This artcle describes step-by-step, how to maxim ize payload of a tide bound hopperdredger. This specialised article requires some knowledge of terms like "squat" and "UKC" .
Sooner or later, every hopperdredger is squeezed through a shallow navigation channel, with tide-restrictions.
Every idiot can load a hopperdredger to the gunwales, but to get that load safely to the discharge area, time and again; a thousand times... and to maximize that load, with minimal risks,... need some reflection.
Through calculation and assesment, tidebound dredgers can both have minimal risks and maximal loads, under the prevailing conditions.
The method described below is rather specific for hopperdredgers. Merchant vessels will make an occasional port call, having limited info on tides, waterdepths, etc... Merchants often wait for the next high water to make a safe passage over shallow area's.
We developed this method with "Leiv Eiriksson" in Malaysia, and fine-tuned it in Oman.
It's just a squat-formula, turned inside out, and further accounting for all possible errors and uncertainties.
We attempt 10 cm vertical accuracy... with modern sensors and systems, this accuracy can be reached.
On a dredge project, tides may be measured, and transmitted realtime to the dredgers.
If realtime tidal data is missing; we can use the method with harmonic tidal constituents, resulting in calculated ("astronomical") tides.
Astronomical tides introduce three problems:
1. Astronomical tides have no corrections for wind, rainfall or barometric pressure. Every mariner knows that tidal lows or highs may be completely flattened out or peaking, due to meteo conditions.
Tidal data receiver display onboard: black graph is calculated tide, red and green graphs are measured tides. A tidebound dredger can load 20 cm less draught on the first low tide of the day, if he enters that time. By presenting measured tidal data in graphs, anomalies can be easily detected.
2. Global warming causes sea levels to rise; most of the harmonic tidal analysis need reviewing. Example: a recent article in Knack states that -over a timespan of 40 years, for the port of Zeebrugge- tidal heights were found to have risen by 13 cm.
3. Accuracy is a major problem, especially outside Europe. British Admiralty has -historically- always been interested in its own turf first. This is reflected in coverage of nautical charts, but also in tidal data. Example: in Duqm, Oman, tidal heights had a fixed correction of 20 cm, compared to Admiralty data. By datalogging tidal data over a longer period of time (say: one year), harmonic constituents can be recalculated and compared with historical data. Differences found are often significant.
Admiralty Tide Tables are arranged in 4 volumes: one volume serves the British Isles, the other three volumes cover the rest of the world.
The benchmark was to have an accuracy of 10 cm, and Admiralty tidal data are simply not able to deliver that accuracy.
Another method -however not yet commonly used on dredgers- is a GPS-receiver, working in RTK-mode. The vertical resolution of this system may be accurate enough (centimeter-level) for tidal height measurements (read here).
RTK-technology is also a solution for jobs requiring exact knowledge of tidal heights, but located far away from reference ports or tidal height transmitters.
The problem with GPS-antenna's heigth is that ship's draughts have to be taken into the calc, introducing errors from that side.
Survey data accuracy can vary wildly, from 0.05 m up. Accuracy mostly depends on survey quality assurance.
Here are some things to consider:
Survey & navigation display onboard TSHD Leiv Eiriksson. Navigation with a 15m draught is always a 3-D-, and often a 4-D- mind game. Draughts and sailing speed must be strictly controlled in shallow area's.
If we allow for all possible errors and uncertainties, as explained above, there remains an important question: "What dynamic ("nett") UKC do we choose ?"
|Muddy seabed|| UKC = 0.3 m|
| Sandy seabed|| UKC = 0.5 m|
| Rocky seabed||UKC = 1.0 m|
Small high spots in the seabed do not affect the ship hydrodynamically. But how large may be a shallow patch, before the ship experiences squat over that position ? This is beyond knowledge.
In the end, it is still the mariner who picks a nett UKC, observes the ship, the tell-tale signs of squat, the vibrations in the ship, the changing wave patterns at the stern...
Running aground is not an option.