Influence of pipeline specifications and support conditions on naturalfrequency of free spanning subsea pipelines

Abstract

Free spanning is a critical problem of subsea pipelines due to lying down of uneven seabed and scouring phenomena around the unburied pipelines. Analysis of free spanning is important to avoid resonance which leads to damage of pipelines. Frequency of vibration due to hydrodynamic forces should not be closer to the natural frequency of the pipelines. Pipeline specifications, support conditions, effective axial force, seabed conditions etc. affect the natural frequency of subsea pipelines. This paper focuses on the effect of geometry of pipelines and support conditions in determination of lowest natural frequency elaborately. The analytical and numerical studies are performed. This paper tries to quantify the influence of pipelines specifications such as outer diameter to thickness ratio, concrete coating around the pipes and support conditions such as fixed-fixed, fixed-pinned and pinned-pinned conditions on the natural frequency. In this paper wide range of ratio of outer diameter to thickness is considered. It is seen that lowest natural frequency of the pipeline increases if the value of ratio of diameter to thickness decreases. It is also observed that the lowest natural frequency of the pipelines is high for fixedfixed support condition and for less amount of concrete coating. These findings can be used while choosing pipe specifications for designing subsea pipelines.