Analysis of eccentric wear defect of annular BOP

Scientific Journal of Intelligent Systems Research Volume 3 Issue 3, 2021

ISSN: 2664-9640

370

Analysis of eccentric wear defect of annular BOP

Ling Feng a, Bin Li b

School of Southwest Petroleum University, Sichuan 610500, China;

[email protected],[email protected]

Abstract

In the process of BOP operation, the drill pipe will lose stability under the action of axial pressure and buckle, and the drill pipe will be close to the inner wall of BOP, resulting in contact wear. In this chapter, aiming at the problem of eccentric wear in the inner cavity of annular BOP, the finite element model of annular BOP after wear is established, and the Mises stress under its working condition and hydrostatic pressure is analyzed.

Keywords

Annular BOP; eccentric wear; defect; stress.

1. Introduction

When the BOP is worn, the failure mode is static strength failure under static load, and fatigue failure under alternating load. The degree of wear has a great impact on the safety of the annular BOP. With the increase of the wear depth, the bearing capacity of the shell decreases. When it is lower than the tensile strength limit, it will fail and affect the service life of the annular BOP. Eccentric wear defects will lead to the increase of stress and decrease of stiffness of annular BOP shell, so it is necessary to study the influence of eccentric wear on the stress of annular BOP shell.

2. Establishment of calculation model for eccentric wear of annular BOP

According to energy wear theory, the friction process of two interacting components can be described as energy consumption and transfer, that is, one part of friction work is transformed into friction heat, which is dissipated with the flow of drilling fluid, and the other part of friction work is metal wear, which is consumed in the form of debris. In this paper, the wear efficiency E is introduced to describe the ratio of this part of work to friction work:

∆𝑉 = 𝐸𝜇𝐹𝐿

𝐻 (1)

Considering the working condition of the BOP, the friction surface on the diameter wall of the BOP is regarded as a semi elliptical surface with concave middle and flat sides

(1) The wear of drill pipe is not considered, only the wear of BOP is considered, and the wear is uniform;

(2) The rotation of drill pipe is not considered;

(3) During the wear process, the contact force between the annular BOP and the drill pipe remains unchanged;

(4) The wear cross section is approximately crescent shaped.


ISSN: 2664-9640

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Fig. 1 Schematic diagram of BOP wear calculation

As shown in Figure.1, the relationship between SABCD and he is

𝑆𝐴𝐶𝐵𝐷 = (𝑆𝑂2𝐴𝐷 − 𝑆𝑂1𝐴𝐶 + 𝑆∆𝐴𝑂1𝑂2) × 2 (2)

𝑆𝑂2𝐴𝐷 =1

2𝑟2arctan(

√𝑅2− y2

𝑦 − ℎ𝑒) (3)

𝑆𝑂1𝐴𝐶 =1

2𝑅2arctan(

√𝑅2− y2

𝑦) (4)

𝑆∆𝐴𝑂1𝑂2 =1

2ℎ𝑒√𝑅2 − 𝑦2 (5)

y =𝑅2 +ℎ𝑒

2 − 𝑟2

2ℎ𝑒 (6)

∆𝑉 = 𝑆𝐴𝐶𝐵𝐷 × 𝐿 (7)

By combining the above formulas, the distance he of the drill pipe from the center of the annular BOP can be obtained, and then the actual wear depth h of the annular BOP can be calculated. It can be seen from Fig. 1 that the actual wear depth of BOP

ℎ = 𝑟 + ℎ𝑒 − 𝑅 (8)

3. Effect of different wear depth on stress of BOP shell

3.1. Establishment of model

Assuming that the worn part is located at the orifice of the lower shell of the annular blowout preventer, the solid model of the annular blowout preventer after eccentric wear is established. The friction surface on the diameter wall of the BOP is regarded as a semi elliptical surface with concave middle and flat sides, and prefabricated as shown in Fig. 2 Wear surface, the top cover and the lower shell of the blowout preventer are regarded as a whole. In the Creo modeling process, some small structures are omitted, such as lifting lug, sealing groove, connection thread between the top cover and the shell, etc. the shell of the blowout preventer is a symmetrical rotating body. During the analysis, 1 / 4 of its structure can be taken for modeling analysis, and the model as shown in Figure 2 is obtained.


ISSN: 2664-9640

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Fig. 2 1 / 4 Model of annular BOP

According to the data and calculation method provided by API 16a, the structural parameters of annular BOP are determined (as shown in table 1) .

Table 1 Technical parameters of FH35-35/70 annular BOP

Diameter 346mm

Dimensions ϕ1270× 1225mm

wall thickness 101mm

working pressure 35MPa

Strength test pressure 51MPa

3.2. Material properties

The material of blowout preventer is 25CrNiMo carbon structural steel, which is cast, with high hardness and good machinability. After quenching and tempering, the surface of annular BOP still has good wear resistance, while the toughness and plasticity of low carbon steel are still maintained in the central part. The material parameters required for finite element analysis are shown in table 2.

Table 2 Material parameters of 25CrNiMo

Material name Elastic modulus

(GPa) Poisson's

ratio

Yield strength

(MPa)

Tensile strength

(MPa)

25CrNiMo 216 0.3 421 724

3.3. Meshing and load setting

The model drawn in Creo is imported into ABAQUS. After the model is assembled, it is meshed. As shown in Fig. 3, the hexahedral shape element is used to divide the mesh. The global size is set to 20, and a total of 29820 nodes and 25461 elements are divided.

As shown in Fig. 4, the following boundary constraints are applied to the meshed model: constraints are imposed on the flanges of the BOP top cover and lower shell to limit their rotation and displacement; symmetrical constraints in X and Z directions are applied to the two cutting planes of the model. During the finite element analysis, the working condition of hydrostatic test is simulated. The internal pressure of BOP shell is uniformly distributed, and the pressure is perpendicular to the surface of BOP cavity, and the applied load is 35MPa.


ISSN: 2664-9640

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Fig. 3 Mesh generation model of annular BOP

Fig. 4 Stress and restraint model of annular BOP shell

3.4. Result analysis

As shown in Figure 5, in order to better observe the stress distribution of the damaged part and reduce the complex transfer area in the solid model, a sub model is established based on the established model.

The eccentric wear model of annular BOP with different wear depth is established, and the stress distribution nephogram and the maximum equivalent stress of the damaged part can be obtained by finite element analysis. As shown in Figure 6, the maximum equivalent stress distribution on the inner wall of annular BOP with wear depth of 1 mm, 2 mm, 3 mm, 4 mm, 5 mm and 6 mm is shown respectively.


ISSN: 2664-9640

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Fig.5 Sub model and mesh generation of annular BOP

a) Wear depth 1mm b)Wear depth 2mm

c) Wear depth 3mm d)Wear depth 4mm


ISSN: 2664-9640

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e) Wear depth 5mm f)Wear depth 6mm

Fig.6 Stress nephogram of annular BOP with different wear depth

From the stress nephogram of different wear depth of annular BOP, the maximum equivalent stress corresponding to different defect depth can be obtained, and the stress concentration factor can be calculated, as shown in Table 3.

Table 3 Stress concentration factor of annular BOP with different wear depth

Defect depth(mm) 0 1 2 3 4 5 6

Maximum stress(MPa) 69.47 80.47 84.50 87.61 91.10 97.96 101.70

Stress concentration factor

1 1.16 1.22 1.26 1.31 1.41 1.46

4. Summary

With the increase of wear depth, the maximum equivalent stress of inner wall of annular BOP diameter increases, the stress concentration factor increases, and the degree of stress concentration increases. It shows that eccentric wear defects will lead to new stress concentration points of annular blowout preventer, and its depth has a positive correlation with the structural safety of annular blowout preventer.

(1) Based on the geometric relationship, the calculation model of eccentric wear of annular BOP shell is established;

(2) By analyzing the equivalent stress of annular blowout preventer shell with different eccentric wear depth, it is found that the influence of different eccentric wear depth on the stress concentration factor of annular blowout preventer shell is positively correlated;

References

[1] Dang Zhigang, Jin Yequan, Jin Jingyang, Li Cheng, Ji Yongqiang, Liao Hualin. Quantitative risk analysis of failure of blowout preventer system in deepwater drilling[J]. Science Technology and Engineering, 2020, v.20; No.536(31): 122-127.

[2] Zhu Chuanyu, Chen Wenke, Xu Lixin, Miao Chen, Yuan Yuan. Discussion on stress analysis methods of underwater blowout preventers at high pressure levels[J]. Chemical Equipment Technology, 2020, 41(02): 67-69.

[3] Chen Wenke, Xu Lixin, Zhu Chuanyu, Miao Chen, Yuan Yuan. Fatigue life analysis of underwater ram blowout preventer assembly[J]. Ship and Ocean Engineering, 2020, 36(05): 38-41+78.


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[4] Yu Jiamin, Chen Wenbin, Zhao Qiyue, Xu Guikai, Guo Mengmeng. Simulation study on the structural strength of ram blowout preventer based on metal loss[J]. Innovation in Science and Technology, 2020(30): 143-144.

[5] Vujasinovic, Ado N.. "How Blowout Preventers Work." J Pet Technol 38 (1986): 935–937 [6] Shanks, Earl, Dykes, Andrew, Quilici, Marc, and John Pruitt. "Deepwater Bop Control Systems - A

Look At Reliability Issues." Paper presented at the Offshore Technology Conference, Houston, Texas, May 2003.

Analysis of eccentric wear defect of annular BOP

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