EXTERNAL PIPELINE COATING SELECTION FOR NEW AND EXISTING BURIED PIPELINES. Mick D. Brown, Ph. D.

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1 International Pipeline Conference Volume 1 ASME 1996 IPC EXTERNAL PIPELINE COATING SELECTION FOR NEW AND EXISTING BURIED PIPELINES Mick D. Brown, Ph. D. Charter Coating Service Ltd. # 6, Street N.E. Calgary, Alberta, T2E 6P1 Canada Phone: (403) FAX: (403) EXTERNAL PIPELINE COATING SELECTION FOR NEW AND EXISTING BURIED PIPELINES The majority of existing and new pipelines are externally coated. The opportunity to examine buried pipelines has shown that selection of both shop and over-the-ditch field applied coatings has resulted in many failures. Coating selection in 1996 has become more complex because of Lhe abundance of available products. Not only are there many available coating types but there are also competitive products within each category. The safe approach is to select a coating that will perform well under the most severe conditions but this approach can be very costly and often a lesser coating is selected with the realization that it affects the risk of failure. This paper addresses the criteria that need to be considered during coating selection and provides an outline for the decision making process. Examples are used to illustrate the effect of different factors on coating performance. GENERAL CONSIDERATIONS There are specific codes that apply to corrosion mitigating coatings on buried pipelines such as CSA Z In addition to ensuring conformance to these codes there are other preliminary questions that should be addressed before considering specific details of the conditions under which the coating is to perform. These considerations and the effects of the answers to the questions are shown in Table 1. This analysis is crucial to the coating selection process since it targets in on the objectives for the coating and on the risks that may or may not be taken in the coating selection for this particular line. It is important in this analysis to bear in mind the variety of environments that exist on the line. Not only does a pipeline differ between main line, risers, joints, and tie-ins; it also varies in temperature and soil type along the line. Once the coating selection process is complete, this analysis should be revisited in order to identify future requirements for inspection and to ensure that all of the main objectives have been met. TABLE 1 General Considerations CONSIDERATION EFFECT Costs associated with failure environmental and other Higher costs due to failure mean less risk can be taken Importance of coating cost Will limit coating choice Life expectancy Longer life requires better coating Inspection - ease o f or requirement for More or easier inspection means more risk can be taken Is there a history of coating performance on this pipeline Field performance indicates stresses on coating To what standard will coating application be conducted Poor application procedures increase the risk of failure Quality of technical support May improve coating application and performance Copyright 1996 by ASME

2 An example that illustrates the impact of this process was coating selection for a pipeline to be installed in a remote part of Africa. Although keeping project costs low was important, the difficulty of inspection or maintenance resulted in a decision to use a relatively expensive coating with a predicted long life. Again because of the concern with maintenance and inspection, it was decided that an FBE coating would be used rather than a polyolefin based system because of concerns o f potential cathodic shielding by the polyolefin. Coating selection for the African line was also influenced by the high ambient temperature in this area. Normally the epoxy coating selected for the girth welds would have too long a cure time for this application but the high local temperature, typically 45 C/113 F, meant that the coating set up sufficiently quickly. The coating of joint areas o f pipelines illustrates the importance of these "general considerations". Until recently the coating of joint areas has been considered a job to be conducted by relatively unskilled labour with low supervision and little training. Also joint coatings were often chosen based on ease o f installation rather than on performance requirements. Largely as a consequence o f the low priority given to this area of pipeline coaling, the oil and gas industry has seen many pipeline failures at the joints o f their lines while the main body is left uncorroded and intact. High costs associated with these failures is leading to a reprioritization of joint coatings. M ore skill and training is now being required of the contractors applying the coatings and coating selection is being reviewed more carefully. shop than in the field. An example from recent work was the cathodic disbondment and hot water soak testing o f an epoxy modified urethane. This coating was potentially going to be applied both in the field and under shop conditions. Therefore two sets o f samples w ere prepared, one over steel blasted with a silica abrasive as would be used in the field, and one over steel grit blasted steel as would be used in the shop. The sam ples prepared over the silica blasted steel showed superior performance despite the lower profile on the steel. If the coating had only been tested over the grit blasted steel it would not have been selected for this line. Various application conditions are shown in Table 2 together with the potential effect of these conditions on coating performance. It is because o f these effects on coating performance that testing should be conducted on samples prepared under conditions that simulate those anticipated for the line. The review of application conditions helps identify the coatings that need to be considered for a particular job. This results from looking at the requirements o f the coating in respect of substrate preparation, application techniques, temperature, and cure. If these application requirements cannot be m et for this job then the coating is not suitable. TABLE 2 Application Conditions PERFORMANCE REQUIREMENTS Application Conditions An initial step in the coating selection process is to identify the conditions under which the coating will be applied. This is most relevant to the field application o f coatings. The reason to consider this at the outset is that it influences the performance of the coating. Thus all tests that are used to generate performance data on the coalings should be conducted on samples that have been prepared under the anticipated application conditions. This aspect is important in examining existing data from the manufacturer or other sources since the performance of a coating applied under ideal conditions over a blasted substrate may vary considerably from the coating performance when applied to the pipeline. The first question to be answered is whether the coating will be shop or field applied since conditions can be better controlled in the CONDITION Pipe temperature during application; can preheat be used Ambient conditions (temperature and humidity) Surface preparation - cleanliness, blast media, and profile Delay between surface preparation and coating application Existing coating on line Time before coating will be immersed EFFECT Influences flow and cure characteristics of coating Influences cure and possibly moisture on pipe surface Effects adhesion properties of coating Possible flash rusting or other surface contamination Requires that new coating adheres to existing coating Early immersion may slow cure and reduce coating performance

3 TABLE 3 Pipeline Conditions SITUATION POTENTIAL EFFECTS RELATED TESTS SOIL TYPE Hard, angular, heavy, or clumped backfill Damage during construction and operation of the line Impact and shear/soil box Peel adhesion Wet - immersion Moisture permeation of coating Cathodic disbondment (CD) and hot water soak Wet/dry cycling Acidic / basic / microbial activity / dissolved salts / organic matter / chemical gradients Moisture permeation and cathodic shielding Soil movement and air/oxygen access to soil and pipe Coating degradation Corrosivity of soil Cycling in CD and hot water immersion Chemical/microbial resistance O PERATING CO NDITIO NS Cathodic protection (CP) on line Cathodic disbondment of coating. Cathodic shielding of pipe. Cathodic disbondment (CD) Maximum temperature (T C) expected for line Fluctuations in operating temperature of line Will the pipe be bent after coating application Softening and degradation of coating Increased movement of the line and related soil stresses Coating damage Use T C as minimum temperature for CD, shear, and water soak Shear or soil box Peel adhesion Bend/Flexibility CD of strained coating LO C AL A ND S P E C IA L C O NDITIO NS Proximity of anchor points Soil stresses may be lower near anchor points Shear or soil box, peel adhesion Availability of coating and application equipment Cost to bring in equipment and coating Tests should be conducted on realistic coating choices Severe corrosion of pipe to be coated Structural support required (from coating or elsewhere) Hydrostatic Storage of coated pipe prior to installation Coating degradation Accelerated weathering PERFORMANCE TESTS Various laboratory lests are conducted in order to determine expected performance o f coatings in service. The importance of each test varies according to the performance criteria required of the coaling. Most tests have standard methods that are typically used since they give data that can be compared with information from other sources. Some standards have the benefit of pass/fail criteria but these criteria need to be reviewed in light o f the performance requirements for a given line. In some instances conditions on the line are better simulated and performance o f the coating better predicted by using a modification of a standard test. Some commonly used tests are described below. Comments have been made on desired coating performance based on industry standards and consideration o f service criteria for external pipeline coatings. Cathodic Disbondm ent 1CD1 Test The use o f cathodic protection (CP) on buried pipelines has significantly reduced pipeline corrosion but at the same time it creates an environment that is hostile to many coatings. In the cathodic disbondment (CD) test, a holiday is drilled through the coating and a stress potential is applied that simulates the potential in a cathodically protected system. The test m easures the resistance

4 Pipeline Conditions Soil Types. Different soil conditions make different demands on coatings. For instance a soil that is subject to cycling between wet and dry may have more effect on the coating than a constantly wet soil. The effects of variations in soil type are reviewed in Table 3 on the following page. A recent study examined the effect o f rapid backfilling onto a low temperature line. Immediately after application of a tape coating the line was being buried with a cold moist soil. A concern was raised as to whether the tape would have less adhesion as a result of reduced opportunity to cure, since low adhesion might result in damage to the coating as a result of soil stresses. The test was conducted by immersing samples o f pipe 20 minutes after tape application, in 10 C water. After 48 hours immersion the peel adhesion of the candidate coatings was tested and was compared to the peel adhesion of normally cured coating. The two main candidates showed considerable variation with one being unaffected and the other having half the normal peel strength after the cold water immersion. - fluctuations in operating temperature resulting in contraction and expansion of the line - freeze thaw cycles in the soil - shifts in the water table - heavy soils Local and Special Conditions. There are many local factors that should influence coating selection. A few of the more com monly encountered factors are included in Table 3. As an exam ple consider the selection o f a coating for risers on a line. Typically risers are situated near or at anchor points and this means that soil stresses may be low and shear resistance not important. However, one practice is to use "S" shaped risers that act as a main point of give in the line during expansion and contraction. In this situation, soil stresses will be high near the bottom o f the "S" and shear resistance is important. Operating Conditions. When considering the temperature of the line, account should be made as to possible increases in the tem perature of the pipeline contents. The influence of operating temperature on coating performance is important and it is critical that tests to determine coating performance reflect this fact. Bear in mind that pipeline temperatures may vary along the line. See Table 3 for detail on the effects o f different operating conditions. In the field we have seen that polyethylene based joint coatings often move off the joint area during service, exposing that area to the sunounding soil environment and frequently resulting in significant corrosion. Based on this failure mode shear tests were conducted at elevated temperature using the Alyeska test as described in the Aramco specification 09-AMSS-096. This test examines movement of a coating along the axis of the pipe as a result of an applied force. These tests have shown that the adhesive or mastic components of many coatings have flow properties at normal operating temperatures that allow the coating to move easily during line movement. This movement is not predicted by the ring and ball softening point that is typically used for rating the service temperature limits for these coatings and so it is possible for them to have low shear resistance below their rated service temperature. The importance o f shear resistance is particularly high where a buried line is subject to:

5 of the coating to disbondment around a damage site where CP is employed. Variations that are introduced into the test protocol include duration, applied potential, holiday size, and heat. The predictive nature o f the test is improved by using a long duration, e g. 30 and 90 days, and a tem perature that simulates the highest expected service temperature. It is suggested that the applied potential is kept close to but higher than the potential on the line; a frequently used test standard is -1.5V with respect to a C u/c us 04 electrode. One variation that causes difficulty in com paring test data from different sources is the method of heating used for these tests. The ASTM G42 and related standards immerse the samples in heated electrolyte. Other standards such as CAN/CSA Z and 21 allow for the use o f heat applied directly to the sample with the contacting electrolyte only being heated as a result o f it's contact with the hot sample. Heating o f the sample rather than the electrolyte solution generally results in a less severe test but it may be a better representation of the situation in service. Another variation appropriate for many environments would be to include cycling between wet and dry for the samples. This would likely increase the severity o f the test and better simulate field conditions for some pipelines. Evaluation of cathodic disbondment results should be based on measurement o f the disbondm ent around the original hole drilled through the coating. The most typical measurement is radius of disbondment. W hile lower is better, a guideline would be that for a 30 day test at -1.5 V at the upper temperature for the line, the disbondment radius should be less than 10 mm for situations where disbondment is considered a central problem and 20 mm where it is less crucial. In addition to the disbondment measurement samples should be examined for signs of channelling since this is also a cause o f failure. Hot W ater Soak Test This test measures the effect of immersion on the adhesion of the coating. The degradation o f the coating, particularly it's adhesion, in test indicates the expected breakdown in service. The test is typically conducted at a higher temperature than used for the CD lest. Test duration can range from 24 to in excess of 1,000 hours. The 24 hour test as in CAN/CSA Z M 92 is particularly useful for FBE coatings since it can be used in the plant to pick up problem s in surface preparation and in the coating. If, as a result of test, the coating can easily be removed in strips or large pieces, the coating is considered to have a tendency to disbond that make it less suitable for use in damp environments. S h e a r This test examines the resistance o f the coating to movement along the pipe as a result o f soil stresses created by movement o f an underground pipeline. The test is applicable to polyethylene and related systems and it's importance is determined by consideration of soil conditions and pipe movement. This test is a better predictor o f service performance when conducted at the upper range o f the expected temperatures for the line. The Alyeska shear test as described in the Aramco specification 09-AM SS-096, has a pass/fail criteria o f less than 1" movement in 50 hours. To pass this test the coating must have good resistance to shear. For some environments it may be acceptable to use less stringent criteria such as less than 1" movement in 3 hours or 18 hours. Im p a c t The impact test examines the resistance of the coating to damage as a result o f handling and backfilling. It is worth bearing in mind that impact damage can be reduced by increasing care in handling and care and quality o f backfill, however both these measures may increase costs. For w inter construction it is important that this test be conducted at the minimum expected ambient temperatures since this will usually be the more severe test o f the coating. A guide to performance is that a coating that does not fail when impacted at 3 joules/26 in-lb is considered to have acceptable impact resistance for most uses. B e n d This test is particularly relevant to coatings that are being applied to line pipe in the shop that will be bent in the field. The test is a better predictor if it is conducted at temperatures that reflect field conditions. An additional advantage of this test is that it can identify problems in the cure of FBE coatings since these coatings develop improved flexibility once they are fully cured. During recent Aramco qualification tests conducted to 09- SAM SS-091 the bend test revealed adhesion problem s at the steel/primer interface. It was found that o f two prim ers being used under an FBE coating, one resulted in test passes and the other in failures between the prim er and the blasted steel. A coating that will pass a bend test at 3 per pipe diameter length is considered to be flexible and acceptable for m ost situations.

6 Peel The peel test for polyethylene and related systems gives a good measure o f the adhesion o f the coating and also shows the weakest point within the coating system. The point of failure in the peel test is significant since it may show weakness in the coating system. Recent tests resulted in failure at the steel/primer interface in a three layer tape system. This demonstrated the potential for this system to fail in the field in such a way as to expose the pipe to corrosion. The problem was solved by changing to a different prim er that had better adhesion to the steel. These tests were conducted on samples prepared over wire brushed steel. The same system that failed at the steel/primer interface was tested over a blasted substrate and demonstrated much better performance in a series of tests. This emphasises the importance of surface preparation in tests and in practice. Interpreting peel resistance data is complicated by the different standards that are employed which vary in coating width, angle of pull, and rate of peel. As a guide, a coating with moderate peel resistance would be expected to show peel strength higher than 20 N per inch coating width at a 10 mm/minute peel rate. A peel strength greater than 150 N per inch would be a coating with good peel resistance. COATING SELECTION There are likely to be a number of different coatings applied to a pipeline in order to cover variations between the main line, joint areas, tie-ins, and risers. There may also be differences along the line as a result of variation in temperature, terrain, and other factors. The process of coating selection in each case, however, can be made using the same framework as detailed below I Determine overall objectives for the coatings. This should include the degree of risk that can be taken and cost constraints. 6. Analyze the coatings for expected performance based on the performance requirem ents and test data. 7. Determine the costs associated with selected coatings. 8. Ensure that selected coatings and coating application methods m eet the criteria determined in step 1 and that all risks taken are acceptable. If this has not been achieved, repeat the steps above with appropriate changes to application methods and/or candidate coatings. 9. The process described above results in selection o f the coatings that best m eet the overall objectives for the pipeline. 10. Ensure that the coatings are applied in accordance with the standards used in the selection process. SUMMARY The selection of external coatings for pipelines must be compatible with the risks and cost constraints for that pipeline. To achieve this the selection process must consider many different variables including application and pipeline conditions. Differentiation of coaling options can be achieved based on test data and cost comparisons. The test information must be reviewed with careful scrutiny o f it's relevance with respect to test conditions and preparation of the test samples. The importance o f each com ponent of the test data must be considered in light of the performance requirements for each of the coatings used on the pipeline. This paper provides a framework that can be used to lead to coating selections that will most closely meet the cost and performance requirem ents for a given pipeline. 2. Identify application conditions or constraints for the various parts of the line. 3. Determine perform ance requirem ents for the coatings 4 Evaluate the relevance of existing data on prospective coatings. 5. Conduct additional tests on appropriately prepared samples.