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The semi-submersible with three circular columns is an original concept of efficient multifunctional platform, which can be used for marginal oil, gas field, and floater of wind turbines (FOWT). However, under certain flow conditions, especially in uniform current with specific velocities, the eddies will alternatively form and drop behind columns, resulting in the fluctuating lift force and drag force. Consequently, the semi-submersible will subject to the flow-induced motions (FIM). Based on the detached eddy simulation (DES) method, the numerical studies were carried out to understand the FIM characteristics of the three-column semi-submersible at two different parameters, i.e., current incidences (0°, 30°, and 60°-incidences) and reduced velocities ( 4 ≤ U r ≤ 14 ). The results indicate that the lock-in range of 6 ≤ U r ≤ 10 for the transverse motions is presented, and the largest transverse non-dimensional nominal amplitude is observed at 60°-incidence, with a value of A y / D = 0.481 . The largest yaw amplitude A y a w is around 3.0° at 0°-incidence in the range of 8 ≤ U r ≤ 12 . The motion magnitude is basically the same as that of a four-column semi-submersible. However, smaller responses are presented compared to those of the three-column systems revealing the mitigation effect of the pontoon on FIM.

Vortex-induced motion (VIM) phenomenon is a great challenge for design and operation of offshore structures subjected to ocean flow. Semi-submersibles with three columns are often applied to the field of wind turbine, suffering VIM motions probably. In recent years, it is showed that many factors have more or less influence on VIM of platforms. A comparison of circular columns with square columns on VIM characteristics of three-column semisubmersibles is carried out using the detached eddy simulation (DES) method via Star-ccm+ software. This paper analyzes motions in the transverse direction and yaw, as well as fluid forces including drag force and fluctuating lift force. The results show that transverse amplitudes of semisubmersible with three square columns are much lower than those of semisubmersible with three circular columns at all incidences. Besides, the authors conclude that the semisubmersible with three square columns do not experience obvious resonance behavior, which is different from the semisubmersible with three circular columns. Besides, galloping phenomenon occurred at large reduced velocities at 0°-incidence for the semisubmersible with three square columns, which is likely not induced by lift force directly. Meanwhile, in these cases, yaw amplitudes are also larger than the others. This may be due to the galloping behavior, which is just a conjecture.

Flow-induced motions (FIM) can cause fatigue damage to the mooring and riser systems of deepwater offshore structures, especially for cylindrical platforms, such as Tension Leg Platform(TLP). Previous experimental and numerical studies have showed conservative FIM prediction compared to the field measurements, due to many significant factors such as the Reynolds number, the nonuniformity of the incident current, the unsteadiness of the current velocity and platform heading, the coupled current and wave presence. Besides, the appendages covering the hull surface of platforms also played an important role in the smaller FIM responses of the field measurements. In order to specifically examine the influence of position of pipe-type appendages, this paper experimentally investigated the FIM of a TLP with different positions of appendages at 0°-incidence and compared the FIM responses, energy transformation, and average drag coefficients with those of the bare hull. An air-bearing system and a horizontal mooring system were employed to provide vertical pretension and horizontal restoring forces, respectively, and to ensure the low-friction horizontal motions. The results showed that for 0°-incidence whatever the appendages position, the transverse and yaw motions were all mitigated and the TLP with appendages-45° was the most effective configuration, with a reduction of 55% for the transverse motion and 52% for the yaw motion, mainly owing to the changes of the boundary layer separation point. However, the analysis of energy transformation indicated the development trend of the galloping phenomenon for the TLP with appendages-45°. In addition, the influence of the FIM behavior on the drag force of the TLP at 0° 22.5° and 45°-incidences was also studied. It showed that the FIM could contribute to larger drag force and the increase of drag force is the most significant at 0°-incidence, with a value of around 39%.

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.

Flow-induced motion (FIM) is an important issue for deep-draft floating platforms, reducing the fatigue life of mooring and riser systems. Tension leg platforms (TLPs) with a smaller mass ratio ( m r , the ratio of the platform mass to the displacement), may experience more pronounced FIM responses in current. To investigate the FIM behavior of TLPs in towing tests, this research adopts an air-bearing system, mainly composed of low-friction air bearings and a smooth flat plate, to accurately simulate the mass ratio. The results demonstrated that in the range of 0 . 64 ≤ m r ≤ 0 . 90 , the transverse amplitudes did not decrease with an increase in the mass ratio because the Scruton number S c did not exceed 0.1. Through model tests, the effect of appendages on the FIM of TLPs was also examined. The results of the screening tests from 0 ∘ to 360 ∘ with an interval of 22.5 ∘ indicated that the transverse response is characterized by symmetry with a cycle of 90 ∘ , while the yaw motion exhibits asymmetry owing to the effect of appendages. The largest transverse amplitude was obtained at 90 ∘ , with a value of A y ∕ D = 0 . 408 occurring at U r = 7 . In comparison with the bare hull platform, the transverse amplitude of the TLP with appendages significantly decreased by approximately 82% in the range of 6 ≤ U r ≤ 8 at 45 ∘ . Overall, these substantial results are expected to improve the understanding of the FIM characteristics of TLPs, and to provide benchmark data for design and numerical prediction as well.