The dynamic behavior of two steel platforms in the Adriatic Sea has been investigated by environmental vibration tests using wind and wave actions. The vibration data was acquired by accelerometers installed at different heights of the platform topsides and positioned in different directions to allow the measurement of either the translation or the rotational motions. The data was recorded at several times; the time duration of each record is from 3 to I0 minutes, during one day, In the day a preliminary elaboration of the data is carried out on the platform. The acceleration records were used to estimate the natural frequencies and modal damping ratios, by the peaks of the associated spectrum. The structural behavior has been confirmed by a procedure that correlates the experimental data with the accelerations relative to the main degrees of freedom a simplified dynamic model of the structure. Also an alternative technique of the spectral estimation, named the Maximum Entropy Method, was employed to compute the natural frequencies and damping ratios. The results were compared to the conventional methods.
The exigency of a continuos structural check of offshore platforms, either in term of response to forces or in terms of variation of design parameters, has shown the great importance of the experimental analysis. Experimental researches in the structural engineering have a fundamental basis and can provide a very important information for the structural diagnosis in each moment of the platform life (R.N. Coppolint, 1980) and (O. Spadaccini, 1990). The new acquisition and elaboration data techniques and the improved numerical structural analysis allow a good check on the behavior of the offshore structures.
