232 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
guidelines has largely replaced API RP14E erosion rec-
ommendations for this application, because many com-
panies fi nd the API guidelines to be overly conservative.
This leads to perceived unnecessary reductions in pro-
duction rates. The economic consequences of reduced
production rates must be balanced against the costs of
production shutdowns and possible reduced reliability.
Production wellheads have a variety of high - strength
components necessary to support the weight of the
casing and tubing. This places major limitations on the
type of materials that can be used. The restrictions of
NACE 0175/ISO 15156 usually determine which materi-
als can be used except in the most benign onshore appli-
cations.
16,34,35
API Spec 6A prescribes strength and
impact properties for wellhead materials. Required
strength levels depend on the pressure ratings of the
equipment, for example, equipment utilized to 10,000 psi
(69 MPa) must be made from materials having a
minimum yield strength of 60,000 psi (414 MPa), and
higher pressure equipment must be made from materi-
als using materials with a specifi ed minimum yield
strength of 75,000 psi (517 MPa). Hardness levels and
Charpy impact properties are also specifi ed.
76
Other
similar specifi cations cover subsea wellheads and
Christmas trees.
77
The ISO standard for wellheads and
Christmas trees is slightly different.
78
The wellhead and Christmas tree standards also
specify various classes of service depending on pressure,
corrosivity, and temperature. It is common for suppliers
to supply valves with different materials selected for:
•
Body and bonnets
•
Flanges
•
Wetted internals
•
Valve stems
•
Various types of seals
•
Trim components including both wear and nonwear
components
Valves are supplied depending on different API or ISO
service classifi cations, which vary depending on pres-
sure, corrosivity of the internal environment, and
temperature.
Charpy impact toughness requirements in API speci-
fi cations are intended to minimize brittle behavior.
Many wellhead and Christmas tree components are
massive pieces of metal that are heat treated after
forming, usually by forging. If the heat treatment is not
correct, then brittle fracture can result.
79
Installations of
subsea completions, and the tremendous expenses asso-
ciated with their retrieval, maintenance, or repair, mean
that subsea wellheads and Christmas trees are now
intended to last for the life of the producing well. This
Wellheads, which support downhole tubing, casing,
and other components, are connected at the top of wells
to Christmas trees, which control production rates and
fl uid fl ows out of the well and may also direct fl uids and
equipment into the well. The primary material consid-
erations for wellheads are strength, erosion resistance,
and corrosion resistance. The relatively small size of
wellhead and Christmas tree equipment compared to
tubing, casing, pipelines, and so on means that mater-
ials costs are secondary considerations. It is common to
use whatever material is necessary to insure reliable
production and minimal maintenance for wellhead
components.
Christmas trees, which regulate fl ow rates and also
change the primary direction of fl ow from vertical to
horizontal, have fewer strength requirements, because
they are not limited by downhole space limitations. The
requirements for pressure containment suggest the use
of low - alloy heat - treatable steels for the bodies of these
components. Lining the fl uid - exposed surfaces with
hard, erosion - corrosion resistant alloys provides the
necessary environmental resistance, but all components
must be made from H
2
S - resistant alloys, because no
coating or hard facing is without defects that can allow
gas penetration. While a composite structure of high -
strength steel with erosion - corrosion lining is possible
for the main components, the smaller internal compo-
nents (gates, springs, etc.) are not coated and are made
from erosion - corrosion resistant alloys. Figure 8.27
shows erosion on a Christmas tree gate valve. The use
of hard corrosion - resistant materials and control of pro-
duction to fl uid fl ow rates that minimize erosion are
means of controlling this erosion. The use of company
Figure 8.27 Erosion of a gate valve component producing
loss of a sealing surface. Photo courtesy of NACE International.
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