Fellow’s Corner

Overcoming Plant Isolation Issues within Cathodic Protection Design

Dr Ahmed Mahgoub, FICorr

Dr Ahmed Mahgoub is cathodic protection subject matter expert for Saudi Arabian Oil Company (Saudi Aramco) in Dhahran, Saudi Arabia. He has more than 19 years of experience in the consulting, engineering, constructing and commissioning of different cathodic protection structures. He is a an AMPP CP Specialist, ICorr CP specialist, Fellow of ICorr, ICorr CPGB Member and AMPP Snr. Corrosion Technologist.

Introduction

Cathodic protection (CP), when applied properly, is an effective means to prevent corrosion of underground plant piping. For many underground applications, such as pipelines, CP system design is relatively straightforward. Plant and facility environments, however, are not simple applications. Plants have congested underground piping systems such as process drains and other utilities in a tightly spaced footprint. The presence of copper grounding systems, foundations with reinforcing steel embedded in concrete, conduit, utility piping and structural pilings (either bare or concrete with reinforcing steel) can greatly complicate the task of designing a pipe CP system.

For simple plant facilities, it is possible to isolate the piping and utilise a conventional galvanic corrosion prevention system. This works only if the plant piping is electrically isolated from other underground structures for the life of the facility. For most plant and facility applications, it is not practical to isolate the piping from the grounding system for the life of the facility. In these cases, an impressed current cathodic protection (ICCP) anode system is the only alternative as a galvanic system does not normally have sufficient capacity to overcome plant earth connections. This paper represents the linear mixed metal oxide (MMO) anodes, which in suitable conditions can be an optimal method of providing adequate CP protection criteria as specified in [1-4] to the piping network in crowded areas of oil and gas plants.There are two conventional approaches to cathodically protecting underground plant piping using impressed current anodes – deep vertical anode bed and shallow / distributed anode bed. However, there are three principal industry challenges to consider for any CP system within a plant and described as shortages in utilising a conventional ICCP system.

• First is the current distribution issue due to highly congested underground environment that is common to most plants.
• The second critical factor is isolation in the presence of a pervasive copper grounding network, often applied for safety reasons to protect rotating equipment and security fences.
• Third is the probability of DC interference due to stray currents from multiple sources.

Figure1: A Typical Process Piping Layout Where Reinforced Concrete Foundations are Restricting The Flow of Protective Current.

MMO System

The linear MMO Anode is a long-line, flexible, cable-like anode, which is placed in continuous close proximity to (typically 0.5m to 1m from) the piping network. In conditions of similar backfill/resistivity, uniform distribution of CP current is therefore achieved on applications where many conventional anodes ground beds do not work or would cause excessive interference. In contrast to conventional anode ground beds CP systems, Linear MMO Anode is placed in the ground parallel and in close proximity to the plant piping to be protected and provides uniform distribution of protective current to the entire steel surface as demonstrated in various studies [4-9], thereby maintaining the steel-to-soil “instant-off” potential in the required protection criteria.

Figure 2: UG Pipeline CP System with Single Linear Anode Layout.

Backfilled Linear Anode

The MMO based anode represents the second generation of backfilled linear anodes. The platinum based catalytic anodes were quickly replaced with MMO based wire anodes as they were more cost effective, less prone to failure, allowed for a longer anode system life and a larger range of current outputs, and provided a far more robust material.

Figure3: MMO Linear Anode Composition.

Below are the key elements that contribute to the MMO linear anode composition,

• Core, is crafted from high-quality titanium wire. This core is meticulously coated with a catalyst blend derived from Mixed Metal Oxides, predominantly featuring Iridium and Tantalum
• MMO Coating, on one hand incorporates an electrocatalytic conductive element, which acts as a catalyst to drive the essential reactions for current generation. On the other, it embeds bulk oxides that serve as a protective shield, ensuring the substrate material remains resistant to corrosion.
• Acid Resistance Fabric, this fabric is designed to provide an additional layer of protection against acidic environments. Its unique composition ensures that the MMO linear anode remains safeguarded from potential corrosive effects of acids, thus enhancing its longevity.
• Protective Braid, surrounding the anode is a protective braid which offers mechanical protection. This robust braid ensures that the anode is shielded from external wear and tear, making it more durable and resilient to external forces.
• Coke Breeze is a common backfill material for ICCP systems. It not only enhances the conductivity of the anode but also aids in distributing the current This ensures efficient operation and reduces potential hot spots.

The MMO linear anode functions as a distributed system including an infinite number of continuously spaced anodes. This system offers the optimal technical CP solution while minimising the required current output as detailed below:

• Electrical isolation is not necessary, Because the MMO linear anodec is closely located next to the piping being protected, electrical isolation as illustrated in [4] is not a significant concern. The anode is “closely coupled” to the piping and operates with a very low anode gradient that minimises any losses to nearby structures including grounding equipment.

• Maintains uniform current distribution by positioning the anode parallel and in very close proximity to the piping being protected, the linear anode CP system design eliminates any requirement for supplemental anodes to address areas where remote anodes may be shielded after the CP system is commissioned. Wherever the piping goes, the linear anode follows in the same trench. This also makes it very easy to adapt the design during piping revisions that may change the piping system routing as the plant construction Sufficient care must be taken during its installation of course.

• Elimination of stray current risks, close proximity to the piping being protected significantly limits current losses to other structures and virtually eliminates shielding and stray current This also significantly reduces the total current requirements for the system, reducing the rectifier requirements.
• Access restrictions, the MMO linear anode is installed in very close proximity to the piping that is to be protected. This minimises the risk of third-party damage and reduces trenching required for buried cable and drilling required for distributed anodes. If installed in conjunction with the piping, the anode can be placed in the same trench as the piping affording the anode protection by the piping itself from external damage. This is a very cost-effective CP installation when installed concurrently with the piping and to correct spacing.
• Ease of installation, when installed alongside the piping during pipelay a matter of laying the anode cable in the trench with no further drilling is needed.

Case Study

• This section outlines the proposed CP system by linear MMO anodes for new 8” & 6” underground Header and branch underground piping’s network in Saudia The total length of the gas grid distribution piping network in this instance is 4900m and the piping will be protected by a permanent (ICCP) system for a lifetime of (25 years) according to project specifications by applying a new CP system of 25V/15A rating.

• The MMO Linear Anodes comprise a continuous MMO/Ti wire with copper cable packaged in fabric jacket fulfilled with calcinated coke installed parallel to the pipelines in the same trenches by maintaining a minimum distance of 0.5-0.8m from pipeline.
• A header cable is attached at the factory via a high-pressure crimp connector to ensure a low resistance That connection is then sealed in a splice kit with epoxy resin to prevent water intrusion.
• Finally, the header cable to anode feeder cable connection is performed on site as per the manufacture specification and standard

Figure 4: Installation of the MMO Linear Anode.

Figure 5: Installation of CP Cables in The Same Trench of the MMO Linear Anode.

The CP system must then be examined to confirm the proper installation of all components. It is crucial to verify the connections and continuity of the positive anode cables. Resistance measurement between various anode feeders is required to ensure electrical continuity, as well as between different main positive cables. The resistance reading should be less than 1 ohm. Sample results of the continuity test are illustrated in the tables below.

Table 1. Anode Feeders Continuity Test Sample.

Table 2. Main Positive Cables Continuity Test Sample

During CP commissioning activities any temporary sacrificial anodes must be disconnected and the piping given 96 hours to depolarise before measuring the native potentials which were found in this instance to lay in the range -410 to -635mV copper/copper sulphate (CSE) with an average of -550mV. Thereafter the permanent CP was commissioned and energised and the piping allowed to polarise for 48 hours at 4.5Volt/2.2Amp setting. ON ‘pipe to soil’ potentials were measured at all test stations and the Instant-Off pipe to soil potentials were then measured at DC coupon test stations. Both ON & Instant-Off potentials did meet the required CP criteria as specified in Saudia Aramco COMPANY specifications. The records of Native, ON and the Instant-Off pipe to soil potentials are illustrated in the tables below.

Table 3. Native, ON and the Instant-Off Pipe to Soil Potentials at DC Coupon Test Stations.

Table 4. Native and ON Pipe to Soil Potentials at Test Stations

Summary

Process equipment reinforced concrete foundations and electrical plant grounding are usually an integral part of the plant. Shielding of underground piping network in congested oil and gas plants is one of the major problems when CP is applied for its protection from external corrosion. Not only is a significant amount of protective current consumed by these elements, but also, they restrict the flow of protective current to the intended structures from a conventional, distributed or remote ground bed. CP of plant piping where current leakage and stray current caused by reinforcement concrete foundations and grounding rods are a real problem.

MMO Linear anodes are a modern solution that can simply be laid alongside a new pipeline and current distribution and polarisation formation are normally better in case of anodes installed close to the pipeline, MMO linear anode is an effective method to protect the plant piping against soil side corrosion.

The utilisation of MMO linear anode for plant piping protection will also lead to a substantial decrease in project installation and maintenance expenses, as well as improved performance in comparison to the traditional method of CP distributed anode. In this particular case study, the approach of Linear MMO anode to be utilised for plant piping protection resulted in revising the initial plan to drill and install 50 distributed anodes. As such, significant cost savings were realised resulting in approximately 3850 meters of cable trenching and 50 drill anodes with total depth of 250 meters were eliminated in addition to avoiding the need for anode bed replacement for an extended period, ranging from 25 to 40+ years, can result in significant cost savings in terms of capital cost.

References

1. ISO 15589-1, Petroleum, petrochemical and natural gas industries — cathodic protection of pipeline systems, Part 1: On-land pipelines, ISO, 2015.
2. NACE/AMPP SP0169, Control of external corrosion on underground or submerged metallic piping systems, NACE International, Houston, 2013.
3. EN 14505, Cathodic protection of complex structures, BSI 2005
4. NACE/AMPP SP0286, Electrical isolation of cathodically protected pipelines, NACE International, Houston, 2002

4. A Nordquist, “Cathodic protection design considerations in congested area facilities,” Paper No. 10900, in Proceedings of the NACE/AMPP Corrosion Conference & Expo 2018, NACE/AMPP, Houston, TX
5. A W Al-Mithin, “Effectiveness of cathodic protection system for buried flow lines near gathering centres using continuous linear anodes,” Paper No. 0001530, in Proceedings of the NACE/AMPP Corrosion Conference & Expo 2012, NACE/AMPP, Houston, TX
6. M Attarchi, “Simulation of linear anode-pipe cathodic protection system: primary and secondary current and potential distribution analysis,” NACE/AMPP Journal of Science and Engineering, 75(9), 1128–1135.
7. Z Chaudhary, “Cathodic protection of piping network in congested area of a petrochemical plant,” Paper 05050, in Proceedings of the NACE/AMPP Corrosion Conference & Expo 2005, NACE/AMPP, Houston, TX.
8. T Huck, “Linear anode for pipeline rehabilitation – thirty years later,” Paper No. 19993, in Proceedings of the 18th Middle East Corrosion Conference and Exhibition (MECC) 2023.
9. A H Mohammad, “Problems associated with remote anode beds in very low resistivity soil for protection of piping networks in congested areas of a petrochemical complex,” Paper No. 03713, in Proceedings of the NACE/AMPP Corrosion Conference & Expo 2003, NACE/AMPP, Houston, TX.