Low Frequency Energy in Measured Waves in a Narrow Norwegian Fjord

Abstract The E39 Coastal Highway Route project undertaken by Norwegian Public Roads Administration aims at replacing existing ferry crossings by constructing permanent road connections using bridges along the western coast of Norway. One of the bridge designs being developed is a beam bridge resting on floating pontoons. In contrast to conventional bridges, a floating bridge rests on pontoons floating on the water surface and are affected to a much greater extent by waves and currents in the waters it spans. In order to have a solid basis for taking the environmental loads into account in the bridge design a wave measurement campaign was initiated at the bridge fabrication location. The wave measurement was carried out using a small diameter wave buoy with a proven track record in the offshore industry. The location of the wave measurement campaign is a narrow fjord surrounded by relatively high terrain. The fjord, being sheltered from swells from the North Sea and considering the available fetch length, it was expected that the measured waves would consist of purely wind generated waves with peak wave period, Tp, between 2s to 7s. However, the observed wave signal showed spurious long period waves with Tp in the range of 20s to 30s interspersed seemingly at random throughout the measured data. This paper discusses possible reasons for the observed low frequency energy. Harbor resonance in the fjord, resonance in the mooring system of the buoy, and passing ship interaction with the buoy were investigated as possible reasons for the presence of the low frequency energy. A method for identifying the low frequency energy and filtering them out using wavelet analysis of buoy heave signal is proposed.