State of Technology Report for Force Main Rehabilitation, Final ...

content; soil and groundwater pH; chlorides; sulfates; redox potential; and known corrosive
environments.
Continuous corrosion monitoring systems can be installed when new pipelines are laid, or can be added
retroactively to existing pipelines. Monitoring implies a series of surveys, planned and organized, to
obtain more comprehensive information on conditions over time. By defining changes in corrosion
conditions over time, this represents a longer-term commitment compared with one-time corrosion
inspections or surveys. It also represents a deeper commitment to quantifying the rate of corrosion and
determining underlying causes of corrosion damage. It has been noted that the rate of corrosion
ultimately determines how long a process can be operated usefully and safely. Thus, there is value in
knowing this rate to identify when maintenance action needs to be taken. Most importantly, to generate
real value from corrosion monitoring initiatives, the information gathered over time has to be translated
into effective corrosion mitigation or control programs.
Corrosion monitoring is mainly used in installations such as critical operations in chemical processing
plants and oil and gas wells and offshore pipelines. Its use in force mains could not be documented at this
time, but that may change in the future. GE Inspection Technology has acquired the technology for an
ultrasonic sensor that is actually made up of a series of 14 thin transducers, all in a flexible band that is
240 mm long by 40 mm wide. This band can be wrapped around the outside of a ferrous pipe in either
the circumferential or axial direction. The accuracy of the transducers is reported to be 0.2 mm. A
coaxial cable is led to a box at grade level where periodic measurements of the 14 transducers can be
made with the DL datalogger. The Rightrax M2 has been used on steel pipe, but not yet on DI pipe.
Another approach is the use of a corrosion potential indicator called, LPR (Figure 6-2). Developed in
Australia by Tyco Water, this is used to evaluate external corrosion in the form of pitting based on soil
aggressivity. This is not a new concept, but Tyco Water has refined the approach. LPR soil testing is an
electrochemical soil testing technique using soil samples taken from the pipe depth to obtain a
quantitative measure of soil corrosivity and rate of corrosion. Previous attempts at measuring soil
corrosivity by a range of chemical and physical
parameters have not been very successful because what
is obtained is the thermodynamic tendency for
corrosion to occur. The LPR approach provides a
corrosion rate that can be extrapolated to allow a
quantitative time to failure to be calculated. The
process involves the use of a specially developed cell
that provides a measure of the combined effects of
several soil parameters. Comparison of the results from
LPR surveys and direct measurement of pitting have
shown a close relationship. The assessment and time to Figure 6-2. LPR Ferguson-Nicholas Cell
failure calculation takes into account the pipe class, age,
length of main, and type and life of the pipe coating.
The method is stated to have the advantages of being: relatively low cost; non-intrusive, able to be
undertaken with the line in operation; and proven on a number of inspections. The interpretation of
results requires specialist skills. Results can be extrapolated in time and space to provide projections of
potential failure. The LPR is currently used in Australia and Hong Kong, but has not been used in North
America to date.
6.1.4 Pressure Monitoring. Stress in a pipe wall is a function of internal pressure. When a pipe is
known to be deteriorated, reducing the operating pressure can extend its service life and reduce the risk of
its failure. Transient pressure monitoring is becoming an accepted O&M procedure in water supply and
75