required the use of a field joint coating process that
could successfully and efficiently apply the selected
FBE DPS system with accurate control of temperature
and without damaging or degrading the insulation or
existing linepipe coatings. The pipeline owner allows the
use of liquid coatings on the girth welds but prefers and
recommends the use of single- or dual-layer FBE to match
the respective FBE coating system on the linepipe.
Coating application process
The planned pipeline network in central Asia involved
pipes of varying diameters, from 4 - 28 in. OD. For the
Procedure Qualification Test (PQT) described, the pipeline
owner selected pipes from a 11.4 km pipeline with 219.1
mm OD and 9.75 mm WT. It was very important to ensure
that the field joints were given the same degree of
corrosion protection as the linepipe and so the Nap-Gard
Gold DPS FBE system was selected. The use of a field
joint coating identical to that on the line pipe along the
length of a pipeline ensures the optimum compatibility,
and the use of DPS FBE in particular overcomes the
issues of delamination seen with other field joint coating
technologies.
6
The thermal insulation on the field joint areas of the
pipes selected for the PQT was cut back and sealed with
heat shrink sleeves. Each field joint was preheated with
the XIOM MWIR heating unit to 60˚C, and grit blasted
to a surface roughness profile of 75
±
10
μ
m and a surface
cleanliness of Sa2½ (ISO 8501-1:2007). The MWIR heating
unit was again moved over the field joint area and the joint
was heated to 155˚C. The heating unit was moved to one
side and the field joint was flame spray coated first with
300 - 350
μ
m Nap-Gard 7-2500 and then with
300 -350
μ
m Nap-Gard Gold 7-2504. The total film
thickness over a series of three field joints coated was in
the range 635 - 680
μ
m. The MWIR heating unit was again
moved over the field joint area and the applied coatings
were post-cured at 203
±
3˚C for three minutes. Visual
assessment indicated that the resulting coating system was
smooth with an even golden colour.
Results and discussion
Sample panels and pipe ring samples were cut from the
coated field joints and were tested in accordance with
the procedures of the Canadian Standards Association
specification CSA Z245.20-14
7
and ASTM International test
methods. The range of tests in the project specification
to be carried out on the field joint coating are designed
to highlight the adhesion and damage resistance of the
coating system and comprise the following:
)
)
Cathodic disbondment at 95˚C, to reflect the elevated
operating temperature of the pipeline.
)
)
Hot water immersion resistance at 95˚C, for similar
assessment.
)
)
Flexibility at temperatures between -30 - 25˚C, to
reflect the range of local winter/summer temperatures.
)
)
Hardness, Shore D.
)
)
Penetration resistance at 25˚C.
The PQT test results are summarised in Table 2. The
degree of cure of the Nap-Gard 7-2500 plus Nap-Gard
Gold 7-2504 dual powder system was evaluated by
measuring the difference in glass transition temperature
(
Δ
T
g
) between successive scans (T
g
3 and T
g
4) of the upper
layer of Nap-Gard Gold 7-2504 on a TA Instruments
Q100 differential scanning calorimeter. The average
Δ
T
g
value over three separate scans was -0.03˚C indicating
a very high degree of cure and well within the specified
Δ
T
g
5˚C range. The cathodic disbondment and hot
water immersion results were also well within the
specified limits and again indicated a very high level of
conversion.
The Nap-Gard Gold Dual powder system is designed for
high performance in very harsh conditions and its excellent
mechanical properties are conclusively demonstrated in
the PQT test results. The system exhibits an impressive
combination of low temperature flexibility and mechanical
damage resistance as shown by the excellent values for the
Shore D hardness and penetration resistance.
Conclusions
The use of Dual Powder System FBE powder coatings on
pipeline field joints to match the same external coating
system on mainline pipe, and applied and cured by a
combination of flame spray and MW infrared heating, is
demonstrated to offer a high performance solution to
address the issue of long-term corrosion protection. The
practical flexibility of this coating and coating application
system presents a viable and cost-effective alternative
to existing field joint coating technology, particularly on
thermally insulated pipelines.
References
1.
KEHR, J., “Fusion Bonded Epoxy (FBE): A Foundation for Pipeline Corrosion
Protection”, NACE International, 2003, Houston, USA.
2.
International Standards Organisation publication, “Petroleum and natural
gas industries – External coatings for buried or submerged pipelines used in
pipeline transportation systems - Part 3: Field joint coatings”, ISO 21809-3:
2008.
3.
MODARSKI, J. G., and WONG, D., “New Developments in Joint Coating and
Field Repair Technology”, NACE International, CORROSION 98, 22 - 27
March 1998, San Diego, USA.
4.
KEHR, J., “Fusion Bonded Epoxy (FBE): A Foundation for Pipeline Corrosion
Protection”, NACE International, 2003, Houston, USA.
5.
MALLOZZI, M. and PEREZ, M., 18
th
International Conference on Pipeline
Protection, Antwerp, 4 - 6 November, 2009.
6.
MELOT, D., PAUGA<, G., and ROCHE, M., “Disbondment of Pipeline Coatings
and their Consequence on Corrosion Risks”, 17
th
International Conference
on Pipeline Protection, Edinburgh, 17 - 19 October 2007.
7.
Canadian Standards Association publication, CSA Z245.20-14, “Plant-applied
external coatings for steel pipe”, Toronto, Canada, 2014.
Acknowledgement
Ian Archer, Managing Director, XIOM.
138
World Pipelines
/
AUGUST 2015
