2. From rough
to smooth
Craig JR Thomas and
Yoshiro Matsubara
(Kansai Paint Co., Ltd.,
Japan) explain coating surface
roughness in a new way.
T
he efficient, effective and safe movement of natural
gas from region to region, from country to country,
onshore or subsea, requires an extensive, well-
designed and well-managed pipeline transportation
system. In a global society, where energy resources are bought
and sold internationally and transferred by the most efficient
means to their point of use, natural gas may have to be moved
over very large distances.
The operating costs of a gas pipeline are significant, and
the volume of gas that can be delivered by the pipeline largely
depends on three key design parameters: pipeline diameter,
length and the internal surface roughness of the steel or
coated pipe.
Recognition of these critical design factors led to
internally coating non-corrosive natural gas transmission
pipelines in the late 1950s, to provide flow efficiency and thus
reduce operational costs. This concept brought about the
product development of a two component, solvent-based,
red oxide epoxy. The next six decades have witnessed a step
change from solvent-based materials to high solids products
World Pipelines / REPRINTED FROM COATINGS & CORROSION 2017
3. with the future, it is predicted, being the wide-ranging use of
ultra-high solids flow efficiency coatings (FEC).
This article will explore the primary benefit of a FEC –
‘flow efficiency’ as the generic product name suggests – in a
way never documented or published before. Flow efficiency is
heavily influenced by surface roughness. The article will seek
to bring together relevant historical fact and figures from this
area of study, before injecting new information and data into
the narrative.
Flow efficiency in pipelines
Flow enhancement in gas pipeline transmission systems can
be created by using a thin film epoxy coating; this coating has
a lower surface roughness than the uncoated steel pipe. The
advent of high(er) solids and ultra-high solids materials has
seen (low) surface roughness catapulted to a key flow coating
feature, even to a point when levels of surface roughness are
now appearing in pipeline design specifications.
A multitude of historical data serves to illustrate that a
reduction in the roughness of the steel pipe surface can lead
to increased flow of gas in the pipeline and can, therefore,
improved capacity.
Leading oil and gas companies have identified that
increases in throughput of 5 - 25% are achievable by internally
lining gas transmission pipelines with a specifically developed
FEC. It is generally accepted that a 1% increase in throughput
provides justification to apply an internal coating from a cost
perspective, and the opinion of the author that increases in
throughput of 15 - 20% are typically experienced.
An effectiveness study was conducted by NOVA
Corporation1
, creating justification for the use of an internal
flow coating for a new 152 km x 48 in. high pressure gas pipeline.
It estimated savings of CAN$4.2 million (at today’s value) in
operational costs over the economic life of their pipeline asset.
The coating surface roughness value used was 6 μm Rz.
Petrobras2
carried out technical and economic studies,
after which it decided to internally coat 1800 km of 32 in. pipe
and 470 km of 24 in. pipe for its GASBOL pipeline, measuring
3150 km x 16 - 32 in. in size. Petrobras factored in a coating
surface roughness of 9 μm Rz.
One noteworthy conclusion, which can be drawn from a
study carried out by Zamorano in 2002, is that the capacity
of a 530 km coated section of the 20 in. dia. GasAtacama gas
pipeline built between Argentina and Chile was considerably
greater (at high pressure) than the uncoated section. Coating
surface roughness was determined to be 6 μm Rz.
Fogg and Morse3
stated that the flowrate seen in a 145 km x
24 in. dia. pipeline with a compressor arrival pressure of 194 barg
and discharge pressure 119 barg could be 60 million ft3
/d higher
if an internal flow coating with a 4 μm Rz surface roughness
was used, compared to uncoated steel (at 50 μm Rz). It could
be further argued that even greater improvements in flowrate
could be achieved if an ultra-high solids FEC was applied with a
surface roughness of 2 μm Rz or less.
Research by Charron4
et al. drew specific attention to
the fact that “the use of relatively smooth [internal pipe]
coatings (2 μm) provides a considerable saving in capital
and operating costs compared to relatively rough coatings
(20 μm).” Comments were made that savings would be even
more significant at a higher pressure. A surface roughness of
20 μm Rz can also be equated to a (slightly) corroded steel
pipe, which can increase to around 50 μm after storage;
a blasted steel surface before coating having a profile of
35 - 75 μm.
Surface roughness of internal flow coatings
It is important to note and understand the word ‘relative’
and the order of magnitude of flow enhancement relative
to surface roughness. The most significant improvements in
flow efficiency will be seen in coated compared to uncoated
pipelines, as is evidenced above.
Elevated levels of flow enhancement can now be delivered
using a higher solids product compared to the original solvent-
based systems, with even greater increases in flow efficiency
now possible by using an ultra-high solids FEC, such as the one
developed by Mirodur S.p.A. and manufactured by Kansai Paint
Co., Ltd.
As previously mentioned, the capacity of a gas pipeline
is predominantly determined by its diameter, length and the
internal surface roughness. It is a well-known and widely
reported fact that internal flow coatings reduce the friction
in gas transmission pipelines due to the creation of a smooth
internal pipe surface, which increases throughput. Viscosity,
density and velocity of the gas can also play a role in terms
of their effect on frictional resistance, as well as pipeline
geometry (e.g. presence of welds, bends and valves) which, in
turn, impact fluid flow.
Figure 1. 3D surface analysis: uncoated (blasted) steel
(50 μm Rz).
REPRINTED FROM COATINGS & CORROSION 2017 / World Pipelines
4. Surface roughness and pressure drop data pertaining to
coated and uncoated pipe have shown the “drag reducing
effect of [a] pipeline coating,” according to Sletfjerding5
et
al. At a Reynolds Number (Re) of 1 x 107
, the pressure drop
in a coated pipe with a surface roughness of 6 μm Rz was
31% lower than in an uncoated steel pipe (surface roughness:
22 μm Rz). This translated into an increase in transport
capacity of the pipeline of 21%. The study also showed that
pressure drop in a 40 in. pipeline with a flowrate characterised
with a Re of 2 x 107
could be reduced by 18% if a FEC was
applied, leading to an increase in transport capacity of 11%.
What is less widely known within the industry is the
‘appropriate value’ for coating surface roughness to be used
in the various stages of planning, design and operation of
a pipeline. What will be the effect on throughput of an
ultra-high solids FEC with a surface roughness of 1 - 3 μm Rz,
compared to a high solids FEC with a surface roughness
4 - 7 μm Rz, compared to a solvent-based, low solids FEC with
surface roughness 6 - 10 μm Rz? This is still a relative unknown,
except to say that an initiative to generate a more detailed
theoretical and practical understanding of these pipeline flow
performance dynamics is currently being undertaken by Kansai
Paint.
Current international standards, such as API RP 5L2,
ISO 15741, EN 10301 and many asset owner specifications,
do not make any reference to the surface roughness of
the applied coating. One key reason is the variability of
application. Other reasons relate to variable operating
conditions (e.g. Re) and the inherent difficulty to link surface
roughness to hydraulic roughness. Notwithstanding these
important facts, the surface roughness of internal flow
coatings has started to be specified by asset and project
owners.
One consequence of a low coating surface roughness
figure being specified is that this then becomes a pass/fail
criterion for that project for every pipe that is produced at
the plant. In order to assist pipe coaters achieve low and
continuously repeatable coating surface roughness in their
production facilities, manufacturers – such as Kansai Paint –
strongly believe in the use of ultra-high solids FEC, which can
provide very low surface roughness characteristics consistently
to each and every pipe of 2 μm Rz or less, and a specular gloss
level of 90 or more.
If a 3D surface analysis of uncoated (blasted) steel (Figure 1)
and steel coated with a standard low solids FEC (Figure 2) is
carried out, it quickly becomes clear that the surface of the
uncoated steel is ‘mountainously’ rough and that, even when
coated, it is not consistently smooth.
However, as can be seen, when an ultra-high solids FEC
is used, such as AlesEpomir PTG Series (Figure 3), the surface
characteristics change completely and the surface becomes
almost entirely smooth.
It can be said that hydraulic roughness required by the
Colebrook-White equation to calculate the friction factor
(Ff
) – which is, in turn, used in the Darcy-Weisbach formula to
ascertain pressure drop – is relatively close to the Rz surface
roughness parameter for steel. Yet, it is argued by some
leading authors that different rules apply to the hydraulic
Figure 2. 3D surface analysis: low solids FEC (~10 μm Rz).
Figure 3. 3D surface analysis: AlesEpomir PTG Series
(1 μm Rz).
World Pipelines / REPRINTED FROM COATINGS & CORROSION 2017
5. roughness of internal coatings. Charron et al. stated that
solvent-free (and, therefore, ultra-high solids) FEC tend to give
a relatively low Ff
. Water-based coatings have a tendency to
give a relatively large Ff
, with values for solvent-based (and
therefore high solids) FEC appearing between these two points
due to the different chemical composition of the formulations
(solids/solvent content and associated properties such as
viscosity and surface tension).
The Ff
is a dimensionless quantity, which is a factor of two
other dimensionless quantities – relative roughness and the
Re. This is why prediction is complicated and a high degree of
accuracy cannot be expected.
Whilst the decrease in surface roughness values from
low solids to high solids to ultra-high solids FEC may appear
small, the percentage point improvements less dramatic, the
pressure drop continues to be reduced and, once these figures
are translated in actual dollar values, the benefit to the asset
owner can become very clear and tangible. The limitation of
pressure drop in a pipeline is considered to be an important
way in which to reduce capital and operating costs. Tools to
accurately predict pressure drop along the length of pipeline,
which can run for hundreds or even thousands of kilometres,
are highly sought after.
At high flowrates with an associated high Re, the Ff
is
dominated by the surface roughness of the coating, but at low
flowrates, the Ff
remains quite independent of the coating
roughness, as “the laminar boundary layer can act as a ‘smooth’
surface,” according to Worthingham.1
This has been experimentally proven by Nikuradse6
et al.,
who glued sand grains of a known size onto the wall of a pipe
to artificially roughen it; the diameter of sand grain attempting
to represent the surface irregularities of the roughness of the
pipe, in reality. This method, whilst working well for uncoated
steel pipe, is not entirely satisfactory for coated pipe that has
a smooth surface, where the FEC surface roughness cannot be
easily equated to uniform sand grains.
The Colebrook-White equation builds on Nikuradse’s
work, attempting to simulate flow behaviour in a pipeline and
to bridge the gap – the transition or critical zone between
laminar and turbulent flow (smooth and rough pipes). It led
to the creation of the Moody7
diagram, but Colebrook-White
correlation still does not fit for entirely smooth surfaces.
The roughness parameter used in such calculations is named
hydraulic roughness, relative roughness or equivalent sand
roughness.
Moreover, the flow in offshore gas pipelines is typically
characterised by a large Re, approximately 107
, due to the low
viscosity and relative high density of the gas at operating
pressures of around 100 - 180 bar. The Re, from which the
Colebrook-White Ff
were developed, were Re ~106
; 10 orders
of magnitude lower.
Since this time, a number of different methodologies
have been hypothesised to solve the issues associated with
theoretical and actual physical roughness. NOVA Corporation
established, after extensive testing and analysis, that theoretical
hydraulic roughness in turbulent flow regimes could be equated
to physical Ra measurements on actual pipe. Total8
used the
‘GERG formula’ to determine that
accurate hydraulic roughness for pipe
coated with a FEC could also be equated
to Ra physical roughness.
A pressure loss simulation model
for natural gas transmission pipelines
is being developed by Kansai Paint,
using calculations developed from the
computational analysis of ship hull
performance in the marine industry.
The model for the prediction of FEC
performance employs both the Darcy-
Weisbach equation and the Colebrook-
White correlation (for methane gas
transportation), using both fixed and
variable parameters to increase the
level of accuracy.
If a scenario is envisaged, where
a 300 km x 36 in. OD pipeline was
constructed to transport methane at
a flowrate of 1060 million ft3
/d using
an inlet pressure of 100 bar, the model
calculates that the outlet pressure at
the end of the 300 km line without the
use of compressor stations and without
the use of a FEC would be 52.3 bar; a
pressure drop of 47.7%. If an ultra-high
solids FEC with a surface roughness of
1 μm Rz was to be used on the same
Figure 4. Pressure loss model: internal surface – uncoated (blasted) steel (50 μm Rz).
Figure 5. Pressure loss model: internal surface – AlesEpomir PTG Series (1 μm Rz).
REPRINTED FROM COATINGS & CORROSION 2017 / World Pipelines
6. line with the same operating parameters, the outlet pressure is
calculated to be 71.0 bar; a drop of 29%.
This model, whilst relatively simple, clearly demonstrates
the need for compressor stations. Yet, it also serves to show
that a FEC with a super low surface roughness can have a
significant impact on pressure drop and, therefore, throughput
efficiency.
Cost reduction for compressor stations
Industry analysis shows that compressor costs can be
dramatically reduced over the lifetime of a pipeline by
applying an internal flow coating.
It may also be possible to engender further project cost
savings by reducing the number of compressor stations,
compressor size or compressor capacity. This can, in turn, have
a positive impact on CO2
emissions, carbon foot print and
boost an oil and gas major’s green credentials.
In 2002, Zamorano determined that fuel gas costs for the
compressor stations alone, which were situated along the
1200 km length of the 20 in. dia. GasAtacama pipeline, were
27% lower on the coated section compared to the uncoated
section, resulting in OPEX savings of 33% over an expected
20 year life.
In Norway, 35% of offshore-generated energy in 1999 was
used to power gas export pipeline compressors, which, as
a result, Sletfjerding et al. reported that the use of internal
efficiency coatings to reduce the operating cost of compressor
stations was therefore important.
A number of different simulations based on the pressure
loss model are being developed by Kansai Paint, to show the
impact of FEC on compressor costs for and compressor station
configuration along a typical gas transmission pipeline.
Conclusion
Surface roughness has a major impact on flow efficiency and
increased throughput, which is why this coating feature is
starting to appear in pipeline design specifications. The use
of ultra-high solids FEC will engender both greater pipeline
flow and application performance, as well as positively
influence pipeline operating costs through the reduction of
costs associated with compressor stations. Tools to accurately
predict pressure drop and analyse the impact of FEC on
pipeline flow and operating performance are now being
developed by the industry.
Bibliography
1. WORTHINGHAM, R. G., ASANTE, B., CARMICHAEL, A. and DUNSMORE, T, ‘Cost
Justification for the use of Internal Coatings’, NOVA Corporation of Alberta,
1993.
2. TAVES, J. M. de V., SANANDRES, S. R., QUINTELA, J. P., FERREIRA, S. R. and
FAZANARO, A. G., ‘Internal Lining of the Bolivia-Brazil Gas Pipeline: a Petrobras
Experience’, 13th
International Conference on Pipeline Protection – BHR Group,
29 September - 1 October 1999.
3. FOGG, G. A. and MORSE J., ‘Development of a New Solvent-Free Flow Efficiency
Coating for Natural Gas Pipelines’, Rio Pipeline 2005 Conference and Exposition,
17 - 19 October 2005.
4. CHARRON, Y., DUVAL, S., MELOT, D., SHAW, S. and ALARY, V., ‘Designing for
Internally Coated Pipelines’, 16th
International Conference on Pipeline Protection
- BHR Group, 2 - 4 November 2005.
5. SLETFJERDING, E., GUDMUNDSSON, J. S. and SJØEN, K.: ‘Flow Experiments
with High Pressure Natural Gas in Coated and Plain Pipes – Comparison of
Transport Capacity’ (PSIG 30th
Annual Meeting, 28 - 30 October 1998); ‘Flow of
Natural Gas in Pipes at High Reynolds Numbers’ (Eurogas-99, 25 - 27 May 1999);
‘Friction Factor in High-Pressure Gas Pipelines in the North Sea’ (SPE/CERI Gas
Technology Symposium 2000, 3 - 5 April 2000).
6. NIKURADSE, J.: Gesetzmessigkeiten der turbulenten Stromung in glatten Rohren’
(Forschungsheft 356, Volume B, VDI Verlag, 1932); ‘Stromungsgesetze in rauhen
Rohren’ (Forschungsheft 361, Volume B, VDI Verlag, 1933).
7. MOODY, D., ‘Friction Factors for Pipe Flow’, Transactions of ASME, November
1944.
8. MELOT, D. and BOTTON, E., ‘Pressure Drop Testing of 12” Pipelines – Influence of
DFT and Surface Roughness’, 17th
International Conference on Pipeline Protection
- BHR Group, 17 - 19 October 2007.
Figure 6. A 56 in. steel pipe coated with an ultra-high solids
FEC. Courtesy of Mirodur S.p.A.
World Pipelines / REPRINTED FROM COATINGS & CORROSION 2017
