A sea state of a specified wave height may exist with a number of different wave periods. For example, a 1.5m Hsig sea state may be found with a very long wave period (12s or more) offshore West Africa. This is due to light winds blowing over a long fetch (distance) in the open ocean.
On the other hand, a 1.5m Hsig sea state can be found in sheltered coastal waters with a short wave period of 6s or less (due to moderate/high wind blowing over short fetch).
The response of your ocean structure depends on the PERIOD as well as the height of the seastate. So you need to know the period range that applies to the sea area of your project.
The WAVE SCATTER DIAGRAM for the oilfield will show the range of wave periods associated with the significant wave height you are working with.
It depends on what you mean, if you are speaking of defining a spectrum to represent the sea state then you are right there are some spectrum that are one parameter spectrum e.g. ITTC and ISSC.
However, from the understanding of significant wave height which is often assumed to be equal to H1/3, there exist a corresponding period for the wave distributions
(H1/3 is defined as the average height of the one-third highest waves and its corresponding period is TH1/3 average period of these waves)
