Kou, Yufeng (Shanghai Jiao Tong University) | Lu, Haining (CISSE - Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Yin, Hanjun (Shanghai Jiao Tong University) | Cai, Yuanlang (CISSE - Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration)
Vortex Induced Motion (VIM) of Tension Leg Platform (TLP) can cause fatigue stresses on the risers and tendons and must be taken into account in design. The VIM response of a TLP hull with 4 round columns is investigated by tow model test, which was conducted by State Key Laboratory of Ocean Engineering in Shanghai Jiao Tong University (SKLOE, SJTU) from August to September of 2015. A 1:50 TLP hull model is built accurately in geometry, including the appurtenances such as anodes, tendon porches, fenders, supports, caissons and other pipe-like structures running on the hull. However, the topsides are neglected and replaced by an aluminum deck which is strong enough to support the air bearing system and horizontal mooring system. The air bearing system composed of six air bearings and a smooth flat-plate is applied to simulate the vertical load of tendons and TTRs, which allows matching the same mass ratio with prototype in model test. The horizontal restoring force and moment of initial tendon and TTR system are modeled by 4-spring horizontal mooring system mounted above water, ensuring the correct natural periods for surge, sway and yaw motion of the TLP. The current is simulated by carriage speed in towing tank. Screening tests for 16 current headings with the interval of 22.50 show different VIM response in every heading. The comparatively larger VIM amplitude in transverse direction, about half of column diameter, occurs in 900 and 337.50 current headings. The lock-in range of reduced velocity between 6 and 8 is also observed in 900 and 337.50 current headings.
Due to the increasing draft, VIM of multi-column floating platforms under sea current may become notable, and cause fatigue stresses on the risers and mooring systems. The interference among columns makes more complex VIM phenomenon than Spars and mono-columns. Liu et al (2015a) experimentally investigated the flow and force on fixed four-square-cylinder array, revealed that the downstream cylinders experience smaller mean drag force but higher fluctuating force than the upstream ones, and that the interaction was highly related to current heading and spacing ratio. Gonçalves et al (2012a, 2012b, 2013) studied a lot on VIM of Semi-submersible with four square columns by tow model test. It was found that not only VIM in transverse direction but also considerable yaw motion oscillation occurred. The largest transverse amplitudes can reach 40% of column width for 300 and 450 incidences. However, the maximum yaw motions were found about 4.50 for 00 incidence. Comparatively, the largest in-line motions were no more than 15% of column width. It was also found that the hull appurtenances had some influence on the VIM response of Semi-submersible, especially the pipes located at columns. The external damping was proven another critical factor to determine the VIM performance, 20% extra damping can decrease 50% of VIM amplitude for some specific response frequencies. Gonçalves et al (2015) also discovered the different behavior of round and square-rounded columns in terms of transverse and yaw response for different incidence angles. Bai et al (2013) studied the VIM of new type Deep Draft Semi-submersible (DDS) with four rectangular columns by tow model test and 2D numerical simulation. The lock-in range of reduced velocity between 6 and 8 was obtained for 1350 incidence. Because of over-simplified model, the numerical simulation predicted broader lock-in range and larger response amplitude. Actually, even the 3D CFD method is hard to accurately predict the VIM performance of multi-column floaters with consideration of the appendages, because of the great difference in size between the hull and attached structures. The model test is the preferred solution to investigate the VIM response of a new platform. The effect of current velocity and heading, hull appurtenance, and external damping should be taken into account in model test, and the transverse and yaw motion must be mainly focused on.