EXPERT TIP #31 – KNOWING CP SYSTEM & STRUCTURE DETAILS

Know the Cathodic Protection (CP) System
When an existing CP system requires review and/or testing, the work will be to determine if the system is working correctly and/or to troubleshoot system issues. Before the fieldwork starts, it is important to review any available documentation or historical data describing the type of CP equipment and itsperformance.

In many cases, the technician may have "inherited" a CP system and is now responsible for the system’scare and maintenance. Valuable information about the system may be available from various sources, butthe technician must make the call regarding the validity, timeliness and usefulness of that information.
When assessing a CP system, several important details must be considered:

1. Is it a galvanic, impressed current, or a hybrid CP system?
2. Is the anode system a deep anode well, distributed or surface system?
3. What are the number, type, and weight of the anodes?
4. When was the CP system installed?
5. Where are the CP components and test locations located?
6. Did the CP system ever work correctly, and is there data to prove that?

When reviewing CP data provided by others, be sure the information makes sense. Understanding the methods and procedures used to collect the data to determine its accuracy is important. If the providedinformation is suspect or insufficient, the actual details and correct information must be obtained to make an informed decision.

When assessing data accuracy, consider the game of “Telephone.” In that game, information is transferredfrom person to person. When the third person in the game delivers the information to the fourth person, the relayed information becomes distorted and unreliable. As in this example, it is time to collect new data and information if the existing information is suspect or deemed unreliable. This is critical in having a sound basis of understanding.

When assessing data, it is essential to know the age of the data, as old data can be useless. Depending on the time involved, many details about the protected structure, most often a pipeline, may have changed. These changes can include new pipeline sections, connections, sensors, or other equipment that can change the operating parameters of the CP system. If there were no structural changes to the pipeline, there may have been maintenance conducted that could have bypassed a dielectric insulator, or an existing insulator could have been replaced with a standard, non-insulated fitting. It happens all too frequently.

To adjust, troubleshoot or formulate proper conclusions, there must be complete and accurate information about a CP system. This is critical to make appropriate corrections, upgrades, and future maintenance recommendations.

Know the Structure
In addition to the various physical changes to a structure or pipeline system, it is important to know the
type of structure that is to be protected. Pipelines will have different CP requirements due to the
characteristics described below.

A well-coated and electrically isolated, dry gas pipeline may only require minimal CP current to fully protect the pipeline. We have designed CP systems that require only milliamps of current to protect miles of pipeline. Example: 8 miles of 16" newly installed gas pipeline was fully protected with 20 mA of CP current.

• A well-coated and isolated produced water pipeline may require a large amount of CP current to protect the pipeline. For instance, an 8" 200-foot pipeline may require over 100mA. This seems exceptionally high compared to the above dry gas pipeline. However, depending on the diameter of the pipeline, significant amounts of CP current can be lost across the internal gap of a dielectric insulating fitting.
• Subsea structures will require a much higher current density than buried structures. When protecting a subsea structure, the initial potential shift will be relatively small, maybe less than ten percent of the final polarization shift. Polarization will take from a few days to many weeks to stabilize.
• Ductile iron pipe (DIP) usually has a complimentary paint coating that keeps it from rusting before it goes into the ground. Electrical continuity is a concern as DIP uses bell and spigot joints, which must be bonded with external bond cables to be cathodically protected. Therefore, it is essential to confirm the electrical continuity of the installed pipe system when it is to be protected.

New DIP systems can tie in with existing pipelines made from other materials, i.e., mild steel or mortarcoated pipelines. This means that galvanic corrosion between the dissimilar metals must be addressed.

Finally, a polyethylene sleeve is often installed over the DIP. The purpose of the sleeve is to eliminate or significantly reduce the contact between the DIP and the surrounding soil, preventing corrosion. While this may reduce the overall corrosion rate of the DIP, the sleeve also serves as a "disbonded"
coating. Therefore, attempting to provide an effective CP system will be extremely difficult.

• A mortar-coated pipeline will react much like a bare pipeline, but in time (days, maybe weeks), the pipe will polarize and stabilize. There are different types of mortar-coated pipelines (listed below), so it is important to know the type. There are both welded and bell and spigot joints for this type of pipe. Again, electrical continuity must be verified if CP is to be effective.

There are four basic configurations of mortar-coated pipelines:
1. Mortar Coated Steel – This pipeline type is often made from SCH 40 or 80 mild steel and the joints are typically welded. The mortar coating is applied over the steel; in some cases, the pipe can also be internally lined with mortar or cement.
2. Mortar Coating Over a Dielectric Coating – In this case, a mortar coating is applied over a traditional coated pipeline and used as a weight coating to prevent the pipeline from floating in marine or marsh installations.
3. Pretensioned Mortar Coated – This pipe type is a thin-wall, bare steel cylinder with a spiral rod (typically 3/16" to 3/8" diameter) tightly wrapped around the pipe cylinder to assist with its ability to work under high pressures. Mortar is applied over the pretensioned rods.

Note: The design and application of CP for the above mortar-coated pipeline examples would be like any other buried steel structure.
4. Prestressed Mortar Coated – This pipe material is a thin-wall, bare steel cylinder initially coated with a thin layer of mortar. Once coated, a high-strength alloy steel wire is tightly wrapped around the thin mortar coating. The tensioned steel wire is approximately 1/8" in diameter and pulled to a point just short of its final tensile yield point. The prestressed pipe and tensioned wires are again coated with mortar as a final product.

Cautions: Forensic tests of failed cathodically protected prestressed pipe have shown that the
prestressed wire is susceptible to hydrogen embrittlement if over protected by CP. This will occur at
potentials above negative 700 mV with respect to a CuSO4 reference electrode. For this reason, we
DO NOT recommend or design CP for a prestressed pipeline. We have witnessed more than a few
failures and resulting legal cases.

It is important to note that a properly installed prestressed or any other mortar-coated pipeline has an inherently high corrosion resistance without using CP.
For projects that require continuity testing of DIP or mortar-coated bell & spigot pipelines, there are different pipe wall thicknesses, defined as pipe “class” or “pressure ratings.” Confirming the pipe's class or pressure rating is important, as it will significantly affect any resistance calculations performed.

Summary
If accurate and factual details about an existing CP system or the type and condition of the structure are unavailable, it is unwise to proceed based on assumptions. It is usually best to base actions on known facts and/or conditions; without those, a way must be found to obtain them.