Parameter adjustment of a model of an offshore platform from estimated eigenfrequencies data

Abstract

A full scale dynamic test which was incomplete because of rough seas and bad weather conditions and a computational model of the research platform “Nordsee” were the starting points of the work described in this paper. The transient excitation used in the test produced vibrations in the lower frequency range with certain identifiable eigenfrequencies and damping ratios. The identified eigenfrequencies were used as the basis for mass adjustment of the computational model. The computational model consists of an 176 degrees of freedom system, in which the flexible constraints and the virtual mass of water are neglected. The most uncertain data were the masses of the deck body. Parameter sensibility investigations, and a priori knowledge of the system and the test conditions led to an adjustment of the mass matrix partitioned corresponding to subsystems. The adjustment was carried out in a factorial global way, in respect to the defined subsystems. The result is an optimum model corresponding to the chosen loss function; the residuum used concerns the inverse eigenfrequency.