Effects of the position of pipe-type appendages on the flow induced motions, energy transformation, and drag force of a TLP

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 degrees-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 degrees-incidence whatever the appendages position, the transverse and yaw motions were all mitigated and the TLP with appendages-45 degrees 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 degrees. In addition, the influence of the FIM behavior on the drag force of the TLP at 0 degrees 22.5 degrees and 45 degrees-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 degrees-incidence, with a value of around 39%.