Its the Inside That Counts June 2013

C
orrosion protection in pipelines
is of the highest priority due to
environmental, health and safety
concerns, and the large impact
a corrosion breach can have given the
remoteness of some of these lines. In the
oil and gas industry, internal plastic coatings
(IPC) have been used on internal pipe surfaces
for corrosion protection and hydraulic
improvement for over 65 years.
IPCs can be categorised based on their
function: flow coats and corrosion coats.
While both coatings provide improved
Michael P. Adams,
Global Line Pipe
Product Line Manager,
NOV Tuboscope, USA,
and Robert S. Lauer,
Director – Corrosion
Control Technology,
NOV Tuboscope, USA,
discuss corrosion
protection for
pipeline
hydraulics by reducing surface roughness,
internals.
corrosion control coatings are engineered to
perform in the expected service environments
through chemistry and thickness, whereas
flow coats are designed to minimise surface
roughness alone. There has been some
hesitation to use IPC systems in the corrosion
protection of the internal surface of flowlines
and pipelines. However, history has shown
that IPC systems have been effective in
protecting the steel substrate from corrosion
in applications that approach and exceed
30 years of service.
Coatings for optimum system performance
To ensure optimum system performance, there are
three variables that need to be considered: selection of the
appropriate coating system, proper coating application, and
a connection system used during installation that ensures a
continuous undamaged coated pipeline surface, including the
field joint, throughout the operation.
Corrosion resistant coatings rely primarily on being a
physical and chemical barrier between the steel substrate
and the internal environment. There are three basic types
of coating systems that are used for corrosion control in
pipeline applications: two component coatings, thermoplastic
coatings, and thermoset coatings. All three types of materials
can be effective, but their overall success is dictated by the
parameters of the operating environment. When evaluating
the operating environment for coating selection, one
should first consider the maximum operating temperature
and pressure of the line. Second, consideration must be
made for the concentration of corrosive species that will
be encountered in the line. The corrosive species that will
typically be encountered in pipeline service is water enhanced
by the presence of CO2, H2S, oxygen, chlorides, added
chemicals, biological byproducts, and acidic compounds.
Elevated temperature and/or pressure can exacerbate the
effects of the corrosive species, thus, all parameters should be
considered together. An additional consideration is needed if
pigging operations will be performed through the line and it is
necessary to know the type of pigging to be used.
A suite of solutions
NOV Tuboscope offers several different coating Tube Kote
(TK™) systems for the internal corrosion protection of
pipelines and has also developed several abrasion resistance
materials that are better suited for mechanical interventions
should frequent pigging runs be required. These coatings have
long histories in downhole environments and are designed
to withstand much more aggressive corrosive environments
than typically seen in pipeline applications. Additionally,
all of these coating systems have a phenolic‑based primer
system. Phenolic primers have been shown to be important
in enhancing the coating’s ability to maintain adhesion to the
steel substrate, even in the severest downhole environment.
As all phenolic primers are not equal, it is important to select
the proper primer to be used with the selected topcoat.
Table 1 shows a breakdown of materials typically used in
pipeline applications.
Table 1. Coating types
TK-15
TK-70
TK-70XT
TK-99
TK-236
TK-505
TK-805
Coating
description
Thermoset
Thermoset
Thermoset
Thermoplastic
Thermoset
Thermoset
Thermoset
Type
Powder-modified
novolac
Powder-epoxy
Powder-epoxy
Powder-nylon
Powder epoxynovolac
Powder-epoxy
Powder phenolicnovolac
Colour
Dark green
Maroon
Red/brown
Black
Green
Tan
Black
Temperature
Up to 300 ˚F
(149 ˚C)
Up to 225 ˚F
(107 ˚C)
Up to 225 ˚F
(107 ˚C)
Up to 225 ˚F
(107 ˚C)
Up to 400 ˚F
(204 ˚C)
Up to 250 ˚F
(121 ˚C)
Up to 350 ˚F
(177 ˚C)
Applied thickness
10 - 18 mils
(250 - 450 µm)
10 - 20 mils
(250 - 500 µm)
10 - 20 mils
(250 - 500 µm)
12 - 25 mils
(300 - 625 µm)
6 - 13 mils
(150 - 330 µm)
10 - 20 mils
(250 - 500 µm)
6 - 13 mils
(150 - 330 µm)
28 mg lost/
1000 cycles
53 mg lost/
1000 cycles
5 mg lost/
1000 cycles
11 mg lost/
1000 cycles
36 mg lost/
1000 cycles
72 mg lost/
1000 cycles
7 mg lost/
1000 cycles
0.38 mils lost/
1000 cycles
0.7 mils lost/
1000 cycles
0.025 mils lost/
1000 cycles
0.2 mils lost/
1000 cycles
0.5 mils lost/
1000 cycles
0.7 mils lost/
1000 cycles
0.065 mils lost/
1000 cycles
1.5%
> 6%
> 6%
> 6%
1%
> 5%
1%
Tabor abrasion
resistance results
Flexibility
(% elongation)
Table 2. Coating case histories
Location
Pipe diameter
(in.)
Length of
line (ft)
Year
installed
Coating
type
Line temperature
(˚F/˚C)
Line pressure
(psi)
CO2
concentration
H2S
concentration
Canada
3
8000
1972
Epoxy
phenolic
250/121
100
Trace
51 ppm
Kuwait
3, 4, 6, 10
400 000
1996
Epoxy novolac
230/110
5000
Trace
Trace
Indonesia
4 and 8
69 000
2000
Epoxy novolac
248/120
2100
11%
Trace
190/80
1000
4%
Trace
Libya
3 and 4
309 200
1984
Epoxy
phenolic
US onshore
3 and 16
42 000
1991
Epoxy
phenolic
200/93
1200
1%
50 ppm
US offshore
6
10 000
2000
Phenolic
200/93
2000
10%
2000 ppm
225/107
1500
2.8%
n/a
200/93
1700
3.6%
n/a
US offshore
6
30 000
1999
Epoxy
phenolic
US offshore
8
30 000
1999
Epoxy
Reprinted from World Pipelines | JUNE 2013
Figure 1. Zap-Lok installation in California.
blasting the internal
steel surface with
a material such as
aluminium oxide. This
is carried out to both
clean the surface
and create an anchor
pattern to facilitate
the mechanical and
chemical bond of the
primer to the steel
substrate. The internal
blast should yield a
NACE number 1 (SA 3)
white metal finish and a
1 ‑ 3 mils anchor pattern.
Proper application
is not just about
marrying the proper
prime with the correct
topcoat, but it is also
about the application
of the proper prime
thickness as it can
range 0.5 ‑ 4 mils
(12.5 ‑ 100 µm). For
thermoset materials, a final bake process is important to
ensure the coating is properly cured, thus maximising the
materials performance. Once again, the coating is a barrier and
needs to possess the physical properties necessary to maintain
its integrity while in service.
Ensuring an internally coated connection
Figure 2. Internally coated Zap‑Lok connection system.
Table 2 provides case history information on a sampling of
land and subsea lines that have used internal plastic coating
technology for corrosion control successfully. All applications
listed are still in service and have no reported internal
corrosion issues.
Steps to proper coating application
Proper coating application is critical to an ultimately
successful performance of the coating system in any
environment. There are several key steps that are needed to
ensure proper application. The first step of this process is to
remove any residual organic species from the surface of the
steel. This can be accomplished by either chemical or steam
cleaning of the internal surface, or by thermally cleaning
the pipe. Thermal cleaning consists of a high temperature
bake cycle of up to 750 ˚F (398.9 ˚C), depending on the pipe
metallurgy. Some grades of line pipe should not be subjected
to a thermal clean process as it can negatively affect the
mechanical properties of the steel. The second step is grit
All of the time and effort in the world can be spent in the
coating selection and application process, but if there is
not an effective way of joining the line pipe together and
still ensure corrosion protection through the connection
area, then the coating will be weakened. There are currently
four main ways of ensuring an internally coated connection
for line pipe: flanged connection, welded connection with
the use of an internal sleeve, welded connection used in
conjunction with a robotic coating application system, and
mechanical interference connections. Flanged connections can
be effective in the joining process, but overall costs can tend
to make this a viable solution only with small piping system
projects.
Welding has long been the primary choice for the
joining of line pipe for energy transportation. Welding
internally coated line pipe will leave an area at the weld
zone where the coating has either been damaged or cutback
and is; therefore, without protection. The NOV Tuboscope
Thru‑Kote™ UB sleeve system is manufactured from cold
drawn mechanical steel that is internally coated for corrosion
mitigation after machining. The sleeve is held in place either
by tabs that become consumed in the weld or by a weld
backing ring that is tied into the root pass of the weld. Use
of the Thru‑Kote UB sleeve system does not require any
alterations to the pipe itself meaning the pipe does not
JUNE 2013 | Reprinted from World Pipelines
need to be belled or sized for use. The pipe will need to be
internally plug gauged prior to internal coating to ensure
that the sleeve will fit properly during installation. This
method of joining allows for higher performing IPC systems
to be used that do not have the flexibility to withstand the
mechanical interference connection process described below,
but are required to handle the aggressive environment. Field
installation of the sleeve uses standard welding practice
Figure 3. Zap-Lok 8012 ZPress.
and only impacts the construction process by adding
approximately 10% to the total time of installation. This
connection process is not limited by pipe size or by steel
grade.
The role of mechanical interference connection
systems
Some mechanical interference connection systems can be
used to join internally coated line pipe. The NOV Tuboscope
Zap‑Lok™ connection system, when coated internally and
externally, allows for an internally and externally holiday‑free
connection. The basic concept behind the mechanical
interference connection is that a bell, or expanded area, is
formed on one end of a joint of pipe, and a rolled groove is
formed on the opposite end. In the field, the belled end of
one length of pipe and grooved end of another are forced
together by a hydraulic joining press, with a thin layer of
air‑dried epoxy serving as a lubricant to prevent galling. The
Zap‑Lok process takes on average 90 secs per connection,
which is a significant reduction in time over standard welding
practices. Current sizes and grades currently utilising this
connection process range from 2 ⅜ ‑ 12 ¾ in. in outer diameter
(OD), to schedule extra strong, and up to grade API 5L X‑60.
As the pipe is pressured during hydrostatic testing, the
pin and bell share the load. The pin is in compression and
any high internal pressure will cause a circumferential tension
in the pin wall. The compression in the pin wall is reduced
but because the pin and bell share the load, the pin always
remains in compression even if the internal pressure is
equivalent to 100% SMYS. The bell is in tension even without
internal pressure in the pipe. When the pipe is pressured
internally, this tension in the wall of the bell is increased and
the bell wall may yield further. Although the bell yields, it still
retains its strength (the stress strain curve) and the amount of
yielding is limited to the strain experienced by the pin. The
strain experienced by the pin is minimal (because the material
of the pin is always in the elastic range); therefore, the strain
experienced by the bell is minimal. Third party independent
testing has shown that a properly manufactured and installed
mechanical interference connection in a tension test will only
separate at a load higher than the minimum specified yield for
the pipe.
Conclusion
Internal plastic coatings have an over 40 year proven track
record of making flowlines and pipelines the safest, most
cost‑efficient method of transporting energy products. The
keys to their success have been and continue to be reliant on
the combination of:
FF Coating selection based on the operating environment.
FF Adherence to application practices that stress proper
cleaning and suitable curing to maximise coating
performance.
Figure 4. Thru-Kote UB Sleeve.
Reprinted from World Pipelines | JUNE 2013
FF Selection of a connection system that will ensure a
holiday‑free corrosion protection throughout the length of
the pipeline.