Sharply Peaked Wave Energy Spectra In the North Sea

Abstract

ABSTRACT Recent research results on the growth mechanisms of a wind-wave field, combined with actual wave measurements of extreme sea states in the North Sea, show that the Pierson-Moskowitz type of spectrally may not always be adequate for the description of extreme and operational sea states. Evidence is presented which shows this statement to be true, and a more sharply peaked wave spectrum is proposed for supplementary use. The importance of the spectral shape of a wave field for structural behavior is considered. INTRODUCTION For offshore operations, an adequate description of the environmental conditions is important. A better understanding of the wind, wave and current conditions experienced in the hostile areas of the North Sea will provide improved criteria for offshore activity. This paper deals with the results from recent studies of the North Sea wave conditions and of their implications for the North Sea exploration. Empirical data and the conceptual idea of a lively developed ll sea state, led Pierson and Markowitz (1964) to propose a formula for a spectral description of the wind generated sea state. This spectrum has been adapted for a great variety of engineering applications?; i. e. testing of offshore structural behavior in irregular waves, and for the statistical description of severe sea states. Wave recordings from the North Sea however, show that the Pierson - Markowitz spectrum may not always give a realistic description of the sea states; in particular duping the wave growth stage. In addition, scientific results, both theoretical and field experiments, show that the wind wave spectrum during wave growth is much more sharply peaked and contains more energy than the correspond ing Pierson - Moskowitz spectrum., Further, the existence of a-"fully developed" sea state questioned THE "JONSWAP" EXPERIMENT In order to improve-the understanding of the processes that develop a wind-way field, a multi-national experiment was conducted in the North Sea. Thirteen wave stations spaced along a 160 km profile extending westward from the Island of Sylt (North Germany) were in operation, recording over 2000 wave spectra. They found traitor-wave growth conditions, the source function S in the spectral energy balance equation (available in full paper)