Digitalized Quality Assurance In Line Pipe Coating
Abstract: In the oil & gas pipeline industries, pipeline coating is always been concerned in every single project because of its importance against the different types of corrosion. During the last few decades, many international standards, the specification had been developed, like ISO 21809 series, CSA 245.20-22, EN30670, EN12068, Shell specs, TOTAL specs etc, these specs give a very clear picture of the technical requirement on the coating material, coating performance and the requirement of the quality control, procedure, Inspection Test Plan (ITP), nevertheless, the coating is still failed during the pipeline operation. With 20yrs plus experience on pipeline coating, this article gives some expertise on the quality control and quality assurance on the factory plant coating in a digitalized way.
1. General
The pipeline coating industry is a service provider and applicator, every single coating plant does not produce anything during the operation, the activities of the application is surface preparation and heating process generated by the gas/electric during the different stages.
Every coating plant has its operation process and coating line adjustment w.r.t different pipe size. Adjust the heating process and line speed to gain the necessary heat profile on the pipe surface is the primary requirements of the coating materials application. Therefore, the coating line adjustment is the most important parameter for the quality application of the coating materials, it raises some questions such as How to heat, the capacity of the heat? What shall be the meaning of heat quality requirements? The answer to these questions and the ability to implement on the coating plant is the key to having a high-quality line pipe coating. Inspectors need to derive an inspection criterion which can ensure the high-quality coating application.
2. Digitalized Data Base Inspection (DBI)
1.1 Materials Testing
Pipeline coating considered as a first barrier against the corrosion. Usually, design engineers consider coating service life as similar to project life. Therefore, it's necessary to design the coating material specifications very carefully, considering the pipeline operating conditions, environmental conditions, wet/dry cycles of soil, seismic activity, application parameters and so on.
Pipelines coating materials design has been diversified from Coal Tar Enamel, Cold Applied Tape, Mastic Adhesive Type Tape to 3LPE/3LPP. Modern-day pipelines are being coated with the 3LPE/3LPP to protect against the corrosion. Considering the HSE and environmental effect of pipelines, 3LPE/3LPP is considered a good solution to satisfy the technical requirements.
Usually, the main materials of the 3LPE/3LPP line pipe coating consist of Fusion Bond Epoxy (FBE), Adhesive (modified polymer treated by chemical) and the top coating (polyethylene or polypropylene). ISO 21809 and other codes & standards provide a clear detailed requirement on the properties of each material. There is no doubt that most of the materials supplied to the coating plant can meet the specification and standard, but these materials are coming from different manufacturers and each manufacturer has different types of materials with different formulations.
All the testing needs to be done as per international codes & standards. The test results shall include the graphs, photo of the test performed and obtained results with some clear remarks about the material behaviour, only writing compliance shall not be acceptable. If different materials supplied to the coating plant, meet the codes & standards requirements, then the formulation difference between each supplier arise some questions about the evaluation criteria for quality consistency of the materials. Many of the materials worldwide are selected based on the compliance of the specification and low-price bidding, that's why if the specification is weak, suppliers may change the formulation to save the cost. Some of the cost-saving tactics implemented by suppliers are:
Fusion Bond Epoxy
- Saving the cost from the raw material, such as the amount/quality of filler. Many different non-organic fillers can be used as the filler in FBE. The international standards do not specify the requirement on the filler or the percentage of the addition of fillers to formulate the FBE.
- Cost-saving from the production procedure such as FBE grinding process. This process indirectly implements effect on FBE performance. It affects the cost of the efficiency of the equipment and production of FBE. The international codes & standards mentioned about the particle size but it does not give the requirement of particle size distribution.
Fig 1. Particle Size Test as per ISO 21809-2
Fig 1 Particle size test from the above picture shows that after sieving (filtration), the FBE fines were adhered to the wall of the sieve because of their high surface energy. These fines were small than 250µm and easy to aggregate. So, the fines left on the sieve. The test result is not cleared but it complies with the specification requirement.
Laser diffraction method as per ISO 13320:2009 was used to further verify the test result, which shows in fig 2. the particle size distribution analysis
Fig 2. Particle Size Analysis by Laser Diffraction Method as per ISO 13320:2009
These extra testing out of the traditional specification needs to be done to verify the materials quality consistency. These testing provide the preliminary data which need to be collected and utilized by the inspector as one of the QC/QA tools to perform random quality checks on the batch of the materials delivered to coating plant
Adhesive (grafted modified)
- To have a high peel strength requires to have a high crystallized adhesive, it requires a high capacity extruder. When a low crystallized adhesive is selected, it will lower the cost of the material (adhesive) as well as the cost of power (electricity).
- The recent specification ignores the crystallinity of the adhesive and also ignore the grafting modified level of the adhesive, it opens the door to have a temporary bonding to meet the peel strength, but it will be disbonded during the operation of the pipeline.
Fig 3. DSC Scan of Adhesive
Crystallinity level of adhesive is very important for long-term bonding. It is critical to understand the correlation between the thermal behaviour of an adhesive and its bonding properties. To understand the crystallization kinetics of adhesive, need to perform DSC analysis on the adhesive. DSC analysis will give the relationship between crystallization rate and crystallization temperature, the data collected from the scan (graph) can be utilized during the application of the coating to gain long term bonding. But it needs a skilled inspector to interpret the data to use as a QA/QC tool.
Polyethylene / Polypropylene
- PE/PP recycle material utilized for saving cost. It’s difficult to evaluate the material from visual inspection regarding the quality of materials. Testing like Strain Hardening Modulus which provide evidence of correlation with long term Stress Cracking Testing Method such as ESCR, FNCT, NPT, PENT, CREEP and so on can be applied as QA/QC tools.
Fig 4. Strain Hardening Modulus Graph vs Recycled Material Content Increment in Sample
The above picture clearly stated that once the recycle material increased in the material, strain hardening modulus will decrease. Therefore, as a QA/QC measure, it advised setting a limit on strain hardening modulus requirement in the specification. Strain hardening Modulus followed by the DSC Scan and other testing criteria will verify the material consistency.
Fig 5. DSC Scan of HDPE
Combining the test results including additional testing will be collected as a data model which can be analyzed with the available equipment to verify the material’s performance. Equipment and its capacity need to be evaluated to draft the critical point matrix for QA/QC.
1.2 Equipment and it's capacity
Coating Line parameters at the plant need to be adjusted based on the heating, speed, time and cooling. The distance from one station to another concerning the heat loss and temperature at the surface of the steel pipe at the time of FBE and Adhesive application is critical for the quality application. Following parameters need to be considered before the application of coating:
· The distance between FBE Gun and Adhesive Extrusion Point
· Speed of line pipe movement to the Pipe O.D.
· Area covered in Sq. meters to one rotation of pipe with different O.D.
· Angles adjustment of FBE Gun to the pipe
These parameters analysis will contribute to understanding the Equipment capacity and its compliance with the properties of the material for quality application
Heat Profile
Heat profile shall be drawn by using a sensor which can draft a graph between temperature, time and position on the steel surface.
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Fig 6. Temperature Heating Distribution |
Fig 7. Temperature Cooling Distribution |
Heat Profile results shall be evaluated and compared with the material test results to analyze the coating application. Visual inspection can be done without data analysis, but it will not be enough to understand the material’s application for long term performance. Therefore, material test results (including consistency testing results) shall be utilized as an important tool for bonding condition analysis. Bonding condition tool requires a comparison of materials with temperature requirement on the steel surface at the time of application. For instance, FBE Gel time and Cure rate need to be considered to achieve the temperature on the steel surface when it will reach to the FBE station, similarly adhesive and PE/PP application require certain temperature on the surface to get a good bond.
Surface Preparation
Surface preparation is one of the important requirements for the long term FBE performance. FBE considered a major barrier against the Corrosion. FBE application dependency on surface preparation can be considered in the same way as its dependency on the heating profile.
· Surface Profile
Most of the project specifies the surface preparation as Sa 2.5 with some surface profile requirements between 50 – 100 microns. Surface profile requirements are good enough for FBE application, but how to make sure that surface profile has been achieved. Testex tape has been utilized to get the surface profile, but it’s a manual method and its limitation is related to only one point evaluation at a time. Digitalized devices shall be utilized to get the surface profile graph of different sections of pipes to get the mean value of surface profile roughness.
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Fig 8. Traditional Surface Profile Measurement |
Fig 9. Digital Surface Profile Measurement |
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Fig 10. Surface Profile Roughness Test Report from Digitalized device
· Salt Contamination and Dust Level
Salt contamination and dust level need to be recorded as per ITP requirements. Some modern Salt Contamination measuring devices have been developed to ease the work of inspectors. These modernized digital devices shall be utilized to speed up the inspection process. Because it's time-consuming to evaluate the salt contamination in a long area of 12 m per pipe with traditional devices and method.
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Figure 11 Traditional SCM meter for salt contamination |
Figure 12 Digitalized Salt Contamination evaluation meter |
Surface preparation has its effect on the long-term performance of the coating. But this article major focus on digitalization quality control plan for the inspectors, so surface preparation effect has not been discussed in details.
3. The inspection required during the application of Line Pipe Coating
The inspector responsibilities can be described in the following points:
· Test Report Review and create application inspection matrix from the data obtained from the testing results
· Execute short term testing such as DSC Scan, FTIR Scan, Strain Hardening Modulus, Equivalent Epoxide Content and other related quality control testing method to confirm the consistency of materials supplied at each batch
· Record the coating line equipment and pipe movement parameters
· Analyze the coating line parameters against the material testing results to ensure the coating material application quality
· FBE, Adhesive and HDPE application need to be examined and record for the pipe movement and rotation.
· Adjust the pipe movement to achieve the required temperature at each station to get a quality application.
· Record parameters of surface preparation, temperature, time and application
· Mark the pipe as required by the client.
4. Conclusion
In General, the most application of line pipe coating is based on temperature and time, the curve of temperature changes on the steel and coating layer shall consider as a vital understanding for the application. The top 4 key factors to have a high-quality line pipe coating:
1. The FBE shall have enough gel time that allows the melted FBE to be “wet-out” on the profile of the anchor pattern.
2. The adhesive shall be applied on the FBE in “Gelling” period.
3. The adhesive shall be fused with topcoat (PE or PP) when it is allowed to fuse.
4. When the pipe reaches to the quenching station, the FBE shall be cured already and the adhesive shall be fused with topcoat already.
In a specific project, when the pipe parameter is fixed (wall thickness and pipe diameter), heat profile evaluation will be convenient to evaluate and record, regarding the heat required to increase every degree centigrade on the steel, and it will also allow us to investigate the heat loss time frame on steel. After proper aligning/understanding of above 4 factors during the application of line pipe coating, then parameters of application will only be dependent on the time frame of heat consuming on steel after heating up at the induction station of the coating line. The following figure will give insight for the inspectors to understand the temperature requirements at each different station.
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Fig 13 The application of coating materials with time
Based on the above 4 key factors to have a high-quality and consistent quality of line pipe coating, when these parameters can be determined and meet the following requirement:
1. T1 shall not cause the fresh dust on the steel surface;
2. The Gel time of FBE at T2 shall be longer than S2;
3. T3 and T4 shall be above the fusion bond temperature between Adhesive and Topcoat;
4. The total time consumption (S2+S3+S4) shall be less than the FBE cure time at T5.
The above information gives a clear picture of how to have a digitalized quality control in the coating plant. This information can be collected by the temperature sensor as recorded by the computer with each pipe identification.
