Introduction

The casing head hanger, which consist of casing head, supporting ring and seals, functions as connecting the surface casing and sealing the annular space between the sealing casing and the casing1,2,3. The casing head hanger suspends and seals multiple layers of casings in order to satisfy the special operational requirements of drilling, cementing, downhole pressure testing, well flushing, acid fracturing during well testing and completion4. Well strength performance of the supporting ring ensures the safety of the casing head hanger system. With development of oil and gas exploration, extra deep oil wells have been frequently explored, and thus, much more complex working condition the drilling facilities should be satisfied5,6,7. The casing head hanger is one of the critical facilities that ensue the successfully drilling operation for such extra deep oil wells.

The literature offers many research studies on designing and optimizing the casing head hanger. Zhang et al. analyzed the reasons for failure of a casing head slip hanger by applying the techniques of optical microscopy, scanning electron microscopy, and energy dispersive spectrometry. Suggestions, such as controlling the heat treatment process of the slip material and correctly installing the slip hanger to ensure the slip and casing were tightly held in place, were proposed to prevent the failure of the casing head hanger in their research8. Mou et al. analyzed the failure causes of connecting threads and put forward improvement measures based on a typical case of a well accompanied by a hanger seal failure. The results indicate that both the hanger material and casing material were characterized with significant ductile fracture9. Li et al. established a mechanical analysis model for slip casing hanger based on the specific size and operation process of casing head and casing hanger in the oil field10,11,12,13. Based on the archived documents available to the authors, it seems there are very few research studies investigating the supporting ring of casing head hanger in extra deep oil wells, although the importance of these structures is recognized by researchers and engineers. The aim of the research reported here was to study the mechanical behavior of the supporting ring based on an actual extra deep oil well, and optimize its structure according to the numerical simulation researches done in this article.

Axial load on the casing head hanger

Structure and analysis model

The casing head hanger system, shown as in (Fig. 1a), consists of casing hanger, supporting ring and wellhead. Both the casing head and supporting head are used to suspend the casing in the downhole of the oil well. Here in this research, R is the chamfered corner of the supporting ring where mates the casing head. R2 is the chamfered corner of the supporting ring where mates the casing. h is the height of the supporting ring. β is the inclination angle of the supporting ring.

Fig. 1
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Structural and analytical model of casing head supporting ring system.

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Due to space limitation, the supporting ring of the casing head hanger is nonetheless capable of bearing large axial loads, despite its compact size. The sealing performance of it is well and its load-bearing is strong, also work stably in complex working conditions. The plane axisymmetric model of the casing head hanger system has been taken in this research because of its symmetric structure, shown as in (Fig. 1a).

The contact vices were established between casing, supporting ring and casing head based on their working condition. Shown as in (Fig. 1b), the casing equivalent load down in the oil well and internal pressure equivalent load upper the casing caused by the test pressure were loaded below (B) and upper (A) casing. And the fixed constrain was applied at lower casing head in axial direction.

Axial load

To analyze the mechanism behavior of the supporting ring, an actual deep oil well was selected. The casing head hanger was loaded with 9 5/8″ casing (2120000N), 7″ casing (3150000N) and tubing (7500000N) during its working condition. Due to the annular space between each casing have been sealed, there has no internal pressure inside the casing head hanger. Meanwhile, the casing head hanger was loaded with 9 5/8″ casing, 7″ casing and equivalent axial internal pressure load (9600000N) during its test condition. Therefore, the casing head hanger was loaded with 602t, 960t and 1487t axial loads in working condition, indoor test condition and engineering test condition, respectively.

Mechanical performance of casing head hanger

Basing on the traditional design of the casing head hanger, the initial inclination angle (β), chamfered corners (R and R2) and height (h) of the supporting ring were set as 45°, 1mm and 45mm, respectively. The casing equivalent load (212t) was loaded at lower part of the casing (point B in (Fig. 1b)) and different internal pressure equivalent load were applied at upper part of the casing (point A in (Fig. 1b)) to analyze the mechanical behavior of the supporting ring in different loads. (Fig. 2) shows the overall stress of the casing head hanger (a), stress performances of supporting ring (b), wellhead (c) and casing hanger (d) under axial load of 1160t. (Fig. 3) shows the stress variation of the supporting ring under different axial loads.

Fig. 2
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Stress distribution casing head hanger system (axial load: 1160t).

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Fig. 3
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Stress variation of supporting ring under different axial loads.

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The results indicate that both the maximum stress and the average shearing stress of the supporting increase gradually with axial load applied on the casing head hanger system. The average shearing stress reaches 895MPa when the system was applied with 870t load, and thus, fracture phenomenon appears on the supporting ring. Therefore, the initial design of the hanger ring support system failed to meet the strength requirements, and thus it is necessary to make structural improvements and optimizations to it.

Inclination angle of supporting ring

Effect of inclination angle on height

The inclination angle which affects the structure of the supporting ring is one of the most important parameters. To optimize the structure of the supporting ring, effect of the inclination angle of the supporting ring on its height was analyzed.

The structural sketch of the supporting ring is shown as in (Fig. 4). Here in this research, F1 is the effective mating surface of the hanger and the supporting ring. R1 and R2 are the small and large diameters of the support surface F1. RA is the equivalent effective diameter of the action surface of F1, and point A is the bisecting point of the area of the conical surface F1. F2 is the effective action surface of the supporting ring and the wellhead. R3 and R4 are the small and large diameters of the support surface F2, and RB is the equivalent effect diameter of the action surface of F2. Point B is the bisecting point of the area of conical surface F2. h is the midpoint height of the inclined plane (longitudinal height difference between points A and B), and h1, h2, and h3 are the diameter differences which can be expressed as h1 = R2 − RA, h2 = RB − R3, and h3 = R3 − R2. L1 is the length of the busbar of the conical platform F1 which can be expressed as L1 = (R2 − R1)/cosβ. L2 is the length of the busbar of the conical platform F2 which can be expressed as L2 = (R4 − R3)/cosβ. In the established structure, R1, R2, R3, and R4 are known parameters which were set as 138mm, 149mm, 158.5mm, and 167mm, respectively.

Fig. 4
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Structural model of supporting ring.

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According to the analysis mentioned above, variation between the midpoint height of the inclined plane and the inclination angle of the supporting ring can be obtained. Shown as in (Fig. 5), the midpoint of the inclined plane decreases gradually with the inclination angle. To ensure the equivalent action force of the two inclined surfaces remain in the same line, the thickness of supporting ring decreases with the inclination angle on the basis of the original design. The midpoint height of the inclined plane should be remained in the minimum thickness to avoid the cantilever shearing of the supporting ring.

Fig. 5
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Variation of the midpoint height of the inclined plane.

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Effect of inclination angle on mechanical behavior

The inclination angle of the supporting ring is the main factor affecting the mechanical performance of the structure in the supporting ring structure. The change of inclination angle affects not only the midpoint height of the inclined plane as analyzed in the previously section, but also the structural stress of the supporting ring. Supporting ring with large inclination angle leads to excessive wedge tightness and results in excessive structural stress. Supporting ring with small inclination angle leads to insufficient effective support surface and results in structural collapse. Effects of inclination angle (setting from 25° to 50°) on mechanical behavior were analyzed in this research.

Since the casing head hanger was applied the largest load under the engineering test condition, the casing suspension gravity was applied on the casing hanger while the axial equivalent load was applied on the upper casing hanger in this research. Shown as in Figs. 6 and 7, the supporting ring has the largest maximum stress, film stress (Pm) and bending stress (Pb) among casing head hanger system and failure of the supporting ring would be caused if its material was not selected properly or its structure was not optimally designed. Due to the effective contact area decrease with the inclination angle of the supporting ring, the supporting ring was loaded larger contact pressure when the supporting ring has larger inclination angle and potential local plastic collapse might be caused. The change in inclination angle β has a relatively small impact on the maximum stress of the body, and has a certain influence on the combined stress of the film and bending of the body; the change in inclination angle β has the least impact on the stress. The supporting ring has well mechanical performance hen the inclination angle was selected from 40° to 46°, shown as in Fig. 6. Moreover, the excessive wedging effect, which induces significant radial compression and bending stress, might be caused and then the supporting ring was transformed from a pressure-bearing component to a bending-bearing component when the supporting ring was designed with excessive inclination angle. Shown as in Fig. 7, it would be better if the inclination angle was selected as 45° for the reason that the supporting ring has small wedging force and stress under such condition.

Fig. 6
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Maximum stresses of each structure.

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Fig. 7
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Values of “Pm + Pb” of each structure.

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Optimization design of supporting ring

Structural improvement

Based on the analysis done in the previously section, initial design of the supporting ring has been carried out with its basic parameters including the height h = 45mm, chamfered corner R = 1mm, chamfered corner R2 = 1mm, inclination angle β = 45°. The initial height of the supporting ring was adjusted to analyze the effect of the thickness of the supporting ring. The effects of inclination angle and thickness of the inclined plane on the mechanical performance of the supporting ring are shown as in (Fig. 8). The stress of the supporting ring increases with the inclination angle and the thickness of the supporting ring increases. The maximum working stress of the original thickness increases first and then decreases as the inclination angle increases. Moreover, new stress concentrations might be caused due to the changes in stiffness altering the load path. And therefore, the increase of supporting ring’s thickness sometimes leads the increase of the stress loaded on the supporting ring. Additionally, the inclination angle and thickness of the inclined plane should be restricted in a proper range to avoid the superposition of high stress areas in the supporting ring.

Fig. 8
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Effects of inclination angle and thicknesses.

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For the reason that it has small effective support area of the support area due to the restrict of the annular diameter of the casing head hanger system, the effective support cross-sectional area of the supporting should be carefully considered during improving the supporting ring. The optimized structure of the supporting ring is shown as in (Fig. 9). (Fig. 9a) shows the initial design of the supporting ring and (Fig. 9b) shows the optimized supporting ring based on the previously research.

Fig. 9
figure 9

Comparison of the optimized supporting ring.

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Mechanical behavior of the optimized structure

Mechanical behavior under working condition

Figure 10 shows the analytical model (a), overall stress (b), overall deformation (c) of the casing head hanger system and the danger paths of the supporting ring (d) under working load condition. The safety verification of the supporting ring was done basing on ASME VIII Division 211,12,13 in this research. And the results, shown as in Table 1, indicate that all 3 danger paths of the supporting ring satisfy the design requirements under working condition.

Fig. 10
figure 10

Mechanical behavior of the optimized structure under working condition.

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Table 1 Safety verification of the optimized supporting ring under working condition.
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Mechanical performance under engineering test condition

Figure 11 shows the analytical model (a), overall stress (b), overall deformation (c) of the casing head hanger system and the danger paths of the supporting ring (d) under engineering test load condition. Shown as in Table 2, all danger paths of the supporting ring satisfy the verification requirement under engineering test load condition. Basing on the analysis done in this section, the optimized structure of the supporting ring satisfies the operation requirement in extra deep oil wells.

Fig. 11
figure 11

Mechanical behavior of the optimized structure under engineering test condition.

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Table 2 Safety verification of the optimized supporting ring under engineering test condition.
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Conclusion

The supporting ring of the casing head hanger in extra deep oil wells was investigated in this research. Effects of the inclination angle of the supporting ring that dominates the load capacity of the casing head hanger was analyzed basing on the working and testing condition and an optimized structure was proposed. Based on the work done in this research, the following results and conclusions were obtained.

  1. (a)

    The axial load that applied on the casing head hanger was firstly analyzed. The casing head hanger system was applied with different loads in working condition, indoor test condition and engineering test condition, respectively. And the numerical analysis results indicate that the traditional casing head hanger can satisfy the working condition load only in deep oil well.

  2. (b)

    Effects of the inclination angle on the structural morphology and the mechanical behave of the supporting ring were analyzed. And the results indicate that: (i) the inclination angle affect both height and thickness of the supporting ring; (ii) the supporting ring has the largest stress compared with casing head and well head; (iii) it would be better if the inclination angle was selected as 45° for the reason to guarantee the supporting ring has small wedging force and stress under axial load.

  3. (c)

    According to the safety verification of the supporting ring basing on ASME VIII Division 2, the proposed optimized structure satisfies the load capacity requirement in both the working condition and engineering test condition