In Deep Sea Mining, material will be excavated at the sea floor and transported to the surface. This transport always consists of horizontal and vertical transport and can be carried out mechanically or hydraulically. If the transport is hydraulically, during the horizontal transport there is a danger of bed formation and plugging the line. This is similar to the horizontal transport in dredging with the difference that in deep-sea mining the line length is much smaller, but also the line speeds may be smaller. To avoid plugging the line, the line speed has to be higher than a certain critical line speed. In literature there are many theories about this critical line speed and about bed forming in the pipe, but these theories are usually empirical and cannot be applied under all circumstances. Different particles sizes, pipe diameters and line speeds require different equations, although a generic theory should cover everything. For the critical velocity different definitions exist. Some researchers use the definition that above the critical velocity no bed, either stationary or sliding, exits. This definition is also referred to as the limit deposit velocity. Others use the transition between a stationary bed and a sliding bed as the definition of the critical velocity. Whatever definition is used, the Moody friction factor on the bed always plays an important role. Since in literature no explicit formulation for this Moody friction factor exists, an attempt is made to find an explicit formulation for the Moody friction factor for the interface of the fluid flow and the bed, where this interface consists of sheet flow.

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