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 Gulfof Mexico, have been operational since 2007 and 2009 respectively. The underwater hull geometry isbasically the same for both vessels. Both Independence Hub and Thunder Hawk are moored by twelvepolyester mooring lines. Independence Hub supports many more flowlines than Thunder Hawk. Each semisubmersibleis equipped with a measuring system which records, amongst other data, the semi-submersible'sposition, 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 VIMamplitudes are much smaller than what is observed under typical model test conditions and there is asignificant variation in oscillation period. Yet and evaluation of oscillation periods between IndependenceHub, Thunder Hawk and model tests reveals very similar characteristics when normalizing the periods bythe natural period in surge/sway. The determination of the surge/sway natural period for field conditions isnot as straightforward; different natural period in surge/sway have been observed for the same vessel underdifferent historical conditions. The natural period in surge/sway for those field conditions was identifiedthrough an iterative procedure. The important aspect of the procedure is that at lock-in 1) the variationin natural period in surge/sway is minimal, 2) that the motions are nearly sinusoidal, and 3) the motionamplitudes are the largest observed. The Independence Hub and Thunder Hawk field observations indicatethat the frequency (response period) behavior of a semi-submersible in the field is rather well duplicated bythe 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 lockinand post lock-in conditions are included.