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A weather window occurs when weather forecasts indicate that a given set of marine operations or
offshore construction Offshore construction is the installation of structures and facilities in a marine environment, usually for the production and transmission of electricity, oil, gas and other resources. It is also called maritime engineering. Construction a ...
activities can be performed within their maximum limits for
wave height In fluid dynamics, the wave height of a surface wave is the difference between the elevations of a crest and a neighboring trough. ''Wave height'' is a term used by mariners, as well as in coastal, ocean and naval engineering. At sea, the te ...
,
wind speed In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer. Wind speed ...
s etc. Critical elements for offshore operations are use of time and expected weather. Planning of marine operations should be based on an operation's reference period defined as:
Operation reference period (TR) = Estimated time of operation + Estimated unforeseen time
If unforeseen time has not been identified, the operation reference period is normally set at twice the estimated operation time. Marine operations with a reference period less than 72 hours can be defined as weather-limited operations. These operations can be planned independently of statistical data and based only on weather forecasts. For weather-limited operations, critical factors such as forces, movements and acceleration shall be calculated in a rather worse weather condition (design criterion) than the weather in which it is planned to perform the operation (operational criterion). This is due to the unreliability of weather forecasts. The longer the planned operation lasts, the greater the difference between the operational criterion and the design criterion. The relationship between the operational criterion and the design criterion is defined as the "alpha factor". The variations in the alpha factor also take account of the fact that it is harder to estimate the wave height for small sea conditions than for larger sea conditions. As an example, operation planned to take 20 hours with a design criterion of
significant wave height In physical oceanography, the significant wave height (SWH, HTSGW or ''H''s) is defined traditionally as the mean ''wave height'' ( trough to crest) of the highest third of the waves (''H''1/3). Nowadays it is usually defined as four times the ...
(Hs) 2.5m will yield an operational criterion of 2.5 * 0.71 = 1.8m. The maximum wave is about 1.86 times Hs (depending on the period). Examples of alpha factors from the DNV Rules for Marine Operations, Part 1, Chapter 2, Planning of operations:. {, class="wikitable" , - ! rowspan=2 , Operational period (hours) ! colspan=3 , Design wave heigh (m) , - , Hs = 1-2 , Hs = 2-4 , Hs > 4 , - , TR < 12 , 0.68 , 0.76 , 0.80 , - , TR < 24 , 0.63 , 0.71 , 0.75 , - , TR < 48 , 0.56 , 0.64 , 0.67 , - , TR < 72 , 0.51 , 0.59 , 0.63


References

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Official Norwegian Report 2008:8 "The Loss of 'Bourbon Dolphin' on 12 April 2007"
p. 38 Inclement weather management