Flexible pipe with staggered buoyancy elements such as lazy wave riser and drilling riser has been widely used in ocean engineering. Under the influence of sea current, both of the buoyancy elements and the riser may experience vortex induced vibrations (VIV). However, when VIV occurs, hydrodynamic characteristics of the buoyancy elements and its influence on hydrodynamic force of the bare pipe still need investigation. The purpose of this paper is to reveal the hydrodynamic characteristics of flexible pipe with staggered buoyancy elements undergoing VIV. The crossflow (CF) hydrodynamic coefficients of the flexible pipe with 25%, 50%, and 100% coverage of staggered buoyancy are obtained from model tests, using hydrodynamic forces and coefficients identification method. Then, the characteristics of added mass coefficients and excitation coefficients in CF direction are analyzed. The results show that the added-mass coefficients of bare pipe are relatively larger than those of buoyancy module, while the total mass per unit length (sum of structural mass and added mass) is consistent along the pipe. Similarly, the range of excitation coefficient on the buoyancy elements is smaller than that on the bare pipe, and their ratio is equal to the reciprocal of diameter ratio 2.5.

Different from the previous studies of the vortex-induced vibration (VIV) dominated by first mode of flexible pipe in an oscillatory flow, the features of a higher mode dominated are experimentally investigated in the ocean basin. The flexible pipe is forced to harmonically oscillate with different combinations of a period and amplitude. The design dominant mode consists of first and second modes under the maximum reduced velocity (V_R) of approximately 5.5 with a KC number ranging from 22 to 165. The VIV responses between only the excited first mode and the excited higher mode are compared and studied using displacement reconstruction and wavelet transform methods. The discrepancies of spatial and temporal response between smaller and larger KC numbers (KC = 56 and 121) are first observed. The strong alternate mode dominance and lock-in phenomena occur in the case of larger KC numbers, while they cannot be observed in the case of smaller KC numbers under higher modes. The VIV dominant frequency in the in-line (IL) direction is found to be always triple the oscillatory flow frequency and not twice that in the cross flow (CF) direction. The dominant frequency in the CF direction can be predicted by the Strouhal law, and the Strouhal number is approximately 0.18 under V_R = 5.5, which is not affected by the excited mode. Moreover, differences of response motion trajectory are also revealed in this paper. The present work improves the basic understanding of vessel motion induced VIV and provides helpful references for developing prediction methods of VIV in an oscillatory flow.

Vortex-induced vibration of single pipe in isolation has received lots of attention in the past few decades. With the massive use of buddle risers and pipelines in deepwater oil production industries, the demand to focus on the research of interference effects of dual pipe has been greatly enhanced. This paper presents interference experiments of dual pipes with different diameters