(19)
(11) EP 0 400 921 A3

(12) EUROPEAN PATENT APPLICATION

(88) Date of publication A3:
11.09.1991 Bulletin 1991/37

(43) Date of publication A2:
05.12.1990 Bulletin 1990/49

(21) Application number: 90305734.7

(22) Date of filing: 25.05.1990
(51) International Patent Classification (IPC)5E21B 4/16
(84) Designated Contracting States:
AT BE CH DE DK ES FR GB GR IT LI LU NL SE

(30) Priority: 30.05.1989 GB 8912396

(71) Applicant: WEIR PUMPS LIMITED
Glasgow G44 4EX Scotland (GB)

(72) Inventor:
  • Ryall, Michael Leslie
    Killearn, Glasgow, G63 9LG, Scotland (GB)

(74) Representative: McCallum, William Potter et al
Cruikshank & Fairweather 19 Royal Exchange Square
Glasgow G1 3AE Scotland
Glasgow G1 3AE Scotland (GB)


(56) References cited: : 
   
       


    (54) Drill bit assembly with concentric rotatable cutting elements and method of drilling


    (57) The invention relates to a drilling system and a drill bit assembly therefor, said drill bit assembly incorporating at least two concentric rotatable cutting elements (9,11 and 30,33) of which one cutting element (9,30) is adapted to be rotated at a rotational speed which is different from that of the other cutting element (11,33).





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