The Art of Dredging

Dredging and shipping

Capsizing of TSHD "Spauwer"

THE ACCIDENT 

November, 14th, 1995, the Dutch trailer dredger “Spauwer” capsized during sandwinning operations on the “Kwintebank”, off the Belgian coast.

Four crewmembers survived for 11 hours in the upside down turned wreck, until rescued by Belgian navy divers. The skipper-owner died in the wheelhouse.

The wreck of the ship was salvaged on November, 26th, and delivered to the owner in Hansweert, the Netherlands one day later.

 

 

 Trailer dredger "Spauwer", after salvage of the wreck.

 

THE SHIP

 

The “Spauwer” was originally built as the “Vesta”, an incinerator ship, incinerating toxic waste in a designated area in the North Sea, until this practice was outphased.

 

 

The “Vesta” was rebuilt in a hopper dredger, under supervision of Germanischer Lloyd.

This rebuild included widening of the ship's hull, to increase stability.

 

 

 

The ship was operated for sandwinning off the Dutch and Belgian coast.  

The Dutch council for Shipping (“Raad voor de Scheepvaart”) investigated the accident; and came to some remarkable conclusions.

 

 

Investigation into the capsing of the "Spauwer" (download pdf 78 kB)

  

 

LESSONS LEARNT

    

The “Spauwer” capsized –apparently- at the end of the loading process, and the Dutch Council for Shipping stated that:

 

  • there were no draught indicators in the wheelhouse present, although they should have been
  • the density of the sand was underrated by the skipper; thought to be 1,6 ton/m3,  but in reality > 1,9 ton/m3. This could have lead to a wrong perception of the stability status.
  • the “Spauwer” capsized most probably due to a combination of a very small GM and the draghead stuck on a bottom obstacle.

 

What the Council did not mention, but –to my personal opinion- may have been contributory cause to the capsizing:

 

  • the bridge was one-man-operated - at night; look-out, navigation and dredging operations were all done by one man. (From the context of the report can be concluded that the layout of the wheelhouse was not adapted for this.)

Stability is a crucial and underestimated point in the operation of trailer dredgers.

Particularly dangerous are:

  • any malfunction on the overflow(s), or environmental throttle valve(s) may quickly lead to dangerous situations, especially at the end of the loading process.
  • any malfunction on the draught sensors may equally lead to dangerous situations
  • older dredgers, built before the nineties have a high ratio draught / beam. Nowadays trailers are built with a much wider beam, which vastly improves transverse stability.
  • loading untill water enters the maindeck. The water on deck is weight shipped onboard. One major Belgian company build dredgers in a completely different fashion nowadays: maindecks are raised, which makes it impossible to ship water on the maindeck, and which adds a lot of buoyancy to the equation.
  • loading of liquid with high density. Free surface effect of such a cargo is much underestimated.

 

What is always hammered home during the training of dredge officers is  -when the dredger starts to capsize- to dump the cargo. (The skipper of the Spauwer did exactly that.)

While this is basically correct, yet another reflex should be trained:

During loading, a layer of water and soil is always on top of the loaded material.     

By simply shutting down the dredgepumps, (even if that would mean that the dredgepumps would block with material), this layer quickly flows away through the overflow(s). 

Since this layer is mostly quite large, and the effect of shuttingdown the dredgepumps immediately, the positive effect on stability is enough to avert capsizing, in most cases.

 

Most trailer dredgers have their suction pipes arranged in trunnion slide(s) in the hull. If a draghead gets stuck on the seabed, the resulting forces is applied to the bottom of the ship.

However: there are still some trailer dredgers around (see photo above), who have the connection ship-sucion pipe on deck. These are mostly found on ships converted into trailers.

This configuration may prove very dangerous when the suction head gets stuck on the bottom. The force applies much higher and may lead the ship to capsize.

 

Marc Van de Velde

 

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