A column stabilized floater with chain-polyester-chain mooring system in deep water recently exhibited two events of noticeable oscillatory lateral motions, known as Vortex Induced Motions (VIM). Metocean, platform motion and mooring tension are monitored by a modern environmental and facility monitoring system (EFMS). The measurements from the EFMS project provide valuable information to understand the VIM performance of the floater. Even though during design study, VIM issues have been extensively addressed through model tests, numerical simulations, and CFD modeling, the presented field data reveals that the actual severity and persistence of VIM is significantly less than forecasted. In particular: The occurrence of VIM, for the duration of loop current at site, is much less frequent than expected The range of reduced velocity to have VIM lock-in response is much narrower than that in design The nominal amplitude of VIM lock-in response is much smaller than that in design These findings are relevant for the fatigue life of moorings and risers, indicating that the originally adopted design criteria might be overly conservative. The paper discusses specific parameters associated with the measured VIM phenomenon such as in-line and cross-flow A/D, Ur, relative current direction, mooring quasi-static restoring forces and dynamic mooring tensions. The paper also addresses the presence of no VIM events though metocean conditions favoring VIM were present. Correlation of the field data versus model tests as well as the design criteria, which were developed based on the model tests are also discussed.
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