DE3721655

A drill string has a bulge equipped with a system of n injection ducts connecting the interior of the string surrounding annular space and opening upwardly into the space to divert a part of the drilling fluid supplied to the bit. This arrangement makes it possible to produce below the bulge a pressure decrease H markedly greater than 10 meters of water by setting the flow of drilling fluid in the system of injection ducts at a value, expressed in m3/s, equal to approximately: 2ROOT 10HxSxs33 n, S and s being the cross-sections, expressed in m2, respectively, of a part of the portion of the annular space included between the periphery of the bulge and the drill string and of the group of n injection ducts.