4th ANSA & μETA International Conference

OIL PIPE ANALYSIS OF LOW CYCLE FATIGUE AND FRACTURE UNDER RECIPROCATING BENDING LOAD 1 Wei Liu, 2Qinglin Hou Beijing FEAonline Engineering Co.,Ltd. Beijing, China KEYWORDS –Oil pipelines; Coiled tube; Low-cycle fatigue; ANSA; ABAQUS ABSTRACT –Oil pipelines often subjected from the cyclic loading during its services, such as bending loading. So it is very significant for the oil pipe to carry out the fatigue life prediction under the reciprocating bending load and to investigate the damage and fracture in different cycle times by using of numerical analysis in the fields of oil and chemical industry. Low-cycle fatigue analysis using the direct cyclic approach was implemented in two different FEM models whether to consider the inner pressure in the pipe or not. The result indicated that there are almost the same cycle times, damage state and fracture morphology for the analysis and the experiments. 1. Introduction The coiled tubing Drilling (CTD) is an advanced drilling technology by the use of the continuous tubes. Its development of the application began in the 90's. According to the different drilling methods, CTD is classified into directional well drilling and vertical shaft drilling. There are many advantages for coiled tubing Drilling, such as small footprint to suit for the many unlimited regions or operation on the sea, especially good in the slim hole and a method of drilling with low-cost and high-quality. The continuous tubes are also used in petroleum transmission pipeline. So its mechanics performance especially the low fatigue property is wildly concerned in the field of oil or chemical industry when suffering from the large bending and torsion deformation.

2. Experiment methods The experiments of low fatigue of continuous tubes are commonly carried out before it is used. An experimental facility is designed and manufactured to test the low fatigue properties of coiled tube. The length and high of coiled tube experimental facility is 1300mm and 1500mm, respectively. The vertical frame support structure is used in this mechanical system (Fig. 1). There is "V"-shaped jig and fixture to fix the freedom on the facility, which consists of straightening module and flexural module. By changing the different flexural module, we can test the fatigue life of coiled tube with different diameters. In addition, because it is hard to remove, so a flange is designed to drive the flexural module along the horizontal direction. When the experiment is carried out, the bottom of coiled tube is fixed by the left and right holder, which can protect this region not suffering from the bending shear force and improve the reliability of experimental data. The top of coiled tube remove along the designed track by the hinged shaft installed on the frame, which consists of plate, sheet stiffener and base beam. An axial actuator is fixed on the piston rod. The coiled tube is installed on the position of hinged connection to make sure that the motion of piston rod is coincident to the linear track. Two experiments of testing the fatigue of coiled tube were carried out. They are the bend along the designed track without or with the inner pressure in the coiled tube. The length of coiled tube is 1500mm, and the diameter of coiled tube is 38.1mm, the deflection of bending is about 30°. The experimental results indicated that the number failure cycling for the coiled tube without inner pressure is about 1000 times, and the number failure cycling for the coiled tube with inner pressure about 34.47MPa is about 264 times, respectively. There are almost positions of failure for the two different coiled tubes.

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4th ANSA & μETA International Conference

A direct cyclic analysis is a quasi-static analysis; uses a combination of Fourier series and time integration of the nonlinear material behavior to obtain the stabilized cyclic response of the structure iteratively; avoids the considerable numerical expense associated with a transient analysis; is ideally suited for very large problems in which many load cycles must be applied to obtain the stabilized response if transient analysis is performed; can be performed with linear or nonlinear material with localized plastic deformation; can be used to predict the likelihood of plastic ratchetting; assumes geometrically linear behavior and fixed contact conditions; uses the elastic stiffness, so the equation system is inverted only once; and can also be used to predict progressive damage and failure for ductile bulk materials and/or to predict delamination/debonding growth at the interfaces in laminated composites in a low-cycle fatigue analysis. A low-cycle fatigue analysis is characterized by states of stress high enough for inelastic deformation to occur; is a quasi-static analysis on a structure subjected to sub-critical cyclic loading; can be associated with thermal as well as mechanical loading; uses the direct cyclic approach to obtain the stabilized cyclic response of the structure directly; models progressive damage and failure in bulk ductile material based on a continuum damage approach, in which case damage initiation and evolution are characterized by the accumulated inelastic hysteresis strain energy per stabilized cycle; models progressive delamination growth at the interfaces in laminated composites, in which case the onset and growth of fatigue delamination at the interfaces are characterized by the relative fracture energy release rate; uses the damage extrapolation technique to accelerate the low-cycle fatigue analysis; and assumes geometrically linear behavior and fixed contact conditions within each loading cycle. It is well known that after a number of repetitive loading cycles, the response of an elastic-plastic structure, such as an automobile exhaust manifold subjected to large temperature fluctuations and clamping loads, may lead to a stabilized state in which the stress-strain relationship in each successive cycle is the same as in the previous one. The classical approach to obtain the response of such a structure is to apply the periodic loading repetitively to the structure until a stabilized state is obtained. This approach can be quite expensive, since it may require the application of many loading cycles before the stabilized response is obtained. To avoid the considerable numerical expense associated with a transient analysis, a direct cyclic analysis can be used to calculate the cyclic response of the structure directly. The basis of this method is to construct a displacement function u(t) that describes the response of the structure at all times t during a load cycle with period T as shown in Fig. 3a. The direct cyclic analysis capability in Abaqus/Standard provides a computationally effective modeling technique to obtain the stabilized response of a structure subjected to periodic loading and is ideally suited to perform low-cycle fatigue calculations on a large structure. The capability uses a combination of Fourier series and time integration of the nonlinear material behavior to obtain the stabilized response of the structure directly. The direct cyclic low-cycle fatigue procedure models the progressive damage and failure both in bulk materials (such as in solder joints in an electronic chip packaging) and at material interfaces (such as in laminated composites). The response is obtained by evaluating the behavior of the structure at discrete points along the loading history. The solution at each of these points is used to predict the degradation and evolution of material properties that will take place during the next increment, which spans a number of load cycles,ΔN. The degraded material properties are then used to compute the solution at the next increment in the load history. Therefore, the crack/damage growth rate is updated continually throughout the analysis. The elastic material stiffness at a material point remains constant and contact conditions remain unchanged when the stabilized solution is computed at a given point in the loading history. Each of the solutions along the loading history represents the stabilized response of the structure subjected to the applied period loads, with a level of material damage at each point in the structure computed from the previous solution. This process is repeated up to a point in the loading history at which a fatigue life assessment can be made. In bulk material the cyclic loading leads to stress reversals and the accumulation of plastic strains, which in turn cause the initiation and propagation of cracks. The damage initiation

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Gomaz, and ng, vol. 15,

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son of the e

ons on low ff experimenessure. Thessure are aAccording

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from the fessure of ahe geometr

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e Effects oSociety of

Materials uchigan Engi

W. Anders1961.

experimenta

fatigue life ntal samplee experimenbout 1000, to the expe

by the softw, direct cyclinner pressat by comn good agretubes with

troyed eleme status of

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h our numerner pressurebeen killed hown as a

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]. World Sc

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‘Life’[J].

ngapone,

Metal [J]. 51, 1954. Transfer

pp. 9–75,

[M]. The