FIELD: measurement technology.
SUBSTANCE: method is disclosed for estimating downhole speed and force variables at an arbitrary location of a moving drill string based on surface measurement data of the same variables, wherein the method comprises the steps of: a) using geometry and elastic properties of said drill string to calculate transfer functions describing frequency-dependent amplitude and phase relations between cross combinations of said speed and force variables at the surface and downhole; b) selecting a base time period; c) measuring, directly or indirectly, surface speed and force variables, conditioning said measured data by applying anti-aliasing and/or decimation filters and storing the conditioned data in data storage means which keep said conditioned surface data measurements at least over the last elapsed base time period; d) when updating of said data storage means, calculating the downhole variables in the frequency domain by applying an integral transform, such as Fourier transform, of the surface variables, multiplying the results with said transfer functions, applying the inverse integral transform to sums of coherent terms and picking points in said base time periods to get time-delayed estimates of the dynamic speed and force variables, also a system for implementation of this method is disclosed.
EFFECT: method and apparatus are proposed for estimating downhole variables of a drill string.
15 cl, 7 dwg
