تجزیه و تحلیل شبیه سازی میدان جریان و توزیع تنش برشی در منطقه داخلی و انتقال آشفته از لوله مته

Perforation in internal upset transition zone is one of the main failure styles for drill pipes. Due to the flow channel specificity of upset transition zone, in this paper, drill pipe failure analysis from the point of flow erosion was put forward. Flow field of drilling fluid in the internal upset transition zone of Φ127 mm API IEU S135 drill pipe under various conditions were obtained using finite volume CFD solver FLUENT 13.0. Pressure, velocity and wall shear stress distribution in upset transition zone indicated the erosion of flowing fluid on drill pipe. The type and displacement of drilling fluid and the structure of internal upset transition zone exert great effects on the flow field. Under the same displacement of drilling fluid, mud shows the most severe erosion effect. And the larger the displacement, the more obvious is the washout. An increase in length or radius of the transition can weaken the flow erosion.

Drill pipe failure is an outstanding problem in drilling engineering, which gives rise to great financial losses. Perforation in internal upset transition zone is one of the main failure styles for drill pipes (Fig. 1). From Li’s surveys [1], 65.7% failure occurred in upset transition zone in 108 failure accidents of 16 oil fields in China. Taking Tarim oil field for example [2], the failure frequency caused by perforation in upset transition zone has been increasing each year. Many Φ127 mm API IEU S135 drill pipe failures of perforation occurred after 2000 h of pure drilling time with 80–120 rpm rotary speed in wells, whose depths are more than 2000 m. Just in 2004, such drill pipe failure accidents were more than 170. Previous drill pipe failure studies have mainly focused on finite element stress analysis or failure statistics [3], [4] and [5]. The impact of drilling fluid on drill pipe is generally ignored. However, sudden change of flow field presents in the internal upset transition zone due to the change of flow channel. The fluctuation of pressure and the existence of local low-pressure aggravate the flow impact on drill pipe. Therefore, it is very urgent to do drill pipe failure analysis from the perspective of flow erosion. Several studies have been performed in attempts to explain flow erosion. Ranjbar [6] reported that the changes of fluid velocity inside tubes may cause severe erosion. Arefi et al. [7] proposed that erosion can be reduced by decreasing the fluid velocity. Ferng [8] pointed out that the geometry of flow channel presents obvious effect on flow erosion. Thus, flow field of drilling fluid in drill pipe may be one of the important factors governing erosion. In this paper, simulation analysis of flow erosion in internal upset transition zone of Φ127 mm API IEU S135 drill pipe has been performed. Based on computational fluid dynamics (CFD), finite volume method was employed to obtain pressure, velocity and wall shear stress distributions under different drilling methods, different drilling fluid displacements or different structures of upset transition zone. Then the impact of each factor was analyzed concretely.

A numerical procedure is employed to study the flow erosion in internal upset transition zone of drill pipe. The following conclusions can be formulated: (1) Sudden change of flow field presents in the internal upset transition zone due to the change of flow channel, giving rise to severe flow erosion. Especially, relatively obvious pressure fluctuation and large local low-pressure area present in the internal upset transition zone near box thread, which induce eddy scouring the pipe wall repeatedly. So the perforation of drill pipe basically locates at the internal upset transition zone near box thread. (2) Due to the high viscosity, mud shows the most severe erosion effect on the internal upset transition zone of drill pipe under the same inlet velocity. And the larger the displacement, the more obvious is the washout. Thus, a reasonable drilling fluid displacement should be selected so that not only the drilling rate can be improved effectively but also the flow erosion of drill pipe can be reduced to the minimum level. The calculation results agreed well with the actual situation. (3) Flow field has a favorable change by modifying the structure of transition zone. An increase in the fillet radius or the length of the transition can reduce the scouring effect of drilling fluid on the pipe wall effectively, leading to a longer service life of drill pipe. And this structural optimization method has been proved to be effective in practice.