The chief portion of this rig is the derrick, Oil which consists of four strong uprights or legs held in Derrick position by ties and braces, and resting on strong wooden sills, which are preferred, as a foundation, to masonry.
The drilling of a well is commonly carried out under contract, the producer erecting the derrick and providing the engine and boiler while the drilling contractor finds the tools, and is Drill ing the responsible for accidents or failure to complete the well.
Derrick cranes are made of all powers, from the timber I-ton hand derrick to the steel 150-ton derrick used in shipbuilding yards.
Obviously, nearly every kind of crane can be made portable by mounting it on a carriage, fitted with wheels; it is even not unusual to make the Portable Scottish derrick portable by using three trucks, one under the mast, and the others under the two back legs.
The needles are put in place one by one against the raised frames, or trestles, by a derrick on a barge lifting them by their ring, whilst a man on the foot-bridge, taking hold of the eye at the top, arranges them in position close together.
The derrick crane introduces a problem for which many solutions have been sought, that of preventing the load from being lifted or lowered when the jib is pivoted up or down to alter the radius.
The whole derrick is set up by keys, no mortices or tenons being used, and thus the complete rig may be readily taken down and set up on a new site.
Somewhat allied to the derrick are the sheer legs (fig.