STORM WAVES IN THE NORTHERN NORTH SEA

Offshore measurements have been made at the Tern platform in the northern North Sea during three severe winter storms. Surface spectra for all three storms are similar, all showing an -4 tail. The wave fields are directionally spread with lowest spreading at a frequency below the spectral peak. After adjustment for 2nd order nonlinearity, the largest crests and troughs are Rayleigh distributed and the most probable shape for an extreme crest or trough is close to the correlation function for surface elevation. This supports the idea of a design wave for use in the design or reliability assessment of offshore structures.