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British Gas uses ultrasonic vehicle for assessing pipeline integrity.

Assessment of a pipeline for fitness-for-purpose quantitatively relates severity of a defect conditions evaluating and ductile fracture, defect growth, and crack propagation behavior using fracture mechanics. Consequently, assessment of this may allow larger defects beyond those permitted by good workmanship requirements of a fabrication code without reducing safety or integrity. Acceptability of fitness-for-purpose is recognized by U.S. Standards API 1104 and ASME XI; British Standards BS 4515 and BS 5500; and Canadian Standard CSA-Z284-M192.

British Gas' Pipeline Integrity International uses a tool which detects longitudinal stress corrosion cracks and fatigue cracks which often are not detected by magnetic or other nondestructive testing. The inspection vehicle travels through the pipeline and transmits ultrasonic signals that are pulsed circumferentially around the inside diameter wall. Longitudinal and radial defects become reflectors which send the pulsed sound waves back to instrumentation on the vehicle. All ultrasonic data is recorded by the vehicle. Transducers are mounted in couplant-filled wheels so the vehicle can work in natural gas or liquids pipelines without requiring a liquid interface or slug.

Defects Which Cause Failure

Since pipeline steels are ductile, pipe body defects generally do not pose brittle fracture risk. Failure mode is usually by plastic collapse. Severity of a defect is related both to its depth in the pipe wall and its length along the pipe axis.

Defects in pipeline girth welds require assessment for both brittle and ductile failure. Since the defects are usually oriented circumferentially, stresses in the axial direction of a pipeline also must be used in determining severity of defects. For older pipelines, defects in longitudinal welds should be evaluated with fracture and fatigue assessments.

Plain, smooth dents with no associated metal loss and up to 8% of pipe diameter have little effect on failure pressure of line pipe. However, plain dents can exhibit short fatigue lives and should be assessed for fatigue failure. Dents containing welds can have low failure pressure and fatigue lives and should be repaired.

Dents with gouging and its associated cracking are the most severe defect. They are unstable and usually fail at low pressures and have very low fatigue lives. Assessment of failure stress can be estimated by a semi-empirical relationship which is being updated by British Gas and will be included in a new European CEN standard.

Will A Pipeline Defect Leak or Rupture?

Axial length of a defect and hoop stress determine whether failure through wall will be a stable leak or an unstable rupture. Ruptures tend to occur with long defects or at high hoop stresses. In North America, there were eight reported ruptures between 1984 and 1990 which were caused by corrosion defects. Stress corrosion cracking and hydrogen induced cracking tend to congregate in forms which interact and cause rupturing rather than leaks.

Depending on toughness of the pipeline steel, a rupture will either arrest within the joint length or propagate as a brittle or ductile crack. Propagation in the brittle fracture mode is related to crack speed and arrest occurs if speed falls below de-compression speed. Brittle cracks propagate at speeds up to 2,000 ft. per second. This speed is slower than decompression speeds of liquids but higher than decompression speed of natural gas (1,400 ft. per second). Therefore, brittle cracks can propagate almost indefinitely because initial gas pressure is continuously maintained at the crack tip.

A running ductile crack travels at a speed of 200 to 800 ft. per second which is lower than initial gas decompression speed. However, the complex nature of the decompression process and transfer of gas energy to the crack tip by bulging cause a ductile crack to propagate at a constant speed for a long distance. Arrest occurs if the Charpy upper shelf energy exceeds a value which depends on pipe geometry and operating conditions.

Defect Assessment

Assessments were made to: (1) evaluate detected defects, (2) formulate safe operating strategies for corroding pipelines, and (3) determining if the pipelines were suitable for uprating or life extensions. High resolution internal inspection of onshore and offshore gas and liquid pipelines has detected defects which were not acceptable to existing pipeline codes.

In the North Sea, 105 miles of 32-in. gas pipeline were inspected and external metal loss features investigated. All features were similar and of depths to a maximum of 48% wall thickness and within 16 in. of a girth weld. These features were associated with the cutting off of field-applied concrete at double joints before replacing it with stronger reinforced concrete. Fitness-for purpose calculations showed that all features were insignificant with respect to current operating conditions and no repair was necessary.

Five cracked girth welds were found in an inspection of 265 miles of Ekofisk-to-Emden 36-in gas pipeline for Phillips Petroleum Co. The most extensive crack was reported onshore and was removed to confirm both presence and size of the crack. A program was initiated for designing and building a test rig for full-scale testing of defective girth welds subjected to internal pressure, external bending, and low temperature. As a result, it was decided that the four remaining cracked girth welds would have no effect on pipeline integrity and no repairs were necessary.

Another inspection focused on assessing a cracked tie-in weld in the expansion pipework at the bottom of a riser. Since there was no available data relating to static and environmental stresses or toughness of the weld, a fitness-for purpose assessment could not be done immediately. Finite element analyses under saturated diving conditions in a hyper-baric chamber and subsequent toughness testing were used to assess fracture and fatigue significance.

Other defect assessments have included evaluation of manufacturing and fabrication defects, mechanical damages, stress corrosion cracking, and pitting or general corrosion.

Methods for internal corrosion assessment usually rely on peak depth of corrosion and minimum wall thickness of the pipe. In this case, probability of failure increases as the pipeline ages since corrosion increases in size. However, as the corrosion rate declines, probability of failure also decreases. Studies have shown that there is a rapid increase in probability of failure at 20 years.

New assessment methods have been necessary since internal spiral corrosion grooves have been detected in offshore pipelines due to preferential corrosion of spiral welds. External spiral corrosion has been seen in onshore pipelines due to failure of the joint in tape applied coatings. Existing assessment methods were empirical and tended to be overly conservative. British Gas has conducted rigorous analytical and finite element studies for its development of simple failure criteria for this spiral corrosion.

With these assessments and data gained from the ultrasonic inspection vehicle, pipeline operations should become much safer in the future. In some cases, pipeline operators will be able to save unnecessary repair costs and line replacements.

With over 25 yrs. experience with pipeline operations and research, British Gas has gained a precise understanding of the types and sizes of defects which can cause failure and the nature of such failures. These parameters have been established for every pipe type, diameter, wall thickness, grade, and weldment used in gas and liquid pipelines.
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Publication:Pipeline & Gas Journal
Date:Dec 1, 1994
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