The helical strakes are now widely being used in offshore riser design for the suppression of vortex induced vibrations (VIV). The purpose of this paper is to investigate the responses and suppression effectiveness of a straked pipe in a more real working conditions of the risers which will endure a kind of “oscillatory flow” due to the relative motions between the fluid around and the risers induced by the top platform motions. Experiments are performed on a flexible straked pipe with pitch length/height of 15D/0.25D in ocean basin. The pipes are forced to harmonically oscillate in various combinations of amplitude and period with Keulegan-Carpenter (KC) number varying between 5 and 165, and maximum reduced velocities from 4 to 12. Responses in both in direction are firstly investigated. Inverse analysis method and the Least Square method are adopted to identify drag coefficients and added mass coefficients. The results show that strakes can reduce the higher frequency responses in both CF and IL direction. Suppression efficiencies of the strakes and are not ideal as expected in oscillatory flow. Moreover, the hydrodynamic coefficients change dramatically under the small KC number and stabilize under the large KC number. The drag coefficients obviously magnify at KC ∼ 20.

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