Vortex induced vibrations, a swinging problem

Pipelines at the bottom of the sea are susceptible to ocean currents. Even relatively calm currents can induce turbulences in the wake of the pipeline, which results in the pipeline to start 'dancing'. Pipe vibrations can trigger fatigue, with catastrophic fracture as a result. Consequently, when designing submarine pipelines, caution is being paid to avoid such vibrations. Our research engineers use powerful software to predict submarine pipeline stability.

“The engine of our Western economy is continually being driven by oil and gas”, says Filip Van den Abeele, Research Engineer at OCAS. “The steadily increased demand for fuel, however, obliges the oil companies to explore and produce new oil and gas fields at remote locations. Wells are currently being drilled sometimes more than two thousand metres deep." "Such extreme conditions lead to very high standards with respect to the design of submarine pipelines. The toughness of the material, as well as the wall thickness of the tube, has to be adequate in order to withstand the very high internal pressure of the transported gas. Moreover, they have to cope with the immense water pressure at great depths.”

Dancing at great depth “Even relatively calm currents can induce turbulences in the wake of the pipeline, resulting in pipeline oscillations. The pipeline vibrations can trigger fatigue, causing accelerated damage. Since fatigue damage can give rise to complete fracture with catastrophic consequences, extreme caution is being paid in order to avoid such vibrations when designing submarine pipelines. Flow patterns around submarine pipelines greatly depend on the velocity of the sea currents and on the tube diameter. When the current becomes too strong, turbulences show up in the wake of the pipeline. This vortex shedding exerts an alternating force on the pipeline. Consequently, the pipeline is being subjected to cyclic loading. The pipeline starts to dance, following a characteristic ‘number-eight’ path. Under cyclic loading, the pipe is being exposed to fatigue, which could cause the pipe to fail under surprisingly modest stresses.” “We use powerful computer programmes so as to predict submarine pipeline stability. COMSOL Multiphysics enables designers to tackle very complex and strongly coupled problems elegantly and to solve them efficiently. With respect to the dancing pipeline, a set of complicated equations has to be solved in order to calculate the flow pattern. Next, the force of the wave currents on the tube is estimated. Finally, the computer predicts the trajectory of the pipeline as a direct consequence of these forces. Multiple mitigation measures to reduce the effect of vortex induced vibrations, such as helical strikes, fairings, splitters or spoilers, are currently being investigated with regard to cost optimisation and efficiency.” (source : http://www.ocas.be/Vortex-Induced-Vibrations)