Crown Blocks and Traveling Blocks

Operating principle

The crown block is fixed on the crown frame. The traveling block hangs below it on the drilling line. The line runs from the drawworks drum to the fast line sheave on the crown, then back and forth between crown and traveling sheaves, and ends at the deadline sheave and the deadline anchor. The crown carries one more sheave than the traveling block, for the deadline.

A 14-line string-up on an 8-sheave crown and 7-sheave traveling block is a common offshore arrangement. API RP 9B shows the standard reeving patterns and efficiency factors. More lines give more hook load for the same line pull, but slower block speed and lower reeving efficiency.

Configurations

• Traveling blocks with an integral hook, or a separate hook on a block-to-hook adapter.
• Traveling blocks with a direct adapter for the top drive.
• Guided traveling blocks with a dolly that runs on derrick rails, common on floating rigs.
• Crown blocks that sit on a crown-mounted compensator on some floating rigs.
• Split crown blocks with a separate fast line cluster.

Key specifications

Standards and certification

API 8C covers sheaves, traveling blocks, hook blocks, hooks and adapters, including the proof load test at manufacture that supplementary requirement SR1 adds. API RP 8B covers crown-block sheaves and bearings, blocks and hooks in service, through its four inspection categories and its rules for repair, remanufacture and load testing. API RP 9B gives recommended sheave diameters and the sheave groove gauges used to check wear.

Failure modes and inspection

• Groove wear. A groove worn tighter than the worn-groove gauge allows pinches the line. A groove worn oversize leaves the line unsupported and flattens it.
• Corrugation in the groove, where the line pattern has printed into the sheave.
• Sheave bearing wear, shown as wobble and side play with the line slack.
• Cracked or chipped flanges, which can cut the line or let it jump.
• Cracks in the main pin, side plates, bail, hook body and adapter, found by MPI.
• Hook latch and positioner wear on hooks.

Gauge every sheave, check bearing play, and run MPI on the load path. Category IV takes the block apart for NDT of the pins and bearings.

Documentation

• API 8C nameplate data, rated load and serial numbers of block and sheaves.
• The last Category III and IV reports.
• Sheave and bearing replacement records, with groove gauge readings.
• Load test certificates after repair or remanufacture.
• Overload, crown strike and block drop incident records.

Integration and compatibility

The block rating, sheave count and groove size must match your derrick, drawworks and line. A traveling block designed for a guide dolly needs matching derrick rails. The lower connection must fit your top drive or hook without an untested adapter. Check overall height, because a taller block shortens the usable travel and can limit stand handling.

Refurbishment and recertification

Sheaves and bearings are replaceable, so worn grooves alone do not rule out a used block. A typical refurbishment fits new or regrooved sheaves within OEM limits, new bearings and seals, runs NDT on the frame and pins, and ends with a load test under RP 8B. A block with a clean Category IV report and good sheaves is often a sound used buy. Unused surplus blocks need a bearing and grease check after long storage.

Common OEMs include NOV and HMH, with older National, Continental Emsco and IDECO blocks still in service. Match grooves to your drilling line and rigging and line pull to your drawworks.