Vortex Induced Motion (VIM) of an offshore platform is a phenomenon caused by synchronized vortex shedding that triggers resonant responses of the platform global motions. As these resonant responses are typically characterized by large amplitudes of motions at relatively low frequencies, VIM is a significant contributor to fatigue damage for mooring lines and risers.
There is a rich set of numerical studies related to VIM of offshore platforms in relatively shallow waters, most of which have been successfully validated against model tests results. In contrast, there is a dearth of studies and consequently, insufficient understanding of the VIM phenomenon for platforms in deep and ultra-deep waters. The resultant uncertainties in design are mitigated by applying quite large safety factors for the VIM parameters, both in met-ocean, and in the platform hydrodynamic analyses.
Relying on historical as well as the most current full-scale field data obtained during the Eddy Lazarus event in the Gulf of Mexico, we discuss two situations: One when VIM occurred and another when it was expected to occur but it did not. We situate these findings in the broader context of VIM persistence and conservatism. In so doing, we aim to sensitize the O&G Industry for the potential benefits of informing design practice with field data about the platform VIM.