Green water velocity due to breaking wave impingement on a tension leg platform

The present study employed the image-based bubble image velocimetry (BIV) technique to investigate the flow kinematics of a plunging breaking wave impinging on a geometry-simplified, unrestrained tension leg platform (TLP). A high-speed camera was used to record images for the BIV velocity determination for both fluid and structure velocities. The plunging breaker was generated using a wave focusing method, and repeated measurements were acquired to calculate the mean flow and turbulence intensity using ensemble averaging. BIV measurements were performed on two perpendicular planes: side view and top view. The flow measurements were compared with those of a similar experiment on a fixed structure by Ryu et al. (Exp Fluids 43(4): 555-567, 2007a). The maximum velocity occurred in the run-up stage with a magnitude reaching 2.8C with C being the phase speed of the breaking wave. The dominant velocities for three distinct phases-impingement, run-up, and overtopping-are very close to those found on the fixed structure. Turbulence intensity was also examined, and the ratio among the three components was quantified. Ryu et al. (Appl Ocean Res 29(3): 128-136, 2007b) reported that Ritter's dam-break flow solution agrees surprisingly well with the measured green water velocity on the fixed structure to a certain degree. The present study followed the same approach and confirmed that Ritter's solution is also in excellent agreement with the green water velocity on the unrestrained TLP model. Based on the self-similar behavior, the prediction equation proposed by Ryu et al. (2007a) was used to model the green water velocity distribution. The results show that the prediction equation is applicable to not only a fixed structure, but also the unrestrained TLP for green water velocity caused by extreme waves.