STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

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Drilling and Well Completions
DERRICKS AND PORTABLE MASTS
Derricks and portable masts provide the clearance and structural support
necessary for raising and lowering drill pipe, casing, rod strings, etc., during
drilling and servicing operations. Standard derricks are bolted together at the
well site, and are considered nonportable. Portable derricks, which do not
require full disassembly for transport, are termed masts.
The derrick or mast must be designed to safely carry all loads that are likely
to be used during the structure’s life [l]. The largest vertical dead load that
will likely be imposed on the structure is the heaviest casing string run into the
borehole. However, the largest vertical load imposed on the structure will result
from pulling equipment (Le., drill string or casing string) stuck in the borehole.
The most accepted method is to design a derrick or mast that can carry a dead
load well beyond the maximum casing load expected. This can be accomplished
by utilizing the safety factor.
The derrick or mast must also be designed to withstand wind loads. Wind
loads are imposed by the wind acting on the outer and inner surfaces of the
open structure. When designing for wind loads, the designer must consider that
the drill pipe or other tubulars may be out of the hole and stacked in the
structure. This means that there will be loads imposed on the structure by the
pipe weight (i.e., setback load) in addition to the additional loads imposed by
the wind. The horizontal forces due to wind are counteracted by the lattice
structure that is firmly secured to the structure’s foundation. Additional support
to the structure can be accomplished by the guy lines attached to the structure
and to a dead man anchor some distance away from it. The dead man anchor
is buried in the ground to firmly support the tension loads in the guy line. The
guy lines are pretensioned when attached to the dead man anchor.
The API Standard 4F, First Edition, May 1, 1985, “API Specifications for
Drilling and Well Servicing Structures,” was written to provide suitable steel
structures for drilling and well servicing operations and to provide a uniform
method of rating the structures for the petroleum industry. API Standard 4F
supersedes API Standards 4A, 4D, and 4E thus, many structures in service today
may not satisfy all of the requirements of API Standard 4F [2-51.
For modern derrick and mast designs, API Standard 4F is the authoritative source
of information, and much of this section is extracted directly from this standard.
Drilling and well servicing structures that meet the requirements of API Standard
4F are identified by a nameplate securely affixed to the structure in a conspicuous
place. The nameplate markings convey at least the following information:
Mast and Derrick Nameplate Information
a. Manufacturer’s name
b. Manufacturer’s address
c. Specification 4F
d. Serial number
499