The dynamic behavior of a tripod steel jacket installed in 65 m water depth in the North Sea has been investigated. The analysis method is a time-domain simulation of the platform exposed to irregular sea. Similar to monotower platforms, the so-called “ringing” phenomenon caused by higher-order harmonics in the wave loading is detected. Further, the analysis discovered another dynamic feature, “double frequency response,” caused by the interaction between the spatial distribution of the wave load and the shape of first vibrational mode. The dynamic response showed that the top of the platform vibrates notably more in first mode than found by a quasi-static analysis with incorporation of standard dynamic amplification factors. However, the resulting extreme stress level and accumulated fatigue damage are found quite similar and the consequences to human comfort small. The basis for this finding is the fact that first mode contributes very little to the stress level in critical points.

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