Scale Modelling The Dredging Process

Author : G. L. M. van der Schrieck
Publisher :
ISBN 13 :
Total Pages : 49 pages
Book Rating : 4.5/5 (671 download)

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Download or read book Chapter 6: Settling Sand in a Hopper Dredge written by G. L. M. van der Schrieck and published by . This book was released on 2020-11-18 with total page 49 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book is derived from chapter 6 from my main book `Dredging Technology Book 1`. In order to keep track of things the section and formula numbers are kept the same with the 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 , 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 transport distance making the suction time the mayor part of the whole cycle. 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 grainsize 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 direct overboard as soon as the overflow level has been reached. In this case one better stop the suction process when the overflow level has been reached and start sailing.The first part of is 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 pumpsystem is explained. The second part deals with explaining and define the relevant parameters like : Restload, Loading rate, Degree of loading , Overflow loss, Hopper load parameter, Grain fall velocity , Hindred settling, Constant tonnage or volume system, Hopper design. From these the Hindred 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 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 years ! Lucky for us the large scale tests in the PhD promotion research carried out by prof. ir. C. van Rhee gave a very clear and well proven description of what really happens in the hopper. Moreover , his computational model resulted in an easy to handle correlation formula for the estimation of the overflow loss.