256 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
pered martensite can occur in carbon steels, 13Cr pipe-
line steels, and selected other ferrous metals.
Hard spots can be detected by MFL inspection.
135
U
nfortunately, it is often necessary to do this after fi eld
failures have indicated that entire shipments of ques-
tionably processed pipe have been delivered, installed,
and placed in service.
The most common grade of API line pipe is X65 with
specifi ed minimum yield stresses of 65 ksi. Grades as
high as X80 have been accepted and used with minimal
concerns about environmental hydrogen embrittlement,
which can come from internal fl uids; from H
2
S - rich soils,
for example, due to anaerobic bacteria; or from cathodic
protection. Steel producers are developing higher -
strength pipelines, and the claimed advantages are
reduced material shipping costs and lower weights in
transport.
136
For most construction, the weight of pipes
is a secondary concern, and offshore and other sub-
merged pipelines frequently require concrete weight
coatings (Figure 8.62 ) to provide negative buoyancy.
The steels used in pipelines have changed over the
years, even though most pipelines are still constructed
from carbon steels. In the 1970s, hot rolling followed by
normalizing was replaced by thermomechanical pro-
cessing. Continuous casting processing has resulted in
an unfortunate tendency for the “ cleaner ” steels pro-
duced in recent years to have more segregation and
inclusions near the middle of plate steels. Varying minor
alloying additions produce higher - strength fi ne - grained
steels with improved weldability.
137,138
The results of
these improvements are generally better steels, and the
introduction of ductility testing requirements to API 5L
in 2000 means that newer pipelines are less likely to
•
They can also become electrically shorted, invali-
dating cathodic protection on the carrier pipe.
There are many problems associated with installation
and maintenance of pipeline casings, and most authori-
ties recommend against their use, except where
required by regulatory agencies. Deeper burial of con-
ductor pipes or the use of thicker steel at crossings are
recommended to avoid the need for casings except
where required by regulatory agencies, railroad opera-
tors, and so on. Developments in onshore directional
drilling have enabled many pipelines to be installed at
greater depths, eliminating requirements for pipeline
casings.
Pipeline Materials
Most pipelines are constructed from carbon steel in
accordance with API 5L or similar standards, although
there has been a tendency for subsea pipelines to be
constructed from martensititc stainless steels (13Cr)
in recent years because of concerns with internal
corrosion.
The installed costs of subsea pipelines and fl owlines
are divided approximately equally between materials
(assuming carbon steel), lay barge and welding, seabed
preparation, and insulation and weight coating. Thus,
doubling the cost of materials by changing from carbon
steel to martensitic stainless steel (13Cr) is sometimes
considered justifi ed. The doubling of the costs for mate-
rials and welding only increases the total costs by about
25%. Compared to the costs of lost production and
repairs, this is often considered to be a justifi ed
expenditure.
All stainless steels have problems with crevice corro-
sion, so subsea or buried stainless steels must have the
same external corrosion control measures — coatings
supplemented by cathodic protection — that are used for
carbon steel pipelines. Unfortunately, welding problems
have been reported for in - service stainless steel
pipelines.
132 – 134
Martensitic stainless steels offer increased corrosion
resistance to CO
2
corrosion but only limited resistance
for mild H
2
S service. For high H
2
S applications, it is
necessary to use much more expensive CRAs. This is
why sour gas is usually processed relatively close to the
source whereas CO
2
- rich fl uids are sometimes trans-
ported for long distances in 13Cr multiphase pipelines
before onshore or centralized processing.
Untempered martensite (called “ hard spots ” in pipe-
line terminology) can occur due to improper thermome-
chanical processing in pipe fabrication mills or due to
improper welding procedures. Magnetic fl ux leakage
(MFL) inspections can identify hard spots. This untem-
Figure 8.62 Concrete weight coating on the exterior of pipe
intended for submerged service.
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