Designing Tees on the China-Russian Eastern Pipeline
Designing structural dimensions
1.1 Design method
Among the standard pipe fittings, the tee is the most difficult to process. The structure of the tee destroys the continuity and integrity of the shell due to the opening of the hole. Under the action of external loads (mainly internal pressure), partial high stress will be generated in the intersection area, that is, the so-called stress concentration effect. To reduce the stress concentration near the intersection area to the allowable range and prevent structural damage due to excessive stress in the intersection area, it is necessary to carry out opening reinforcement.
The reinforcement design methods for openings of tees mainly include the equal-area reinforcement method, the blasting test verification method, and the limit analysis design method (Table 2). In the current design standards for extruded tees, the equal-area reinforcement method is still the main method. The basic idea of these methods is to fill the area weakened by the opening around the opening, and the stress concentration effect near the opening cannot be considered. According to the previous research, the equal-area reinforcement method is too aggressive for the reinforcement design of the tee with a large opening ratio. The blasting test verification method to determine the wall thickness of the tee requires multiple sets of blasting tests, which is costly and has a long test period. At present, it is generally believed at home and abroad that the tee’s bursting pressure is greatly affected by the wall thickness and length of the connected nipple, but there is no standard for the quantitative index of the influence level. The limit analysis design method is based on the plastic limit failure, considering the ultimate bearing capacity under internal pressure and branch load. This method is suitable for opening ratios between 0.75 and 1.0 and diameter-thickness ratios between 10 and 30.
Table 2 A comparison of reinforcement design methods for openings of pipe tees
1.1 Design method
Among the standard pipe fittings, the tee is the most difficult to process. The structure of the tee destroys the continuity and integrity of the shell due to the opening of the hole. Under the action of external loads (mainly internal pressure), partial high stress will be generated in the intersection area, that is, the so-called stress concentration effect. To reduce the stress concentration near the intersection area to the allowable range and prevent structural damage due to excessive stress in the intersection area, it is necessary to carry out opening reinforcement.
The reinforcement design methods for openings of tees mainly include the equal-area reinforcement method, the blasting test verification method, and the limit analysis design method (Table 2). In the current design standards for extruded tees, the equal-area reinforcement method is still the main method. The basic idea of these methods is to fill the area weakened by the opening around the opening, and the stress concentration effect near the opening cannot be considered. According to the previous research, the equal-area reinforcement method is too aggressive for the reinforcement design of the tee with a large opening ratio. The blasting test verification method to determine the wall thickness of the tee requires multiple sets of blasting tests, which is costly and has a long test period. At present, it is generally believed at home and abroad that the tee’s bursting pressure is greatly affected by the wall thickness and length of the connected nipple, but there is no standard for the quantitative index of the influence level. The limit analysis design method is based on the plastic limit failure, considering the ultimate bearing capacity under internal pressure and branch load. This method is suitable for opening ratios between 0.75 and 1.0 and diameter-thickness ratios between 10 and 30.
Table 2 A comparison of reinforcement design methods for openings of pipe tees
| Design Methods | Design Bases | Advantages | Disadvantages | Design standards |
| Equal area reinforcement methods | Elastic design methods | The design method is simple, and based on the empirical formula. | Prefer aggressive for large openings. | ASME VIII-1-2017 "Code for Boilers and Pressure Vessels", GB 150-2011, ASME B31.8-2018, CSA-Z662-2015, GB 50251-2015, GB 50253-2014 "Code for Design of Oil Pipelines" |
| Burst test verification method | Tests |
Being intuitive, ultimate verification methods | The cost is high, and the influence of piping stress cannot be considered. | It is mentioned in the relevant standards of pipe fittings. |
| Limit analysis design methods | Elastic-plastic design methods | Compared with the blasting test verification method, it is safer and greatly reduces the design cost; compared with the equal area method, it can solve the problem of large opening ratio extrusion tee reinforcement design. | The results are greatly affected by the structural dimensional accuracy of the tee analysis model. | SY/T 0609—2016 |
1.2 The structure design of the tee based on the ultimate load
The structural design of the tee with a diameter of 1422 mm x 1219 mm in the China-Russian East pipeline is determined by the limit analysis design method specified in Appendix A of SY/T 0609—2016, and the design is verified by scanning model finite element calculation method and blasting test. Its specific calculation steps are as follows:
(1) Calculate the opening ratio p of the tee with a diameter of 1422mm x 1219mm as
In the formula: db is the outer diameter of the tee; Dmn is the outer diameter of the tee
(2) Put 57mm for Ts, and calculate the geometric dimensionless parameter λ:
In the formula: Ts is the thickness of the upper part of the tee.
(3) According to the values of ρ and λ, find the generalized stress concentration factor map of the corresponding opening ratio, and obtain the generalized stress concentration factor Kp to be 2.32 through the difference.
(4) Calculate the reference pressure P0 according to formula (3):
Where σs is the material yield strength, MPa.
(5) Calculate the ultimate pressure P1 according to formula (4):
(6) P is the design pressure of the stations on the China-Russia East pipeline, which is taken as 12MPa; F is the design coefficient of the China-Russia East pipeline. Take 0.5 according to the fact that P1 is greater than or equal to the checking limit pressure
, if the checking result p1 is less than , the Ts value taken cannot meet the requirement for structural design. (7) Take the value of increasing Ts again, and repeat steps (2) to (6) until p1 is greater than and equal to 24MPa. Calculate that when Ts is equal to 68mm, the ultimate pressure is 24.8 MPa, which meets the requirement for the design structure. The thickness of the lower part of the tee T0 is greater than and equal to 56.6 mm, and the diameter of the Sino-Russian East pipeline is 1422 mm × 1219 mm. The thickness of the lower part of the tee is 57mm.
According to SY/T 0609-2016, design the dimensions of the tee with a diameter of 1422 mm X 1219 mm for the China-Russia East pipeline (Table 3). These tees are all pigging tees. To prevent the tee from being stuck, the inner diameter of the main pipe of the tee should not be less than 1340 mm.
Table 3 Dimensions of tees with a diameter of 1422 mm x 1219 mm for the China-Russia East Pipeline (the minimum size mm)
| The length of the main pipe | The length of the branch pipe | The thickness of the lower part of the tee | The thickness of the upper part of the tee | The thickness of the branch pipe | The radius of the shoulder fillet |
| 1060 | 1090 | 57 | 69 | 48 | 140 to 160 |
1.3 Experimental verification
In order to verify the reliability of the design size of the tee, a small batch of tees with a diameter of 1422 mm x 1219 mm was trial-produced for burst stress and burst pressure tests, and the scanning tee model was used to complete the test based on the ultimate load. The structural stress of the tee and the finite element calculation of the burst pressure.
Figure 1 The finite element calculation results of stress and burst pressure of a tee with a diameter of 1422 mm × 1219 mm based on the ultimate load.
The wall thickness of the main tube of the blasting tee is 33.8 mm. The length is 1.5 m; the yield strength is 700 MPa, and the tensile strength is 710 MPa; the wall thickness of the branch pipe is 33 mm. The length is 0.7 m; the yield strength is 590MPa, and the tensile strength is 670MPa; The yield strength of the main body is 581MPa, and the tensile strength is 664MPa. The test burst pressure is 40MPa, and the calculated pressure is 38MPa based on the ultimate load, both of which are greater than the standard requirement of 29.89MPa.
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