Patent GB-1493054-A (GB)
Title: Methods and apparatus for measuring the rate of penetration in well drilling from floating platforms
IPC: E21B47/00
Filed: 1975-01-23
Abstract: 1493054 Measuring displacement electrically, photo-electrically SCHLUMBERGER Ltd 23 Jan 1975 [23 Jan 1974] 2950/75 Headings G1A and G1N [Also in Division G4] Apparatus, for determining a function of penetration by a drill bit suspended from a drill string carried by a floating platform, comprises: respective means for providing signals representing the magnitude and sense of (a) the displacement of the drill string relative to the platform, (b) tensional and compressional changes in the length of the drill string, and (c) displacement of the platform relative to the earth's surface; and processing means for providing output signals representing only net advances in the drill bit displacement. In a floating drilling rig, Fig. 1, optical encoders 23, 27, 40 each comprise a rotatable slotted disc co-operating with a pair of photodiodes to produce a pair of phase-separated pulse trains whose frequency and phase-order represent the magnitude and sense of rotation. The encoders respond respectively to (i) the rotation of a crown block 13  representing the vertical displacement of a travelling block 16; (ii) the rotation of a worm drive (not shown) representing the amount of extension of a heave compensator 17; (iii) the rotation of a pulley 41 representing the length of a portion of cable 42 anchored to the sea bed. A temperaturecompensated strain gauge bridge 36, attached to the top of a kelly joint 21 just below a swivel 18, supplies an analogue W of the strain in a drill string 19 to a potentiometer 38 Fig. 2 settable to a position K corresponding to the total length, crosssection and elastic modulus of the drill string 19 and providing an output analogue KW representing elastic changes in length of the drill string. Circuits 26, 35, 39 and 46 co-operate with respective transducers 23, 27, 38, 40 to provide, on separate outputs, alternative trains of narrow positive or negative pulses V s , V h , V a , V w , each pulse representing a uniform downward or upward increment of a drill bit (20) Fig. 1 resulting from any one of the measured increments. Positive and negative pulses V s , V h  and V a  are combined in respective OR gates 47, 48 to produce total trains of signals +Vd and -Vd representing the resultant displacement of the drill bit (20 Fig. 1) relative to the platform (11). These signals are applied to respective AND gates 53, 54 which in normal operation stay enabled; but when the drill string (19 Fig. 1) is disconnected to allow a further joint of pipe to be added to the drill string, the strain gauge signals 37, Fig. 2, fall short of a reference voltage 56, causing a comparator 55 to inhibit the AND gates 53, 54 until the strain gauges 36 are again loaded by the weight of the drill string. At the same time the increased overall length of drill string may necessitate a resetting of the potentiometer 38. Signals from the AND gates 53, 54 are combined in OR gates 59, 60 with platform displacement signals V w  to provide total bit displacement signals V t . A reversible counter 62 Fig. 2 detects when the drill bit is retracting, by accumulating a net excess of negative pulses -V t  over positive pulses +V t . Any output from the counter 62 is passed by an OR gate 65 to enable an AND gate 64 and to disable an AND gate 63. When an excess of positive pulses +V t , routed to the counter 62 via the gate 64, has restored the net contents of the counter 62 to zero, indicating that the drill bit has again touched bottom, the OR gate 65 is turned off, so disabling the AND gate 64 but enabling the gate 63 to pass positive output pulses Vp indicating further penetration by the drill bit. Further processing is provided by a frequencyto-voltage converter 68 which generates an analogue of the penetration rate, and by recorders 71, 74 charting penetration rate with respect to depth and time, and a penetration-rate indicator 75.