Drilling Operations

Derricks and Masts

Derricks and Masts are the steel structures that hold the crown block, carry the hook load to the substructure and support the racked setback. The Category covers bolted offshore derricks, dynamic derricks built for floating rigs, dual-activity derricks, and masts raised as a unit on platform and smaller rigs. Racker rails, fingerboards and torque tracks are often part of the structure.

How the structure carries load

The crown block sits on the crown frame at the top. Hook load passes from the crown through the four legs into the drill floor and substructure. The setback load of racked stands bears partly on the drill floor and partly on the fingerboards, which transfer wind and motion loads into the derrick. On a drillship or semisubmersible, vessel roll, pitch and heave add inertial loads to all of these.

Types you will meet offshore

Static derricks on jackups and fixed platforms, designed for wind and the rig's move conditions.
Dynamic derricks on floating rigs, designed for vessel motions as well as wind.
Dual-activity derricks with two well centres, so one side can build stands or run casing while the other drills.
Masts that are raised or telescoped as a unit, more common on platform and land rigs.
Modular platform derricks, built in sections to suit crane lift limits.

Key specifications

Parameter	Why it matters	Typical classes or units
Static hook load	Upper limit of the whole load path	short tons or kN, stated with number of lines
Setback capacity	Sets how much pipe you can rack	short tons or kN
Wind ratings	Separate limits with and without setback	knots or m/s
Motion criteria	Floating rigs only	roll and pitch in degrees, periods in seconds
Clear height	Sets stand length and block travel	ft or m
Base size	Must match the substructure	ft or m

Standards and certification

API 4F covers design, manufacture and marking of drilling and well servicing structures. The 4F data plate states the maximum rated static hook load with its number of lines, the rated setback and the rated wind speeds. Note the edition the structure was built to, because load and wind requirements changed between editions.

API RP 4G covers inspection, maintenance and repair of structures in service, and uses the same four inspection categories as RP 8B. On a classed unit, DNV-OS-E101 or the ABS drilling systems guide also apply. A derrick moved to a different rig type needs a new structural analysis against the new rig's loads, and class review.

Failure modes and inspection

Fatigue cracks at welded gussets, brace ends and the crown frame, found by magnetic particle inspection (MPI).
Internal corrosion in closed tubular members, measured by ultrasonic thickness readings.
Lost bolt preload at bolted splices, found by torque checks against the design value.
Bent or dented members from pipe, racker or crane impact.
Corrosion at the base connections and water table, where water collects.
Loose fittings, lights and secondary items, found by a dropped-object survey.

A Category III inspection covers all load-bearing members with NDT of exposed critical areas. Category IV adds disassembly or cleaning so that hidden critical areas can be tested.

Documentation

The API 4F data plate details and the design calculations or load charts.
As-built drawings and the member mark list.
The latest Category III and IV reports and the repair procedures used.
Engineering sign-off for any overload, impact or modification.
Bolt grade and torque specifications.
Class design approval and survey records where the rig was classed.

Integration and compatibility

A derrick has to match the substructure footprint and drill floor layout. Check the crown frame against your crown block or crown-mounted compensator. Racker rails, torque tracks for the top drive and fingerboard positions must line up with the machines you plan to run. A dynamic derrick from a drillship has bracing and connection details a jackup does not need, and the reverse also applies.

Refurbishment and recertification

Recertifying a dismantled derrick usually means blasting and coating, replacing all structural bolts, repairing members to an approved procedure, and a Category IV inspection after re-erection. Every member must carry its mark so it returns to its original position. Unused surplus derrick steel still needs a check for storage corrosion and handling damage before erection.

A used structure makes most sense from a sister rig of the same design. For the equipment that hangs from the crown, see crown blocks and traveling blocks. For the machines that run on the derrick's rails, see pipe racking systems.

FAQ

Frequently asked questions

What is the difference between a derrick and a mast?

A derrick is a four-legged lattice structure assembled in place, while a mast is raised or telescoped into position as a unit. Offshore floating rigs and jackups mostly use derricks. Masts are more common on land and some platform rigs. API 4F covers the design of both and API RP 4G covers their inspection.

What inspection does a used drilling derrick need before it goes back into service?

A used derrick needs at least a recent API RP 4G Category III inspection, and usually Category IV after dismantling and re-erection. Category IV adds disassembly or cleaning so hidden critical areas can get NDT. Ultrasonic thickness readings on closed tubular members check for internal corrosion that visual inspection misses.

Can a derrick from a jackup be installed on a semisubmersible?

Only after a new structural analysis shows it can carry the semisubmersible's motion loads. A jackup derrick is designed for wind and static loads, while a floating rig adds roll, pitch and heave accelerations. Expect reinforcement, new calculations and class approval before such a transfer.

What does the API 4F data plate tell me?

The API 4F data plate gives the structure's maximum rated static hook load for a stated number of lines, its rated setback and its rated wind speeds. Use it to check the derrick against your planned string-up and setback, and confirm the edition of 4F the structure was built to.

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