Weather window
Encyclopedia
A weather window occurs when weather forecasts indicate that a given set of marine operations or offshore construction
activities can be performed within their maximum limits for wave height
, wind speeds
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:
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
(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:
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....
activities can be performed within their maximum limits for wave height
Wave height
In fluid dynamics, the wave height of a surface wave is the difference between the elevations of a crest and a neighbouring trough. Wave height is a term used by mariners, as well as in coastal, ocean and naval engineering....
, wind speeds
Wind speed
Wind speed, or wind velocity, is a fundamental atmospheric rate.Wind speed affects weather forecasting, aircraft and maritime operations, construction projects, growth and metabolism rate of many plant species, and countless other implications....
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
Significant wave height
In physical oceanography, the significant wave height is defined traditionally as the mean wave height of the highest third of the waves , but now usually defined as four times the standard deviation of the surface elevation...
(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
Det Norske Veritas
Stiftelsen Det Norske Veritas is a classification society organized as a foundation, with the objective of "Safeguarding life, property, and the environment". The organization's history goes back to 1864, when the foundation was established in Norway to inspect and evaluate the technical condition...
Rules for Marine Operations, Part 1, Chapter 2, Planning of operations:
| Operational period (hours) | 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 |