The Art of Dredging

Dredging and shipping

Incident with TSHD Breydel - damage to suction pipe

 

TSHD "Breydel", the Jolly Green Giant of DEME, at work on the Westerscheldt river, close to the Zandvliet- and Berendrecht locks, May 2008. 

During trailing, an anchor and anchorchain got stuck in the draghead. The crew lifted the draghead, hoisted the suction pipe onboard and -most probably- started sailing towards the dump area, the chain still stuck in the draghead and anchor still on the seabed. 

And this was the result: the trunnion gantry was torn apart. Since this ship has only one suction pipe, it was inoperational for the duration of the repairs.

A trunnion gantry is designed to withstand vertical forces, not horizontal ones. It's perfect for hoisting a suction bend. But this construction cannot take the horizontal dragging force of a suction pipe, with a heavy anchor attached. 

The root cause of this incident may be found in the one-man-operated bridge design onboard new DEME dredgers. A forward looking operator may not have noticed the chain in the draghead, his attention may have been fragmented between shiphandling, traffic on the river, operating the dredge installation.

A single operator does not have sufficient time to check wether the draghead is free before speeding up the vessel.

Personally, I do not believe a single operator can handle all those tasks, process all that data, without overrelying on technology. It would be worthwhile to assess this from an ergonomical viewpoint, not from a mere technical. If an operator has a career of ten to wenty years, you cannot simply expect that he will be infallible. Having a second operator in the wheelhouse would decrease the number of errors tenfold.

Captain James Cook already had: "One man's navigation is always wrong."  The design of breydel goes beyond one-man-navigation; it's one-man-operation.

Technology, computers, sensors are prone to failure, for a multitude of reasons. When technology fails, a human operator can step in and intervene to avoid damage.

 

Photo: wheelhouse desk on "Brabo", a similar design.

One man handles the whole ship, wheelman, officer of the watch and pipe operator. Hope there is a lookout ?

 

 

 

 

 

The culprit: a heavy anchor with anchorchain was lodged in the draghead, later recovered.

 

 

 

 

 

Damage in some details

 

The whole trunnion gantry is pulled aft.

This can only happen with the suction bend in highest position, and with the ship making headway.

 

 

 

 

 

 

 

 

 

 

Suction pipe was pulled back by about half a metre.

From the marks on the suction pipe, it's clear this happened with the suction pipe already in the saddles. 

  

 

 

 

Apron completed separated from the gantry.

 

 

 

 

 

 

 

 

 

 

 

 

   

Distortion of gantry padeyes, probable damage to bearing bushes and cilinder bearings.

  

Marc Van de Velde

10/2008

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