Study On Point-the-bit Rotary Steerable Tool With Inner Bias Unit And Its Control Method

Automated closed loop rotary steerable system, dated from the end of 20 century, is the world leading-edge drilling technology. China yet doesn’t fully grasp the critical technology of rotary steeable tool (RST) now. Base on comprehensive analysis of foreign current rotary steerable systems, extensive laboratory drilling tests and BHA modeling show the point-the-bit mode rotary steerable tool (RST) provides better hole quality,longer bit life, low bit vibration,and more efficient rock cutting. And also point-the-bit RST has better performance than Push-the-bit mode while drilling in soft formations or overgauged holes. Therefore, point-the-bit RST are preferd by oilfield technology Service Company and Oil Company.Base on the point-the-bit mode, referring on rather limited information about the point-the-bit RST, related mechanical model and mechanical theory about the point-the-bit RST, mechanical structure desgin of the related drlling tool and steerable vector control method were systematically researched. This paper provided a siute of technical solution for the mechanical drived point-the-bit rotary steerable tool with inner bias unit. The total mechanical structure design solutions about the tool and its steerable vector control method wer presented. Main achievements were showed bellow:1) Base on the point-the-bit mode rotary steerale system, the mechanics model of rotary shaft was established under the static bending condition, while considering the feasibility of practical engineering design and increasing the service life of rotary shaft. The geometric equations and mechanical equations of bias rotary shaft were derived. And by setting the minmum bias force as the optimization objective, action spot of the bias force were solved.2) Bending strength, shear strength and the combined strength of bending and shear of the bias rotary shaft were calculated under its maximum flexural displacement of the shaft axis. Calculated results provided theoretical support for the shaft material selection, shaft dimension design, the level of shaft machining accuracy and shaft material heat treatment. 3) The geometric equations and mechanical equations of bias rotary shaft were better validated under the static bending condition by lab test while the shaft did not rotate. And two shaft supporting bearing units and bias unit exerting bias fore on the shaft were tested while the shaft continuously rotated at 110r/min. By the shaft rotation test and its supporting bearing units, some critical proplems were discovered, which provide good design references for the prototype of the rotary steerable tool.4) By setting the bias vector as the control object, a suit of fully innovative bias mechanism with two opposition-placement sliding blocks(drived by leading screw) and bevel blocks were degined base on principle of the sliding blocks and bevels. And the mechanical structure design scheme and bias mechinsm scheme base on optimum control of the steerable vector were presented in this paper. And also, bias displacement vector control method is provided.5) In order to solve the high pressure rotary sealing, cut down the battery power consuming and improve the service life of the shaft upper end supporting bearing unit, this paper provided pressure compensation system design scheme, anti-rotation device design scheme for the tool housing and the design scheme for increasing the service life of upper cantilevered supporting bearing unit.6) Base on mechanical drived bias mechanism and bias displacement vetor control method, the position relationship between the geodetic coordinate system and coordinate of the control plane of the rotary steerable tool were established by the inertia measurement unit. So base on the inertia measurement unit installed on the rotary steerable tool, the tool were controlled by the steerble vetor.This paper presented the servo drived control scheme for 4 uniform opposition-placement sliding blocks(drived by leading screw) and bevel blocks with the synchronizer, while providing bias displacement vector solving method base on the steerable tool control plane and the installation angle of the inertia measurement unit.