{"operation":"document","citation":"PI-73-0100","title":"Cathodic Protection Service — Pipeline Safety Interpretation","source_type":"guidance","agency":"Pipeline and Hazardous Materials Safety Administration","status":"guidance","official":true,"published_on":"1973-01-03","effective_on":null,"summary":"PI-73-0100 response to Cathodic Protection Service concerning 192.457.","machine_formats":{"json":"https://regulus.evalyn.ai/document/phmsa-interpretation-pi-73-0100.json","markdown":"https://regulus.evalyn.ai/document/phmsa-interpretation-pi-73-0100.md"},"app_url":"https://regulus.evalyn.ai/document/phmsa-interpretation-pi-73-0100","source_url":"https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/legacy/interpretations/Interpretation%20Files/Pipeline/1973/g73-01-03_Doremus_192.457-nlmx.pdf","body":"<<<PAGE 1>>>\n\nPI-73-0100\nJanuary 3, 1973\nMr. E. P. Doremus, P.E.\nService Division\nCathodic Protection Service\nP.O. Box 66387\nHouston, Texas 77006\nDear Mr. Doremus:\nThank you for your letter of November 2, 1972, in which you forwarded information and comments concerning the two-\nelectrode surface potential survey method for locating areas of active corrosion. Your comments were made with\nreference to an interpretation contained in the OPS Advisory Bulletin 72-8 of August 1972 in which it was stated that\nOPS does not feel that the “leap frogging” survey method will provide any useful information in determining where\nactive corrosion is taking place on Dresser-coupled pipelines (insulated joints). The Bulletin went on to say that there\nare other types of electrical equipment that will do this job.\nYour letter suggests that the “leap frogging” technique is no longer in use and that, therefore, it may not be appropriate\nto refer to it. However, the interpretation quoted in the Bulletin was in response to a specific question, and, as indicated\n“leap frogging” is not useful for Dresser-coupled pipelines with insulated joints. On the other hand, the “leap frogging”\nsurface potential survey method is being successfully used for locating areas of active corrosion on electrically\ncontinuous pipelines. As your letter recognizes, the purpose of “leap frogging” is to compensate for electrode potential\ndifferences. While we recognize that there is equipment available today which makes it unnecessary to use “leap-\nfrogging,” the leap-frogging technique remains available for use on electrically continuous pipelines.\nYour letter also states that the two-electrode surface potential survey method is the most satisfactory approach for\nproviding Dresser-coupled pipelines with adequate cathodic protection. To the extent that you are referring to the use\nof two electrodes in continuous contact with the earth, we agree. The proper technique provides for a continuous\nvoltage difference measured between one electrode placed over the pipeline and the other electrode located five feet\nfrom the pipeline and perpendicular to the pipeline. The survey is normally conducted on both sides of the pipeline to\nidentify existing galvanic anodes.\nYour interest in this important aspect of pipeline safety is appreciated. If you have further questions or comments,\nplease contact us.\nSincerely,\nSIGNED\nJoseph C. Caldwell\nDirector\nOffice of Pipeline Safety\n\n<<<PAGE 2>>>\n\nCathodic Protection Service\nP.O. Box 66387\nHouston, Texas 77006\nNovember 2, 1972\nOffice Of The Secretary Of Transportation\n800 Independence Avenue SW\nWashington, D.C. 20590\nAttention: Mr. Joseph C. Caldwell Director\nOffice of Pipeline Safety\nGentlemen:\nWe are attaching a copy of a letter from your office which has been brought to our attention by several of our clients.\nThe contents of this letter were also published in the DOT Advisory Bulletin No. 72-8. As a result, we have received a\nnumber of inquiries as to the meaning of the contents.\nIn the interest of clarifying some confusion in the minds of the readers, and to dispel certain erroneous conclusions\nwhich could be drawn from the letter, we consider it important to bring the following information to your attention.\nThe letter refers to \"the use of the two-electrode 'leap frogging' surface potential survey method for locating areas of\nactive corrosion on Dresser coupled pipelines (insulated joints)?\" First, we wish to point out that the term \"leap\nfrogging\" in connection with the surface potential survey method refers to a procedure which was utilized in the early\ndevelopment stages of the two-electrode surface potential survey some 20 years ago. The purpose of \"leap frogging\" at\nthat time was to accommodate electrode imbalance by averaging out the error between the two electrodes. The two-\nelectrode surface potential survey technique was refined many years ago to avoid this awkward and confusing\nprocedure of \"leap frogging\". Today's surface potential surveys are accomplished using very stable copper/copper\nsulfate electrodes with the electrodes being balanced against each other at maximum 1,000' intervals. If the imbalance\nbetween the two electrodes exceeds two millivolts, the electrodes are cleaned, and the copper/copper sulfate solution\nreplaced to return the electrodes to a balanced condition. Such being the case, we feel that the usage of the term \"leap\nfrogging\" was unfortunate in that it refers to a procedure which has not been utilized in connection with the two-\nelectrode surface potential survey, with possible rare exceptions, for many years.\nThe conventional two-electrode surface potential survey procedure has been successfully used for many years to locate\nand define localized and gross corrosive areas on bare Dresser coupled lines. For many years, our contracts with clients\nhave specified that when corrosion is detected at or on a Dresser coupling, the center ring and follower of the coupling\nwill be bonded to the adjacent joints of piping and the anode installed at the coupling location. Over the years, and over\nmany hundreds of miles of Dresser coupled lines surveyed, we have encountered many of these Dresser couplings which\nwere corroding and at which bonding and anode installation was accomplished.\nIt should be pointed out here that the reference in the DOT letter to \"insulated joints\" is considered to be somewhat\nmisleading in that the standard Dresser coupling rarely functions as an insulating coupling. For a standard Dresser\ncoupling to serve as an insulating coupling, the alignment of both joints of pipe and the coupling would have to be\nabsolutely perfect. It is possible but a rare case indeed when such a condition exists after the line is backfilled and\nsubject to soil stresses, and when taking into consideration the expansion and contraction characteristics of a bare steel\npipeline. It is quite true that Dresser couplings can and frequently do develop high resistance contacts. However, this\nsituation is entirely different than that of an insulating Dresser coupling. During a well conducted surface potential\nsurvey, an underground insulating Dresser fitting is detectable from surface potential data. It is in some cases difficult to\ndetermine from surface data that the Dresser coupling per se is actually subject to concern able corrosion attack.\nHowever, in the majority of cases, if the corrosion process is taking place at or near the coupling, this fact will be\nreflected by surface data. If indeed there should be any doubt from surface data, then the standard procedure is to\nexcavate and to confirm by visual inspection.\nIn summary, we submit to you that, based on our many years of experience and successful corrosion control results with\n\n<<<PAGE 3>>>\n\nDresser coupled pipelines surveyed by the two-electrode surface potential survey method, and followed by \"hot spot\"\nprotection with magnesium anodes, the two-electrode surface potential survey method is the most thorough, practical\nand economical approach to providing bare Dresser coupled pipelines with adequate cathodic protection.\nThis information is furnished to you on the basis that we do not feel that it was the intent of DOT to publish information\nwhich by inference might be misunderstood or misleading to the industry as a whole, or to rule out a proven procedure\nwhich has served industry well for many years.\nWe would appreciate your comments on this matter in the near future. A copy of this letter is being furnished to the\nclients who have contacted us with questions concerning the Advisory Bulletin No. 72-8.\nVery truly yours,\nCathodic Protection service\nE.P. Doremus, P.E.","truncated":false,"body_characters":7671}