Evaluación de fallas durante la fatiga de tuberías enrollables usadas en pozos de petróleo

F. DÍAZ, S. HEREÑÚ, J. MALARRÍA, I. ALVAREZ, A. ARMAS, J. PEREZ IPIÑA

San Nicolás (Argentina)

Conferencia; 2nd IAS Conference on Uses of Steel; 2004

Instituto Argentino del Acero

QT-800 coiled tubing (CT) is used to perform a wide variety of production and service operations in oil wells. The material used to manufacture QT-800 CT is a modification of ASTM A-606 Type 4 high strength low alloy (HSLA) steel. The most dominant factors controlling the deformation behavior of CT in the field are the bending/straightening cycles associated with the spool and gooseneck. In spite of the fact that strain fluctuations are larger at the outer surface of the tubing, the primary cracks always nucleate along the inner surface. This could be due to the nature of the manufacture process, which tends to produce an inner surface less resistant to fatigue than the outer surface.In this study, hardness measurements and transmission electron microscopy (TEM) observations are performed to evaluate the differences between the internal and external surfaces of a sector of CT, which has consumed less than 5% of the service life.The structure of the studied material is composed of ferrite and pearlite. The ferrite corresponding to the internal side of CT is 4% harder than that of the external side. The pearlite also shows a highest hardness in the internal side, but in this case 6%.The micrographs obtained by TEM show dislocation cells in the ferrite in both the inner and outer side. It was reported that this substructure is developed at the beginning of the cyclic life for the case of high deformation amplitudes. The cells generated in the inner surface appear with a more regular size and shape than those generated in the outer surface. The equiaxial cells of the interior show broad edges and high dislocation density. On the other hand, the appearance of disordered cells and broad walls in the outer surface could indicate a previous state of cyclic deformation. The evolution from broad dislocation walls to ordered cells has been observed in other materials, which contain a high percentage of free ferrite.The conclusions of the study are as follows:- In spite of the external side of CT supports higher cyclic deformation amplitudes than the internal side, this latter side shows a most accumulated fatigue damage.- This anomalous behavior could be due to the formation of different dislocation substructures at both surfaces during the manufacture process of the tubing.