Page 102 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

4/80 Corrosion Index
rectifiers and bonds must be maintained. Inspections of these surveys where readings are taken at short intervals such as
pieces of equipment are usually performed at shorter intervals every 3 to 15 feet.
than the overall check of the potential levels. Because a rectifier An annual test lead survey is the cornerstone of many opera-
provides the driving force for the cathodic protection systems, tors’ CP verification programs. A pipe-to-soil measurement
the operator must not allow a rectifier to be out of service for taken at a test lead measurement indicates the degree of
any length of time. Monthly or at least bimonthly rectifier cathodic protection on the pipe because it indicates the ten-
inspections are often the norm. dency of current flow, both in terms ofmagnitude and direction
Use of a risk assessment adjustment factor that could be (to the pipe or from the pipe) (see Figure 4.6). Uncertainty
called something like a rectifier interruption factor is one way increases with increasing distance from the test lead because
to account for the effects of inconsistent application of CP. An the test lead reading represents the pipe-to-soil potential in only
interruption or outage can be defined as some deviation from a localized area. Because galvanic corrosion can be a localized
normal rectifier output (probably in amperes) once sufficient phenomenon, the test leads provide only limited information
data have been accumulated to establish a baseline or normal regarding CP levels distant from the test leads. A test lead read-
output for a rectifier. The tracking of kilowatt-hours may also ing is therefore an indicator of cathodic protection only in the
be useful in determining outage periods. The hours of outage immediate area around the lead-a lateral distance along
per year can be tracked and accumulated to assign risk assess- the pipe that is roughly equal to the depth of cover, according to
ment penalties for both high outage hours in any year and the one rule of thumb. Closer test lead spacings, therefore, yield
accumulation of outage hours over several years. Each indi- more information and less chance oflarge areas of active corro-
cates periods during which the pipeline might not be adequately sion going undetected. Because corrosion is a time-dependent
protected from corrosion. process, the number oftimes the test leads are monitored is also
A potential difficulty in application of such rectifier inter- important.
ruption factors is that each rectifier must be linked to the spe- Using these concepts, a coarse point schedule can be devel-
cific pipeline lengths that are influenced by that rectifier. The oped based on general criteria such as:
adjustment factor is derived from each rectifier, but the penalty
applies to the actual portions of the pipeline that suffered the All buried metal in the vicinity of the pipeline is monitored
inconsistent application of CP In a complex system of recti- directly by test leads, and test lead spacing is no greater than
fiers and pipelines, it is often difficult to ascertain which 1 mile throughout this section Best
rectifiers are influencing which portions of pipeline. Test leads are spaced at distances of 1 to 2 miles apart (maxi-
Tracking the equipment performance might also provide the mum) and all foreign pipeline crossings are monitored via
ability to better quantify the benefits of remote monitoring test leads; not all casings are monitored; there may be other
capabilities. If a system is installed to monitor and alarm (in a buried metal that is not monitored Fair
control center as part of the SCADA system, perhaps) rectifier Test lead spacing is sometimes more than 2 miles; not all poten-
malfunctions and perhaps even pipe-to-soil readings at test tial interference sources are monitored Poor
leads, then outage times and inadequate protection times can be
minimized. Depending on the corrosion rates and economic A more robust assessment can use actual distances from the
considerations such as labor costs and availability, adding such nearest test lead to characterize each point along the pipeline.
monitoring capabilities might be justified. This would penalize (show higher risks), on a graduated scale,
those portions of the pipeline that are farther away from a test
Test leads Often, the primary method for monitoring the lead or other opportunity for a pipe-to-soil potential reading.
effectiveness of a cathodic protection system is through the use The frequency of readings at test leads is rated as follows.
of test leads, fixed survey points for taking pipe-to-soil voltage Pipe-to-soil readings are taken with the IR drop understood and
readings. A test lead is normally a wire attached (usually compensated at intervals of
welded or soldered) to the buried pipeline and extended
above the ground. A test lead allows a worker to attach a volt- <6 months Best
meter with a reference electrode and measure the pipe-to-soil 6 months-annually Fair
potential. >annually Poor
Placement of test leads at locations where interference is
possible is especially important. The most common points are Notes: As previously explained, lack of proper IR drop com-
metal pipe casings and foreign pipeline crossings. At these pensation may negate the effectiveness of readings. For our
sites, careful attention should be paid to the direction of current purposes here, test lead can be any place on the pipeline where
flow to ensure that the pipeline is not anodic to the other metal. an accurate pipe-to-soil potential reading can be taken. This
Where pipelines cross, test leads on both lines can show if the may include most aboveground facilities, depending on the
cathodic protection systems are competing. type of coating present.
Readings taken at longer intervals such as greater than one
survevs Several survey types are commonly used to verify year do have some value, but a year’s worth of corrosion might
that effectiveness criteria are being met. These include varia- have proceeded undetected between readings.
tions in how pipe-to-soil readings are taken and where they are
taken. Examples of the former include on readings, instant-off Close interval surve-vs A powerful tool in the corrosion
readings, voltage gradient, and polarization measurements as engineer’s tool bag is a variation on test lead monitoring called
previously described. Variations in where readings are taken close intenialsurveying (CIS) or closespacedsurveying. In this
include readings at test lead stations only versus close interval technique, pipe-to-soil readings are taken (and IR compensa-