 |
 |
One such material that has been successfully qualified for use with high temperature insulation systems is an epoxy-based coating. A variety of methods were used for this determination, which included elevated temperature cathodic disbondment testing, dynamic mechanical analysis (DMA), accelerated heat ageing, and also standard coatings tests specified by the Canadian standard CSA Z245.20.
The results of this investigation support the recommendation that the selected coating should be capable of withstanding operating temperatures of at least 150oC for the expected 30-year life of a typical insulated pipeline.
Insulated pipe service
Insulated pipes for buried service have a long history of use in Canadian oil and Gas developments. In general, due to limitations in coating materials suitable for high temperature service, insulated pipeline temperatures have until recently been limited to operating temperatures less than 110˚C. However, for many oil sands developments, the use of thermal recovery methods has increased the temperature requirements for the bitumen pipelines, and the transportation of the bitumen to the processing facilities now often necessitates the use of insulation coatings capable of withstanding service temperatures as high as 150˚C.
Insulation coating components
The coating materials typically used on insulated pipes for buried service include the following (Figure 1):
Anticorrosion coating
The use of anticorrosion coatings applied to the external diameter of the steel carrier pipe is now commonly performed for most factory applied pipeline insulation systems produced in North America. In the past, foam insulation materials had often been applied directly to bare steel pipes, but due to the potential for corrosion and rupture of the pipelines caused by water ingress through damaged outer jacket or field joint coatings, such practices were mostly abandoned. Fusion bonded epoxy (FBE), a commonly used anti-corrosion coating for pipelines, is typically only rated for use at temperatures of less than 110˚C since there have sometimes been concerns about the long-term performance of these materials at high temperatures. A key property that influences a material usage temperature is glass transition (Tg) temperature. Tg plays a key role for influencing the performance of a polymeric material in a number of ways.
It is typically desirable to have the Tg value of the anticorrosion layer to be equal to or greater than the operating temperature of the pipeline in order to ensure that the coating remains mechanically stable at high temperatures and also retains its maximum corrosion resistance. For use under insulation, resistance to mechanical damage such as impact is secondary to achieving good performance under potentially corrosive/wet condition. In recent years there had been work performed by several coating manufacturers to develop fusion bonded epoxy coatings with higher Tg values and capable of operating at the elevated temperatures being required for many of the insulated pipelines used for offshore/marine service. Much of this research was focused on using the FBE solely as a primer that would allow the specialized insulation/water barrier materials such as solid or foamed polypropylene to be bonded to the steel pipe. Some of the resulting insulation coating systems produced using this concept have subsequently been rated for operating temperatures as high as 155˚C.
Recent developments in the formulation of the FBE coatings have now resulted in materials with higher Tg’s that not only have improved flexibility at low temperatures, but also provide better resistance to direct exposure to water or other corrosive conditions. However, the performance at high temperatures was still not well understood for these coatings. Thus, for the purpose of evaluating the suitability of the new FBE materials for use with insulated pipelines operating at temperatures of up to 150˚C, a number of different methodologies were used. These included:
Procedures
For this study, test pieces and other specimens of a candidate FBE coating were produced in the following configurations:
FBE and insulation coated pipes
These samples were produced in a commercial coating factory by first applying the FBE onto abrasive blasted and preheated 114.3mm (4.5-in.) OD steel pipes at a nominal thickness of 400mm (15mils). The pipes were subsequently insulated with approximately 50mm of heat resistant spray-applied polyurethane foam and then extrusion coated with a high-density polyethylene jacket, which is used to provide mechanical and environmental protection. All coating application processes were performed using standard production techniques.
Laboratory coated panels
Test panels of mild steel were cleaned to remove oil and grease and then abrasive blasted using G-25 steel grit. The panels were then preheated in a laboratory oven prior to application of the epoxy powder coating at a nominal thickness of 400mm using electrostatic powder spray equipment. Each coated panel was then returned to the oven for several minutes to complete the curing reaction before being finally quenched in water.
Free film material samples
Specimens of the epoxy coating not attached to a substrate were prepared by applying the FBE onto a preheated non-stick steel pan and then curing and water quenching the material in a manner similar to that used for the laboratory coated panels. once quenched, the free film sheet was easily removed from the pan.
Glass transition temperature
A dynamic mechanical analysis (DMA) instrument was used to measure Tg. This device applied an oscillating strain to the specimen while it was exposed to temperature ramping conditions and then measured the physical response of the material in relation to the temperature. A 25mm long x 12.5mm wide x 0.8mm thick test specimen was prepared from a flat sheet of free-film material. A ramping rate of 2˚C/minute was used from 25˚C to a final temperature of 200˚C and the sample was tested in the torsion rectangular position in an air atmosphere at a frequency of 1 rad/s. The Tg value was determined from the peak tan-delta (tan d) value, which is the ratio of the loss modulus to the storage modulus for the material. For comparison purposes and also for testing FBE samples collected from the pipes subjected to thermal ageing, Tg determinations were also made using a differential scanning calorimeter (DSC) instrument.
Qualification of unaged FBE coating
In 2006, a new edition of Canadian Standards Association document, CSA Z245.20 “External Fusion Bond Epoxy Coatings for Steel Pipe” was issued which included within its scope FBE pipeline coatings for higher service temperature. These types of coatings are defined in the standard as “single-layer FBE with a glass transition temperature greater than 110˚C”.3 Therefore, prior to conducting the CCOT ageing, the coating was first qualified in the new (unaged) condition in accordance to this standard to ensure it met these base requirements. Such testing is required by to comply with section 9.2.7 of CSA Z662 “oil and Gas Pipeline Systems” which is used as the main reference document for the material and installation requirements for all such pipelines in Canada. The qualification testing of the FBE coating was conducted on both laboratory coated plates as well as factory-coated pipes and based on the requirements outlined in the CSA Z245.20 standard. The coating thickness was a nominal 400mm (15mils). The FBE coating was found to meet all of the requirements of the CSA Z245.20 standard for the system 1B classification.
Cathodic disbondment testing
Cathodic disbondment (CD) testing of the FBE at 150˚C was used to evaluate the resistance of the coating to loss of adhesion under temperature conditions similar to those being considered for operation of the high temperature pipelines. Although for most buried-service insulated pipes the external surface of the steel carrier pipe is unlikely to come into contact with large amounts of water, cathodic protection is still used in many cases as a contingency measure.
Test panels consisting of a nominal 400mm thick layer of the FBE coating applied to steel were used for this study. The panels were tested for durations of 28 and 56 days using a potential of –1.5V with the steel temperature of the coated panels being maintained at 150˚C. After the 28 days of testing at 150˚C, the average disbondment radius values for the three specimens ranged from 5-7mm, while for panels exposed for 56 days they ranged from 10-15mm. There is no direct reference to this particular equipment setup or testing temperature within the CSA Z245.20 standard, however the results for both exposure periods were well within the maximum 20mm disbondment radius permitted for 28-day testing at 95˚C.
Calculated continuous operating temperature
Accelerated heat ageing of insulated pipes was conducted based on the procedures outlined in the European District Heating Insulated Pipe standard EN 253. This document specifies a method referred to as Calculated Continuous Operating Temperature (CCOT) used for predicting the long term performance of the insulation coating based on shorter term testing using internal pipe temperatures much higher than the actual intended service conditions. This procedure relies on using the Arrhenius equation for calculating the activation energy of a material by testing for chemical reactions occurring in relation to temperature. The activation energy is defined as the minimum amount of energy required to initiate a chemical change in a material, and determining this value allows one to predict the effects that heat and time will have on materials like the insulation or FBE coating. The Arrhenius equation can be expressed as:
K = Ae –E / RT (1)
Where
K= Rate constant
A = Pre exponential rate factor
e = Natural exponent
E = Activation energy
R= Gas constant
T= Temperature in K
This relationship may in turn be used to allow for predicting the time to failure as follows:
Tf = Ae [ΔH/KT] (2)
Where
Tf = Time to failure
A = Scaling/pre exponential rate factor
e = Natural exponent
ΔH = Activation energy
k = Boltzmann’s gas constant of 8.617 x 10-5 eV/˚K (1.380,658 x 10-23 J/˚K)
T = Temperature in K.
Determining the thermal life of the insulated pipes was based on selecting a critical property, in this case the shear strength of the bond between the insulation to the FBE coated pipes, and testing for this value on several pipes exposed to different ageing temperatures that accelerated reduction in this property over time. By using substantially higher ageing temperatures, this allowed to greatly reduce the total testing time required. Based on similar previous work, the temperatures selected for this study were: 180˚, 185˚, 188˚ and 193˚C. Four insulated pipes, each approximately 6m long, and all produced under similar conditions in a pipe coating factory were used, with one pipe being tested per ageing temperature.
After the initial inspection and testing of the new (unaged) properties of the coating system, the pipes were each assembled into individually controlled flow loops that used pressurized hot water to raise the internal temperature of the pipes to the specified values. The pipes were each maintained at the required ageing temperature for varying periods of time at which point the internal water temperature was then temporarily lowered to 140˚C and a shear adhesion test was conducted on the coating system. This was performed by cutting into the entire coating to isolate a 100mm (4-in.) wide ring section of the insulation exposed down to the surface of the pipe, then attaching a mechanical clamp to the test section and applying a tangential force using a controlled loading system equipped with load sensors. The force required to shear or break the bond of the insulation coating to the epoxy coated pipe was recorded and the shear stress calculated. once each test was completed, the pipe was reheated back to the original set point and the ageing continued until the next required testing interval.
The life for each accelerated ageing temperature was determined based on the time at which the floating mean shear strength value of the insulation coating first crossed the minimum specified acceptance value of 0.13Mpa (18.9psi) listed in the EN 253 standard. A plot was then constructed of the natural logarithm of the ageing time required to reach the acceptance value versus the reciprocal of the ageing temperature in units of K. By extrapolating from the ageing data points plotted on this graph, a prediction of the maximum continuous operating temperature was obtained based on the commonly specified service life of 30 years.
Testing of the insulated pipes determined that the coating system should be able to withstand 150˚C for 30 years of service life as shown in Figure 2. It should be noted that the foam insulation has considerably lower shear strength than the epoxy coating and so all failures typically occurred within the foam itself, as it would typically fracture cohesively, leaving residual material bonded to the epoxy coating.
Evaluation of aged FBE coating
Since the CCOT testing on the insulation system required that the pipes remain intact for the entire ageing period, it was not possible to perform testing for corrosion resistance on these same samples at periodic intervals. Instead, it was decided to complete all the CCOT ageing and then section the pipes for the purpose of evaluating the coating in accordance to the procedures for cathodic disbondment and hot water soak (HWS) adhesion outlined in Tables 2 and 4 of CSA Z245.20. Tests for flexibility and impact resistance of the aged coating were deemed unnecessary as these are of primary importance for installation of new pipes rather than for assessing in-service performance. Although the testing listed in the CSA standard is only required for newly produced and unaged coatings, it was decided that still meeting the requirements for corrosion resistance after the ageing period would demonstrate that the FBE could retain its effectiveness in protecting the pipe against any possible corrosion.
Specimens were cut from each of the four ageing temperature pipes and the bonded foam insulation was removed from the surface of the FBE coating by carefully scraping with a sharp tool, avoiding damage to the coating. The coating specimens were then tested for:
The Tg values of the coating for each ageing condition were measured using a differential scanning calorimeter (DSC) instrument, as even though it might not posses the accuracy of the DMA, it was better suited for testing the small coating chips removed from the pipe sections and could thus still give a relative comparison for any changes in the Tg value.
Conclusion
From the testing of the aged coatings it was determined that all of the specimens met the cathodic disbondment and hot water adhesion requirements of both Tables 2 and 4 of CSA Z245.20 for new condition high temperature FBE coatings. The average disbondment radii for the cathodic disbondment specimens ranged from 2.5-3.5mm, while the hot water immersed specimens had ratings of 1. Both tests met the maximum CSA acceptance criteria of 6.5mm disbondment radius for the CD testing as well as a maximum number 3 rating allowed for the HWS evaluation (Figure 6). The glass transition temperature of the coating measured by the DSC method was determined to increase from an unaged value of approximately 143˚C, to nearly 155˚C after the CCOT ageing.
Glass transition (Tg) temperature
The evaluation for the Tg of the unaged epoxy using a DMA instrument showed that the material possessed a Tg of approximately 150˚C based on the peak of the tan delta plot. This result was approximately 7-10°C higher than for what was obtained by the less sensitive DSC method, however differences in sample preparation and thermal history could play a role in affecting the results. From this testing, the coating should be capable of service temperatures as high as 150˚C, however excursions beyond this value do not appear to be detrimental based on the CCOT ageing study conducted at much higher temperatures.
Qualification of unaged FBE coating
The initial qualification testing of the new condition FBE in accordance to CSA Z245.20 showed that the FBE met all the requirements for system 1B. This coating is therefore suitable for stand-alone service which is likely more severe compared to the relatively protected conditions when used under foam insulation where there should be much less chance of exposure to water and other corrosive conditions.
Cathodic disbondment testing
Cathodic disbondment testing with a substrate temperature of 150˚C for 28 and 56 days indicated that the coating had good performance for these severe test conditions. Even after 56 days of testing, the average disbondment radii of 10, 14, and 15mm on each of the three specimens were well within the maximum allowed value of 20mm for the 28 day testing at 95˚C and –1.5V based on CSA Z245.20.
CCOT of insulation system
From the shear strength testing of the four pipes subjected to accelerated heat ageing, it was determined that the insulation system comprised of the FBE coating and polyurethane foam could be rated for an operating temperature of at least 150˚C for 30 years of service. Even though the heat ageing was conducted at temperatures well above the glass transition temperature of the FBE, there did not appear to be any detrimental effects on this coating as all the shear fractures occurred within the foam material itself and not in the FBE.
Evaluation of aged FBE coating
Testing the FBE coating from the CCOT aged pipes for 24-hour cathodic disbondment and hot water soak adhesion in accordance with the CSA Z245.20 standard demonstrated that the coating retained very good resistance to these conditions even after being aged well in excess of the maximum life for the foam insulation. Both the CD and HWS results for the aged epoxy were within the maximum acceptance criteria rating of 6.5mm for the CD testing and a maximum rating of 3 for the HWS evaluation according to the CSA Z245 standard.
Summary
An FBE coating material with a glass transition temperature of approximately 150˚C that complies with the requirements of CSA Z245.20 for high temperature service (system 1B) has been identified and qualified for a continuous service life of at least 30 years at 150˚C when used as part of an insulation coating system. Additional cathodic disbondment and hot water adhesion testing of the accelerated heat aged FBE coating taken from insulated test pipes has further demonstrated that the material can retain its effectiveness as a corrosion barrier even after exposure to combined conditions of temperature and time which exceed the equivalent service life of the insulation foam itself.
Acknowledgments
The authors wish to thank Research Technician Janet Zhou at the ShawCor Technology and Development Laboratory (Calgary, Canada) and also the additional researchers at ShawCor CR&D (Toronto, Canada) who contributed to this project. Special thanks to Winston Chand at CR&D for his valuable assistance with the TGA and DMA studies. Based on a paper at the NACE CORROSION 2008 Conference & Expo, held in New Orleans, Louisiana, March 16-20, 2008.
|
Loading...
|
|
Copyright© 2010 Hart Energy Publishing, LP 1616 S. Voss, Suite 1000, Houston, Texas 77057-2627 Tel: (713) 260-6400; Fax: (713) 840-8585 |
|
|
Visit Our Other Sites:
Hart Energy |
Oil and Gas Investor |
E&P |
PipeLine and Gas Technology |
Unconventional Gas Center |
A&D Center |
IFQC |
World Fuels |
ISEE |
Hart Conferences |
BioFuels |
FUEL |
WRFS
All rights reserved. Reproduction in whole or in part, in any form or medium without express written permission is prohibited.
|
||
