The Art of Dredging

Dredging and shipping

Dredging freeboard

Must read for everyone involved in the operation of trailer dredgers:

IMO Circular Letter nr. 2285:





For outsiders, a dredging freeboard is close to black magic.





Why normal ships have to load to their Plimsoll mark, and dredgers can load deeper ?


Dredging was, is and will always be a business strongly anchored in the Netherlands and the northern part of Belgium.


For many decennia, the Dutch authorities granted dredging freeboards to dredgers.

This practice has also infiltrated in some classification societies and other flag states. 

Namely Bureau Veritas has experience with it.


Essentially: trailing hopper dredgers are allowed to load deeper than their Plimsoll marks, restricted to certain conditions (waveheights, ship's design,...), and up to certain distances from the coast.


The “classic” dredging freeboard was fifty percent of the international freeboard

(so called DR-50).


Because of the special nature of the cargo (high density liquid cargo, liable to shifting) some special requirements had to be in place: the most important was that the dredger must be able to dump his cargo quickly.


In the nineties, some developments in international legislation (on damage stability for cargoships) and in the construction of jumbo dredgers have lead to a review of the rules for dredging freeboard.


An international workgroup, consisting of classification societies, government , dredging companies and shipyards studied the problem.

They attempted to find a new set of rules for a reduced freeboard for dredgers, altogether complying with higher safety requirements in international regulation under I.M.O..


The dredging companies asked to further reduce the dredging freeboard to allow a more economical exploitation of their ships.


It took years – and some scientific research, including simulations-  before the new regulations for dredging freeboard came available at the end of the year 2000, marrying economics with safety.

These regulations were proposed by the Dutch Government to I.M.O. as the “Guidelines for the Construction and Operation of Dredgers Assigned Reduced Freeboards” (I.M.O. circular letter 2285) to I.M.O. and the European Union. This is the downloadable .pdf file on top of this article; a must-read for everyone involved with the operation of trailer dredgers.



The old and the new; old lady Sanderus, built 1967, and Gerardus Mercator, built 1997.



The new rules for dredging freeboard.


The new rules put forward that a dredger should comply with all the internationally accepted rules for normal ships (environment, safety, stability).


The new rules offer the possibility to reduce the freeboard to one third of the statutary freeboard (the “Plimsoll mark”).


Specific requirements for hopper dredgers are:


  • no closed bulwarks are allowed between the hopper and the ship’s side (so that spoil flowing over from the hopper is not retained on deck). This is particluarly important when a ship rolls over to a large list and cargo shifts in the hopper.
  • overflow capacity has to be provided, this capacity is calculated


This rule (on overflows)  is -by some measure- offset when an environmental throttle valve is fitted in the overflow.


Operators can close this valve completely.

Or these valve(s) may fail and block the overflow entirely.


This can quickly lead to dangerous situations during loading of the hopper.

At least one major Dutch dredging company changed the design of the throttle valve to allow a flow of at least 30% at all times.








  • a safe passage way should be provided between fore-and aftship (mostly this is a catwalk above the hopper, but can be an underdeck passage also)
  • when the dredger experiences stability problems, it must be possible to dump the spoil so that the ship is back to international freeboard in eight minutes  (And that system must be operational. If a dredger has –for example- a hydraulic problem, and sails on a dredging freeboard with his bottomdoorcilinders blocked, he’s really in harm’s way, and way out of line.) 
  • an emergency dump mode must be provided, the dredger must be able to return to normal freeboard in 8 minutes
  • excess water above the spoil must be evacuated through sufficiently large overflows
  • the ship must be equipped with automatic draught sensors
  • the suction inlet valve(s) must have a remotely operated hydraulic valve(s)
  • watertight doors under deck must be hydraulically operated (in most cases)
  • all watertight compartments should kept closed, including watertight doors and hatches on deck, to preserve buoyancy in case of stability problems

Due to the smaller freeboard, restrictions to the working area apply, typically 15 miles from the coast.


Stability requirements


In the past, cargoships were required to comply with intact stability requirements (with the exception of tankers and some bulkcarriers, which were granted also a reduced freeboard, having higher requiremnents).


Only passengersships had to comply with rules for stability in damaged condition.


New damage stability requirements were defined for cargo ships; these rules also apply for dredgers, in the most unfavourable loading conditions, with the highest possble draught, (including the free surface effect, which can be considerable.)


The new rules define all necessary intact stability calculations over a wide range of loading conditions, and have requirements on damage stability on the international freeboard.


Damage stability index on reduced (dredging freeboard) should be at least 70% of requirements for international freeboard.



Other requirements


During dredging there must be weather- and waveheight data available onboard, in relation to the operational restrictions as mentioned on the Freeboard Exemption Certificate.

(“Normal” cargoships do not know such limitations.)


Existing dredgers were also allowed to switch to the new reduced freeboard (so called DR-67), but each ship had to be examined in detail concerning watertight compartimentation, bulkhead penetrations, stability, etc…





The new rules lead to the newbuilding of wider ships, improving stability.


For all that, some serious accidents happened with dredgers sailing on reduced freeboard, mainly because of a shortsighted tendency to overload the vessel, with a total neglect, or ignorance of stability theory.



An important factor in some of these accidents were the access points to accomodation, hatches on deck, etc …

Often these openings are left open, even obstructed and impossible to close in case of emergency.


Sailing on reduced dredging freeboard is intrinsically safe; few accidents happen, but everyone involved should realize that not following a strict set of rules may cause a ship to capsize quickly.




Marc Van de Velde

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