Effects of column shape and configuration on the vortex-induced motions of semi-submersibles

Abstract

The vortex-induced motions (VIM) of offshore floaters occur when the frequency of the vortex shedding is close to the natural frequency, and this phenomenon can severely affect the fatigue life of risers and moorings. During the design and operation stage of the semi-submersible (SS), VIM behavior has emerged as an important issue in offshore engineering. The shape of the columns, circular or square, and the wake interference, characterize the VIM of the SS. Numerical studies using the validated Detached Eddy Simulation (DES) method are carried out for parametric analysis of the VIM performance of various SS models consisting of different column shapes: with four rounded square columns, four circular columns and the mixed square-circular columns. The results show that the most significant transverse responses at 0∘ current incidence are observed for the SS with four circular columns and the one with two circular upstream columns. At a diamond configuration (i.e., 45∘ and 135∘), the SS models with a square upstream column all present the most pronounced VIM. The analysis shows that the vortex shedding characteristics of each column at different current incidence govern the VIM of the SS models. The square and circular columns both do large positive work on the transverse motions, and thus improve the VIM response at a certain current incidence. Therefore, the design of SS with square, circular and multi-shape columns can not mitigate the VIM behaviors efficiently. Designing a SS model with irregular polygon-shaped columns or twisted columns to avoid the strong vortex shedding may work.