Vortex Induced Motions and Surge/Sway Oscillation Periods of the Independence Hub and Thunder Hawk Semi Submersibles

摘要

The Independence Hub semi-submersible and Thunder Hawk semi-submersible, both located in the Gulf of Mexico, have been operational since 2007 and 2009 respectively. The underwater hull geometry is basically the same for both vessels. Both Independence Hub and Thunder Hawk are moored by twelve polyester mooring lines. Independence Hub supports many more flowlines than Thunder Hawk. Each semisubmersible is equipped with a measuring system which records, amongst other data, the semi-submersible's position, and the current speed and direction. The measurements indicate that Vortex Induced Motion (VIM) behavior occurs, and VIM behavior occurs under increased current velocities. However, observed VIM amplitudes are much smaller than what is observed under typical model test conditions and there is a significant variation in oscillation period. Yet and evaluation of oscillation periods between Independence Hub, Thunder Hawk and model tests reveals very similar characteristics when normalizing the periods by the natural period in surge/sway. The determination of the surge/sway natural period for field conditions is not as straightforward; different natural period in surge/sway have been observed for the same vessel under different historical conditions. The natural period in surge/sway for those field conditions was identified through an iterative procedure. The important aspect of the procedure is that at lock-in 1) the variation in natural period in surge/sway is minimal, 2) that the motions are nearly sinusoidal, and 3) the motion amplitudes are the largest observed. The Independence Hub and Thunder Hawk field observations indicate that the frequency (response period) behavior of a semi-submersible in the field is rather well duplicated by the traditional model test setup, though significant differences exist regarding the magnitude of response. The field measurements contain information for reduced velocities up to approximately eleven; hence lockin and post lock-in conditions are included.