Engineering and selecting the most adequate and appropriate pipeline coating depends upon a multitude of factors. Careful analysis of several critical factors must be considered, such as the pipe diameter, the service pressures or operating temperatures of the transport materials, the soil conditions if installing a landline, construction techniques for installation, cathodic protection (CP), and whether impressed current CP or sacrificial CP will be used. These are all factors that will determine the material selection for pipeline protection; and it is important to understand that the critical field joint zone and girth weld will be subjected to precisely the same factors, stresses, and service conditions.
What is a Field Joint?
The area where two pipe spools or pipe joints are welded together is known as the field joint. This is a significant area because the pipe is welded here and its surface is uncoated. Subsequently, the field joint is exposed to the environment and susceptible to corrosion. Field joints are often considered the weakest point within a pipeline primarily due to compatibility issues between the factory-applied or mainline coating and the selected material used to protect the field joint. Some of the most important properties that field joint coating systems must provide are:
• Long-term corrosion protection and thermal insulation performance
• Excellent adhesion to the substrate that is to be protected
• Exceptional compatibility with the factory or mainline coating system
• The ability to be applied under extreme environmental conditions
• Ease of application—to ensure rapid application and reduced field joint cycle times
Challenges when engineering the most suitable anticorrosion protection systems for the transport pipeline include cost effectiveness, safety of operators and applicators, the proposed lifetime of the structure, local legislation, and coating in conjunction with CP. There are many other factors as well that depend greatly upon the pipe itself.
Corrosion is one of the leading causes of failures of subsea and land-based pipeline transportation systems, with both internal and external corrosion recorded as significant factors in pipeline coating failures. Significant catastrophic failures recorded in recent history have linked corrosion failure to health, safety, and environmental consequences for operators and the public, and fines for negligence have cost hundreds of millions of dollars. The criticality of engineering a suitable field joint coating to cope with extreme environmental and service conditions is imperative.
Coating a Field Joint
In recent years there have been significant changes to the way field joint coating systems have been engineered. Traditionally the field joint coating was often overlooked and protection for this crucial zone was based upon compatibility with the mainline coating, regardless of whether or not the field joint coating requirements were different.
Several factors differ between the mainline factory coating and the field joint coating. For example, the mainline coating is often applied in ideal factory conditions using specialized equipment, and time spent on surface preparation and application is not critical. Since field joint coatings are applied in the field, they are subject to extreme environmental factors with differences in ambient conditions. Historically, the application of field joint coating systems has been rushed to adhere to critical cycle and production times, which has subsequently led to quality issues and premature failures. Simply put, the field joint coating system must be integrated into the pipeline construction conditions.
To develop a successful field joint coating system, the engineer needs to consider additional factors such as specifying the correct product to ensure compatibility of the material to the mainline coating, and the overall coating system’s performance with respect to pipeline operating conditions. The corrosion rate depends on various parameters, including but not limited to pipeline operating temperatures, soil or water conductivity, external accelerators, acids and alkalis, microbiologically influenced corrosion, chemical attack, stray current corrosion, and pipeline stresses that lead to stress corrosion cracking.
Based on the nature of the parent coating or factory-applied coating, the field joint coating system may consist of single or multiple layers of selected protective coating materials. These are applied for numerous purposes that include corrosion control, mechanical protection, thermal insulation, and adhesion. While there are a variety of materials used as a pipeline field joint coating, the following are the most predominantly used and specified systems for pipeline protection: cold applied polymeric tape, polyethylene (PE) and polypropylene (PP) heat shrink sleeves used in conjunction with or without a primer, fusion-bonded epoxy (FBE), liquid epoxy, PP coatings applied on FBE, PE coatings applied on FBE, polychloroprene coatings, polyurethane (PUR), three-layer polyethylene coatings (3LPE), three-layer polypropylene coatings (3LPP), and three-layer polyolefin coatings (3LPO).
Standard for Field Joint Coatings
All coatings have distinct advantages and disadvantages, with many property and characteristic variables that should be taken into consideration. There have been many developments in recent years regarding standards for field joint coating systems. In 2016, the second edition of ISO 218091 was published. This standard specifies the requirements for a field joint coating on seamless or welded steel pipes for buried and submerged sections of pipeline transportation systems used in the petroleum, petrochemical, and natural gas industries as defined in ISO 13623.2
ISO 21809 has clear definitions, scopes, requirements, and recommendations for engineers to follow. For example, it elaborates on surface preparation techniques; material selection; material testing; material field testing; acceptance and rejection criteria requirements for joint preparation and repairs; and testing methods such as thickness checks, holiday detection, peel strength, adhesion testing, hot water immersion testing, degree of cure, cathodic disbondment, impact resistance, indentation resistance, oxidation induction times, and flexibility.
It is essential that the pipeline is fully protected from corrosion so petroleum products are contained, and this standard is a major leap in achieving this.
1 ISO 21809-3:2016, “Petroleum and natural gas industries—External coatings for buried or submerged pipelines used in pipeline transportation systems—Part 3: Field joint coatings” (Geneva, Switzerland: ISO, 2016).
2 ISO 13623:2009, “Petroleum and natural gas industries—Pipeline transportation systems” (Geneva, Switzerland: ISO, 2009).