This block is a set of sheaves (pulleys) that move up and down in the derrick. The drilling line is threaded (reeved) over the sheaves on the crown and through the sheaves in the traveling block. This provide.
[pdf] In a drilling rig, the drill line is a multi-thread, twisted wire rope that is threaded or reeved through in typically 6 to 12 parts between the traveling block and crown block to facilitate the lowering and lifting of the drill string into and out of the wellbore.
[pdf] Drill stems must be designed to transfer drilling torque for combined lengths that often exceed several miles down into the , and also must be able to resist pressure differentials between inside and outside (or vice versa), and have sufficient strength to suspend the total weight of deeper components. For deep wells this requires tempered steel tubes that are expensive, and owners spend considerable efforts to reuse them after finishing a well.
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