Study On The Vibration Response Of A Separated Drilling Riser Against Typhoons

Vortex-induced vibration is the main reason of fatigue and structural damage of the riser.As offshore oil and gas exploration moves to deep and ultra-deep water,the vortex-induced vibration of flexible risers with large aspect-ratio is increasingly concerned.The study of vortex-induced vibration of a riser mostly focused on its working condition that the riser normally connected with the platform.While,when the riser system against typhoons,the riser is disconnected with the platform and the tension force supplied by the buoyant tank enables the disconnected riser to stand in the sea by itself.Under the current load,the riser is free to swing,performing like the fixed support rigid body.In the meantime,the buoyant tank imposes a dynamic constraint on the top,so that the riser has the quasi-constrained features with approximately both ends restrained.Therefore,the vibration response of the riser in this condition is more special and complicated.In this paper,the vibration response of the riser in this special condition is studied by the model test and the three-dimensional numerical simulation.The main work and results are listed as follows:(1)The vibration model was designed according to the flow rate condition and the size of the experiment sink in the first place.The model test was carried out in 15 working conditions.The results show that after disconnecting,the vibration of the riser performs like the fixed support rigid body when the flow rate is low.With the increase of flow rate,the riser exhibits a vibrational behavior that approximates the constraint at both ends with multi-frequency involved in vibration.With further increase of flow rate,beginning from the lower part of riser,this kind of vibration gradually occupy the leading position,the primary and secondary relationships have changed which corresponds to the transformation of the vibration shape,but the maximum amplitude position trends to move upwards comparing to the connecting state.Then,with the flow rate continuously increasing,this vibration shape will extend from the lower to the up end until the entire riser presents a vibrational behavior that approximates the restraint at both ends.The vibration transformation in in-line direction and cross-flow direction are not synchronous,that in the in-line direction requires a larger flow rate.(2)The density ratio of the cylindrical buoyant tank to the surrounding seawater is as low as 1.3 ×10-3,so it is necessary to consider the effect of its additional mass.In this paper,five buoyant tank models with different sizes were 3D printed using resin material which has a very small density,and the additional mass of each buoyant tank model was analyzed by simple damped vibration test in still water.The results show that when the buoyant tanks with equal volume providing same tension force,the greater the overcurrent section,the greater the additional mass of the buoyant tank,and the harder for the up end to move freely(closer to the connecting state),therefore the smaller the flow rate required for the vibration transformation.(3)The model test was difficult to achieve the external flow similarity and the structural dynamics similarity for the limitation of the experimental conditions.Therefore,the fluid-structure coupling analysis was finished by ANSYS numerical simulation software for quantitative study purpose.The real-size three-dimensional numerical model was established on the basis of the availability of the simulation scheme to simulate the vibration response of the riser under the actual working conditions.The vibration response data with a certain engineering reference value were given,and the flow field analysis which did not be accomplished by the model test was supplemented.