Chapter 6: Settling Sand in a Hopper Dredge: Dredging Technology Book 1 - Softcover

Inhaltsangabe

This book is derived from chapter 6 from my main book `Dredging Technology Book 1`. In order to keep track of things in both books the section and formula numbers are kept the same with prefix 6. The Trailing Suction Hopper Dredger (TSHD) is the mayor workhorse of the modern Dredging Industry. During the last decades the load capacity of a single dredger has been increased to over 30-40 thousand cubic meters. This increase of scale kept the price per cubic meter low and thus enabled large scale projects such as: “Chek Lap Kok” Hong Kong Airfield, The Dutch Beach Replenishment Program and The “Maasvlakte 2” Rotterdam Harbour Extension. The sand production of a TSHD not only depends on the load capacity of the ship but also on the suction capacity of the pumps and dragheads. This is especially the case if there is a short sailing transport distance making the suction time the mayor part of the whole dredging cycle: dredging-sailing-discharge/dumping-sailing-dredging. However, there is one more important production factor: the settling velocity of the sand in the Hopper. To give you an idea of the problem: When the average grain size of the sand is as low as 100 microns or less most of the sand will not even settle in the hopper but will flow directly overboard as soon as the overflow level has been reached. In that case one better stop the suction process as soon as the overflow level has been reached and start sailing.The first part of this book explains how to optimize the cycle production of the TSHD. The use of a variable overflow height is described. Also the difference in optimizing between a cycle with a dump of the load and a cycle using the self emptying pump system is explained. The second part deals with explaining and define the relevant parameters like : Rest load, Loading rate, Degree of loading , Overflow loss, Hopper load parameter, Grain fall velocity , Hindered settling, Constant Tonnage or Volume System, Hopper design. From these the Hindered settling caused by the near bed sand concentration is one of the most important phenomena to reckon with. As shown above it is of great importance to know in advance what the max settling rate and overflow loss will be during the loading phase of a dredging cycle. Therefore the last part of this book describes the development over the last decades of the “insight” in this settling process. It is amazing to see what completely different sedimentation models have been suggested over the many years ! Lucky for us the large scale tests in the PhD promotion research carried out by prof. ir. C. van Rhee (2002) gave a very clear and well proven description of what really happens in the hopper. His computational model resulted in an easy to handle correlation formula for the estimation of the overflow loss.

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