Overview
Sea-state tables provides limiting sea states for a given activity or activity type defined by wave direction, wave peak period (Tp) and significant wave height (Hs). The significant wave height limitations are described in a tabulated format with wave peak periods as rows and incoming wave direction as columns.
See also Creating Sea-state Tables for how to add and edit sea-state tables.
Selecting Hs from sea state tables
The tolerance parameters defines the relative vessel-wave heading(s) and peak period(s) to be evaluated.
Heading tolerance = 0 deg - Only the nominal vessel-wave relative heading is evaluated
Heading tolerance > 0 deg - Three vessel-wave headings are evaluated (nominal, nominal + uncertainty and nominal - uncertainty) and limiting Hs is selected as the lowest
Peak period tolerance = 0 sec - Only the nominal Tp is evaluated.
Peak period tolerance > 0 sec - Three periods are evaluated (nominal, nominal + uncertainty and nominal - uncertainty
If the forecast vessel-wave relative heading is between the provided columns, or the peak period is between the provided rows has two options:
Interpolation = "Linear" - Limiting Hs is selected by interpolating between the two surrounding vessel-wave relative headings or peak periods
Interpolation = "None" - Limiting Hs is selected as the lowest Hs between the two surrounding vessel-wave relative headings or peak periods.
Warning:
If the forecast peak period is outside the provided wave periods, the tool will treat this as follows:
For periods smaller than the minimum period in the table, limiting Hs for the minimum provided period is selected.
For periods larger than the maximum period in the table, limiting Hs of 0m is always returned to indicate the need for further analysis.
If the forecast vessel-wave relative heading is outside the provided relative heading, the tool will treat this as follows:
The limiting Hs will be chosen from the closest relative vessel-wave heading column.
If the forecasted peak period or relative wave-vessel direction is outside the sea state table range, a yellow exclamation ❗ will pop up in the plan. Hold your mouse over it to read the message.
Examples:
Example 1 (Heading tolerance = 0 deg):
Vessel-wave relative heading is 195 deg, wave period is 11.6 s. +/-0 deg wave direction uncertainty is selected so only 195 deg heading is considered to select limiting Hs.
If "linear" heading interpolation is selected:
For wave period 11 s, Hs is linearly interpolated between (180 deg, 1.0 m) and (210 deg, 1.50 m) - Hs_11s=1.25 m
For wave period 12 s, Hs is linearly interpolated between (180 deg, 1.5 m) and (210 deg, 2.0 m) - Hs_12s=1.75 m
Selected limiting Hs is the lowest between the two periods, i.e. 1.25 m
If "None" heading interpolation is selected:
For wave period 11 s, Hs is the lowest between (180 deg, 1.0 m) and (210 deg, 1.50 m) - Hs_11s=1.00 m
For wave period 12 s, Hs is the lowest between (180 deg, 1.5 m) and (210 deg, 2.0 m) - Hs_12s=1.50 m
Selected limiting Hs is the lowest between the two periods, i.e. 1.00 m
Example 2 (Heading tolerance = +-15 deg, heading interpolation = Linear, Tp tolerance = 0 sec, Tp interpolation = None):
Vessel-wave relative heading is 195 deg, wave period is 11.6 s. +/-15 deg wave direction uncertainty is selected so 195 deg, 180 deg and 210 deg headings are considered to select limiting Hs. "Linear" interpolation has been selected.
For 195 deg vessel-wave relative heading:
For wave period 11 s, Hs is linearly interpolated between (180 deg, 1.0 m) and (210 deg, 1.50 m) - Hs_11s=1.25 m
For wave period 12 s, Hs is linearly interpolated between (180 deg, 1.50 m) and (210 deg, 2.0 m) - Hs_12s=1.75 m
Selected limiting Hs is the lowest between the two periods, i.e. 1.25 m
For 180 deg / 210 deg vessel-wave heading:
For wave period 11 s, Hs is 1.00 m
For wave period 12 s, Hs is 1.50 m
Selected limiting Hs is the lowest between the two periods, i.e. 1.00 m
Selected limiting Hs is the lowest between the three headings i.e. 1.00 m
Example 3 (Relative vessel-wave heading is outside the defined range):
The sea state table is defined for head sea (180deg) +- 15 degrees.
If the relative wave-vessel heading is > 195 degrees, the limiting Hs will be taken from the "195" column.
If the relative wave-vessel heading is < 165 degrees, the limiting Hs will be taken from the "165" column.
Because of this, you can use this sea state tables for weather vaning operations, where you assume that the vessel heading is always positioning itself after the sea state table (in this case with bow towards the wave)
Example 4 (Heading tolerance = 0 deg, heading interpolation = None, Tp tolerance = 1 sec, Tp interpolation = None):
Vessel-wave relative heading is 195 deg, wave period is 11.6 s. +/-1 sec peak period uncertainty is selected so 10.6 sec, 11.6 sec and 12.6 sec periods are considered to select limiting Hs. "None" interpolation has been selected.
Tp = 10.6;
For wave period 10 s and 11 s, Hs limit is the same, and lowest at 180 deg relative vessel-wave direction. Hs_11s = Hs_10s =1.00 m
Tp = 11.6;
For wave period 11 s and 12 s, Hs limit is the lowest for Tp = 11s at 180 deg relative vessel-wave direction. Hs_11s =1.00 m
Tp = 12.6;
Since the forecasted Tp is larger than the table upper bound (12.6 > 12 sec), an Hs limit does not exist. In this case, further analysis is required, and thus, Hs = 0 is set.
Selected limiting Hs is the lowest between the three periods i.e. 0.00 m
