See also article on creating alpha factor tables.
Overview
Alpha factors are applied to activity criteria according to DNV-ST-N001. In practice, this implies that each criteria is multiplied by the calculated alpha factor.
References in brackets relates to DNV-ST-N001
The alpha factor can be assumed to vary in time for one operation. (2.6.10.2) At each instant t, the alpha factor is calculated based on (2.6.10.1):
The selected table
The operational limiting criteria, OPlim
The lead time, Tlead
For wave-based criteria, the operational limiting criteria is the Hs limit. For vessel motions-based criteria, the operational limiting criteria is the forecasted Hs.
The lead time is the duration between the latest weather forecast issue to instant t. The lead time is capped when it exceeds the planned operation period, Tpop. When the lead time is capped, the alpha factor will not change in time (static).
Planned operation period
The planned operation period is defined in two ways:
Operation cannot be halted: Tpop is the time between the issuance of the weather forecast (Twf) to the operation is completed (2.6.9.2).
Operation can be halted: Tpop is the time between two weather forecasts (Tissue) plus the required time to safely cease the operations (Tsafe) (2.6.9.3).
Tpop must be less than 72 hours for an operation to be considered as weather restricted (2.6.7.1).
If Tpop > 72 hours, the operation is considered weather unrestricted, and the environmental criteria should be based on extreme value statistics (2.6.6.1)
A warning is issued if Tpop > 72 hours. The operational support should then not be considered as valid. (see warnings explained)
Alpha factors vs lead time
Lead time | Hs <= 1 | 1 < Hs < 2 | Hs = 2 | 2 < Hs < 3 | Hs = 3 | 3 < Hs < 4 | Hs >= 4 |
<= 12 | 0.93 | Interp. | 0.95 | Interp. | 0.95 | Interp. | 0.95 |
<= 24 | 0.89 | Interp. | 0.92 | Interp. | 0.93 | Interp. | 0.95 |
<= 36 | 0.90 | Interp. | 0.90 | Interp. | 0.92 | Interp. | 0.95 |
Examples:
If lead time is 13 hours and Hs=1.5 m, then an alpha factor of 0.905 is selected.
If lead time is 28 hours and Hs=5 m, then an alpha factor of 0.95 is selected.
If Hs=2m, time between forecast issues is 12 hours and point of calculation is past 36 hours after forecast issue, then an alpha factor of 0.90 is selected.