The Art of Dredging

Dredging and shipping

 

I published this article because, during dredging operations, everybody gets involved with tugs, occasionally.  It's the word "occasionally" that has all the poison. Tugs may be manned with locals, with or without experience. Operations may be planned with experience and knowledge or rather hush-hush, and equipment may be reliable or not.

It makes a difference at the end of the day.

On the dangers of tug use

 

"Then and Now" 

Late 2008, "Gerardus Mercator" was in alongside in SIngapore.

One afternoon, I saw a small tug struggling to dock a reefer. There was a squall, with some wind gusts, typically Singaporean weather, and the little tug really  "hung in".  

That scene made me remember an accident I eyewitnessed, some twenty years ago.

I add this sketch to clarify the story:

During some complicated manoeuvre a small twin-screw tug towed the dredger backwards. 

One moment, the tug was at a straight angle with the fore-and aftline of the dredger, and the dredger was on dead slow ahead. The resulting arm was enough to capsize the tug, in a few seconds.

Two crewmembers died in this accident. 

 

So, when I see twin- or single screw tugs performing like this, it makes me anxious all over again, especially when you notice that no stopper-lines are used, and that no emergency-release on the towing hook is installed.

 

 You see that lack of good "towmanship" a lot in backwater tugs.

It becomes almost impossible for the tug's skipper to manoeuvre his tug back in line with the towline and the tug is very liable to capsize. The tug's captain can apply high steering forces, but this adds to the heeling moment already on the tug. The skipper's only option is to release the towline with an emergency quick-release to avoid capsizing ("girting")..

A stopperwire is a (steel)wire attached to the towline, which prevents the towline to move sideways. The stopper may be adjustable -as seen on this photo of  "President Hubert"-, rigged to a runnerwinch.

 

 

  

 

The more manoeuvrable tugs are, the less likely they will get involved in this kind of tricky situation. Tractor-tugs, (such as Voith-Schneider-tugs) and the like do not have this risk, since their propulsion is well forward of the attachment point of the towline onboard.

Left an Azimuth-Stern-Drive tug, used as a tractor, no risk of girting here, left a combi-tug (a conventional tug, with an extra azimuth thruster in the bow), used in a conventinal way. Remark the stopperwire on the right tug.

(Both photo's up courtesy A. Grande, U.R.S.)

 

"BOURBON DOLPHIN"

A very similar accident -but far more publicised- happened to the anchor handling tug "Bourbon Dolphin", april 2007.

 

Anchorhandlers are equiped with sets of hydraulically retractable towpins on their aftship, to prevent shifting of the towline to an undesribale angle. 

(Photo: aftship of "Bourbon Dolphin" with tcamforks forward, towpins aft.)

Again: force (weight of anchor and mooring) was applied at an angle to the fore-and aft line of a tug, causing it to heel, then to capsize. The whole chain of events, leading to the capsizing can be found in the accident report here.

 

A simulation of "Bourbon Dolphin" capsizing.  

More details on this accident, along with comments can be found here and here.

The animation shows clearly what happens if conventional tugs are not in line with the forces on their tow line. Tugs do capsize quickly. On the "Bourbon Dolphin" eight crewmembers never made it out alive.

It made me wonder if every new generation of seafarers need to make the same mistakes all over again.

 

Marc Van de Velde

15 / 3 / 2009

 

Recommended reading: "Tug use in port - a practical guide" by Capt. Henk Hensen, published by The Nautical Institute

 

 

 

 

 

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- a sailor's fifth column