Stress Analysis and Fatigue Life Prediction of Pseudoelastic NiTi Shape Memory Alloy Tapered Stress Joint via Finite Element Method

Abstract

This research concerns Tapered Stress Joints (TSJ), which connect the offshore platform and the top tensioned riser transporting the produced oil. In this paper, the TSJ applying the pseudoelastic effect of Shape Memory Alloys (SMAs) is proposed to reduce the stress concentration at the hang-off point, thereby increase the fatigue life of the TSJ and allowing the TSJ to survive extremely large rotations due to storms, explosions, and collisions. To this end, the finite element analysis (FEA) of the pseudoelastic NiTi TSJ system is implemented using the Abaqus/Standard program and with a user subroutine (UMAT). The FEA assumes the applied top tension at the top of the TSJ and the cyclic rotation to the right and left. By using the result of the FEA, the stress analysis and the fatigue life prediction is implemented.

In the stress analysis, whether the pseudoelastic NiTi TSJ satisfies the design criteria based on the API Standard 2RD was examined. The elastic and pseudoelastic behavior of the NiTi TSJ was also analyzed by comparison to the steel and the titanium TSJ. The plastic yield angle and the maximum allowable angle of rotation for the operating temperature are obtained. The stress data in a cyclic rotation is used for the fatigue life prediction, which is based on the SWT and Morrow equation.

In conclusion, it is demonstrated that the pseudoelastic SMA TSJ shows the better performance than the steel and titanium TSJ regarding the stress and the fatigue life. Particularly, the conclusion of this thesis suggests that martensite transformation of iii the NiTi TSJ is useful to reduce the stress, improve structural integrity, and extend the service life.