A movable block with four to six pulleys is used to raise or lower the drill string and casings in deep wells. It can handle loads over 1,000,000 pounds and requires robust and reliable components.
A movable block is a multi-sheave sheave used to raise or lower the drill string and casings in a well. The blocks typically consist of four to six individual pulleys over which the steel cables used to suspend the traveling black are passed. The cables are then secured to the fixed crown block at the top of the tower, leaving the lower block free to move up and down as the cable falls. A shock absorber and crane hook are attached to the underside of the moving block and are used to suspend the drill string. These block assemblies are most frequently encountered in oil drilling applications and are often capable of handling loads in excess of 1,000,000 pounds (454,000 kg).
Lowering, raising and controlling the drill string in deep wells generally requires an extraordinarily strong lifting device. These tackles usually consist of a crown block mounted in a fixed position at the top of the well tower and a movable block at the bottom of the rope drop. The movable block is a multi-pulley device, usually featuring four to six individual pulleys. These pulleys are flat discs with a deep groove machined around their circumference. When grouped together, as they are in the movable block, they are collectively referred to as a pulley.
The function of the sheaves is to allow the drill line ropes to pass through the block with minimal friction. In the case of a floating block, four or more sheaves are used to distribute the often considerable loads carried by the block. They also serve to increase the overall mechanical advantage of the hoist system, as well as a measure of redundancy in the event of a rope break. The pulleys are positioned on a central shaft as an assembly enclosed in a steel casing fitted with a bracket on its lower surface. The bracket is used to suspend a shock absorbing crane hook which, in turn, is used to support the designated loads.
The drill string and casings often extend significant distances below the surface and exert extreme loads on the drill lines. This requires moving blocks to be rated to safely suspend loads often exceeding 1,000,000 pounds (454,000 kg). These extreme loads, combined with the stresses placed upon it by the drilling process, require the use of highly robust and reliable components in the construction of the block.
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