The Art of Dredging

Dredging and shipping

Intro

Ships become bigger and faster all the time. 

VLCC's and ULCC's are common, containerships above 10.000 TEU are all the new fashion.

In the dredging industry, hopper dredgers follow the trend, with the birth of the jumbo-dredger in the mid-nineties. Draughts of 13 tot 15 meters are nothing special, and the speed of hopper dredgers increased, from -say- 12 knots, to 17 knots and beyond.

 

The size of the channels and fairways have not been scaled up in the same proportion. 

 

Nowadays big ships sail in small ditches with miniumum underkeel clearance (UKC), and "squat" has become very much a thing of our time.

 

"Squat" is the collective noun for all interaction effects between ships and seabed, sides of channels, or beween ships. 

Horizontal squat

The effect of shallow water on a ship's turning circle is enormous.

In shallow water, the diameter of a turning circle can double or triple diameter ! 

The ship's speed with which a turning circle is made, has NO effect on the diameter of this turning circle.

If you need to turn in shallow water; you may think it prudent to slow down, and turn at slow speed.

Well: forget it: your turning circle will be the same at low or high speed...

In 2005 "Gerardus Mercator" entered Palm Island II, Dubai, fully loaded and made a 200 degrees turn, to enter another channel, for the first time during that project.

Initial ship's speed was 7 knots. Gerardus made an unexpected wide turn, due to horizontal squat, and bumped into the side slope of the channel with a forward speed of 4 knots.

Due to the shallowwater effect, the diameter of the turning circle was much larger than anticipated. 

Damage was extensive, as you can see on the photos.

 

 

 

 

 

 

 

 

 

 

 

That's horizontal squat in real life.

Refer also to our topic on vertical squat

Marc Van de velde

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- a sailor's fifth column