Non-linearities of sea waves and wave crest properties in a sea state (height distribution, symmetry, steepness) are receiving the attention of the offshore industry due to the implications on structure loading and deck clearance. A characterisation of these aspects of the Mediterranean sea wave environment is attempted by analysing directional wave measurements collected in three locations with different geographical exposure and fetch lengths: Sicily Strait, southern Tyrrhenian Sea and Gulf of Valencia in the Balearic Sea. The first two sets of data have been collected to support the construction of the new two pipelines of the Transmed gas transportation system, the Spanish data have been collected in the framework of MAST2-GSM. All instruments (directional wave buoys) have recorded a number of significant storms from the major directional sectors of each site in deep sea conditions for the Sicily Strait and Tyrrhenian Sea and in deep to intermediate conditions for the Gulf of Valencia. A statistical approach is used to define the properties of wave crest steepness and symmetry, growth and decay rates, height distribution and wave grouping. Non-linear behaviour of wave trains is investigated by a third-order approach, already applied with success to shallow water waves collected in laboratory experiments (Petti, 1994). By this methodology bound long wave and high peak frequency components can be easily investigated, and non linear third order autocorrelations can be found. Finally, a comparison between linear and non linear synthetic parameters is given, in order to estimate the discrepancies that arise using a simple linear approach. THE WAVE MEASUREMENTS The wave data have been collected at three sites in the Mediterranean Sea (figure 1): the Tunisian side of the Strait of Sicily in the proximity of Cape Bon, the southern Tyrrhenian Sea in the proximity of the Italian town of Palmi (30 km north of the Messina Strait), and the Gulf of Valencia (Spain) in front of El Saler beach.

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