Abstract
The fatigue fracture of a drillstring could cause drilling disturbances and some negative impacts (e.g., economic loss) will be brought when restoring the drillstring to functionality. In order to evaluate the effects of the fatigue damage of the drillstring during drilling, a new apparatus, which could monitor the load level in real-time, was built to perform the four-point bending fatigue test on 35CrMo steel, a typical material of drillstrings. Such an apparatus is based on metal magnetic memory (MMM) technology and can acquire the tangential and normal components of MMM signals. Based on the analysis of the change of surface morphology and MMM signals, it was concluded that the variation of MMM signals could be divided into four stages, which are used to accurately describe the fatigue damage process of the drillstring. Additionally, the MMM signal characteristics are introduced to especially evaluate the fatigue damage of the drillstring, including crack initiation. Furthermore, the scanning electron microscopy (SEM) results demonstrated that morphologies of fatigue fracture were related to the variation of MMM signals. Linear fitting results indicated that fatigue crack length had a good linear relationship with the characteristics, so it is feasible to monitor fatigue damage and predict the residual life of a drillstring by using MMM technology.
Highlights
A drillstring is a long, hollow tubular structure used to drive the drill bit from the surface to the oil and gas area [1]
The magnetic properties of ferromagnetic materials would be affected by fatigue
The specimen was forced through the impact and the residual stress of the specimen began to accumulate with the increase of plastic deformation
Summary
A drillstring is a long, hollow tubular structure used to drive the drill bit from the surface to the oil and gas area [1]. The drillstring often suffers from early failure under large stress mutation caused by alternating loads [2]. Investigation shows that 14% of oil and gas wells experienced underground fracture accidents of drillstrings to different degrees [5], and the average costs of a drilling accident caused by drillstring fracture is as high as $250 million [6,7]. Monitoring stress concentration or microcracks at the early stage during drilling and promptly taking precautions before crack propagation is of great significance to prevent fatigue failure of drillstrings
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