Truss Spar VIM Correlation Between Model Test and Field Measurement

Abstract

Vortex Induced Motions (VIM) of a floater are the result of the exciting forces by vortex shedding on the hull, causing response near the resonant period of any of the six degrees of freedom motions. VIM is currently a critical design component for risers and moorings on floaters, particularly in the deepwater Eastern and Central Gulf of Mexico (GoM).There have been a number of publications on spar VIM numerical modeling and on physical model testing procedures and results. Due to complexity of the VIM phenomenon, model tests are still considered the best approach to validate other tools, such CFD models or simplified numerical models dependent on empirical coefficients. However, little work has been reported on assessing the differences between field data and model test predictions.This paper examines field measurements of motions of a truss spar when the spar experienced multiple high current events in the GoM between the years 2006 and 2011. The motion data are studied to see if VIM lock-in occurred during any of these periods of high current events. Assumptions and procedures for estimating the reduced velocity (U R ) and A/D ratio using field observations are summarized. Comparison between the design guide based on the model tests and the field measurements are also presented. SummaryThe paper examines possible VIM events for the Constitution truss spar based on several hindcast GoM loop-current events over the duration of five years. The events are selected to match the design practice when VIM is expected to occur.The field data for these events are processed and the full-scale results compared with the model test results and design basis. The conclusion of this study is that both the model test results and VIM design parameters are conservative for the platform responses observed in the first five years of operation. It is recommended that an industry wide study is needed to establish industry guidelines for VIM prediction that more accurately reflect calibration to measured floater VIM response. These guidelines can then be used in design process not only for spars but also for other types of offshore structures.