Reference: Thompson, N. G., "Technical Proposal No. TP419-1355, The Use of Coupons for Estimating Off-Potentials, Phase II", PR-186-9220, PRC of AGA, JAN 1994.
Reference: Thompson, N. G., Lawson, K. M., "1994 Annual Report On the Use of Coupons for Estimating Off-Potentials, Phase II", PR-186-9220, PRC of AGA, MAY 1995.
Reviewer's Note: These three papers are grouped because they all related to the same research project sponsored by The Pipeline Research Committee (PRC) of the American Gas Association (AGA). Moreover, there is significant redundancy in the three papers.
A final report for Phase II was not available at the time of this review.
Review: The feasibility study was a 15 month effort in 1992 and 1993. Phase II was a 9.73 person month effort conducted in 1994 and 1995.
The incentive for performing this project was to provide an additional means of evaluating the polarized potential of a pipeline in areas where interruption of all current to the pipeline is difficult or impossible to perform.
The feasibility study was conducted at a well controlled site at Sugar Grove, OH and an Operating Pipeline site near Elkins, WV. Both sites are owned and operated by Columbia Gas Transmission.
Data collected during the feasibility study was certainly not conclusive but provided strong promise for the methodology. It was suspected that the effects of long-line currents caused some of the discrepancies.
Phase II was divided into four tasks. Each of these tasks is discussed below.
Task 1: Review of Coupon Studies
Reviewer's Note: It appears they reviewed six papers and talked to persons at Algonquin Gas Transmission, Alyeska Pipeline Company, ARCO Oil and Gas Company, East Australian Pipeline Limited, and US Steel. This reviewer concurs with the following five points as a good summary of the GERG report.
The following five points are the author's summary of the literature review.
- Most coupons were installed as a tool to monitor IR-drop free polarized potential measurements.
- Many coupons were installed to monitor sites requiring special attention such as stray current (AC and DC) sites and direct connect sacrificial anode locations.
- Many coupon designs incorporated a permanent reference electrode as part of the design; however, most of the problems associated with the use of coupons were related to either the failure of the permanent reference or the contamination of the soil near the coupon by the permanent reference.
- Several of the coupon designs incorporated coupons of different surface areas. The range of surface areas of the coupons was from 1 to 300 cm2.
- The majority of the sites were installed with the coupon facing away from the structure or facing up towards ground level. Many of the installations were cylindrical in nature, i.e., with bare surfaces exposed for 360°.
Task 2: Operating Pipeline Site Evaluations
The following condition is frequently stated in this task, Task 3, and in the feasibility study.
"The tube is backfilled with the soil removed by the auguring operation to insure that the coupon is exposed to environmental conditions that closely match those of the nearby structure."
The following five sites were used because the pipe off-potential can be accurately measured at each site.
Victorville, CA Southern California Gas Co. Elkins, WV Columbia Gas Transmission Co. Greenup, KY Tenneco Gas Rockville, MD Columbia Gas Transmission Co. Ferndale, WA ARCO, Cherry Point Refinery
The test sites were constructed to examine the effect of coupon size and coupon location.
Data collected at these sites were not conclusive, but showed strong promise for the methodology.
The author's conclusions are:
"No preliminary conclusions are even attempted at this time because of the short-exposure time of the collected data. Many of the trends are likely to change over the twelve-fourteen month exposure planned for this project. It is obvious that in a few cases, coupons connected to a pipe may become anodic due to the long-term potentials existing on the pipe. This net anodic current to the coupon may prevent the coupon from ever experiencing cathodic protection required to simulate conditions existing on the pipeline. These conditions will be followed closely in the 1995 research."
Task 3: Test Site Evaluations
Except for the bare pipe tests at Sugar Grove they found good agreement between the coupon off-potential and the pipe off-potentials. The author's attribute this to the fact that they could control the effect of long line currents.
The investigators suspect that it is not likely (under these conditions) that coupon potentials will ever agree well with the bare pipe potentials. However, they reserve final judgement until steady-state conditions are achieved.
Reviewer's Note: It appears that their phrase "under these conditions" refers to a carbonate/bicarbonate buildup at the pipe surface. They attribute this buildup to long-term application of CP. They promise further study of this condition during the remainder of Phase II.
Task 4: Guideline Development
This task was not completed prior to the 1994 Annual Report because the field part of the study was not yet complete.