ABSTRACT In practical applications, it is usually assumed that the wave spectrum is of a single mode form, and well modeled by a JONSWAP or Pierson-Moskowitz spectrum. This assumption is of a reasonable accuracy for severe sea states. However, moderated and low sea states are often of a combined nature, consisting of both wind-sea and swell and should be characterized by a double peak spectrum. The present paper discusses two-peak spectra observed in Norwegian waters, which are particularly affected by swell. The analysis is based on 5-year of directional data from Haltenbanken. A directional distribution for swell is suggested. Further, a procedure for including directional spreading in two-peak spectra is proposed. The method is illustrated for the two-peak Torsethaugen frequency spectrum INTRODUCTION The Pierson-Moskowitz and JONSWAP spectra are commonly used for engineering applications. Both spectra are of a single mode nature, characterizing pure wind-sea conditions. Several studies document that these spectra are of a reasonable accuracy for severe sea states. However, moderated and low sea states are often of a combined nature, consisting of both wind-sea and swell and should be characterized by a double peak spectrum
