A dual wall drill string assembly for subsurface drilling. The drill string assembly includes a metallic outer tube having an outer tube first end and an outer tube second end opposite the outer tube first end. The assembly also includes a flexible, substantially non-metallic inner tube that is substantially...http://www.google.co.uk/patents/US7134514?utm_source=gb-gplus-sharePatent US7134514 - Dual wall drill string assembly
