Page 105 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Scoring the corrosion potential 4/83

nterference potential (10 PtS) I age differential across the coating. Disbondment or arcing may
occur. If the potentials are great enough, the arcing may dam-
age the pipe steel itself.
where: The induction scenario occurs as the pipeline is affected by
Interference potential either the electrical or magnetic field created by the AC power
AC related (20%) transmission. This sets up a current flow or a potential gradient in
Shielding (10%) the pipeline (Figure 4.8). These cases of capacitive or inductive
DC related (70%) coupling are dependent on such factors as the geometrical rela-
Telluric currents (I?’o) tion ofthe pipeline to the power transmission line, the magnitude
DC rail (50%) of the power current flow, the frequency of the power system, the
Foreign tines (49%) coating resistivity, the soil resistivity, and the longitudinal resis-
tivity of the steel [77]. Induced potentials become more severe as
Corrosion is an electro-chemical process and corrosion pre- soil resistivity and/or coating resistivity increases.
vention methods are designed to interrupt that process, often Formulas exist to estimate the potential effects of AC inter-
with electrical methods like cathodic protection. However, the ference under normal and fault conditions. To perform these
prevention methods themselves are susceptible to defeat from calculations, some knowledge of power transmission load char-
other electrical effects. The common term for these effects is acteristics of the power system, including steady-state line cur-
inferjerence. Three types of interference are evaluated: AC rents and phase relationships, is required. Estimations and
related DC related and shielding effects. measurements will be needed to generate soil, coating. and
steel resistivity values, as well as the distances and configura-
AC-related interference (weighting: 20% of inte+rence poten- tions between the pipeline and the power transmission facili-
tial) Pipelines near AC power transmission facilities are ties. The key factors in assessing the normal effects for most
exposed to a unique threat. Through either a ground fault or an situations will most likely be the characteristics of the AC
induction process, the pipeline may become electrically power and the distance from and configuration with the
charged. Not only is this charge potentially dangerous to people pipeline. Fault conditions can, of course. encompass a multi-
coming into contact with the pipeline, it is also potentially tude ofpossibilities.
dangerous to the pipeline itself. Induced AC voltage can also be measured by methods simi-
The degree of threat that AC presents to pipeline integrity lar to those used to measure DC pipe-to-soil voltages for
has been debated, Reference [38] presents case histories and an cathodic protection checks. Therefore, an AC survey can be a
analysis of the phenomena. This study concludes that AC can part of a close interval survey (see earlier section on CIS).
cause corrosion even on pipelines cathodically protected to thereby generating a profile of AC voltages.
industry standards. “The corrosion rate appears to be directly Methods used to minimize the AC interference effects. both
related to the AC density such that corrosion can be expected at to protect the pipeline and/or personnel coming into contact
AC current densities of 100 A/m2 and may occur at AC current with the line, include [53,62]
densities greater than 20 A/m2” [38]. Given specific measura-
ble criteria for the threat, the evaluator might be able to develop Electrical shields
a threat assessment system around AC current levels directly Grounding mats or gradient control electrodes
measured. Otherwise. indirect evidence can lead to an assess- 0 Independent structure grounds
ment system. 0 Bonding to existing structures
A basic understanding of the AC issue will serve the evalua- 0 Supplemental grounding of the pipeline via distributed
tor in assessing the threat potential. Electric current seeks the anodes
path of least resistance. A buried steel conduit like a coated Casings
pipeline may be an ideal path for current flow for some dis- Proper use of connectors and conductors
tance. Almost always, though, the current will leave the Insulating joints
pipeline to another more attractive path, especially where the Electrolytic grounding cells
power line and the pipeline diverge after some distance of par- Polarization cells
alleling. The locations where the current enters or leaves the Lightning arresters.
pipe may cause severe metal loss as the electrical charge arcs to
or from the line. At a minimum, the pipeline coating may be Monitoring should be an integral part of the AC mitigation
damaged by the AC interference effects. effort.
The ground fault scenario of charging the pipeline includes Because so many variables are involved in performing accu-
the phenomena of conduction, resistive coupling, and elec- rate calculations and this is a relatively rare threat to most
trolytic coupling. It can occur as AC power travels through the pipelines, a simplified schedule can be set up for this rather
ground from a fallen transmission line, an accidental electrical complex issue. In terms of risk exposure, one of three possible
connection onto a tower leg, through a lightning strike on the scenarios can exist and be scored from a risk perspective:
power system, or from an imbalance in a grounded power sys-
tem. These are often the more acute cases of AC interference, No AC power is within 1000 ft of the pipeline 3 pts
but they are also often the more easily detectable cases. The AC power is nearby, but preventive measures are
sometimes high potentials resulting from ground faults expose being used to protect the pipeline 1-2 pts
the pipe coating to high stress levels. This occurs as the soil sur- AC power is nearby, but no preventive actions
rounding the pipeline becomes charged, setting up a high volt- are being taken 0 pts