Determining Natural Frequency of Free Spanning Offshore Pipelines by Considering the Seabed Soil Characteristics

Free spans or suspended spans normally occur in subsea pipelines due to the irregularity of seabed and by scouring phenomena around the installed non-buried pipeline. The hydrodynamic around the pipeline can lead to a vibration of the pipeline.This kind of vibration may cause fatigue damage to the pipeline. In order to study the hydrodynamic around the pipeline, calculating the natural frequency of the marine pipeline in free spans accurately is very important. Resonance phenomenon may occur when the frequency of the hydrodynamic forces induced by a vortex shedding approach the natural frequency of pipelines. A resonating span can experience significant deflections and associated stresses. Several parameters such as pipeline profile, axial forces, seabed soil and boundary conditions influence the natural frequency of the pipeline. The soil characteristic is an important factor which should be considered in determining the natural frequency of pipeline in free span. In this paper, attempts have been made to investigate the natural frequency of free spanning pipelines and influence of soil characteristic in support of pipeline in free span. In this regard, various boundary conditions were considered and the results were analyzed.