The safety criteria that can be selected as possible cut off values that indicates the line between safe and not safe.
Safety criteria
The safety criteria that can be selected as possible cut off values that indicates the line between safe and not safe. It is possible to select three types of criteria:
- Under keel clearance (UKC): the gross under keel clearance = channel depth + water level - draft
- UKC and squat:the net under keel clearance = channel depth + water level - draft - squat
- Bottom touch probability: the probability that the vertical downward movement due to waves of the most critical point of the ship exceeds the keel clearance, which is reduced by squat
Absolute and relative cut off values
For the two UKC based criteria, both an absolute as a relative UKC cut of value can be given. Both apply as a minimum UKC. As an example, when a tidal window should be calculated on criterion 1 with minimum relative UKC of 15% and a minimum absolute UKC of 2 meters, then ships can only enter the channel when the UKC is at least 15% and at least 2 meters. For a ship with 15 meters draft, this results in a minimum of 2.25 meters and for a ship with 10 meters draft, this results in 2 meters minimum UKC. For the probability based criterion, only an absolute value can be given of the maximum bottom touch probability.
All applicable criteria are used
Keep in mind that despite the selection of one criterion, PROTIDE uses all applicable criteria. This means that when the gross UKC is selected, then only the minimum absolute and minimum relative UKC values are used, but when the Bottom touch probability is selected, then PROTIDE will use the gross, net and probability based cut off values. This makes it possible to use any combination of criteria. For example a voyage is stated safe when the bottom touch probability is under 0.017%, but we also need 1 meters of absolute UKC for navigation slack.
