Although submarine pipelines are designed with an external coating as the primary system for corrosion control, a cathodic protection (CP) system is normally installed as a back-up for any deficiencies in the pipeline coating, including defects induced during coating application, transportation, installation, and operation. In defining the required capacity of the CP system, primary factors to consider are the detailed design of the applicable pipeline coating systems and provisions for quality control during their application. Results from remotely operated vehicle (ROV) inspections of numerous submarine pipelines for lifetime extension projects have shown that the coating is still in good condition after 20 to 30 years in service and the consumption of the galvanic anodes is much lower than assumed in design.
Despite the advances and improvements coating systems, however, some of these systems may be vulnerable to disbondment due to insufficient quality control of the application process or improper design and application of field joint coatings. This is particularly applicable to pipelines installed by reeling, which makes the field-joint coating and adjacent parent coating vulnerable to damage.
In CORROSION 2016 paper no. 7430, “External Corrosion Control of Submarine Pipelines” by A. Pedersen, E. Skavås, and T. Sydberger, the authors note that reliable corrosion control of submarine pipelines can be ensured by referring to requirements in the newly revised standard, DNV GL-RP-F103 (2016), “Cathodic Protection of Submarine Pipelines by Galvanic Anodes.” This standard addresses CP design in combination with detailed requirements for pipeline coating design and quality control during application of line pipe coating and field joint coating as outlined in other DNV standards. This paper highlights the first revision of DNV GL-RP-F103, the 2016 revision of that standard, and the main differences between DNV GL-RP-F103 and ANSI/NACE SP0115-2015/ISO 15589-2:2012, “Petroleum, petrochemical and natural gas industries—Cathodic protection of pipeline transportation systems—Part 2: Offshore pipelines.” Additionally, the paper compares design current density, coating breakdown factor, and additional anodes near a structure, platform, or shore approach using DNV GL-RP-F103 and ANSI/NACE SP0115-2015/ISO 15589-2:2012. Illustrative examples are also given that compare anode requirements.
The main motivation to update DNVGL-RP-F103 in 2016, the authors say, was to incorporate new experience from DNV GL work with verification and certification of pipelines, experience from work with lifetime extension of pipelines, and the revision of ISO 15589-2 that was issued in 2012.
The CP design in DNV GL-RP-F103 emphasizes the importance of design and quality control of pipeline coatings. Recommended practices for line pipe coatings (DNV RP-F106, “Factory Applied External Coatings for Corrosion Control”) and field joint coatings (DNV RP-F102, “Pipeline Field Joint Coating and Field Repair of Linepipe External Coating”) were developed when the first revision of DNV-RP-F103 was done in 2010. These standards provide detailed requirements for the design and quality control of coating application for the most commonly applied line pipe coating systems and field joint coating systems used on submarine pipelines.
For submarine pipelines, the coating systems applied constitute the main corrosion protection system. For the design of pipeline CP that is intended as a back-up, the coating breakdown factor is a primary design parameter for determining the required CP capacity. The constants for calculations of coating breakdown factors recommended in DNVGL-RP-F103 are based on detailed requirements to both coating design and quality control of the application, as defined in DNV RP-F106 and DNV RP-F102. The requirements, especially for coating systems with fusion-bonded epoxy as the innermost layer, exceed those defined in ISO 21809-1:2011, “Petroleum and natural gas industries—External coatings for buried or submerged pipelines used in pipeline transportation systems—Part 1: Polyolefin coatings (3-layer PE and 3-layer PP).” ANSI/NACE SP0115-2015/ISO 15589-2:2012 refers to coating quality being in accordance with ISO 21809.
According to the authors, applying these three DNV standards together in a project will reduce the need for arbitrary conservatism in CP design. By using stringent quality assurance/quality control requirements for the coating applications on submarine pipelines, the anode mass required for CP of submarine pipelines can be reduced. For transmission lines and flowlines with a typical diameter between 8 to 20 in (203 to 508 mm) and an advanced pipeline coating systems, the reduction is quite substantial.