ABSTRACT

This paper presents the results of a four year field program on the use of coupons to monitor the adequacy of the Trans Alaska Pipeline System’s (TAPS) cathodic protection systems. The objective of the program was to evaluate the use of coupons to correct for IR-drop, anode/pipe mixed potentials, and Tell uric currents.

Coupon monitoring assemblies were designed and installed at over 400 locations in different types of geological strata. Design guidelines, material specifications, and construction and monitoring techniques were developed to efficiently and effectively install and monitor cathodic protection coupon stations. Data evaluation included, coupons connected to the pipe and cathodic protection system (CP coupon), free corroding coupons (corrosion coupon), coupon “on” and instant-off potentials, polarization potentials, soil resistivity and temperature effects, and surface vs close proximity potentials.

As a result of this field and supporting laboratory program, if was concluded that coupons can and will be used to monitor the adequacy of the TAPS cathodic protection systems.

INTRODUCTION

The adequacy of a cathodic protection system is determined by periodic monitoring of a pipeline with respect to industry recognized criteria, such as NACE International Standard RP0169-92. Adequate cathodic protection can be achieved at various levels of polarization depending on the environmental conditions. The most common method to measure a polarized potential, without effects of IR-drop, is to interrupt all sources of current. Additionally, the measurement of polarization decay or formation requires the ability to interrupt current sources. When it is impractical or not possible to interrupt al current sources, sound engineering practices are required to ensure that sufficient cathodic protection has been achieved. In accordance with this, buried cathodic protection coupons may be installed to assess the adequacy of a cathodic protection system.

The technical validity for the use of coupons for cathodic protection assessment was examined and verified through a literature review, laboratory testing program1, and application to TAPS. All of the results indicate that a coupon placed close to the pipe (i.e. in the same environmental conditions) accurately assesses the adequacy of the cathodic protection system at the coupon location.

BACKGROUND

The Trans Alaska Pipeline System (TAPS), is owned by a consortium of seven companies. TAPS transports nearly 25 percent of the nation’s domestically produced crude oil. Since operation began in 1977, the 800-mile (1287km) long, 48-inch (122cm) diameter pipeline has delivered over ten billion barrels of crude oil.

The pipeline traverses continuous and discontinuous permafrost areas in arctic and subarctic regions, three mountain ranges, approximately 800 rivers and streams, and three seismic fault zones. Because the warm oil in the pipe would thaw the supporting soil at many locations along its length, 420 miles (676km) of TAPS is supported above ground on vertical support members (VSM’s). Above ground piping is jacketed and insulated to prevent thermal loss. Below ground piping is coated with a fusion bonded epoxy and tape wrap system. The mainline pipe cathodic protection system consists of two parallel zinc ribbon anodes placed near the bottom of the pipe (one on each side).

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