Experimental Investigation On Lift Coefficient For a Truss SPAR Subjected to Vortex Induced Motions

Zhang, Hui (State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University) | Xiao, Longfei (State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University) | Yang, Lijun (State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University) | Lu, Haining (State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University)

OnePetro 

ABSTRACT:

The prediction of lift coefficient of Truss Spars is the base for investigating added mass coefficient and damping coefficient, which are important in the estimation of fatigue damage rates. The 1:60 scale Truss Spar model tests have been conducted in towing tank at Zhejiang Ocean University (ZOU). The Truss Spar model consists of a hard tank with removable helical strakes, a truss section and a square soft tank. Model tests were carried out with simplified mooring system. In this study, results from the experiment subjected to uniform current were reported. The Reynolds numbers varied from 8E04 to 2E05. The lift coefficient, lift coefficient components in phase with velocity and acceleration and the phase angle were calculated. The results showed that the symbol of the lift coefficient in phase with velocity varied with the reduced velocity, while the lift coefficient in phase with acceleration was negative at the investigated range of reduced velocity. The magnitude of lift coefficient in phase with acceleration decided the magnitude of the lift coefficient. The symbol of phase angle varied with the reduced velocity, which was in agreement with the symbol of the lift coefficient in phase with velocity.



INTRODUCTION


Vortex-induced motions (VIMs) of the Truss Spar would cause considerable fatigue damage level, which is a significant consideration in the structure design (Blevins, 1990; Sarpkaya, 2004; Gabbai & Benaroya, 2005; Williamson & Govardhan, 2008). The vortex-induced motions depend on many dimensionless parameters. The prediction of the vortex-induced motions requires not only the investigation of the parameters, but also the relationships between the parameters (Vandiver, 1993). Lift coefficient is a critical parameter in the prediction of the Truss Spar VIM behaviour. The prediction of lift coefficient of Truss Spars is the base for investigating of added mass coefficient and damping coefficient, which influence the response frequency.