Failure analysis of wellhead casing and optimization of slip hanger in ultra deep wells

Abstract

When replacing the sealing element and lifting the oil production spool in a certain oilfield in Xinjiang, it was found that the casing at the clamping point of the wellhead hanger slip was broken. Based on the Ф232.5 mm W-type slip hanger structure used in the X1 well on site, a three-dimensional finite element model of the casing’s grip by the slip hanger was established. Firstly, based on the third and fourth strength theories, the calculation formula for the ultimate load of the casing under the slip effect was derived. Furthermore, tensile experiments, chemical composition measurements, and microstructure analysis were conducted on the 155 V casing samples used, and it was found that the performance of the 155 V casing material meets the standard requirements, the data suggests that the slip may contribute to casing fracture, warranting further investigation. Therefore, based on the mechanical finite element analysis model, it is calculated that the maximum stress on the outer wall of the casing reaches 1146 MPa, which is higher than the yield strength of the 155 V casing. This will lead to the damage of the casing used on site. Therefore, the structure of the slip is optimized, and the analysis shows that the top of the slip tooth is optimized to have a long axis of 400 mm, the one-fourth elliptical distribution with a short axis of 1 mm and the optimization of the top angle of the slip teeth to 60°–70° will significantly decrease the maximum stress on the outer wall of the casing. However, the optimization scheme of changing the inclination angle of the slip teeth and increasing the clearance of the slip back cone angle cannot effectively improve the stress distribution and maximum value on the outer wall of the casing.