Pipeline & Gas Journal
March issue 2000:

• The cost of excavation is high Underwater
  Environmentally unfriendly areas
• The cost of a failure is high
  Pipeline not looped
  Hazardous product
  Environmentally sensitive area
  High population density area
• A large number of defects is suspected
  Prioritize digs
  Less sizing accuracy

usually is appropriate when:

• There are long pipe runs in remote areas
• There were no previous intelligent pig inspections
• The pipeline is old
• The cathodic protection data is not conclusive
• There is a history of accident free operation of the line
• There are a small number of defects suspected

You plan to dig all indications anyway

Before initiating a smart pig inspection of a pipeline, decisions need to be made on which defects to excavate and inspect, whether the defect can be repaired or needs to be replaced, and which repair method should be utilized. One example of a simple corrosion defect classification for prioritizing inspections is2:

Category 1 Defects—Less than 20 percent wall loss

• Unlikely to fail in service
• Likely acceptable by B31G
• Least critical

Category 2 Defects—Between 20 and 50 percent wall loss

• Small likelihood of failure in n service
• May not be acceptable by n B31G, depending on defect n length

Category 3 Defects—Greater than 50 percent wall loss

• Potential for failure in service n of long defect
• Only short defects acceptable n by B31G
• Most critical

Defect Assessment
The purpose of measuring and characterizing corrosion and blunt defects is to determine the remaining wall thickness of the steel pipeline. Remaining wall mapping measurements may be made directly using an ultrasonic thickness gauge or indirectly by measuring depth. Ultrasonic thickness gauges are used for measuring either internal or external corrosion. To obtain accurate measurements, the tip of the gauge must be able to fit into tight radius curves in a corrosion profile. The basic measurement tools for external corrosion are straight edges or rulers, pit gauges, scales or profile gauges, bridging bars, and depth micrometers. The latest development in defect assessment is the laser profilometer3. Laser and ultrasonic technologies have provided more precise measurements of pitting corrosion than is achievable with manual tools.
Pit gauges (Figure 1) can provide a quick and reasonably accurate measurement of the depth of single isolated pits. However, they provide limited benefit for measuring patch corrosion, circumferential or spiral corrosion, and corrosion near a long seam or girth weld.
Scales or profile gauges (Figure 2) provide the ability to take an imprint or cross section of the shape of the corrosion defect. This is a convenient tool for examining individual pits or small corrosion patches smaller in size than the length of this tool.

A bridging bar allows for examination of longer lengths of corrosion and allows for the presence of a girth weld by bridging over these obstacles. An area of corrosion can be examined by taking depth measurements with a micrometer as the bar is incrementally moved about the circumference of the pipe.

The assessment of corrosion requires that the measurements be spatially organized and representative of the actual corrosion on the pipe surface. This is typically achieved by way of a grid system that is drawn on the corroded area prior to depth measurements being taken. Once corrosion measurements are recorded, a “river bottom path” in the corrosion is selected and used in calculating burst pressure or the remaining pressure capacity of the corroded pipe.
The first solution developed for determining the remaining strength of corroded pipelines was ASME B31G2,4. ASME B31G was developed in the early 1970’s to predict the failure pressure of corrosion defects. The calculation for the remaining pressure carrying capacity of a pipeline caused by a corrosion defect based on maximum defect depth, defect width, and the yield strength of the pipe material. If the remaining pressure carrying capacity exceeds maximum allowable operating pressure (MAOP) by a sufficient margin of safety, the corroded segment can remain in service. If not, it must be repaired or replaced.

The advantages and disadvantages of ASME B31G are:

• Advantages
• Extensive experimental
• validation
• Almost always conservative
• Simple to apply
• Disadvantages
• Can sometimes be excessive
• only conservative, resulting in
• unnecessary cutouts
• Does not address complex
• shapes or axial pipe loads
• Empirical basis prevents
• extension to address prob-
• lems of complex defect
• geometries, and complex
• axial and bending loads

The concern of conservatism of B31G led to the development of RSTRENG (Modified B31G) in the 1980s. RSTRENG is based upon a better estimate of the remaining wall thickness of the corrosion defect. RSTRENG was verified by an empirical fit of expression to an experimental database. It is more complex to compute than B31G, but can readily be implemented in tables and spreadsheets. The widespread use of personal computers has led to the development of RSTRENG software. Features of the software are as follows:

• Requires more detailed defect geometry input
• Automatically addresses complex geometry issues
• Provides less conservative results, but continues to have significant scatter
• Recently was accepted by DOT
• Recommends caution when applying to low ductility pipe

The advantages of RSTRENG are:
Significantly less conservative than ASME B31G and will result in fewer cutouts and repairs validated against the experimental database of B31G

The introduction of a fiber-reinforced composite sleeve repair system called Clock Spring® led to the development of a computer software program GRIWrap® for Windows that enables the user to perform a strength analysis on a pipeline defect. The software duplicates B31G tables and estimates range of defect depths and axial defect lengths of a damaged or corroded pipe section to predict required action: 1) if there is no need to repair; 2) whether a Clock Spring can be used; or 3) if the pipe needs to be replaced.

Repair Techniques
Newer fiberglass composite and epoxy filled shell repair systems compete with older, highly accepted weld repair techniques. Pipeline personnel must fully understand the pros and cons of repair options. Typically, pipeline defects have been repaired in the following fashion:
Removal of the pipe as a cylinder

• Grinding the anomaly to reduce effect of stress concentration
• Depositing weld filler metal to replace the missing wall thickness
• Reinforcing with a full-encirclement, nonpressure-containing sleeve (Type A)
• Sealing defect with a pressure containment sleeve or clamp (Type B)
• Removal of defect via hot tapping
• Reinforcing with newer epoxy filled shell sleeve
• Reinforcing with newer fiberglass reinforced composite materials

Defect variables that must be obtained prior to making the repair decision are pipe dimensions, yield strength, defect depth, defect axial length, geometric shape factor, installation pressure, pipeline MAOP, class location, and any other applicable pipeline company standards. The procedures to evaluate whether or not to repair the pipe, in order of decreasing conservatism, are ASME B31G, RSTRENG, and GRIWrap®5.

Pipeline operators must consider an array of possibilities. Table 1 helps summarize repair applications for defects at differing locations, size, and type. An operator would consider the items in order of the pipeline location, defect location on the pipe wall, type of defect, and the defect size. Repair applications marked conditional and special configuration require modification to the fitting by the manufacturer or detailed application knowledge and experience of the repair application.
Weld repairs have long been accepted. Most operators feel comfortable working with welded fittings or mechanical sleeves for repair of pipeline defects. Welding on to in-service pipelines has risks to the pipeline operator. These risks are:

• Pipe wall burn through/blowout
• Hydrogen cracking
• Metal decomposition
• Previous welder-induced defects

These risk factors need to be evaluated prior to choosing a weld-repair method.
Techniques for depositing weld metal have been proven for cases in which a defect is located on a sharp bend inappropriate for a sleeve. Weld deposition repairs are feasible to 900 psi for minimum 0.125 wall thickness pipe. Small diameter low hydrogen electrodes should be used with limited heat input. The restoration of static strength is accomplished with full-thickness weld deposition.
Prestressed welded reinforcement sleeves reduce bulging and hoop stress at the defect. Two general categories exist for reinforcement sleeves. Type A for non-welded sleeve ends and Type B for welded sleeve ends. The major differences between the two types are that Type B sleeves can be used to repair leaking defects, reinforce internal corrosion defects, and are utilized more frequently on larger defect areas. Training in proper welding techniques is extensive for proper installation of these sleeves. Mechanical sleeves require less operator expertise when installing, however are generally more expensive.

Composite reinforcements, specifically Clock Spring®, have undergone vigorous testing and evaluation. Burst testing of Clock Spring repairs have proven that repairs made upon severely defected pipe, up to 80% wall loss, withstand operating pressures in excess of the maximum pressure ratings. A 36-unit statistical matrix was developed by GRI to determine “in-service” integrity. The statistical matrix verified Clock Spring as a valuable new repair-method for the pipeline industry. The Department of Transportation has approved the Clock Spring repair system for repair and reinforcement of both gas and liquids transmission pipelines. Continuing research verifies composite reinforcements as a permanent repair for corrosion-metal loss and mechanical damage. Clock Spring offers the operator the following benefits.

• Composite reinforcements avoid introducing welding associated risk;
• Eliminates cost and service outages associated with conventional repairs, which require pipeline shutdown prior to welding;
• Restores the pipe’s original pressure capabilities and improves resistance to further structural deterioration
• Repair can be made without line-pressure reduction
• Can be installed easily without highly skilled welders and/or specialty equipment; and
• Typical installation time less than 25 minutes after proper set-up.
  However, training and certification is essential to ensure proper composite installation and performance. The simple design of the Clock Spring reinforcement sleeve is shown in Figure 3.

Economic Considerations
The final repair decision should be based upon sound engineering alternatives and the economic impact of those alternatives. Pipeline operators consider many variables impact the economics of the repair decision. Economic consideration can be categorized as the pipe specifications, the pipe defect to be repaired, buried conditions, operating conditions, environmental impact, and labor, material, and equipment requirements. Table 2 summarizes the categories for considerations of repair economics.

A typical pipeline company may excavate a location identified through smart pigging. The operator finds a large corrosion defect on a 20-inch pipeline and must decide on an appropriate repair method. All or some of the economic factors in Table 2 may be considered. Considering all factors equal, a typical breakdown of the cost associated with repairing the defect by varying methods is shown in Table 3.

The cost saving is significant using the newer technology of Clock Spring. The major cost differences are in the labor and other engineering services. Lower cost personnel, because of differences in training and experience needed to complete the job, can install composite sleeves. Economies of scale, with the repair of many or several defects on a single line, make composite sleeves even more attractive as a repair method. The material costs are not that different from one another. Availability may be a problem when needing several 20-inch full-encirclement sleeves.

Conclusions
The two goals of a pipeline inspection are the identification of defects and the sizing of defects. Corrosion defect and pipeline damage classification for prioritizing inspections is imperative for maintaining pipeline integrity. Once identified and prioritized, assessment of the defect(s) must be made. ASME B31G is commonly used to predict the failure pressure of corrosion defects. Other assessment tools are RSTRENG and GRIWrap. These three methods require detailed defect geometry input and detailed defect profiles. Defect variables obtained prior to making the repair decision are: pipe dimensions, yield strength, defect depth, defect axial length, geometric shape factor, installation pressure, pipeline MAOP, class location, and any other applicable pipeline company standards.

Newer fiberglass composite and epoxy filled shell repair systems compete with older, highly accepted weld repair techniques. A pipeline operator must understand the pros and cons of their repair technique decision and the associated cost. Clock Spring composite reinforcement sleeves has provided an alternative repair method for severely defected pipe, up to 80% wall loss. Operators educated in pipeline repair methods can eliminate costly service outages associated with conventional repairs. A thorough understanding of defect identification, assessment, repair techniques, and economic impact must be a part of the modern day pipeliner’s knowledge. The GRI conducts a Pipeline Repair Methods Workshop to convey this information to engineers, operating personnel, and technical specialists involved in pipeline operations and repair decisions. P&GJ

David J. Boreman is Coordinator Corrosion Control Services at Nicor Technologies, Naperville, IL. He has 22 years of experience in the natural gas distribution and transmission pipeline industry, with particular emphasis in corrosion control. He has an A.A.S. degree in environmental science and is a NACE International certified Cathodic Protection Specialist.

Bradley O. Wimmer is North Region Corrosion Operations Supervisor at Nicor Gas, Rockford, IL. He has nine years of experience in natural gas distribution and transmission maintenance activities, with emphasis on corrosion control and system integrity. Wimmer has a B.S. degree in Organizational Supervision, currently pursuing a M.B.A. degree, and is a certified NACE Corrosion Technician.

Dr. Keith G. Leewis (photo not available) is the Program Manager of the Pipeline Integrity Management and Systems Operations division of the GRI. He has 20 years experience in materials engineering in iron and steel making; fracture mechanics; pipeline inspection, welding and fabrication, and adult education. He is liaison with the Interstate Natural Gas Association of America (INGAA) Pipeline Safety Committee, and member of the Corrosion, Offshore and Design, and Welding Supervisory Committees for the Pipeline Research Council International (PRCI).