Experimental study of the column shape and the roughness effects on the vortex-induced motions of deep-draft semi-submersible platforms

• Published in: Ocean engineering 2018-02, Vol.149, p.127-141
• Main Authors: Gonçalves, Rodolfo T., Fujarra, André L.C., Rosetti, Guilherme F., Kogishi, André M., Koop, Arjen
• Format: Article
• Language: English
• Subjects: Column shape; Deep-draft semisubmersible (DD-SS); Model tests; Roughness; Vortex-induced motions (VIM)
• ISSN: 0029-8018; 1873-5258
• DOI: 10.1016/j.oceaneng.2017.12.013

Experiments regarding the VIM of SS platforms with four columns were carried out to investigate the effects of the surface roughness and the column shape. Two different geometry of columns were experimented: rounded-square and circular-cross sections. Each model had the same arrangement of four pontoons in a closed configuration. The range of reduced velocity was from 4 up to 25 covering Reynolds numbers from 7.000 to 80.000. Three angles of incidences for each model were tested, namely 0, 22.5 and 45°. For each model, three different levels of roughness were tested. Regarding roughness effects the results seem to confirm two aspects: the important role played by the separation point near the rounded corners of the rounded-square columns and the roughness effect on the boundary layers of the circular columns. Regarding the effect of the column shape, the results indicated that the heading effects on the VIM of SS strongly depend on the column shape. The characteristic motion periods upheld the resonance behavior for the motions in the transverse direction and yaw motions for the lock-in range. Heave, roll and yaw motions did not impact the VIM behavior in the horizontal plane, and their amplitudes were not considered relevant. •Effect of column shapes and roughness were studied for a four-column semisubmersible.•Amplitudes in the transverse direction showed to be different for circular and rounded-square cross section columns.•Amplitudes in the transverse direction were higher for the circular than for the rounded-square case except for 45-deg.•Circular column cases presented a clearly resonance of the yaw motion for 22.5 and 45-deg.•Low level of roughness for circular columns showed to be a good practice to minimize Reynolds scaling effects.