Patent GB-1476673-A (GB)
Title: Motion compensation and/or weight control system
IPC: E21B19/09
Filed: 1974-08-28
Abstract: 1476673 Taking up slack in flexible elements DRILLING SYSTEMS INTERNATIONAL Inc 28 Aug 1974 [17 Oct 1973] 52397/75 Divided out of 1476671 Heading B8B [Also in Division E1] A motion compensating device, e.g. for supporting a load from a floating vessel, includes a multiple piston and cylinder arrangement having a primary chamber which is passively supplied with fluid acting to support the load and a secondary chamber actively supplied with fluid which acts in the opposite direction, the device being used in a constant position mode. In a first embodiment (Figs. 1, 2) the motion compensating device 22 is interposed between the travelling block 27 of a derrick 24 carried by a floating vessel 13 and the hook 21 supporting the load, e.g. drill string 14 or a blowout preventer (not shown) to be landed on the well head 17. The motion compensating device 22 consists of a pair of piston and cylinder arrangements 39 and 39<SP>1</SP> as shown, or a single piston and cylinder arrangement (Fig. 5, not shown). The piston and cylinder arrangement 39 includes an outer cylinder 43, an inner cylinder 46 and a piston 47 having a hollow piston-rod 41 which slides over the inner cylinder 46 to define a primary chamber 51 and a secondary chamber 52. The primary chamber 51 is passivly supplied with fluid by means of an oil/air interface tank 35 and a bank of air accumulator tanks 36, the fluid in chamber 51 acting to support the load on the hook 21. The-secondary chamber 52 is actively supplied with fluid by a system 37, the fluid in chamber 52 acting in the opposite direction to the fluid in the chamber 51.  The active fluid supply system 37 (Fig. 2) includes a bidirectional, variable volume pump 61 controlled by signals from a control means 65. The control means 65 may operate in a constant position mode, e.g. when landing a blowout preventor, or in a constant pressure mode, e.g. to maintain a constant weight on the drill bit 15. When operating in a constant position mode the control means 65 responds to signals from position transducer 80 which measures the position of the vessel 13 relative to the sea bed 12 and to position transducer 75 which measures the position of the hook 21 relative to travelling block 27. The control means 65 may also respond additionally to corresponding velocity transducers 81 and 78. When operating in a constant pressure mode the control means 65 responds to pressure transducers 57 and 60 which respectively measure the pressures in primary chamber 51 and secondary chamber 52. In a modification (Fig. 6, not shown) the motion compensating device 22 is mounted on a structure fixed to the vessel 13 and supports a rotary drill head. In a second embodiment (Fig. 7) the primary chambers are defined by piston and cylinders 136, 136<SP>1</SP> and the secondary chambers by piston and cylinders 137, 137<SP>1</SP>, the primary and secondary chambers being-supplied with fluid as described above. In a third embodiment (Fig. 8) the device includes two differential piston and cylinders 138, 138<SP>1</SP>, the primary chambers being defined by the chambers 144 below pistons 139 and the secondary chambers by the annular chambers 145 above pistons 139.