crown block
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07. CROWN BLOCK
7.1 FUNCTION
- Crown block definition
The Crown Block is a fixed set of pulleys(called sheaves) located at the top of thederrick or mast, over which the
drilling line isthreaded.The companion blocks to these pulleys arethe travelling blocks. By using two sets ofblocks in
this fashion, great mechanicaladvantage is gained, enabling the use ofrelatively small drilling line to hoist loadsmany
times heavier than the cable couldsupport as a single strand.
- Sheave characteristics
The number of sheaves on the two Blocks(Crown and Travelling ) can range from 5 to8 and is a function of the
Hoisting systemcapability.The rating of the Crown Block must behigher than the Travelling Blocks.The diameter and
the groove of sheavesdepends on the diameter of drilling line inuse. This values are established by thebuilder based
the recommendations of APIRP 9B.The ratio of sheaves diameter to drilling linediameter should be between 30-40.
Crown Block
- API specifications
The Crown Block, Travelling Block and the Hook are built in accordance with API specifications8A or 8C.
7.2 TYPES AND CHARACTERISTICS
- Groove size
The groove on the sheaves must be samesize as the diameter of drilling line used toprovide the right support. (Fig.
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77)
A groove to wide will flatten the drillingline, while a groove to narrow will causehigh friction and excessive wear on
thedrilling line.
Groove (Fig. 77)
- Typical Derrick Crown Block
7.3 INSPECTIONS
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- Periodic inspections
The Crown Block, as with all Hoisting equipment, must have periodic inspections according to thebuilder's
recommendations and API RP 8B.ENI procedures stipulate that the Crown Block be certified every 5 years, in addition
to themandatory periodic inspections.
- Frequency of Periodic Inspections
The frequency of periodic inspections is:
- Daily
- Monthly
- Semi
-annual
- Annual
- Five-year
- Table: Periodic Inspection and Maintenance Categories and Frequencies
- API Recommended Practice 8B
CATEGORIES
Category I
Observation of equipment during operation for indications of inadequate performance.
Category II
Category I inspection, plus further inspection for corrosion; deformation; loose or missingcomponents; deterioration;
proper lubrication; visible external cracks; and adjustment.
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Category III
Category II inspection, plus further inspection which should include NDE of exposed criticalareas and may involve
some disassembly to access specific components and identify wearthat exceeds the manufacturer's allowable
tolerances.
Category IV
Category III inspection, plus further inspection where the equipment is disassembled to theextent necessary to conduct
NDE of all primary load carrying components as defined bythe manufacturer.
FREQUENCY
The owner or user of the equipment should develop his own schedule of inspections basedon experience,
manufacturer's recommendations, and consideration of one or more of thefollowing factors:
- environment;
- load cycles;
- regulatory requirements;
- operating time;
- testing;
- repairs;
- remanufacture
As an alternative the owner or user may use Table 1.
- Example of DimensionalInspection
a. scheme
b. Measures and Methods
The Drilling Contractors must havea sheave gauge to carry out thechecks and measurements toevaluate wears.
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- Example of NDT Inspection
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