Cathodic protection (CP) current distribution for long parallel pipelines in a common right-of- way, with a common negative junction box, have been ineffective as variables such us temperature, magnitude and direction of current flow, pipeline coating quality, pipeline diameter, and length of pipelines, have resulted in problems to maintain adequate CP levels on the pipelines. To overcome some of these problems a Current Distribution System For Cathodic Protection (S.D.C) was designed and tested in a major gas pipeline right-of-way in Venezuela. Field results are presented and discussed.
The Moron - Barquisimeto (MB) Gas system operates four underground pipelines: two high pressure gas lines, one oil pipeline and one poliduct pipeline, all of different diameter, length and coating quality. The gas pipelines, the lo-inch ( 0.25 m ) MB gasoducto and the 16-inch (0.41 m) MB Lazo, are each 136 km (84.5 mi) long, the 20-inch (0.5lmts) oil pipeline is 350 km (217.5 mi) long, and the 12-inch (0.30 m) poliduct is 152 km (94.4 mi) long . The MB system is protected by 11 rectifiers with remote surface groundbeds. The four pipelines are parallel for 75 Km (46.6 mi) from the start of the right-of-way.
In 1995 on I off potential measurements were made along the right-of-way of the MB system for both Laze and Gasoducto gas pipelines. The two gas pipelines had been cathodically protected by 7 of the 11 rectifiers. That is, the cathodic protection of the oil and poliduct pipeline was separate from the gas pipeline in the common right-of-way. Many problems in the right-of-way were detected requiring the development of a new system to better distribute the existing cathodic protection current.
PROBLEMS
The MB Gas System had the following problems prior to the S.D.C
1 .-.: Inadequate cathodic protection.
2.- Interference between the rectifiers.
3 .- As a consequence of high temperatures at the variable resistors there were constant failures of the Negative junction boxes.
4.- Low potentials because of poor current distribution along the pipelines.
5.- Interference between the pipelines when they are close to each other or cross each other due to inadequate continuity bonding.
6.- Changes in direction of current flow in the negative junction boxes.
From the theoretical standpoint, there are adequate rectifiers to protect the pipelines. There is, however, a need to control the magnitude and direction of current for all the pipelines at the same time. Cathodic protection (CP) current distribution of long parallel steel pipelines is difficult as there are many variables like temperature, magnitude and direction of current flow, pipeline coating quality, diameter of pipelines and soil resistivity, that result in poor levels of protection no matter which cathodic protection criteria are used.
The standard equipment used to distribute current over several buried metallic structures is the well known junction or negative box. This box, which is standard in the CP industry, has the advantage of being easy and simple to manipulate in the field, as one can control and measure the current for each of the pipelines with negative leads within it. Figure 1 is a general schematic illustrating a junction box and some of the optional parts that
