Device for measuring the stresses applied in use to the downhole assembly of a drill pipe

Claim 1

A new device for measuring the stresses applied to the downhole assembly of a drill pipe, said device comprising measuring means operatively associated with at least one resilient member interconnecting two adjacent conduits which can respectively be coupled to an upper and to a lower part of said downhole assembly, a sleeve integral with one of said conduits and for protecting said resilient member against external radial pressure, wherein said conduits have substantially the same diameter at their adjacent ends, and wherein said sleeve and one of said conduits form the lateral walls of an annular space which houses at least one intermediate annular piston, said annular space limited by two annular elements forming the end walls thereof, said annular elements being integral with a first of said lateral walls, and slidable along the second of said walls, while said annular piston is integral with the second lateral wall and is slidable along said first wall, said piston separating two annular chambers, one of which communicates with the space outside the downhole assembly of the drill string, and the other communicating with the inner space of said downhole assembly, the operative annular cross-sections of said chambers on both sides of said piston being selected, in dependence on the values of the internal diameter of said annular space at the respective levels of the end walls thereof, so as to substantially nullify any axial load resulting from a pressure difference between the inner and the outer space of the downhole assembly of the drill pipe.