] Table 4 explains T LC T SC It has a high degree of fit with f5 and a small error.

[0113] The fitted function for the roll acceleration correction coefficient is: f5 = 1.68 + 6.04f T , where f T For the actual draft of the ship (T) LC With structural draft T SC The ratio of .

[0114] Based on the above correlation and regression analysis methods, the fitted functional relationship of the sway acceleration correction coefficient f1 can be calculated as: f1 = 0.52f T +0.19

[0115] The fitted function expression for the sway acceleration correction factor f2 is: f2 = 3.04 + 6.26f BL +5.42f TL

[0116] The fitted function expression for the heave acceleration correction factor f3 is: f3 = 2.3f BL +6.2f TL -0.85

[0117] The correction factor f4 for pitch acceleration and L,B,D,T LC ,T SC ,f BL ,f TL The correlations were all low, but with f BL and C B The correlation between the combined effects of these factors is very significant. The fitted functional relationship is as follows:

[0118] f4 = 6.26(f BL ·C B ) 1 / 5 -3.23

[0119] From the acceleration correction coefficients fitted for each of the above degrees of freedom, the formula for calculating the corrected acceleration at the center of gravity can be obtained, which can be expressed as:

[0120]

[0121]

[0122]

[0123]

[0124]

[0125] Finally, the main FPSO parameters were substituted back into the corrected formula and recalculated to obtain the corrected acceleration values, as shown in Table 5.

[0126] Table 5. Corrected acceleration values

[0127]

[0128] Comparing the error a' after correction for oscillation acceleration with the minimum error d, the magnitude of error a' is 10⁻⁴, while the magnitude of the minimum error d is 10. -4 This indicates that the error a' calculated by the modified acceleration formula meets the requirements; similarly, it can be shown that the modified sway acceleration, heave acceleration, roll acceleration, and pitch acceleration formulas also meet the minimum error d requirement and can be used directly.

[0129] When correcting the acceleration calculation formula at any location, the calculated acceleration value and the software simulation value were obtained for any point on the deck (with coordinates (70, 0, 0)). The results are shown in Table 6.

[0130] Table 6 shows the calculated and simulated acceleration values ​​at any location, along with the errors.

[0131]

[0132] Substituting the accelerations in various directions simulated by the software into the acceleration calculation formula at any position, the synthesized longitudinal, lateral, and vertical accelerations can be obtained, as shown in Table 7:

[0133] Table 7 Software simulation values, formula calculation values, and their errors

[0134]

[0135] As shown in Table 7, there is a large error between the software simulation values ​​and the calculated values, so correction is needed.

[0136] Based on the correction method for acceleration at the center of gravity, expressions for the longitudinal acceleration correction coefficient f6, the lateral acceleration correction coefficient f7, and the vertical acceleration correction coefficient f8 at any position on the ship can be obtained. The longitudinal acceleration correction coefficient f6 and f... BL and f TL The correlation is strong, and the corrected coefficient function expression after regression is: f6 = 0.264 + 2.12f BL +0.78f TL

[0137] Lateral acceleration correction coefficients f7 and f BL The correlation is strong, and the corrected coefficient function expression after regression is: f7 = 0.66 + 2.39f BL

[0138] Vertical acceleration correction factors f8 and f BL and f TL The correlation is strong, and the corrected coefficient function expression after regression is: f8 = 0.82 + 1.25f BL +1.42f TL

[0139] The formula for calculating the acceleration at any position after fitting is:

[0140]

[0141]

[0142]

[0143] By substituting the main parameters of the general-purpose FPSO back into the modified acceleration calculation formula at any position, the error relationship between the new calculated acceleration values ​​and the software simulation values ​​is obtained, as shown in Table 8:

[0144] Table 8. Calculated and simulated values ​​of the corrected acceleration formula.

[0145]

[0146]

[0147] Comparing the corrected longitudinal acceleration error a' with the minimum error d, the magnitude of error a' is 10⁻⁴, while the magnitude of the minimum error d is 10. -4 This indicates that the error a' calculated by the modified acceleration formula meets the requirements; similarly, it can be shown that the modified lateral and vertical acceleration formulas also meet the minimum error d requirement and can be used directly.

[0148] Finally, it should be understood that the embodiments described in this invention are only used to illustrate the principles of the embodiments of this invention; other variations may also fall within the scope of this invention; therefore, as examples rather than limitations, alternative configurations of the embodiments of this invention can be regarded as consistent with the teachings of this invention; correspondingly, the embodiments of this invention are not limited to the embodiments explicitly introduced and described in this invention.

Claims

1. A method for correcting the acceleration load calculation formula for pipeline stress analysis, characterized in that, The specific operating steps are as follows: Step (1): Based on potential flow theory and diffraction theory, establish the ship motion equations; Step (2): Based on the length, beam, depth, design draft, displacement and moment of inertia parameters of the general-purpose FPSO, calculate the six-degree-of-freedom acceleration of the ship using ANSYS AQWA software, and use SESAM software to make long-term predictions of the acceleration. Step (3): Based on the length, beam, depth, design draft, displacement and moment of inertia parameters of the general-purpose FPSO, calculate the acceleration of each degree of freedom according to the calculation formula of ship motion acceleration defined in the classification society's specifications. Step (4): Perform error analysis on the acceleration values ​​calculated in steps (2) and (3) and compare them with the minimum error d; Step (5): Correct the calculated acceleration values ​​with error values ​​greater than the minimum error, and introduce a correction coefficient f. i ; The introduction of the correction coefficient f i The specific method is as follows: When the difference between the calculated acceleration and the predicted acceleration is greater than the minimum error d, the formula is corrected by introducing a correction coefficient f. i The correction factor f i With length L, beam B, depth D, and design draft T SC Actual draft (T) LC Water surface coefficient C W Square coefficient C B Related; Among them, f i In this context, 'i' represents the correction coefficients for sway, roll, heave, pitch, and roll accelerations, as well as the longitudinal, lateral, and vertical accelerations at any position. The mathematical model is expressed as follows: f i =f i (L,B,D,T LC ,T SC ,C W ,C B ); Correction factor f i The specific method for determining it is as follows: The correlation analysis and regression methods used in statistics were employed. Among them, correlation analysis is used to determine the dependent variable f. i With independent variables L, B, D, T LC ,T SC C W C B Is there any connection between them? Regression methods combine highly correlated variables with the dependent variable f. i The relationship between them can be expressed using functions; the functional relationship can be expressed as: f i =f(L、B、D、T LC 、T SC 、C W 、C B ) The correction coefficient f after regression i Substituting the original acceleration calculation formula, we obtain the new acceleration calculation formula; Step (6): Compare the corrected acceleration calculation value with the acceleration prediction value in step (2), analyze the error, and continue to execute step (5) for acceleration values ​​with errors greater than the minimum error d. Repeat this process until the error is less than the minimum error d. Step (7): Substitute the modified acceleration calculation formula into the general FPSO pipeline stress analysis as the acceleration load calculation formula; calculate the actual stress value at the stress concentration point on the pipeline to provide a reference for pipeline design.

2. The method for correcting the acceleration load calculation formula for pipeline stress analysis according to claim 1, characterized in that, In step (1), establishing the ship's motion equations means that, when establishing the ship's motion equations, a coordinate system needs to be defined to describe the ship's motion. The method for defining the ship's coordinate system is as follows: The origin of the coordinate system is the intersection of the waterline and the stern vertical line of a general-purpose FPSO under full load conditions. The intersection of the waterline and the centerline is the x-axis, pointing towards the bow as positive; the intersection of the waterline and the midships plane is the y-axis, pointing towards the port side as positive; and the intersection of the centerline and the midships plane is the z-axis, pointing upwards as positive. The established equations of motion for the ship are shown in the following formula: Where: M jk Let A represent the generalized mass matrix of the ship. jk B represents the added mass. jk C represents the damping coefficient. jk Indicates the static restoring force coefficient. Indicates complex amplitude; in, η k The displacement of the ship from its equilibrium position is represented by k = 1, ..., 6, which represent pitch, sway, heave, roll, pitch and yaw respectively.

3. The method for correcting the acceleration load calculation formula for pipeline stress analysis according to claim 1, characterized in that: In step (2), the specific method of using ANSYS AQWA software is as follows: After hydrodynamic calculations, wave spectrum analysis and long-term forecasting are performed to obtain the acceleration limit values ​​for each degree of freedom, i.e., the acceleration forecast values. In the acceleration prediction process, the following assumptions are made: 31) The wave direction angle is 0° to 180°, the wave direction interval is 15°, there are a total of 13 wave directions, and it is assumed that the probability of each wave direction is 1 / 13; 32) The wave frequency is 0.2–1.8 rad / s, with an interval of 0.05 rad / s; 33) The sea state used is the North Atlantic sea state; 34) The spectrum used is the JONSWAP spectrum; Its expression is as follows: Wherein: T p Indicates the period of the spectral peak, s; H s ω represents the significant peak height, m; γ represents the peak enhancement factor, taken as 3.3; σ represents the peak shape parameter: when ω≤ω p σ = 0.07; when ω > ω p , σ=0.

09.

4. The method for correcting the acceleration load calculation formula for pipeline stress analysis according to claim 1, characterized in that: In steps (2)-(3), the acceleration motion modes of each degree of freedom of the general-purpose FPSO are as follows: oscillation acceleration a surge : The acceleration generated by the swaying of a general-purpose FPSO along the x-axis; sway acceleration a sway : The acceleration generated by the y-axis sway of a general-purpose FPSO; heave acceleration a heave : The acceleration generated by the wobbling of a general-purpose FPSO along the z-axis; Roll acceleration a roll : The acceleration generated by the oscillation along the x-axis of a general-purpose FPSO; Pitch acceleration a pitch : The acceleration generated by the y-axis oscillation of a general-purpose FPSO; The oscillation acceleration a surge sway acceleration a sway heave acceleration a heave Roll acceleration a roll Pitch acceleration a pitch and the longitudinal acceleration a at any position x-env lateral acceleration a y-env Vertical acceleration a z-env The calculation formula is as follows: Where: g represents the gravitational acceleration constant, taken as 9.81 m / s². 2 θ represents the roll angle of the ship's rolling motion; T θ The roll period, which represents the roll motion, is related to the roll radius and the initial stability height. The pitch angle represents the pitch motion; The pitching period, representing the pitching motion, is related to the ship's length and the ratio of draft to structural draft under different loading conditions; B represents the beam; L represents the overall length; fT represents the ratio of draft to structural draft under actual operating conditions; a pitch_x This represents the longitudinal acceleration caused by pitching, in m / s². 2 ;a roll_y The lateral acceleration caused by the roll is expressed in m / s². 2 ;a pitch_z Represents the vertical acceleration caused by pitch, in m / s². 2 ;a roll_z This represents the vertical acceleration caused by the roll, in m / s². 2 .

5. The method for correcting the acceleration load calculation formula for pipeline stress analysis according to claim 1, characterized in that, In step (4), the error analysis process is as follows: calculate the difference 'a' between the calculated value of the motion acceleration formula and the predicted acceleration value, and compare it with the minimum error 'd', where the minimum error 'd' is defined as 10 times the order of magnitude of the calculated value 'b'. -2 times.

6. The method for correcting the acceleration load calculation formula for pipeline stress analysis according to claim 1, characterized in that, In step (5), Substitute the corrected acceleration calculation formula into the length, beam, and depth of the general-purpose FPSO, and recalculate the acceleration value under different working conditions, including the design draft, structural draft, and displacement parameters. By comparing the acceleration prediction value with the value described in step (6), and re-analyzing its error, and comparing it with the minimum error d; for acceleration values ​​with errors greater than d, the error is corrected again and so on, until the difference between the two is less than the minimum error d; If the difference between the calculated acceleration value and the predicted acceleration value is less than the minimum error d, then the two calculated values ​​are considered to be in agreement and can be used directly.

7. The method for correcting the acceleration load calculation formula for pipeline stress analysis according to claim 1, characterized in that, In step (7), the acceleration of the ship's motion, obtained by the modified acceleration calculation formula, is applied to the pipeline as a uniformly distributed load, and its influence on the pipeline stress is analyzed by the stress analysis software CAESARⅡ.

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