(J. Soc. Mat. Sci., Japan), Vol. 52, No. 11, pp. 1351-1356, Nov. 2003 Fatigue Life Prediction of Coiled Tubing by Takanori KATO*, Miyuki YAMAMOTO*, Isao SAWAGUCHI** and Tetsuo YONEZAWA*** Coiled tubings, which are used in drilling and washing up oil wells, receive various bending and straightening plastic deformations with operating internal pressure. Therefore the low cycle fatigue of coiled tubings is an important issue. In this report, as objective to clarify fatigue properties of coiled tubings and to develop a fatigue life prediction method of coiled tubings, we carried out full scale fatigue tests, fatigue tests with small size specimen, and FEM analysis simulated the full scale fatigue tests. As a result, it was clarified that fatigue lives of high strength coiled tubings (T95) were longer than those of low strength coiled tubings (L80) with high internal pressure, and predicted fatigue lives were satisfied general allowable difference, factor of two. Key words: Low cycle fatigue, Fatigue life prediction, FEM analysis, Coiled tubing, Material strength Fig. 1. Schematic illustration of coiled tubing. Fig. 2. Schematic illustration of full scale fatigue test and cross-section of CT. Corporate R & D. Lab., Sumitomo Metal Industries Ltd., Fuso-cho, OCTG Tech. Center, Sumitomo Metal Tech. Inc., Higashikaigan-cho, Amagasaki, 660-0843 Tech. Res. Center, Japan National Oil Corporation, Mihama-ku, Chiba, 261-0028
Fig. 4. Configuration of fatigue test specimen. Table I. Mechanical property of test materials. Table II. Full scale fatigue test conditions and test results. Fig. 5. s-n curves of L80 and T95. Fig. 3. Observed cracks in full scale fatigue tests. Fig. 6. Configuration of crack propagation test specimen.
Table III. Loading conditions of FEM analysis. Fig. 7. Relationship between crack propagation rate and plastic strain range (L80 and T95). Fig. 8. Full scale fatigue test model of FEM analysis. Fig. 9. Cyclic stress and strain curves of L80 and T95.
Fig. 10. Signature adopt to following results. Fig. 11. Distribution of equivalent plastic strain. Fig. 12. Stress-strain hysteresis loop obtained by FEM. Fig. 13. Fatigue life prediction method.
Fig. 14. Distribution of predicted fatigue life. Fig. 16. Comparison of CT fatigue lives of L80 and T95 (Arc radius 72inch). Fig. 15. Comparison of test and predicted results. Fig. 17. Comparison of CT fatigue lives of L80 and T95 (Internal pressure 1500psi).
Fig. 18. Relationship between equivalent plastic strain range of L80 and T95 and internal pressure. 1) T. Urayama, T. Yonezawa, M. Hamada, M. Sugino, H. Takabe, and A. Ikeda, Proc. the 2000 IADC/SPE drilling conferrence, 59164 (2000). Fig. 19. Relationship between mean ratchet strain of L80 and T95 and internal pressure. 3) S. S. Manson, G. R Halford and M. H. Hirschberg, "Design of Elevated Temperature Environment", p. 12 (1971) ASME. 4) S. S. Manson and G. R. Halford, 1976 ASME-MPC Symposium on Creep-fatigue Interaction, 283 (1976).