The Art of Dredging

Dredging and shipping

 

 

How to maximize the load of a hopperdredger.

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.

  • A dredger cycles through the same passage several times a day at the same project -often for months.
  • Dredgers have access to excellent bathymetric data of their site.
  • Dredgers don't wait for highwater, if tides do not rise faster (allowing deeper draughts) than cycle production. Time is money

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.

 

How ?

  1. Check draughts
  2. Check tidal data
  3. Check surveydata of the channel
  4. UKC ?

 

         

Check draughts


  • Check if draught readouts on the bridge are correct, or -if we find an error- is this a fixed error ?
  • Quick draught checks can be done from deck, measuring freeboards. This should be done regularly, to detect any hickup with draught sensors. Paranoia pays.
  • Checking water salinity .... With a draught of 15 meter (like TSHD "Leiv Eiriksson"), the difference between freshwater or seawater amounts to 37 cm of draught.
  • Roll and list are two different things. A vessel can have a permanent list of 1 degree, and roll for another extra six degrees...
  • List: formula to calculate extra draught by list:  shows that list is a decisive factor, especially for the modern wide-beamed dredgers. List must be controlled during loading, or be corrected during sailing. (On Leiv Eiriksson we found out that hanging the 600 ton dredgepipe overboard adjusts for 1 degree of list. Lucky us.)
  • Roll angle of the ship has to be determined before entering the channel. Clearly, no mariner can read the future, but by monitoring previous trips, roll can be reasonably "guesstimated" and accounted for.
  • Trim affects draughts; and a loaded ship with some trim, clearly does not use it's full payload capacity. Trim can be manipulated by shifting weights onboard; bunkers throughout the bunkercycle, or on "Leiv Eiriksson" shifting the travelling deckcrane forward. Every idea may work.
  • Other effects can influence draught: heel angle with turning ship, heaving and pitching motion in waves. All these effects can be observed, and taken in account.

 

Check tidal data

 

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.

 

 

Check surveydata of the channel

 

Survey data accuracy can vary wildly, from 0.05 m up. Accuracy mostly depends on survey quality assurance.

 

Here are some things to consider:

  • Multibeam or single beam ? Clearly: multibeam carries the day. With single beam; shallow spots as wide as the interval between survey lines can remain undetected. This is especially scary in area's with a rocky seabed.
  • Does the survey platform pitches, rolls, etc... ? And what about accuracy of their motion-reference unit (MRU) ?  Top-of-the-line systems, functioning in a strict QA-system, can reach consistent accuracies of 0.15 meter; with small survey boats opearting in waves up to Hs = 2 m.  To reach this accuracy however, interpretation and manual "cleaning up" of survey-data may be necessary, to remove "noise" and "spikes".                 
  • By all means: if you look at a survey chart: check the date of the survey. Channels are liable to sedimentation, sandbanks move around, other dredgers dump loads... Bathymetry is constantly changing, and what you see on the survey chart, is not what you get after a couple of weeks or months.
  • Multibeams gather a lot of depth data per square meter. For site management purposes; survey charts often display mean values per grid cell (typically 1 sq.m.). Mariners are more interested in the shallowest values of a multibeam survey.It may be the difference between a safe passage in shallow water, or the keel of the ship ripped open by a protruding rock.

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.

 

 

UKC ?

 

If we allow for all possible errors and uncertainties, as explained above, there remains an important question: "What dynamic ("nett") UKC do we choose ?"

 

 

  •  In ship manoeuvring textbooks, we often find the rule of thumb: "UKC is minimal 10% of deepest ship's draught". Or sometimes "12.5%", or a very prudent "15 %"
  • Some ports have their own guidelines, as do some shipping companies, often in ISM-procedures.
  • PIANC has developed standards:
     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.

 

 

References

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- a sailor's fifth column