Fatigue Life Analysis Due to Vortex Induced Vibration (VIV) On Pipeline Free Span Section in Operating Condition

Abstract

Subsea pipeline is used for transferring oil and gas. The diverse seabed topography can cause free span conditions in certain areas. Environmental loads like wave and current flows around the pipeline can induce oscillations in the pipeline, known as vortex-induced vibrations (VIV). If the frequency of the vortex shedding approaches the pipeline's natural frequency, resonance can occur, leading to fatigue in pipelines. This study analyze the VIV to understand the flow patterns around the pipeline through numerical simulation method. The flow pattern is identified at a Reynolds number of 3.85 × 105, indicating turbulent flow which can be classified into 2P vortex shedding pattern. The vortex shedding frequency around the pipe is 0.18 Hz. In this study, VIV criteria calculations are performed based on DNV GL RP F101 guidelines. The study finds that in-line VIV is occured, so that the fatigue life analysis is required. The von Mises stress on the pipeline is 21.41 MPa. Fatigue analysis using the rain-flow counting method shows a fatigue life of 38 years for the pipeline with free span. A comparative analysis on the pipe without free span indicates a fatigue life of 117 years. Both results are within the structure's design life of 20 years.