Low-Frequency Forces on Tubular Spaceframe Towers: Analysis of Cognac Data

Abstract

The Cognac field data indicate that the low-frequency force response components are not negligible and are significantly correlated to the wave envelope process. Although this is a well-known phenomenon in floating structures, it had previously not been validated from data recorded on fixed platforms. Simulation studies based on a vertical rigid cylinder subjected to Hurricane Frederic sea states show significant differences in applied low-frequency drag forces obtained using the Wheeler and Delta stretching schemes used to approximate near-surface fluid kinematics. Previous studies focusing on the peak hydrodynamic forces, as used for the design of fixed platforms, have shown a significant difference between predictions based on Wheeler and Delta stretching. The present study reveals that the Cognac field data could be used to discriminate between the Wheeler and Delta stretching schemes in terms of the low-frequency forces, which are potentially important in the design of compliant towers. However, such a discriminatory study would require detailed structural and hydrodynamic modeling of the Cognac tower.