Structural evaluation and lightweight design of 175 MPa ultra-high pressure wellhead 6BX flange

doi:10.3785/j.issn.1006-754X.2025.05.114

Chinese Journal of Engineering Design

2025, Vol. 32

Issue (4): 499-513 DOI: 10.3785/j.issn.1006-754X.2025.05.114

Optimization Design

Structural evaluation and lightweight design of 175 MPa ultra-high pressure wellhead 6BX flange

Xuliang ZHANG^1,^2,^3,⁴(

),Junlin SHI^5,⁶,Ren DONG^1,^2,^3,⁴,Zhanghua LIAN⁵,Lei ZHA^1,^2,^3,⁴,Hongbo JIANG^1,^2,^3,⁴

^1.R & D Center for Ultra-deep Complex Reservior Exploration and Development, CNPC, Korla 841000, China
^2.Engineering Research Center for Ultra-deep Complex Reservoir Exploration and Development, Xinjiang Uygur Autonomous Region, Korla 841000, China
^3.Xinjiang?Key?Laboratory?of Ultra-deep Oil and Gas, Korla 841000, China
^4.Petrochina Tarim Oilfield Company, Korla 841000, China
^5.National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
^6.School of Mechanical Engineering, Sichuan University of Science & Engineering, Yibin 644000, China

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Abstract

To address the issues of volume redundancy, high manufacturing difficulty and potential failure risks in the traditional design of 175 MPa ultra-high pressure wellhead equipment due to the absence of specification standards, a structural evaluation and lightweight design research on the critical component 6BX flange of wellhead equipment was conducted. Taking the 6BX flange with a diameter of 280 mm and employed in 175 MPa wellhead equipment as the research object, the 6BX flange produced by a certain company was evaluated in terms of material properties, gasket sealing strength and other factors. The relationship between the hydrostatic test pressure and the structural strength and dimensions of flange was explored, and the influence factors of flange dimensions were revealed. Under the premise of ensuring safety, an optimization scheme for lightweight was proposed, and the rationality of the optimized structure was verified by the finite element method. The research results indicated that the flange structure designed by the traditional method could meet the actual use requirements, but it had an excessively large structure volume, making installation inconvenient. Excessive wall thickness would make the material processing more difficult and risky, and using standard BX gasket also posed the risk of sealing leakage. The flange structure dimensions was influenced by multiple factors. The main controlling factor for the dimensions of large-diameter flange structures was the hydrostatic test pressure, followed by the neck thickness. It was recommended to design the neck thickness according to the elastic-plasticity theory. For the 175 MPa ultra-high pressure wellhead equipment, the ratio of the inner and outer diameters of flange was recommended to be 2 and a widened BX gasket should be adopted. The proposed flange structure optimization scheme was reasonable, significantly reducing the weight of the flange, and also offering a certain safety margin. It was recommended to reduce the hydrostatic pressure test pressure for the 175 MPa ultra-high pressure wellhead equipment from the original 1.5 times the rated pressure to 1.25 times. The research results provide a theoretical basis for the design of the 175 MPa ultra-high pressure flange structure.

Key words： ultra-high pressure wellhead equipment 6BX flange lightweight finite element analysis

Received: 24 February 2025 Published: 01 September 2025

CLC:

TE 931.1

Corresponding Authors: Junlin SHI E-mail: 821070764@qq.com

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Cite this article:

Xuliang ZHANG,Junlin SHI,Ren DONG,Zhanghua LIAN,Lei ZHA,Hongbo JIANG. Structural evaluation and lightweight design of 175 MPa ultra-high pressure wellhead 6BX flange. Chinese Journal of Engineering Design, 2025, 32(4): 499-513.

URL:

https://www.zjujournals.com/gcsjxb/10.3785/j.issn.1006-754X.2025.05.114 OR https://www.zjujournals.com/gcsjxb/Y2025/V32/I4/499

175 MPa超高压井口6BX型法兰结构评价及轻量化设计

为了解决175 MPa超高压井口装置因规范标准缺失而导致的传统设计中体积冗余、制造难度大及存在潜在失效风险等问题，开展井口设备关键部件6BX型法兰的结构评价及轻量化设计研究。以175 MPa井口装置使用的口径为280 mm的6BX型法兰为研究对象，从材料性能、垫环密封强度等方面对某公司生产的6BX型法兰进行评价，探讨了水压试验压力与法兰结构强度和尺寸的关系，揭示了法兰尺寸的影响因素；在保证安全的前提下，提出了轻量化的优化方案，并采用有限元方法论证了优化后结构的合理性。研究结果表明：采用传统方式设计的法兰结构能满足实际使用要求，但结构体积过大，安装不便，其壁厚过厚会造成材料热处理难度大且存在风险，而且采用标准BX型垫环有密封泄漏的风险；法兰结构尺寸受多因素影响，大口径法兰结构尺寸的主控因素是水压试验压力，其次是颈部厚度；建议根据弹塑性理论设计颈部厚度，对于175 MPa超高压井口设备，推荐法兰内外径之比为2，并采用加宽的BX型垫环。所提出的法兰结构优化方案合理，法兰轻量化效果显著，且有一定的安全裕量；建议降低175 MPa超高压井口装备水压试验压力，从原来的1.5倍额定压力降低为1.25倍。研究结果为175 MPa超高压法兰结构的设计提供了理论依据。

关键词： 超高压, 井口设备, 6BX型法兰, 轻量化, 有限元分析

Fig.1 Christmas tree installation site

Fig.2 Flange structure model

Table 1 Measurement results of flange dimensions

Fig.3 Mechanical model of BX gasket

Table 2 Main chemical composition of two types of martensitic stainless steels

Fig.4 Tensile test data for F6NM （UNS S41500） material

Fig.5 Decomposition of pressure load on gasket

Fig.6 Determination of diameter of bolt hole distribution circle

Fig.7 Relationship between diameter of bolt hole distribution circle and major diameter of hub and number of bolts

Fig.8 Flange diameter ratios calculated by different methods

尺寸参数	数值
垫环外径D_r	368.1 $- 0.13 0$
环高H	23.14 $0 + 0.20$
环宽A	30.1 $0 + 0.20$
平面直径D_p	364.83 $- 0.5 + 0.5$
平面宽C	26.83 $0 + 0.15$
孔径D	3.2 $0 + 0.15$
槽深E	13.88 $0 + 0.50$
槽外径G	373.3 $0 + 0.10$
槽宽N	36.14 $0 + 10$

Table 3 Dimensions of BX gasket and gasket groove after optimization

Table 4 Flange structure dimensions under different schemes

Fig.9 Comparison of flange structure dimensions before and after lightweight

Fig.10 Finite element model of flange structure

Fig.11 Stress-strain curves of gasket material

Fig.12 Stress nephograms of flange structure under various operating conditions

Fig.13 Stress nephograms of flange structure in various directions under rated condition

Fig.14 Axial displacement nephograms of flange structure under various operating conditions

Fig.15 Contact pressure nephograms of convex surface between upper and lower flanges under various operating conditions

Fig.16 Stress nephograms of bolt under hydrostatic test condition

Fig.17 Stress nephograms of bolt middle section under various operating conditions

Fig.18 Axial displacement curves in the middle of flange

Table 5 Calculation results of flange stiffness coefficient

Fig.19 Stress and strain nephograms of flange structure under ultimate internal pressure

Fig.20 load-strain curves at key positions of gasket

Fig.21 Deformation of flange structure under additional bending load under rated condition

Fig.22 Relationship curves between stress of bolt and bending load at ultimate state

Fig.23 Relationship curves between upper and lower flange boss clearance and bending load

Fig.24 Stress nephograms of flange structure under ultimate tensile load


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