Review: This paper is a companion to NACE Paper No. 563.
The stated objective of the field study was:
"... to evaluate the use of coupons to correct for IR-drop, anode/pipe mixed potentials, and Telluric currents."
The stated conclusion of the investigators was:
"... coupons can and will be used to monitor the adequacy of the TAPS cathodic protection systems."
Reviewer's Note: TAPS is an acronym for Trans Alaska Pipeline System.
In the Introduction the investigators imply that the use of buried cathodic protection coupons to assess adequacy of a cathodic protection system falls in the category of "sound engineering practices".
In the Introduction they claim to have conducted a thorough literature review, a laboratory testing program, and application to TAPS. They further claim that:
"... all results indicate a coupon placed close (i.e. in the same environmental conditions) accurately assesses the adequacy of the cathodic protection system at the coupon location."
The Background section of the paper provides a brief description of TAPS and its cathodic protection systems, with emphasis on features unique to TAPS.
In the Regulatory Compliance section they list the regulatory agencies that monitor TAPS. They also give a brief history of corrosion issues which led to an agreement with the Joint Pipeline Office (JPO) that conditionally recognized cathodic protection coupon technology as a valid method of monitoring the adequacy of the TAPS cathodic protection systems.
Contained in the agreement between TAPS and JPO are the following:
"the parties agree that CP coupon technology constitutes sound engineering practice"
CP coupon technology meets the requirements for CP monitoring, as set forth in 49 CFR 195."
In the Use of Cathodic Protection Coupons section they summarize two papers (GERG study and British Gas Research Report by Greenwood) and a statement by Peabody (in his famous book). The summaries corroborate the field study. They claim these papers are representative of the papers found in the literature review.
The Cathodic Protection Coupon Design Basis section enumerates a number of theoretically important issues. These issues will probably be important in those instances where it is required that the use of cathodic protection coupons be explained or defended.
There are two basic theories to support the use of cathodic protection coupons.
"1. The coupon adds an additional "load" to the cathodic protection system. When electrically connected to the pipe, the coupon becomes a part of the cathodic protection circuit and requires current to protect the surface area of the coupon. If adequate current is available to protect the coupon sized to represent a large pipe coating defect (coupon increases the "load" of the system), a sufficient amount of current is available, at the coupon location, to protect the pipe."
"2. The coupon has a similar potential as that of a pipeline "holiday" or coating defect. Coupons are typically sized to represent a large coating defect. If the coupon potential indicates adequate cathodic protection levels, pipeline coating defects, of similar size, have similar potentials, and the pipeline has adequate levels of cathodic protection. This theory assumes that a large pipeline coating defect represents a conservative potential (more positive) as compared to smaller defects. It also assumes that uniform cathodic protection exists around the pipe."
Other important issues are:
"Coupons assess the adequacy of a cathodic protection system and may be used to determine compliance with criteria outlined by Federal regulations and industry standards."
"Size coupons to represent a large coating defect."
"Place the coupon in the same environment as the pipeline."
"Avoid preferential distribution of current to the CP coupon."
"Install a sufficient number of coupons to adequately monitor the cathodic protection system."
"CP coupons assess the adequacy of the cathodic protection system at the coupon monitoring station location and may need to be used in conjunction with other monitoring tools to insure pipeline integrity."
The next three sections contain descriptions of the coupon test site assembly, construction methods, and monitoring procedures used by TAPS. Most of this material is routine specifications type text. A few items are noteworthy. They used three tubes to avoid problems of "uplifting" caused by freezing and thawing. They point out that garnet and bentonite are not good filler for the monitoring tube because high resistance air pockets are created.
Field data indicated that negative potential shifts, in excess of 100 mV, may occur when the input impedance is increased from 10MW to 1GW.
The paper ends with two observations about the data collected on TAPS and the desired conclusions.