Modal Parameter Identification of an Offshore Platform From Earthquake Response Records

Abstract

The Whittier Narrows earthquake on October 1, 1987, was recorded by the earthquake monitoring system on an offshore platform installed in a water depth of 97 m in the Santa Barbara Channel. The maximum seafloor acceleration was approximately 4% g, despite the site's epicentral distance of 129 km. In addition to structure response, soil-motion recordings were obtained at two locations in a well conductor directly beneath the platform (3.6 m and 93 m below mudline). Using these soil motions as input, the output-error approach was used to identify optimal modal parameters. The dynamic behavior of the structure was modeled as a linear system having classical normal modes. The recorded response was adequately modeled by four sway modes plus a torsion mode. Identification of reasonable natural frequency and damping values was straightforward. Selection of plausible mode shapes was somewhat more complicated. This may have been due to the presence of closely-spaced modes and possibly incomplete representation of the input