Abstract
The fatigue life prediction model cannot accurately predict the service life of coiled tubing operations, resulting in waste and accidental fracture of coiled tubing. This article introduces the fatigue life prediction model of coiled tubing and a detection system developed using weak magnetic detection method and eddy current detection method. The detection device is used to detect accurate defect dimension, wall thickness reduction, and diameter deformation to improve the accuracy of fatigue life prediction. Furthermore, the actual defect dimension is included in the fatigue life assessment calculation to eliminate the influence of the existing model on the immobilization of defects and corrosion influence factors. By analyzing field operation cases and measured values, the effects of defects, wall thickness reduction, and diameter growth on fatigue life are studied to improve the accuracy of parameters and the integrity of basic data in the calculation process. The influence of reverse bending and weld on fatigue life is discussed. The residual life can be predicted more accurately by combining field testing technology and fatigue life evaluation method. Field measurement input of fatigue life prediction model is the main means to improve its accuracy and practicability.
Highlights
Coiled tubing (CT) is a pipe made of low-carbon alloy steel
Field measurement values are input into the fatigue model software for calculation to improve the prediction accuracy of the fatigue model
From the comparison of the two figures, according to the predicted wall thickness reduction value, it is found that the fatigue life of the entire working pipe is large
Summary
Coiled tubing (CT) is a pipe made of low-carbon alloy steel. It is a revolutionary technology in the petroleum field. Most current fatigue life models directly input a predicted wall thickness reduction value and apply it to the entire pipe segment. This method has great drawbacks to the accuracy of fatigue calculations. Through the study of the working process of the CT, the CT experienced six bending plastic deformations in each start-up operation This is the main cause of low-cycle fatigue damage in CT. The strain concentration factor is used to express the severity of the defect, and the relationship between the strain concentration factor and the defect sensitivity factor Q is found, and the relationship of the equivalent strain amplitude is obtained This provides a theoretical model to reflect the influence of defects on the prediction of tubing fatigue life. The equivalent strain amplitude of the defective tubing can be calculated by the following formula
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