Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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42 SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM Although the penetration cracks on the tubing string were not created during pro- duction, the tubing might have been dam- a ged , or it s pl a sti c ity an d to u g hn e ss reduced , during inter vals of significant bending. Once cracking occurred from the action of alternating loads and the corro- sive medium, the tubing string would have experienced a leak or break. The tubing string present in the well was mainly composed of six types of tubing. Among them, the 89-mm diameter tubing was the smallest in diameter and com- prised the least amount of section area. Stiffness is determined by the elastic modulus multiplied by the area of the sec- tion. If the elastic modulus of the tubing material is constant, then the tubing stiff- ness is proportional to its section area. FIGURE 3 The microfracture morphology of Segment No. 418 tubing. FIGURE 4 The SCC crack morphology of Segment No. 441 tubing. FIGURE 5 The fatigue fracture morphology of Segment No. 420 tubing. Therefore, the smaller the section area, the easier it is to bend the tube. As mentioned previously, the fractures or cracking of the tubing occurred at well depths of 4,146 to 4,712 m, which is a length of 566 m with the midpoint of this length at a well depth of 4,429 m. The entire 89-mm diameter tubing string was located at well depths between 2,892 m and 6,293 m, which is a length of 3,401 m with the midpoint of this length at a well depth of 4,593 m. The tubing fractures and cracking were located at or near the middle of the 89-mm diame- ter tubing string. This indicates that the fractures and cracking were the result of the bending load instead of the tensile load. The 89-mm diameter tubing had the small- est section area among the entire tubing string and could have easily bent to failure under the loads due to fluctuations in tem- perature that caused tubing expansion and contraction. Analysis of the Fracture and Crack Position on the Tubing The tubing's pin and box connections formed a double layer of material at the joints , 7 so the wall thickness of a premium connection—and its stiffness—was greater than that of the pipe body. The thickness change at the end of the coupling was an area of stress concentration on the pipe body. The ratio of the joint section to the pipe body was only 36.2%. Since the transition point of the tubing section was at the pin's vanish point, defor- mation took place at the pin's vanish point first when the tubing string was subjected to a bending load. It was further shown that the tubing easily failed at the end of the joint under bending load in accordance with the observed transverse fracture or crack position at the pin's vanish point. Additionally, there was a stress concentra- tion at the pin's vanish point according to the results of finite element analysis for the connection. Analysis of the Fracture at the Mill's Pin Vanish Point Since there was little load difference for the mill-prepared connection and field- prepared connection for one joint, the rea- son why 10 lengths of tubing fractured or cracked at the pin vanish point for the con- MATERIALS SELECTION & DESIGN

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