Fatigue Analysis of Risers

Abstract

This paper studies the fatigue problem of risers in marine platforms located in the Bay of Campeche, Mexico. Considering the future wave loads on these systems, the lifetime of these structures is obtained. Possibly, these systems have problems of fatigue in the zones where large stress concentrations appear. The risers fatigue analysis with a regular pipeline section is carried out taking into account operational and environmental load conditions. Due to operation, among others, the risers are subject to internal pressure, thermal expansion, and the type of fluid that transports. In order to take into account the environmental conditions one it is considered that the riser is subject to waves, current and ground movement, which were suitably modeled in the analysis. The supports on the platform, in the riser zone, were considered. They take into account the movements and their corresponding lateral stiffness. To evaluate the load cycles to which the risers are subject, for the study zone, the annual rates of occurrence and all the sea states have been considered. In addition, all the possible heights and directions of the wave that might be present in the Bay of Campeche have been considered the accumulated damage caused by the diverse cycles of stresses by means of the Palmgren Miner rule are assessed. This study includes the analysis of representative number of risers with different diameters and recent inspections. This allows us to know, in an approximated way, the present condition of the risers; it an give us suitable information for the calculation of the fatigue reliability and therefore define the present levels of structural safety of these facilities.