E — FIXED CONSTRUCTIONS; E21 — EARTH DRILLING; MINING; E21B — EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS; E21B41/00 — Equipment or details not covered by groups E21B15/00 - E21B40/00; E21B41/0099 — …