Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02253225 1998-11-10
TITLE
ON-SITE PIPE COATING PROCESS
Field of the Invention
The present invention relates a method for repair of a defective
section of a protective coating on the exterior of a metal pipeline. The
method is carried out in the field and may be carried out on an operating
pipeline.
Background of the Invention
It is common practice to coat sections of new pipelines off-site in a
pipe manufacturing operation or at a site other than in-line on the pipeline,
to
protect the pipeline from corrosion or mechanical damage. It is important for
the long term protection of the pipeline that all sections of the pipeline
have
a viable coating.
Pipeline coatings invariably became damaged or defective, as a
result of damage during installation, movement of rocks or other abrasive
materials in the ground e.g. due to settling of soil or freeze/thaw cycles in
certain areas, or aging of the coatings.
A wide variety of portable field-applied coatings are in use to recoat
previously coated, and partially protected, aged metallic substrates e.g. in
service steel pipeline fittings and assemblies and steel lined pipe. Such
aged substrates are normally coated with e.g. mill-applied fusion bonded
epoxy powder, extruded bitumen/polyethylene tape wraps, heat-shrink
sleeves, two-part liquid epoxies applied by brush, by roller or by airless
spray equipment; and modified liquid urethanes which are also applied
either by brush, by roller or by airless spray equipment.
While each of these field-applied coatings is useful in certain
applications, there are compatibility and pertormance limitations on both
new and aged metallic substrates. Moreover, in situ coating repairs are
difficult or impossible to accomplish in a manner that re-coats the pipeline
with the coating originally applied to the pipe in a plant site.
Fusion bonded epoxy powders generally exhibit excellent adhesion
and excellent cathodic disbandment properties. However, these coatings
tend to be brittle and prone to handling damage and to display high moisture
permeation. The coatings are useful as a new coating on metallic
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substrates, but generally are not chemically compatible with other coatings
such as extruded plastics and plastic film. This excludes them from use with
such dissimilar coated surfaces. For instance, fusion bonded epoxy
powders, by themselves, are not compatible or reliable coatings with two or
three layer, mill-coated composites as found on new steel line pipe.
Thermoset coatings, which include two-part liquid epoxy and two-part
liquid urethanes, generally exhibit excellent adhesion and cathodic
disbandment properties. However, these coatings also tend to be brittle and
prone to handling damage and to display high moisture permeation.
Thermoset coatings work well as either a new or recoat coating when the
already-applied coating is also a thermoset material or a fusion bonded
epoxy powder. However, they are not chemically compatible with other
coatings such as extruded plastics and plastic films, and tend not be usable
with such dissimilar coated surfaces. Thermoset coatings, by themselves,
generally are not reliable with two or three layer mill-coated composites as
found on new steel line pipe.
Tape wraps and heat-shrink sleeves show a much lower adhesion to
metal substrates and to other coatings, when compared to thermoset
materials. Consequently, they do not withstand attacks from soil stress and
soil loads, common situations for buried metal pipe. Tapes and sleeves also
leave a visible seam with the underlying substrate, which is susceptible to
adhesive failure. Adhesive failure can provide a path through which moisture
and other contaminants can migrate to undermine the integrity of the
remaining bond and attack the underlying substrate.
An example of a method for repairing a coating in the field using a
thermospray process is described in U.S. Patent 5 792 518.
Summary of the Invention
Aspects of the present invention provide a new portable and field-
applied, coating process that produces a two layer composite, protective
coating with a finished, seamless bond between its interlayers and to other
adjacent coating materials and which may be used to coat new or to recoat
previously coated, cathodically protected or not, metallic substrates, without
using a thermospray process.
Accordingly, an aspect of the present invention provides a method for
repair of a defective section of a protective coating on the exterior of a
metal
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pipeline in the field, comprising:
(a) stripping a portion of said protective coating from said pipeline,
said portion including the defective section of the protective coating;
(b) forming a clean metal surface on said portion of the pipeline;
(c) applying an undercoat of at least 5 mil of liquid polyepoxide or
polyurethane composition to said clean metal surface, said composition
having a curing agent for said polyepoxide or polyurethane;
(d) optionally applying a non-melted, powder layer of a
thermoplastic polyolefin selected from ethylenel(meth)acrylic acid copolymers
and polyolefins modified with functional ethylenically unsaturated carboxylic
acid groups or derivatives by means of forced air onto the entire surface of
the undercoat and onto the protective coating adjacent thereto;
(e) allowing the undercoat to cure until the coating exhibits no flow
when a stream of heated air is applied; and
(f) applying a topcoat of at least 5 mil of a thermoplastic polyolefin
selected from ethylene/(meth)acrylic acid copolymers and polyolefins
modified with functional ethylenically unsaturated carboxylic acid groups, or
derivative thereof, onto said undercoat, by contacting the undercoat and
protective coating adjacent thereto with said thermoplastic polyolefin in a
non-
melted state, the pipeline being at a temperature such that the polyolefin
melts and undergoes melt flow on contacting the undercoat and protective
coating adjacent thereto.
The present invention further provides a method for application of an
exterior protective coating on a minor section of a metal pipeline,
comprising:
(a) providing a clean metal surface;
(b) applying an undercoat of at least 5 mil of liquid polyepoxide or
polyurethane composition to said clean metal surface, said composition
having a curing agent for said polyepoxide or polyurethane;
(c) optionally applying a non-melted, powder layer of a
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thermoplastic polyolefin selected from ethylenel(meth)acrylic acid copolymers
and polyolefins modified with functional ethylenically unsaturated carboxylic
acid groups or derivatives by means of forced air onto the surface of the
undercoat and onto the protective coating adjacent thereto;
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(d) allowing the undercoat to cure until the coating exhibits no flow
when a stream of heated air is applied; and
(e) applying a topcoat of at least 5 mil of a thermoplastic polyolefin
selected from ethylene/(meth)acrylic acid copolymers and polyolefins
modified with functional ethylenically unsaturated carboxylic acid groups, or
derivative thereof, onto said undercoat, by contacting the undercoat and
protective coating adjacent thereto with said thermoplastic polyolefin in a
non-melted state, the pipeline being at a temperature such that the
polyolefin melts and undergoes melt flow on contacting the undercoat and
protective coating adjacent thereto.
The preferred embodiment of the methods, the thermoplastic
polyolefin is in the form of a powder, film or tape.
Brief Description of the Drawings
The invention will now be described with reference to the
embodiments shown in the drawings, in which:
FIG. 1 shows a schematic representation of the layers of a coating
applied to a pipeline;
FIG. 2 shows a schematic representation of portion of pipe having a
girth weld around its circumference and having a coat by a process of the
present invention;
FIG 3. shows a schematic representation of the pipe of FIG. 2
partially coated during the coating process;
FIG. 4 shows a schematic representation of pipe having a longitudinal
weld;
FIG. 5 shows schematic representation of the pipe of FIG. 4 partially
coated during the coating process.
Detailed Description of the Invention
The method of the present invention is a portable, field applied
method. It is understood that the method is intended for use in the field i.e.
on existing, installed pipelines, which may or may not be operating at the
time the method is used. Suitable removal of soil or the like from around the
pipeline is required, as will be understood by persons skilled in the art.
The nature of the bond between the thermosetting polymer viz. liquid
polyepoxide or polyurethane composition, and functionalized thermoplastic
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polyolefin in powdered form appears to be an intermingling of undercoat
and top coat materials to form a blended material which might be described
as a "plastic material composite". In a preferred embodiment discussed
below, an additional step is taken to form plastic materials composites.
5 Functionalized thermoplastic polyolefin is described herein and is a
ethylene/(meth)acrylic acid copolymer or polyolefin modified with functional
ethylenically unsaturated carboxylic acid groups, or derivative thereof
The undercoat, which is adherent to the metal surface, may be
applied by brushing, spraying or rolling and may form a mechanical and/or
chemical bond with the pipe profile.
The metal surface may be a partially exposed surface of a previously
coated metal pipe or may be a clean surface of an unused manufactured
line pipe. When these pipes or articles have been previously coated with
continuous coatings or wrappings and the inventive coating of the invention
is to be utilized in a previously coated area, it is necessary to clean the
previous coating from a discrete area to be coated accordingly to the
invention. For example, if a defect occurs in a coating of a previously
coated pipe, it is necessary to clean the previous coating away from the pipe
in the region of its defect, and the~immediately surrounding area. The
inventive coating may then be applied, firstly the layer of liquid
thermosetting
polymer onto the exposed metal, and secondly, the functionalized
thermoplastic polyolefin, optionally with the additional powder layer
discussed herein.
If the functionalized thermoplastic polyolefin is applied in the form of a
powder, the powder is applied at ambient temperature or with minimal
heating. The heat to form the layer of molten polymer on the pipe is
provided by the pipe. Thus, the polyolefin does not melt until contact with
the undercoat and protective coating adjacent thereto on the pipe.
However, the functionalized polyolefin must undergo melt flow on contact
with the undercoat and the existing and remaining adjacent protective
coating.
Similarly, if the functionalized thermoplastic polyolefin is applied in
another form e.g. a film or tape, the film or tape is applied at ambient
temperature or with minimal heating. The heat to form the layer of molten
polymer on the pipe is provided by the pipe.
The pipe would normally be heated immediately prior to contact of the
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functionalized thermoplastic polyolefin with the pipe. Any convenient means
of heating the pipe may be used, examples of which are infra red and
induction heating. It is understood however that the heating could be
applied after the functionalized thermoplastic polyolefin is applied,
especially
if the functionalized thermoplastic polyolefin is in the form of a film or
tape.
Fig. 1 shows a metal surface 10 coated according to the invention.
The coating comprises an undercoat 12 of thermosetting polymer, obtained
from a liquid polyurethane or polyepoxide, and a topcoat (overcoat) 14 of
functionalized thermoplastic polyolefin, e.g. a modified polyethylene or
polypropylene or ethylene/acrylic acid copolymer. Intermediate layer 15 of
plastics material composite is shown between the topcoat 14 and the
undercoat 12. It is believed that intermediate layer 14 may be formed when
topcoat 14 is applied to undercoat 12. However, intermediate layer 15 may
be formed in a separate step. As discussed herein with reference to
application of an additional optional layer, it is further understood that the
layers of undercoat 12, topcoat 14 and intermediate layer 15 will, to a
significant extent, not be separate and distinct layers but rather show a
transition in composition between undercoat 12 and topcoat 14.
It should be understood that the thickness of the undercoat,
intermediate layer and topcoat shown in FIG. 1 and elsewhere are not to
scale.
FIG. 2 shows a length of pipe 20 having a girth weld 16 adjoining
adjacent pipe lengths. It will be appreciated that while FIG. 2 illustrates a
weld 16, the present invention will be equally applicable to any type of join
between two pipes or to joins between pipe lengths and a T-junction for a
reducer or an elbow or the like. Moreover, the process of the invention
might equally be applied to any weakened area of the pipe coating and the
description with respect to the embodiment of FIG. 2 would equally apply to
any such weakened area.
When pipe 20 that has already been provided with a conventional
coating is initially laid or when it is uncovered for rehabilitation or
repair, it
may be treated by the process of the invention. One embodiment of such a
process will now be described.
The welded pipes, 20, are thoroughly cleaned in the region of weld 16
and outbacks 18 located on each side of weld 15. Cleaning may be by
sandblasting or by other conventional means. The level of cleaning may be
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SSPCSP2 (simple wire brushing) up to SPS, SPS or SPIO (SPIO is
sandblasting) to remove as much as 4 mils of existing metal surface, It is
understood that a section of existing coating may have to be removed,
especially where it is believed to be defective, to provide a clean surface
where a repair is required. For a new pipeline, less cleaning may be
required. It is understood that chromium, zinc or other metal treatments may
be applied to the pipe after cleaning but prior to application of liquid
undercoats.
After cleaning of cutback 18 and weld 16 between lengths of pipe,
which itself is coated with coating 17 except in the area of the cutback and
the weld, the cleaned section of pipe is coated with undercoat 12 of liquid
thermosetting polymer which will adhere to the metal surface of the pipe and
may also blend with the subsequently applied topcoat 14 (partially shown in
Fig. 3) of thermoplastic polyolefin. The coating 12 may be applied by
brushing, spraying or rolling liquid over the cutback 18 and weld 16. Topcoat
14 may be applied by any of the various methods described herein.
The thermosetting polymer may be a liquid polyepoxide or a
polyurethane composition. The thermosetting polymer is applied as a liquid,
in a thickness of at least 5 mils, e.g. 5-40 mils, especially 10-20 mils, at
ambient temperature. It is not necessary to heat the pipe, although this may
be done. If the liquid coating is a polyepoxide, it is preferred that the
ambient
temperature be at least 1000. If the liquid coating is a polyurethane, it is
preferred that the ambient temperature be at least -20°C. After
application of
the liquid polyepoxide or polyurethane, the resultant undercoat is permitted
to cure to an extent that there is no flow of the coating when a stream of hot
air is applied. It is understood that when no flow of the coating is
exhibited,
application of heat may result in further curing of the undercoat, and even
burning of the undercoat.
The time required for sufficient curing of the undercoat depends on
the ambient temperature, but typically takes 4-24 hours. This is convenient
for operation of the process, as it eliminates the need to apply a topcoat
immediately. A pipeline at ambient temperature also facilitates this, although
a low amount of heat may be applied to the pipeline.
Examples of polyepoxides include KEMAT"" 500 as manufactured by
Kemacoat International Inc., PROTAT""L 5900 as manufactured by Denso
North America, and SPOT"" 2888 as manufactured by Specialty Polymer
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Coatings.
Examples of polyurethanes include VALPIPET"~ 100 as manufactured
by the Valspar Corporation. KEMA T""100 as manufactured by Kemacoat
International Inc., and PROTEGOLT"", manufactured in West Germany.
It is understood that both the polyepoxide and polyurethane
composition as applied to the pipe will contain a curing agent, as is known
for such compositions.
When the undercoat has reached the no-flow condition discussed
above, the overcoat is applied.
The overcoat may be applied in the form of a powder at ambient
temperature or with minimal heating. For instance, the powder may be blown
onto the pipe in a stream of air. However, the pipe must be heated to a
temperature such that the powder will melt on contact with the pipe and
undergo melt flow thereon. Methods of heating are discussed herein.
Alternatively, the overcoat of functionalized thermoplastic polyolefin is in
another form e.g. a film or tape, which would be wound or otherwise applied
to the pipe. In some circumstances, it might be preferred to apply the
polyolefin in the form of a patch.
The heating of the pipeline may be immediately prior to application of
the functionalized thermoplastic polyolefin or subsequent to that application.
The preferred method may depend on the form of the functionalized
thermoplastic polyolefin.
The polyolefin coating is applied in a thickness of at least 5 mils, e.g.
5-40 mils, especially 10-20 mils, onto the undercoat 12 and the adjacent
existing protective coating 17. It is preferred that the pipe be at a
temperature such that the polyolefin becomes molten, and moreover melt
flows on contact with the undercoat. The temperature required depends on
the particular polyolefin, but does not need to be measured as the
acceptability of the applied topcoat is determined by noting the melt flow of
the polymer. It is understood that excess temperatures that degrade the
topcoat should be avoided.
The functionalized polyolefin may be a polyolefin modified (grafted)
with the functional ethylenically unsaturated carboxylic acid or derivative
thereof, or the polyolefin may be formed from olefin monomers that have
been copolymerized with the functional ethylenically unsaturated carboxylic
acid. In embodiments, the polyolefin may be a thermoplastic polyolefin
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modified with at least one ethylenically unsaturated carboxylic acid, or
derivative thereof, especially an anhydride or ester thereof. Examples of
such functional ethylenically unsaturated carboxylic acids include malefic
acid, malefic anhydride and acrylic acid. Examples of polyolefins that may
be grafted include polyethylene, polypropylene, or propylene with other
monomers e.g. acrylic or methacrylic acid and esters, carbon monoxide and
the like, with which a preferred grafting monomer is malefic anhydride.
Examples of copolymers include ethylene/acrylic acid copolymers (EAA) and
ethylenemethacrylic acid.
FIGS. 4 and 5 shows a longitudinal weld treated in the same manner
as that described for the girth weld of FIGS. 2 and 3. Similar reference
numerals refer to similar parts. Pipe coating 17, which in this case is a tape
wrap, is cleaned away from the longitudinal weld 19 and a two step coating
according to the invention is applied.
Although the plastic material composite may be formed in situ by the
method discussed above, it may also be formed as an additional,
intermediate, step in the process. After the liquid undercoat has been
applied, powdered polyolefin may be applied. This is also carried out at
ambient temperature, and is akin to a dusting procedure. On application of
the powdered polyolefin, the powder becomes occluded in the coating, but
excess powder falls from the coating and does not adhere or otherwise
combine with the undercoat, hence the analogy to a dusting procedure.
While the powdered polyolefin so applied may be different from that of the
undercoat, it is conveniently the same powdered polyolefin as subsequently
applied.
The method of the present invention may be applied for repair of a
pipeline having a wide variety of defects, including separation of the
existing
coating from the pipeline, physical damage to the existing coating and
cathodic disbondment of the existing coating. The method may also be
used for applying an original coating to a section of pipeline.
The method of the invention is a portable, field- applied, coating
process. It is simple and economic in operation, and utilizes relatively
inexpensive and less bulky equipment for application of the coatings. As
such, it is well suited for field application, including in remote areas where
other equipment could not be readily located. Moreover, the process can
frequently be utilized on an operating pipeline i.e. it is frequently
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unnecessary to cease operations of the pipeline in order to utilize the
process of the invention to repair the pipeline.
The method of the present invention provides protection that is
believed to be equivalent to that using a thermospray, as disclosed in U.S.
5 5,792,518, but offers the advantages of not requiring use of a thermospray,
of alternate heating methods and of application of different forms of a
coating.
