To calculate a tidal window a probabilistic methodology is used that simulates a set of possible conditions to determine exact times and locations that the probability of touching the bottom is below the safety criterion. Then a route is found through these "safe" moments to determine the tidal window advice.
Find applicable measurements and forecasts of possible circumstances
All measurements and forecasts between the estimated time of arrival and the estimated time of arrival plus one day are retrieved from the database:- water level estimation
- water level variation
- current direction
- current speed
- wave heights (He10)
- wave height variation
- channel depth
- channel bottom elevations
- channel heading / angle
- ship dimensions
Calculate possible vertical motions
OCTOPUS (by Amarcon) is used to calculate the possible vertical motions that can occur for this ship under the estimated circumstances. This is done by:- determining the wave height variation
- creating wave spectra files
- calculating the ship responses for different depth / draught ratios, headings, wave heights and wave directions
Calculate safe time location diagram
The next step in the tidal window calculation is the determination of the time intervals that the ship can be at each kilometer point with the given speed. This is done by finding exact times and the locations that the chance of touching the bottom is exactly equal to the maximum allowed criterion. These "roots" also determine the safe time intervals in the time location diagram.
Exclude times of excess cross current
The safe time intervals calculation only determines that at the given time the chance of touching the channel bottom is lower then the desired safety criterion. An extra criterion is the maximum allowed cross current on harbor entrance. This is set between about 0.4 and 0.6 m/s. The safe time intervals are trimmed, split or fully removed when the cross current is expected to higher then this criterion.
Calculate safe route
After the safe time intervals are determined and corrected for the cross current the algorithm tries to find a safe route with a desired minimum time window width. The algorithm starts at the first available safe time interval on the most restrictive kilometer position in the channel and then tries to reach the berth and start of the channel from there. The algorithm starts with a minimal time window width of 2 hours and on failure tightens the desired width.