The structural strength of the upper block of the offshore converter station is very important to the overall structural safety of the offshore converter station. Based on the standardization of the SACS model data file and the result data file, this paper carries out secondary development of the SACS model, and uses MATLAB software to write programs to extract the stress conditions of all nodes under various calculation conditions. According to the BS EN 1993-1-8:2005 standard, the structural strength of all the nodes of the upper block of the offshore converter station was checked. The calculation results show that using MATLAB to write the structural strength check software of offshore converter station can be used to check the strength of all nodes under all working conditions, which can save the calculation time and greatly improve the calculation efficiency. Through the strength check of the developed software, the nodes that do not meet the requirements of the code can be quickly found out, and the structure that does not meet the requirements of the code can be optimized until the requirements of the code are met, so as to ensure the structural safety of the offshore converter station.
Research Article
Calculation of Ultimate Bearing Capacity of Main Joints of Upper Block of Offshore Electrical Platform Based on Secondary Development of SACS
https://doi.org/10.21203/rs.3.rs-2889638/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 15 Aug, 2023
Read the published version in International Journal of System Assurance Engineering and Management →
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offshore substation
strength analysis
SACS
secondary development
Offshore electrical platform is the fortress of power collection, boosting and output of offshore wind farm, and its structural safety under complex environmental conditions and various combined working conditions directly affects the normal operation of offshore wind farm [1–3]. The upper block of offshore electrical platform is a jacket structure, and the ultimate bearing capacity of its members is very important to the safety assessment of the overall structural strength [4–6], so it is necessary to analyze and check its calculated strength according to the code. Because of the large number of nodes and the number of working conditions to be calculated, it will take a lot of time if manual input is adopted, or typical nodes in special parts can only be considered with reference to experience. Considering that SACS file has fixed data structure, it is convenient to extract data for follow-up research work, which provides the possibility for secondary development. Therefore, developing a simple and convenient strength checking program based on SACS data model and result file will save a lot of calculation time and facilitate checking every node. The calculation and analysis process is simple and efficient, which is convenient for checking the strength of any node under multiple working conditions, thus avoiding errors caused by human input and improving the reliability of calculation results [7–8].
According to the different types of specific components of each main node of the upper module of the substation, the internal components are numbered respectively, as shown in Fig. 1. Reference source not found. As shown in. Select different structural planes of the main node, the right direction of XZ plane is X positive direction, the upper direction is Z positive direction, and the in-plane direction is Y positive direction; The right direction of YZ plane is Y positive direction, the upper direction is Z positive direction, and the out-of-plane direction is X positive direction. The bar comparison is shown in Table 1.
Table 1 Node Component Number Table
| Unit number | Component unit | |
|---|---|---|
| 1 | Left chord | |
| 2 | Right chord | |
| 3 | Upper left brace | |
| 4 | Upper right brace | |
| 5 | Lower left brace | |
| 6 | Lower right brace | |
| 7 | Upper column | |
| 8 | lower column | |
According to "BS EN 1993-1-8: 2005 standard", the strength of joint structure is checked and calculated. According to the upper block drawing of offshore electrical platform, if the main column of the main joint is broken through the beam, the main column is chord, the main beam is brace, and the check formula of the joint of H-section connected to CHS member is given in Table 7.4 of EN 1993-1-8. The failure formulas of different chord surfaces corresponding to different joint forms are shown in Table 2 and Figs. 2–3.
| Node type | Chord surface failure formula | |
|---|---|---|
| 1 (Fig. 2) | \({N_{1,Rd}}={k_p}{f_{y0}}t_{0}^{2}\left( {4+20{\beta ^2}} \right)\left( {1+0.25\eta } \right)/{\gamma _{M5}}\) (1) \({M_{ip,1,Rd}}={h_1}{N_{1,Rd}}/\left( {1+0.25\eta } \right)\) (2) \({M_{op,1,Rd}}=0.5{b_1}{N_{1,Rd}}\) (3) | |
| 2 (Fig. 3) | \({N_{1,Rd}}=\frac{{5{k_p}{f_{y0}}t_{0}^{2}}}{{1 - 0.81\beta }}\left( {1+0.25\eta } \right)/{\gamma _{M5}}\) (4) \({M_{ip,1,Rd}}={h_1}{N_{1,Rd}}/\left( {1+0.25\eta } \right)\) (5) \({M_{op,1,Rd}}=0.5{b_1}{N_{1,Rd}}\) (6) | |
Where, \(\beta ={b_1}/{d_0}\); \(\eta ={h_1}/{d_0}\); For \({n_p}>0\) (compression), there is \({k_p}=1 - 0.3{n_p}\left( {1+{n_p}} \right)\) and \({k_p} \leqslant 1.0\); For \({n_p} \leqslant 0\) (tension), there is \({k_p}=0\); Among them, \({n_p}=\frac{{{\sigma _{0,Ed}}/{f_{y0}}}}{{{\gamma _{M5}}}}\) and \({\sigma _{0,Ed}}\) are the maximum compressive stresses in chords at joints.
Support members under the combined action of bending and axial force shall meet the following conditions:
$$\frac{{{N_{1,Ed}}}}{{{N_{1,Rd}}}}+{\left[ {\frac{{\left| {{M_{ip,1,Ed}}} \right|}}{{{M_{ip,1,Rd}}}}} \right]^2}+\frac{{{M_{op,1,Ed}}}}{{{M_{op,1,Rd}}}} \leqslant 1.0$$
7
Among them, \({M_{ip,1,Rd}}\) is the design in-plane bending moment resistance; \({M_{ip,1,Ed}}\) is the design in-plane torque; \({M_{op,1,Rd}}\) is the design out-of-plane bending moment resistance; \({M_{op,1,Ed}}\) is the design out-of-plane and in-plane torque.
According to the calculation formula of design resistance of CHS support member and H-section chord joint given in Table 7.21 of EN 1993-1-8, considering each node type of substation upper module main structure, T-shaped, Y-shaped and X-shaped node calculation formulas are selected, and the node structure form is shown in Fig. 4.
The formula for calculating the axial bearing capacity of chord web (yield of chord web) is as follows:
$${N_{{\text{i}},{\text{Rd}}}}=\frac{{{f_{y0}}{t_w}{b_w}}}{{sin{\theta _1}}}/{\gamma _{M5}}$$
8
Where \({f_{y0}}\) is the yield strength of chord members; \({\gamma _{M5}}\) is node resistance 1.0 according to the note in Table 2.1 of EN 1993-1-8; Table 7.22 of EN 1993-1-8 gives the calculation formula of design bending moment resistance of welded joints between rectangular hollow cross-section support members and H-section chords. The joint structure form is shown in Fig. 5
.
Figure 5 Joint diagram of rectangular hollow section support member and H-section chord
The calculation formula of bending bearing capacity of joints (yield of chord web) is as follows:
$${M_{ip,1,Rd}}=0.5{f_{y0}}{t_w}{b_w}{h_1}/{\gamma _{M5}}$$
9
The calculation method of design support failure resistance of chord joints with tubular member stiffeners given in Table 7.21 and Eq. (7.6) of EN 1993-1-8:
$${N_{i,Rd}}=\frac{\pi }{4} \cdot 2{f_{yi}}{t_i}\left( {{b_{eff}}+{b_{eff,s}}} \right)/{\gamma _{M5}}$$
10
Table 7.22 of EN 1993-1-8 gives the calculation method of bending bearing capacity of struts with stiffened chords:
$${M_{ip,1,Rd}}={f_{y1}}{t_1}\left( {{b_{eff}}+{b_{eff,s}}} \right)\left( {{h_1} - {t_1}} \right)/{\gamma _{M5}}$$
11
EN 1993-1-8 Type 7.5 gives the conditions to be met for the connection of support members under the combined action of bending and axial force:
$$\frac{{{N_{i,Ed}}}}{{{N_{i,Rd}}}}+\frac{{{M_{ip,1,Ed}}}}{{{M_{ip,1,Rd}}}} \leqslant 1.0$$
12
Among them, \({N_{i,Ed}}is{\text{ }}the{\text{ }}design{\text{ }}axial{\text{ }}force\), \({M_{ip,1,Ed}}~is{\text{ }}the{\text{ }}design{\text{ }}in - plane{\text{ }}torque\).
3.1 SACS Data Extraction
According to the data structure of SACS model file and result file (Table 3-Table 7). The algorithm is written to read the model parameters such as nodes, members and sections, and the calculation results such as internal forces and moments of members with different load schemes of each node, which are used as the basic data of the node checking program.
| JOINT | 527 | 6. | 18. | 26. | 55.000 | 222000 |
|---|---|---|---|---|---|---|
| JOINT | 528 | 17. | 18. | 26. | 55.000 | 222000 |
| JOINT | 531 | 6. | 8. | 26. | 55.000 | 222000 |
| JOINT | 533 | -17. | -18. | 26. | 55.000 | 222000 |
| MEMBER | 114 | 0083 | T15 |
|---|---|---|---|
| MEMBER | 115 | 0084 | T15 |
| MEMBER | 116 | 0317 | T15 |
| MEMBER | 119 | 0202 | T15 |
| MEMBER | 120 | 0282 | T15 |
| MEMBER | 121 | 0054 | T15 |
| GRUP | PIL | PILETOP | 20.607.90035.501 | 1.001.00 | 0.500.7.8490 |
|---|---|---|---|---|---|
| GRUP | T01 | C7 | 20.607.90034.501 | 1.001.00 | 0.500.7.8490 |
| GRUP | T02 | DJ1 | 20.607.90034.501 | 1.001.00 | 0.500.7.8490 |
| GRUP | T03 | DJ2 | 20.607.90034.501 | 1.001.00 | 0.500.7.8490 |
| GRUP | T04 | DJ3 | 20.607.90034.501 | 1.001.00 | 0.500.7.8490 |
| GRUP | T05 | DJ4 | 20.607.90034.501 | 1.001.00 | 0.500.7.8490 |
| GRUP | T06 | LT1 | 20.607.90034.501 | 1.001.00 | 0.500.7.8490 |
| SECT | FZ1 | TUB | 27.3001.600 |
|---|---|---|---|
| SECT | FZ2 | TUB | 21.9001.200 |
| SECT | FZ3 | TUB | 40.2002.000 |
| SECT | H1000 | WF | 40.0003.600100.001.6001.000 |
| SECT | H200 | WF | 10.0000.80020.000.550 |
| SECT | H300 | WF | 15.0000.90030.0000.6501.000 |
| SECT | H400 | WF | 20.0001.30040.0000.8001.000 |
| SECT | H500 | WF | 20.0001.60050.0001.0001.000 |
| S ACS-IV SYSTEM MEMBER FORCES AND MOMENTS | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| MEMBER NUMBER | MEMBER END | GROUP ID | LOAD CASE | FORCE(X) | KN FORCE(Y) | FORCE(Z) | MOMENT(X) | KN-M MOMENT(Y) | MOMENT(Z) |
| 1-133 | 1 | T01 | TOPS | -23.25 | 0.34 | 3.41 | 0.11 | -4.34 | -1.14 |
| L200 | -41.56 | 0.61 | 6.09 | 0.20 | -7.75 | -2.04 | |||
| L215 | -47.79 | 0.70 | 7.00 | 0.23 | -8.92 | -2.34 | |||
| L230 | -54.03 | 0.79 | 7.92 | 0.26 | -10.08 | -2.65 | |||
| 133 | TOPS | -23.25 | 0.34 | -2.47 | 0.11 | -0.90 | 1.38 | ||
| L200 | -41.56 | 0.61 | -4.42 | 0.20 | -1.60 | 2.46 | |||
| L215 | -47.79 | 0.70 | -5.08 | 0.23 | -1.84 | 2.83 | |||
| L230 | -54.03 | 0.79 | -5.75 | 0.26 | -2.08 | 3.20 | |||
3.2 Calculation flow of joint strength checking
According to the SACS model data format shown in section 3.1, Write an algorithm to read parameter information such as nodes, components and sections, screen out the number of main nodes according to node types, screen out the vertical columns connected to each main node according to the direction of components judged by node data, and judge the facade where the components are located and the number type corresponding to Fig. 1 according to the section type of horizontal components of main nodes; According to the data format of SACS calculation results shown in Section 3.2, the internal forces and moments of members with different load schemes of each node are extracted; According to the detailed drawing of joints, the related data of joint strengthening, such as stiffeners and thickened webs, are obtained. According to the related calculation theory of Section 2 specifications, the automatic calculation algorithm is compiled by MATLAB, and the joint strength of offshore electrical platform under various working conditions is automatically checked. The block diagram of automatic calculation program is shown in Fig. 6.
Using the self-compiled program described in the previous section, taking an offshore electrical platform as an example, the strength calculation of the main node structure is carried out, and its structure is shown in Fig. 7. Based on SACS calculation results of various working conditions of offshore electrical platform, a program is compiled according to three basic theories, and seven typical working conditions, namely, in-place static working condition, in-place extreme working condition, earthquake working condition, hoisting working condition, berthing working condition, transportation working condition and loading working condition, are selected to realize automatic checking of node strength under various working conditions; According to the analysis results, the nodes that need to be strengthened locally are automatically screened out.
4.1 Analysis of internal force checking results of joint strut
The internal force check results of main node strut under various working conditions are shown in Table 8. According to the calculation and verification results, The internal force check of strut on each check surface of main nodes of the structure meets the requirements of the code under in-situ static working conditions, in-situ extreme working conditions, berthing working conditions and loading working conditions. There are some nodes whose internal force UC value is greater than 1 under earthquake working conditions, hoisting working conditions and transportation working conditions, and their statistical numbers, nodes and components can be quickly obtained by the program. At the same time, the maximum UC value and the location of nodes and components are listed in the table.
| Working condition | Number of UC values greater than 1 | Node number | Maximum UC Value Component Position | Maximum UC value |
|---|---|---|---|---|
| In-situ static working condition | 0 | 116 | 116 − 33 | 0.6201 |
| In-situ extreme working condition | 0 | 116 | 116 − 33 | 0.8105 |
| Earthquake condition | 15 | 121 | XZ Plane Upper Right Brace | 1.1614 |
| Hoisting condition | 21 | 83 | XZ Plane Lower Right Brace | 1.6727 |
| Boarding condition | 0 | 116 | XZ Plane Upper Left Brace | 0.7532 |
| Transportation condition | 7 | 116 | XZ Plane Upper Left Brace | 1.1010 |
| Loading condition | 0 | 35 | XZ Plane Lower Left Brace | 0.6341 |
4.2 Analysis of checking results of chord shear bearing capacity
The check results of chord shear bearing capacity under various working conditions are shown in Table 9. According to the calculation and verification results, The check of chord shear bearing capacity of each check surface of each main node of the structure under in-situ static condition, in-situ extreme condition, berthing condition and loading condition meets the specification requirements. There are some joints whose UC value of chord shear bearing capacity is greater than 1 in earthquake, hoisting and transportation conditions, and their statistical numbers, joints and members can be quickly obtained by the program. At the same time, the maximum UC value and the location of nodes and components are listed in the table.
| Working condition | Number of UC values greater than 1 | Node number | Maximum UC value component | Maximum UC value |
|---|---|---|---|---|
| In-situ static working condition | 0 | 121 | XZ Plane Right Chord | 0.5926 |
| In-situ extreme working condition | 0 | 121 | XZ Plane Right Chord | 0.7755 |
| Earthquake condition | 16 | 121 | XZ Plane Right Chord | 1.1981 |
| Hoisting condition | 19 | 83 | XZ Plane Right Chord | 1.6345 |
| Boarding condition | 0 | 121 | XZ Plane Right Chord | 0.7233 |
| Transportation condition | 6 | 116 | XZ plane left chord | 1.0865 |
| Loading condition | 0 | 42 | XZ Plane Right Chord | 0.8115 |
4.3 Analysis of checking results of chord gap bearing capacity
The checking results of chord gap bearing capacity under each working condition are shown in Table 10, and the checking of chord gap bearing capacity on each checking surface of each main node under seven working conditions meets the specification requirements.
| Working condition | Node number | Maximum UC value component | Maximum UC value |
|---|---|---|---|
| In-situ static working condition | 4114 | XZ Plane Right Chord | 0.1424 |
| In-situ extreme working condition | 4114 | XZ Plane Right Chord | 0.1341 |
| Earthquake condition | 4006 | XZ Plane Right Chord | 0.1213 |
| Hoisting condition | 4114 | XZ Plane Right Chord | 0.0940 |
| Boarding condition | 4114 | XZ Plane Right Chord | 0.1169 |
| Transportation condition | 4114 | XZ Plane Right Chord | 0.1587 |
| Loading condition | 4020 | XZ plane left chord | 0.1126 |
4.4 Key node analysis
According to the calculation results of the previous section, the UC value of the strut internal force and the UC value of the chord shear bearing capacity of No.121 joint under earthquake conditions are all greater than 1, so the key analysis of this joint is carried out. The upper right strut of 121 node is 121 − 34 member, and its right chord is 121 − 0054 member. Sensitivity analysis is carried out on the profile dimensions of the right upper strut and the right chord, and the structural check calculation results are shown in Table 11–12 and Fig. 8-Figure 9. Under earthquake condition, changing the sizes of strut and chord and combining them one by one, the calculation results show that the UC value of the internal force of strut decreases effectively when the sizes of strut (or chord) are constant and the sizes of strut (or chord) are increased; When it does not meet the standard check, the UC value of internal force check of strut has little change with the increase of chord size, but after meeting the standard check, the UC value of internal force check of strut will decrease significantly with the increase of chord size. For chord shear check, increasing chord size can effectively reduce the shear UC value of the bar under the condition of certain strut size, but increasing strut size has little effect on the shear UC value of the bar under the condition of certain chord size. The analysis reason is as follows: Analyzing the force form of chord, the result of chord shear check is not sensitive to the size of strut.
From the above calculation results, it can be seen that the components that fail to pass the check are concentrated in a single elevation of individual nodes. Take 83 nodes under hoisting conditions with the largest UC value of stay internal force check and chord shear bearing capacity check as an example to optimize the node strengthening scheme and control the decisive factors of node UC. The local details of this node are shown in Fig. 10-Figure 11.
For the local joint structure, the two-section code formulas only consider the influence of stiffened plate, but not the influence of web thickening. When the web of the chord at about 83 joints is thickened to 280mm, the internal force UC of the strut at 83 joints is reduced to 0.8364, and the shear bearing capacity UC of the chord is reduced to 0.8409, which meets the requirements of the code.
According to previous studies, the structural form and size of profiles have great influence on the safety of joint structures of offshore electrical platforms [9–11]. Therefore, the XZ facade of 83 nodes, which has the largest UC value in the internal force check of struts and the shear bearing capacity check of chords, is selected as an example to control and analyze the decisive factors of UC value of nodes.
5.1 Effect of chord web thickening
XZ facade chords of 83 joints are made of H500 and H800 steel, and the web thicknesses are 10.0 mm and 14.0 mm, respectively. Web thickening PL16, PL17, PL18, PL20, PL24, PL25 and PL28 are selected according to the specifications of joint plate, and the influence of chord web thickening on joint strength checking results is explored. The results are shown in Fig. 12. According to the calculation results, when there is no web on the chord, the UC values of the internal force check of the strut and the shear bearing capacity check of the chord are the maximum and greater than 1, so the structure is unsafe. With the increase of the web thickness, the UC value of the internal force check and the shear bearing capacity check of the chord decreases gradually. When the web thickness is 24mm, the UC value decreases to less than 1, and the structure meets the design requirements of the code. Considering the total weight of the structure, when the web thickness is 24mm, the total weight of the structure is the smallest under the condition that the structure meets the safety requirements. Therefore, for the offshore converter stations selected in this paper, the scheme with web thickness of 24mm is preferred.
5.2 Influence of joint stiffening plate
Considering the four situations of web thickening PL18, PL20, PL24 and PL25, five design schemes are selected for 1223-83 struts, such as no stiffening plate, one PL11 stiffening plate, one PL20 stiffening plate, two PL11 stiffening plates and two PL20 stiffening plates. The influence of the number and thickness of stiffening plates on the internal force check results of joint struts is explored, and the results are shown in Fig. 13. The calculation results show that when there is no stiffening plate in the strut, the check results of the internal force of the strut are all greater than 1, and decrease with the increase of the web thickness. At this time, the structural strength is not enough, which does not meet the structural code design. When the stiffeners are installed in the structure, the check result of the internal force of the strut decreases, but for the same web thickness, the check result of the internal force of the strut does not change with the change of the thickness and number of stiffeners. This shows that the thickness of web plate is the main factor to ensure the structural strength, and the thickness and number of stiffeners are the secondary factors.
Based on the standardized SACS model file and result file, Secondary development of SACS file by MATLAB language, The structural strength of the upper block of the offshore electrical platform is checked. The calculation results show that the strength checking software of the offshore electrical platform compiled by MATLAB is suitable for the strength checking of all nodes under all working conditions, which can greatly improve the calculation efficiency, save the calculation time and reduce the calculation errors caused by human factors. Through the developed software to check the strength, we can quickly find out the nodes that do not meet the requirements of the code, and optimize the design of the nodes that do not meet the requirements of the code until they meet the design requirements of the code, so as to ensure the structural safety of the offshore electrical platform. Through the calculation of structural strength check by the factors such as the thickness of web, the thickness and the number of neck plates, it is shown that with the increase of web thickness, the UC values of internal force check of strut and shear bearing capacity check of chord decrease gradually, and the thickness of web is the main factor to ensure structural strength, while the thickness and the number of stiffeners are the secondary factors. For members whose UC value is greater than 1, the scheme of increasing web thickness is preferred for structural strength optimization design.
Data Availability
The experimental data used to support the findings of this study are available from the corresponding author upon request.
Authors’ contributions All the authors have participated in writing the manuscript and have revised the final version. All authors read and approved the final manuscript.
Conflict of interest Authors declares that they have no conflict of interest.
Ethical approval This article does not contain any studies with human participants and/or animals performed by any of the authors.
Informed consent There is no informed consent for this study.
Funding Statement
The study was supported by the National Natural Science Foundation of China (Grant No. 51909103), China.
- Wang Yongfa, Sun Zhenzhou, Tang Qunyi. Strength analysis of grouting connection section of boot-type offshore booster station [J]. Ocean Engineering, 2020, 38 (05): 107-115.
- Sun Zhenzhou, Jin Weiliang, Fang Tao, Yu Huafeng. Study on seismic reserve bearing coefficient of typical offshore booster stations [J]. Ocean Engineering, 2019, 37 (03): 95-101 +142.
- Yuan Jianping, Chen Jiefeng, Sun Zhenzhou. Fatigue damage of jacket booster station under wave and current excitation [J]. China Offshore Platform, 2021, 36 (01): 56-63.
- Wang Mengying. Strength check and influencing factor analysis of pipe joints of jacket platform [J]. Ship and Sea Engineering, 2019, 48 (01): 146-149.
- Zhang Xiaowei, Liu Yuxi, Huang Huaizhou. Reasonable selection of design parameters of jacket tubular joints and its engineering application [A]. China Ocean Engineering Society. Proceedings of the 16th China Ocean (Shore) Engineering Symposium (Volume I) [C]. China Ocean Engineering Society: Ocean Engineering Branch of China Ocean Society, 2013: 8.
- Zhao Kailong, Liu Chengming, Yao Zhiguang. Study on ultimate strength analysis method of offshore platform tubular joints [J]. Petroleum Engineering Construction, 2011, 37 (01): 12-15 +6.
- BS EN 1993-1-8-2005, European Code 3: Design of steel structures. Part 1-8: Design of joints [S].
- Bentley Systems, SACS Verison 10.2.0.1[M], 2015.
- Zhang Feng, Chen Yangji, Chen Yiyi, Lin Yingru, Yao Nianliang.Effect of inner stiffener setting on performance of steel tube joints [J]. Spatial Structure, 2004 (01): 51-56.
- Chen Desheng, Yang Dongping, Cai Yanqing, Long Fengle, Niu Gengqi, Shao Yongbo. Bearing capacity analysis of T-shaped circular steel tube joints strengthened by ring plate under in-plane bending moment [J]. China Shipbuilding, 2014, 55 (03): 84-93.
- Shi Manyu. Ultimate bearing capacity analysis of T/TT tubular joints of offshore jacket platform strengthened with CFRP [D]. Southwest Petroleum University, 2019.