Fatigue Study of Copper Conductors

Abstract

This master thesis was carried out during the spring semester of 2014 at Department of Marine Technology in Norwegian University of Science and Technology. It was a continuous work of the master project during autumn semester 2013. The tasks of this thesis were given by Aker Solutions Umbilicals. The background of this thesis is that the dynamic IPC umbilical was delivered by Aker Solutions in Africa. In this thesis the central concerns were to perform bending fatigue analyses of the umbilical cross section and to propose strategies to calculate the fatigue life given the histogram of curvatures with corresponding frequencies of occurrence. Furthermore, parameter studies including meant tension, tension frequency, bending frequency and lay angle were carried out to find the critical parameter with respect to the fatigue life of the umbilical. This topic is important within the field of oil and gas because umbilicals are always exposed to dynamic(cyclic) loading, and inspection after installation is impossible. Finite element software USAP was utilized to perform bending fatigue analyses. The developed model in USAP was applied in order to obtain the force-time relationship and moment-force relationship. Then, the strategy of fatigue life calculation was proposed based on the results from finite element analyses and Miner's rule. The necessary input data were from Aker Solutions Umbilicals. The overall conclusion is that the model made in USAP is capable of performing bending fatigue analysis taking friction between PVC profile and the other components into account for simple cases. Mean tension and lay angle are critical parameters for the fatigue performance of copper conductors in the umbilical cross section. The results achieved were in agreement with the accepted knowledge. However, more studies are needed in the future on parameter study of lay angle, because there exists some monotonous relation between the lay angle and the stress in a certain small interval of the lay angle, but non monotonous relation in large interval. The sequence of loadings is also required to be studied in the future. Further, the validity of these findings should be verified through full scale tests.