Page 608 - Corrosion Engineering Principles and Practice

570 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 571

In addition to the variable-type stray currents described, steady-
state stray currents may be encountered. These may be caused by an
impressed current CP system on an adjacent, but electrically separate
structure when the groundbed is too close to the unprotected struc-
ture. Under such a condition, the affected structure passes through
the potential gradient field around the groundbed in which the soil is
positive with respect to the system. This positive potential difference
causes current to flow onto that part of the affected structure in the
gradient field.
In severe cases, moving the groundbed that is causing the stray
current condition may be necessary. Figure 7.29 illustrates the general
features of typical stray current conditions that may be encountered in
association with an electrified DC transit system. A pipeline is used, in
this illustration, as a convenient means of illustration. Such situations
may require the installation of metallic bonds between the possibly
affected structure and the structure that is the source of the offending
stray current. In the DC transit system, example, the metallic bound
would be arranged to make a return path to the power house.

13.9 Monitoring Pipeline Cathodic Protection Systems
Monitoring pipeline CP systems can be quite complex and in many
cases, condition-monitoring requirements are specified by regulatory
authorities. Since CP systems are expected to operate in demanding
environmental conditions over long time periods, the reliability re-
quirements of the associated hardware are high. Regular monitoring of
the equipment is therefore an important aspect of any CP program.
An increasing trend toward selective remote rectifier monitoring,
using modern communication systems and computer networks is
emerging to accomplish these tasks with reduced resources. Wireless
cell phone and satellite communication systems are also available for
interrogating rectifiers in remote locations. The other aspect of
monitoring a pipeline CP system is monitoring the actual protection
that is provided to the pipeline. The following sections briefly describe
some of the techniques used to carry out this work.

13.9.1 Close Interval Potential Surveys
The principle of a close interval potentials surveys (CIPS) is to record
the potential profile of a pipeline over its entire length by taking
potential readings at approximately 1-m intervals. A reference
electrode is connected to the pipeline at a test post and positioned in
the ground over the pipeline at regular intervals for the measurement
of the potential difference between the reference electrode and the
pipeline (Fig. 13.35). The CIPS technique provides a complete pipe to
soil potential profile and the interpretation of results, including the
identification of defects, is relatively straightforward.