Philip Medlicott was
project manager for the Oil States Industries (UK) Ltd (OSIUK) Joint
Industry Program that was set-up to develop a methodology for
qualifying fibre reinforced plastic (FRP) lined tubing and flowlines
for high temperature production service. At the time, the
availability of industry wide engineering documentation for downhole
and flowline applications of composites was generally limited to API Specifications that
cover pressure rating axial loading. There was no similar level of
documentation to cover issues such as chemical compatibility, the effects of
rapid decompression and wear resistance.
The JIP addressed the following performance issues
- Chemical compatibility and temperature
- Bending during installation
- Thermal transients during
- External collapse and rapid decompression
- Abrasion from wirelining tools and pigging
- Corrosion rate behind the liner when
the liner is damaged
- Galvanic corrosion and interface with CRA
- Wax and scale deposition and removal
Much of the work concentrated on identifying the application limits of
FRP liner systems and a range of materials were subjected to 160°C at 5000
psi in a production fluid environment over several thousand hours. Other
issues addressed included rapid decompression, abrasion from wirelining
tools and pigging, the interface with CRA materials and wax and scale
The work, which was sponsored by BP, Advantica Technology Limited
(Formerly BG Technology), Kerr McGee, Oil States Industries (UK)
Ltd, Petrobras, Saudi Aramco and Shell Exploration and Production,
was focused on the use of FRP liner systems for flowlines and
downhole tubing. However, the results of the
work are applicable to many other applications where composite and
polymeric materials are exposed to production service environments.
These include drillpipe, coiled tubing, spoolable flowlines,
umbilicals and separator vessels.
Description of FRP Liner Systems
The FRP liner system comprises a thin wall fibre reinforced resin tube, which is inserted inside the steel tubular. The small annular gap is then filled with a grout material to provide the means to transfer the pressure loads directly to the surrounding steel pipe. The grout material conventionally used is cementitious
but the system developed by OSIUK uses a polymer grout. The polymer grout provides a secondary level of corrosion protection and is considered to be suitable for the high levels of bending applied during pipeline installation and in extended reach wells.
The liner, which has no load bearing function, is about 2 mm thick and is usually made from filament wound E-glass fibre in an epoxy resin matrix. However other reinforcement fibres are possible, in particular carbon fibre because of its enhanced resistance to chemical attack compared to glass. The matrix material may also be made from either a thermoset or thermoplastic resin. Epoxy has been the preferred choice of thermoset resin, but other thermoset resins, e.g. phenolic, may provide enhanced benefits, to meet particular corrosion or high temperature requirements. Continuity of the corrosion barrier across the coupling is usually provided by
an elastomeric corrosion barrier ring.
A primary objective of the JIP was to determine the best means of quantifying
the chemical compatibility of the liner in the production environment. In
addition to the hydrocarbon producition fluid the other chemical environments of
interest were identified to be:
- Strong acids used in acidizing treatments
- Strong alkalis used to dissolve scale
- High concentration of aromatics used to disperse wax
- 100% methanol used to remove hydrates at flowline start up after a
- Strong oxidising agents used in biocides
The production fluid was based on a ‘standard hydrocarbon, aqueous and
gas mixture’ comprising of three or four chemical constituents. The
hydrocarbon components (45% of total) included 51.6% heptane, 33%
cyclohexane, 15.4% toluene + 30 ppm corrosion inhibitor. The 10% gas phase
comprised 10% CO2 and 90% CH4. The remaining 45% was brine.
H2S sour service was not addressed.
Eight candidate pipe materials were tested, four with glass fibres and four with carbon fibres. The resin system was either epoxy or phenolic.
In all samples a veil was provided in the inner layer of the pipes. For the glass fibre pipes, a C-glass veil was used.
A set ofscreening tests were carried out that showed that different
methods could be successfully used to quantify the extent of degradation in the
different materials (see right). The results after exposure can be summarised as follows:
- The carbon reinforced materials performed better than the equivalent glass reinforced materials.
- Both the glass reinforced epoxy and phenolic materials are substantially degraded in the 160°C production environment. The glass phenolic is significantly degraded in the 120°C production environment.
- Phenolic materials performed surprisingly well in the alkaline scale dissolver. However phenolic materials may be adversely affected by long term exposure to strong chlorine based oxidising agents for any prolonged period.
- The epoxy and phenolic resins both have their strengths and weaknesses and the selection of material will depend on the particular operating circumstances.
- The testing demonstrated the value of carrying out several different types of tests to gain a proper understanding of the possible degradation mechanisms.
System and long term testing
Long term exposures of system components was carried out which included the grout and corrosion barrier ring.
The FRP material was supplied in two forms:
- Free-standing (double sided exposure), 86 mm ID tube specimens of 7 materials
(including a vinyl ester glass sample)
- Composite tube (single sided exposure), 150 mm ID FRP tubes bonded into standard 7” 29lb/ft C steel tubulars.
After assembly the tubulars were subject to temperature, pressure, tension and bending loads to simulate installation.
In each test, two of the sections were manufactured using a phenolic
grout to provide the high temperature performance. One of the sections was
lined with the glass epoxy liner, the other with the carbon epoxy liner. In
the early tests two additional 2 m long sections had a glass epoxy liner but
manufactured using a cementitious grout. An example of the end connection
detail is shown to the right.
In addition, a sequence of exposures to typical down hole chemicals was included to investigate for possible synergistic interactions.
The exposures were carried out on a set of 86 mm diameter samples for comparison with specimens exposed for a similar length of time to brine and hydrocarbons
but with no exposure to the chemicals.
Further details of the chemicals are given below:
- Chem 1: hypochlorite biocide added to brine at 250 ppm chlorine concentration
- Chem 2: 15%HCl/10%methanol mixture, with inhibitor added to the acidizing treatment at 2%
- Chem 3: Scale inhibitor added to brine at 10% by volume
- Chem 4: Scale dissolver at 50% concentration in fresh water.
- Chem 5: Biocide in brine at 250 ppm
- Chem 6: 9% formic acid solution
On completion of the final exposure, the rig was allowed to rapidly decompress while still at elevated temperature over about 10 minutes.
The glass epoxy materials exhibited severe blistering resulting in exposed fibres. One of the glass epoxy liners also exhibited severe collapse damage indicative of buckling after the first production fluid exposure.
This suggests there may be a limit of axial swelling that can be tolerated.
In contrast, none of the carbon reinforced epoxy or phenolic materials show any consistent deterioration in mechanical properties with time,
although the visual observations indicate that a degree of surface degradation does take place with time. These materials showed no evidence
which would indicate that exposure to the chemical environments contributed
to a further degradation in performance.
The purpose of this task was to determine whether significant galvanic corrosion may occur between a FRP lined carbon steel tubular and a coupling(s) or accessories made using CRA materials. It also considered the possible effect
that the electrically conductive carbon fibre reinforcement in the liner may
have on galvanic corrosion.
The test set-up comprised a composite liner applied to the external surface of a steel tube.
To simulate in-service damage, the FRP liner had a 6mm dia. hole
drilled in it to allow direct contact of the carbon steel with the production
environment. The galvanic cell was set up inside the FRP tubing assembly to avoid possible premature failure of the tubing assembly if the CRA component, e.g. coupling, was applied in the conventional manner.
The results of testing showed that:
- Galvanically enhanced corrosion had not occurred with the material couples and configuration used in these tests.
- The FRP successfully protected the C steel surface for the duration of the exposure.
The evidence of a reducing corrosivity with exposure time shows that
the duplex steel CRA did not significantly affect a carbon steel free
surface only 75 mm from its boundary. The lack of corrosion product on
the sealing face of the PTFE insert suggests the successful protection
of the carbon steel from the test environment. This is also true of the
glass epoxy FRP liner, despite the poor performance of this material in
the mechanical test evaluations of the degradation
Scale deposition/removal and wax deposition on FRP
The experimental programme involved studying the deposition of calcium carbonate (CaCO3), barium sulphate (BaSO4) and wax from supersaturated solutions
onto the surface of types types of fibre reinforced polymers and a stainless steel (UNS S31603). The extent of scale and wax deposited after a given period
was compared for the three different surfaces. In addition, the tenacity of the scale once deposited
was assessed through tests involving an impinging jet of liquid directed at varying velocities at the surface of the samples.
It was shown that the extent of scale is invariably smaller on FRP than on stainless steel and very little difference between the
glass epoxy (amine harderner) and glass phenolic could be determined. The scale
formed on the stainless steel was found to be more difficult to remove than that
formed on the FRP. It can be concluded from these results that it is unlikely that use of FRP liners would promote severe problems due to scaling where SS is already used.
The ease of removal of scale from FRP would be advantageous.
A large difference in the extent of wax deposition between the two types
of FRP was observed with the glass phenolic material showing more deposition than on the stainless steel under identical conditions.
This is opposite to what is often assumed as FRP will undoubtedly be
insulating and will reduce the temperature driving force which is thought to
be the key point associated with wax deposition. The glass phenolic exhibited just slightly more deposition than the stainless
steel. In some final tests the deposition on an IPD cured FRP material was
assessed and it was shown to be comparable with the results of the glass
Testing was carried out to quantify the abrasion resistance of glass epoxy,
carbon epoxy and carbon silica modified phenolic materials from wirelining at conditions typical of service environments at elevated temperature.
Testing was carried out on both virgin and material that had been conditioned in
high temperature production environment and at three temperatures, 25°C, 100°C and 160°C.
However not all materials were tested at all these conditions.
The test rig subjected the inside surface of the 25x20mm rectangular liner test samples to a reciprocating wear action by the 3.175mm diameter (125) 435Hv slick line steel wire.
The total contact force was 80 N equivalent to 4 N/mm and the mean velocity was about 0.5 m/s (to give a peak of about 0.7 m/s).
This compares with typical wireline speeds of 100 ft/min (0.51 m/s).
The virgin carbon epoxy material was subject to 3 sets of wear tests at 25°C. These corresponded to wireline distances of 1,000 m, 5,000 m and 10,000 m.
From this an indication of the wear characteristics was made. Testing of the remaining test samples was carried out using the 1,000 m test duration.
The results are summarised as follows:
- The carbon-epoxy showed little effect of conditioning on the wear resistance.
- The carbon phenolic material showed a significant reduction of wear resistance due to conditioning but still performed better than carbon epoxy FRP.
- The wear behaviour measured suggests initial losses are dominant and are associated with the resin characteristics
The superior wear performance of the carbon phenolic at the 160°C elevated temperature was expected but it is surprising that carbon phenolic also proved more robust at ambient temperature. Past jet impingement tests of glass phenolic have generally shown phenolic to be much more brittle and to have high erosion rates compared to glass epoxy.
On the issue of the severity of the test the most pertinent factor relevant to the short time scale of these tests is likely to be the viscosity and adsorption behaviour onto the surfaces and the effect this would have on wear of the resin. The lubrication properties of a crude oil would be anticipated to be superior to that provided by the test fluid hydrocarbon. The test could therefore contain a second level of conservatism in the result obtained; the first being the possible effect of distance over which the results were obtained.