Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

CFD analysis and size optimization of the overflow duct in a trailing suction hopper dredger
Lei Cao, Tengwei Wu, Jingzhao Hu
CCCC Nation Engineering Research Center of Dredging Technology and Equipment Co.,Ltd.
Shanghai,China

performance. There is one or two overflow ducts in the hopper, which
can discharge the upper low-concentration slurry into the sea. Since
low-density dredged soil such as fine sands is difficult to completely
settle down in the hopper, a part of sands that has been loaded into thehopper will be discharged overboard with the overflow water. Then
overflow loss occurs.

ABSTRACTThe overflow duct is a key part of a trailing suction hopper
dredger, which is directly related to the loading efficiency andoverflow loss. However, the design of the overflow duct usuallyrefers to the previous design drawings and lacks of guiding
theories. This paper takes the hopper system of a 4500 m3
trailing suction hopper dredger as the research object andapplies the Free Surface Model to simulate the water-air twofluid flow during the overflow process. Aiming at reducing thethickness of the overflow layer and meanwhile decreasing the
air entrainment through the overflow duct, two parameters , i.e.,
the inlet diameter of the upper conical structure and the diameterof the lower cylinder, are numerically analyzed and optimized
for a given loading condition. The results show thatappropriately increasing the inlet diameter of the conicalstructure is beneficial to reduce the thickness of the overflow
layer on the premise of that the cylinder diameter is large
enough to tolerate the overflow rate. The air entrainment
linearly rises as the cylinder diameter increases, while it firstrises and then falls as the inlet diameter of the conical structure
increases. In summary, when the ratio of the inlet diameter ofthe conical structure to the cylinder diameter is 2.3, thecomprehensive overflow effect is the best.

Whether the overflow duct is reasonably designed or not is directly
related to the overflow loss and furthermore affects the construction
efficiency. Most of the modern TSHDs are equipped with overflow
ducts, and the size and arrangement position of the overflow ducts areimportant factors affecting the overflow loss. Model test results indicatethat if the overflow duct is located at a distance of 20% to 30% of thelength of the hopper from the front bulkhead, the overflow loss is thelowest (Xiao, 2015). In terms of the size of the overflow duct, thedesign guidelines followed by famous dredging companies such as
JANDENUL and DEME propose that the overflow area should belarger than "0.7L2/1000 (m2)" or "Q/3 (m2)", where L (m) is the lengthof the hopper and Q (m3/s) is the loading flowrate. The size of theoverflow ducts in these existing TSHDs designed by China or imported
from abroad usually much larger than the above requirement, but there
is no obvious linear relationship between the specific size and theoverflow parameters such as hopper capacity and loading flowrate. In
the design of the hopper system for subsequent new dredgers, there isno general theoretical support for how to optimize the size of theoverflow duct more reasonably.
In recent years, the concept of sustainable development has gradually
become the mainstream, and countries around the world are paying
more and more attention to the marine environment. The sediment
discharged from the overflow duct spreads at the stern and is often
suspended on the water surface with the entrapment of entrained air
bubbles, which is harmful for the aquatic environment. Thus more andmore dredgers are equipped with an environmental valve in theoverflow duct. The environmental valve can increase the overflow duct
inflow resistance and reduce the overflow rate, thereby reducing thepressure at the inlet of the overflow duct. Accordingly, it decreases theair entrainment into the overflow duct and simultaneously increases thethickness of the overflow layer above the overflow duct (JAIN, Garderand Raju, 1978). SAREMI and Jensen (2014) calculated the air content
in the overflow duct with and without the environmental valve via CFD

KEY WORDS: Trailing suction hopper dredger; overflow duct; CFD;
Size optimization; Free Surface Model.

INTRODUCTION
Trailing suction hopper dredgers (TSHD) are widely used in dredgingengineering such as port construction and waterway maintenance. The
main equipment on TSHD includes drag head, dredge pump, andhopper system. During construction, the dredge pump sucks the sands
excavated by the drag head and transports them to the hopper. The
hopper system is an unique equipment for TSHD, and it is not only
closely related to the stability of the hull, but also affects the dredging

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