Hull design of hopper dredgers for shallow waters.
Type:
Presented during:
CEDA Dredging Days 2005 - Dredging: The Extremes, Rotterdam
Authors:
Jager A. de and Verheijen L.T.G.
Abstract: Operating in shallow waters is “business as usual” for hopper dredgers. Whether it is the attractiveness of a specific dredging area, or the shortcut in sailing to the disposal area, it doesn’t matter, vessels ability to sail in shallow waters is a necessity to keep dredging profitable. Although shallow water is “normal” for dredging, seamen experience this as extreme. Making the vessel’s design suitable for such circumstances is not only a realistic and understandable demand, but also a real naval architectural challenge.
Ship theories are normally based on the deep water situation. Merchant ships are designed for a long voyage in deep waters crossing seas and oceans. For these ships a water depth is already considered to be “restricted”, when it is 5 times her draught. For big ships with a deep draught, the manoeuvring properties in shallow waters are investigated for approaching and entering harbours, which will always be at low speed. Mostly, tug assistance is required then.
Hopper dredgers, however, are usually operating in estuaries and harbours. Neither speed reduction nor tug assistance is desired, because of their negative effect on the economics of dredging. Consequently, these vessels have to be able to operate fully on their own. Therefore, the properties of the hull of a dredger and its manoeuvrability need to be investigated thoroughly. This applies especially to shallow water conditions i.e. for a water depth of 1.5 or even extremer: 1.25 times her draught! Attention must be paid to propulsion performance, wave making, vibrations, manoeuvring and course keeping ability, squat and dynamic trim. These factors are not only important for the crew and the ship operators, but also for the surrounding traffic.
This paper will highlight the above mentioned points of interest in the design and testing of the dredger (both at model- and full scale) in shallow waters.
Keywords: hopper dredger, shallow water, squat, manoeuvring.