Numerical studies on flow-induced motions of a semi-submersible with three circular columns

Abstract

Abstract 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.