Calculation description

During the development of the prototype a calculation method is figured out. This method is developed in cooperation between DVS (Ernst Bolt) and Charta Software (Marlies de Keizer, Peter-Jan Roes and Karsten Uil). The initial idea was to make a method based on the research of Ruud Bouw ('Admittance policy tidal bound ships'), which was a study to find a truly probabilistic methodology as an improvement on the currently used HARAP model. The main objectives:

1. Introduce a model that can be used real-time
2. Use actual forecast data instead of schematizations for water levels, swell and current
3. Use a continuous/actual data for the channel layout, channel depth reductions and ship dimensions
4. Use state-of-the art ship motion analysis
5. Analyse and use a safety criterion for each individual ship entrance

These objectives where analyzed in an initial feasibility study that resulted in a method reported on June 2007. The report can be downloaded in the Meetings section on this website.

The calculation determines the probability that a ship, going through a channel with a certain speed at a certain time and location, hits the bottom. The times and locations where the probability is smaller than a prescribed safety criterion are suitable for a navigational route.

The probability is determined in a number of steps:

1. The under keel clearance (UKC), the space between the bottom of the channel and the navigating ship in smooth water with squat reduction, is determined.
2. The motion (period and probability distribution of the significant vertical movement) as a result of the waves is determined.
3. The frequency of bottom touches, given the UKC and motion, is determined.
4. The probability of a bottom touch, given the frequency, is determined.

With this method the probability of a bottom touch at one location in the channel at one moment in time, can be determined.

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In order to get a tidal window advice we can apply the above method in a progressive scheme:

1. By applying a search method, which searches for the locations and times where the probability of a bottom touch is just equal to the safety criterion, a time location diagram can be determined with possible navigational routes.
2. Next, the moments in time that the cross current is too strong at the entrance of the harbour are excluded.
3. The last step is determining a navigational route which, given a certain speed, is allowed and safe enough.

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