Experimental investigation on the alteration of natural frequency of a flexible pipe adjacent to a bottom plane with the consideration of pipe-plane impact

The obtainment of natural frequencies of near-wall submarine spanning pipelines is the essential basis of the prediction of vibration response and fatigue life. Nevertheless, published literature has not yet taken into account the pipe wall impact in free attenuation tests and has not offered a method to identify the impact frequency from natural ones. Therefore, the present work experimentally investigates the free attenuation response of a horizontal flexible pipe positioned adjacent to and parallel to a bottom plane with the consideration of pipe-wall collision, aiming to figure out the influences of wall proximity and wall impact on natural frequencies and to provide a measurement method for attenuation tests of such fluid-flexible-structure-solid interaction issues. The experimental results indicate that setting a horizontal initial displacement is easier than setting a vertical initial displacement to get the vertical natural frequencies of a near-wall flexible pipe. The natural frequency of near-wall flexible pipe is a function of the added mass coefficient, decreasing with the reduction of pipe-to-wall distance. Three main alterations of natural frequencies are introduced in the presence of pipe-wall impact. With the imposition of a horizontal initial displacement, the impact and natural frequencies can be distinguished from the wavelet time-frequency contours because of their participations during different time periods.