The Art of Dredging

Dredging and shipping

Dredging with a trailer dredger in heavy weather

 

This article gives an analysis of the dangers and limits of operations of a trailer hopper dredger in foul weather conditions.

Most is own experience, gathered during the Korean campaign, 2003-4, and in Taiwan, onboard Gerardus Mercator.

We dredged offshore, in open sea, in typhoon-infested waters. By pushing the weather envelope, we improved operational time of the vessel.

 

We subdivide the cycle of a hopperdredger in different phases (for the purpose of this article):
  1. Sailing to dredge area (empty ship)
  2. Arrival on dredge area, lowering suction pipes
  3. Lowering suction pipe to the seabed
  4. Dredging (loading) the hopper
  5. Turning the vessel during dredging
  6. End of dredging / hoisting of suction pipe
  7. Sailing to discharge / dump area (loaded schip)
  8. Discharging / Rainbowing / Dumping

Let's get into detail:

 

 Sailing to dredge area (empty ship)

Sailing with an empty hopperdredger means sailing with a ship with a very large positive stability arm, and with a low total mass. This leads to a violently moving ship. Rolling is even further enhanced by the huge free surface effect in the hopper.

The ship's movements can be made more gently by filling up the hopper with water.

On most hopperdredgers the width of the hopper is smaller at the top, compared to half height of the hopper. Thus with a hopper filled with water; the free surface effect is considerably less.

 

 

Filling up the hopper reduces freeboard, and this may be prohibitive. But normally; the limits of the dredging process of a trailer are not to be found in sailing with an empty hopper. 

The only real problem can be the violent rolling and pitching, with risks for deckcrane(s), spare parts and suction pipes to break loose.

With modern hoppers the engine power surplus is an extra liability. If a vessel steams in heavy weather; the excess power is not simply lost, but stresses the ship construction. This may result in structural damage.

Filling up the hopper is:

  • unfavourable for sailing time (empty) to dredge area)
  • safer for lowering suction pipes in the water
  • an underwaterpump hangs deeper in the water, which is favourable for production

 

Arrival on dredge area, lowering suction pipes

On arrival on the dredge area, a possible strategy is to position the ship, taking the waves sharply on the bow, creating a leeway for the suction pipe.

 

Another idea is to perform a full turning circle, at full speed. Due to the centrifugal forces, the ship will roll less. Once the circle completed, the ship is almost stopped in the water, and the area covered by the circle has temporarily abated waves, smashed as they were, by the propeller wash. In this lee, the dredgepipe has to be lowered as quickly as possible.

 

 

If one is in doubt if dredging operations can start safely, one has to observe the waves on the dredge area, 10-15 minutes, with the suction pipes still safe on deck. At night, this assessment is particular difficult. (One would do wise and go to the main outer deck, and observe the waves from there; paints a completely different image than from the enclosure of a bridge high up).

Putting overboard the dredgepipe(s) is most critical, since they can start swinging uncontrollably, hitting the hull. Once the suction pipes are underwater, they must be lowered under the keel of the ship, at once.

 

Getting the pipes in the water is one thing, but you should be aware of changes in weather conditions. Will you still be able to recover the suction pipes on deck within a few hours. Will this gale gather strength ?

Your decision on go / no go should depend on the weather forecast, and on the consideration what the weather might look like in a few hours from now.

 

Lowering the pipes into the water, or hoisting them up is critical because the top layer of the water is very turbulent, with wave action, packing a lot of energy. This layer of water is a dangerous “interface” to cross with the suction pipes.

Once the suction pipe is in position on the hull, the worst risk is taken, except for the serviceframe. The serviceframe remains standing in the turbulent air/water interface and is subjected to wave action during the whole dredge process. Loosing a serviceframe during dredging –especially on hoppers with underwaterpumps- is not an option. The serviceframe dictates often workability in heavy weather.

          

Lowering suction pipe to the seabed

While lowering the draghead to the seabed, there is a distinct danger that the ship will heave or pitch on the wrong moment, so that the draghead hits the seabed with a too large speed.

Increasing the swell compensator pressure may help prevent damage in this situation.

 

Dredging (loading) the hopper

The main danger from trailer dredging in heavy weather comes from a combination of:

  • Too slow ship’s speed
  • Heaving ship
  • Steep angle of suction pipe

These three circumstances combined can give rise to buckling of the suction pipe. The mass of the heaving ship will push on the pipe, causing damage.

  

(Examples of buckled pipes, due to dredging in heavy weather, with a steep pipe.)

During dredging, one should asses the maximum desired draught.

Max. draught is determined by: 

  •    height of the waves
  •    direction of the waves, especially in relation to the ehading towards the discharge area

On hoppers with a large freeboard, this limitation is less of an issue.

But Gerardus Mercator was still designed with a smal freeboard. Due attention had to be given to loaded draught, and finally we put experience in a polar diagram, to assist decisions.

(This polar diagram shows max. allowablle draughts versus wave heights versus heading.)

 

 

 

 

The effect of swell on a ship must not be underestimated.

A loaded hopper dredger already “sags” considerably, since the weight of the (heavy) load is concentrated midships.

Swell can easily lead to constructional damage , especially if one did not gave good attention to the ship’s “Loadline – “ or “Exemption for Loadline Certicate”, where there is often a remark towards the max. wave height in which the ship is allowed to operate.

Generally, when a ship sails in high waves, there is a lot of spectacle, spray water, noise, waves roll over the deck, minor damage... 

Swell, on the other hand is more sneaky: the ship gently rides on the swell, the sun shines and all seems to be ok with the world. At the same time, the ship’s construction is submitted to bending forces..

 

 

The worst situation:

  •  wavelength same length as ship’s length
  • ship loaded, sagging condition
  • fore- and aftship on wavetop

The resulting bending force is huge.

 

Turning the vessel during dredging

During dredging in heavy weather, while turning the vessel, the draghead should be lifted, always. And it should be lifted high enough. When the ship rolls heavily during turning, the suspended draghead may touch the seabed, with heavy damage as a consequence.

To hoist the draghead faster from the seabed, the operator can hoist the draghead, simultaneously lowering the intermediate part.

 

End of dredging / hoisting of suction pipe

This is the next critical phase during heavy weather operations; the suctionpipe(s) have to cross again the turbulent interface water / air.

Firstly, the ship should be headed to prevent rolling. Most captains do this with the bow in the waves. This is wrong.

The worst case scenario is that the draghead surfaces, while at the same time a wave “jumps” on top of it. (A small calculation: if the wave height would be one meter, rolling over a draghead with a flat area of  e.g. (5x7m =) 35 m2, this would add 35 tons to the weight of the draghead, and this is only viewed statically; the draghead could break off, and not a thing you can do about it.)

 

The better option is to sail the ship before the waves, with the suction pipes still fully submerged. Once the ship’s speed equals the propagating speed of the waves, the pipes can be lifted safely to deck. No waves can roll over the draghead(s) then.

This manoeuvre is not for the faint-at-heart; sailing speeds (with pipes submerged) may be 7-9 knots. It just looks very radical, less so in theory.

 

Sailing to discharge / dump area (loaded ship)

Sailing with a fully loaded hopper is about the least risky part of a cycle, when operating in heavy weather, provided there is enough freeboard.

 

Discharging / Rainbowing / Dumping

Once the ship arrives at the discharge area, there remain some dangers.

A factor that should be given good attention is the change in the ship’s sail area when discharging. During dumping, this is most dramatic; within a few minutes, the ship changes draught, from fully loaded, to empty, and suddenly floats on the water like a balloon. A great many operators (including myself) have been surprised by this, and have seen their ship blown into the scenery.

 

During rainbowing (or pumping ashore), this change in draughts (and the increasing effect of wind) is more slowly, giving more time to counter its effects.

 

 

If the discharge site is well chosen, with an eye to prevailing wind conditions, the TSHD should be able to remain operational, but workability will be dictated by the auxiliary craft(s).

 

 

 Marc Van de Velde

September 2010

 

 

 

 

 

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