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Paper Information

Journal:   JOURNAL OF FACULTY OF ENGINEERING (UNIVERSITY OF TEHRAN)   NOVEMBER 2007 , Volume 41 , Number 4 (106); Page(s) 415 To 430.
 
Paper: 

AN ANALYTICAL MODEL FOR THE PREDICTION OF DRILLPIPE FATIGUE LIFE

 
 
Author(s):  RASHED GH.R.*, GHAJAR R.A., HASHEMI SEYED JALAL ALDIN
 
* AHWAZ FACULTY OF PETROLEUM ENGINEERING, PETROLEUM UNIVERSITY OF TECHNOLOGY, I.R. IRAN
 
Abstract: 

An analytical model for the prediction of drillpipe fatigue life has been proposed. The model utilizes a physically based critical plane approach. This approach uses two damage parameters: one is based on normal strain amplitude and the other is based on shear strain amplitude. Included in both parameters, the effect of the normal stress on a specific critical plane has been considered. Meanwhile, stress concentration and fatigue strength reduction factors are also taken into account in this analysis. A drillstring is used to transmit rotation to the drill bit and provide fluid circulation in hole bottom while drilling. As these functions are being performed, the drillpipe experiences a combination of cyclic loads that include bending, torque, axial force and pressure. This situation is very common in directional drilling operations.
Axial force and pressure can be assumed to be non-cyclic for short intervals. However, a torque load, like bending, is cyclic in nature. Due to common slip/stick effect, drill pipes experience small torque fluctuations with high frequency. Also, torque fluctuations occur due to making connections. The torque changes from zero to a relatively constant value soon after rotation resumes. In a directional well with high friction, torque can be significant factor and may cause long term fatigue damage.
Since drillpipes are generally thin-walled tubulars, it is assumed that they are in a plane stress condition. An analytical model is developed in this paper to calculate the stresses and strains that drillpipes experience during drilling operations. Then a proposed analytical model has been applied to predict drillpipe fatigue life. Two computer programs were also accomplished and used. The first one allows the calculation of the loads, and stresses along the drill string. The second one allows the prediction of the fatigue damage in several planes along the drill string for a certain drilled interval. It has been shown that there is a good agreement between test results and the fatigue model predictions. It is also shown that torque and stress concentrations are very important and must be considered in drill pipe fatigue analysis. The drillpipe fatigue model is compared to the present API approach. It is observed that the drillpipe fatigue model is more conservative than the present API approach. This is because the API model does not account for torque.

 
Keyword(s): DRILL PIPE, CRITICAL PLANE MODEL, API APPROACH, DAMAGE PARAMETER, FATIGUE, DOG LEG
 
References: 
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