### Definition(s)

#### Mean Zero-Upcrossing Period

average period of the (up or down) zero-crossing waves in a sea state

NOTE In practice the mean zero-crossing period is often estimated from the zeroth and second moments of the wave spectrum as.

*Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards*

#### Mean Zero-Upcrossing Period

average intrinsic period of the zero-upcrossing waves in a sea state.

NOTE 1 In practice, the mean zero-crossing period is often estimated from the zeroth and second moments of the wave spectrum as given by Equation (3.41-1):

where

f is the frequency in cycles per second (hertz);

m_{0} is the zeroth spectral moment and is equivalent to σ2, the variance of the corresponding time series;

m_{2} is the second spectral moment;

T_{2} and T_{z} are the average zero-crossing period of the water surface elevation, defined by the zeroth and second order spectral moments, (T_{2} = T_{z});

ω is the wave frequency in radians per second.

NOTE 2 Adapted from ISO 19901-1:2005, definition 3.17.^{
}

*Source: ISO 19905-1:202, Petroleum and natural gas industries – Site-specific assessment of mobile offshore units – Part 1: Jack-ups. Global Standards*