Testing for fitness of service: conversion of products pipeline for compressed gas service.
Consider that 77% of oil and products pipelines operating today were constructed prior to 1970 with 45% constructed in the 1950s and 1960s, as reported by Kiefner & Associates, December 2001. The US fuel and products transportation system relies on a delivery system that is, on average, 45 years old. Regardless of the age of the infrastructure, pipeline transportation has been and still is the safest and most economical method for the movement of liquid products.
The decline in domestic oil production has shuttered old refineries and production facilities, leaving thousands of miles of oil and products pipelines idled, abandoned or underutilized. Natural gas fired power generation, cogeneration and expanded residential and commercial use is driving demand above existing pipeline capacity. High construction costs are forcing operators and investors to consider conversion of idle pipelines to meet growing demand.
The first steel pipe was produced in the late 1890s. Early pipe manufacturers used furnace butt welding where the seam was formed by heating the edges red hot and forcing them together under pressure to form a bond. Larger diameter mills used furnace lap welding where the long edges of the plate were scarfed or tapered, heated, and forced together to form the bond. The quality of the seam was dependent on proper control of the exothermic heating to maintain a proper weld temperature to extrude all the oxides from the bondline, a difficult task to control in the early 1900s. In the mid-1920s, the electric resistance weld (ERW) seam process was commercialized and became the standard for pipe manufacture. By 1970, most mills had converted their ERW processes from direct current or low frequency alternating current to high frequency current welders, greatly improving the quality of the pipe seams.
In the early 1960s, basic oxygen process for steel manufacture was in place and the Bessemer process, furnace butt weld and lap weld pipe were discontinued. API specification 5L became the mandatory specification for inspection and testing of line pipe, greatly improving steel quality control among mills and between heats in the same mill.
Except for a handful of mills, early pipe steel prior to the 1960s contained high sulfur and coarse grain structure resulting in low ductility and poor weldability. To make matters worse, the early low frequency welding process produced further grain coarsening in the heat affected weld zones, making them prone to brittle fracture, hook cracks and grooving corrosion. This phenomenon also held true for the girth weld zones as well.
What affect do the aforementioned mechanical properties have on conversion from liquid to gas service? A pipeline transporting a non-compressible fluid contains much less energy than a pipeline transporting compressed gas at the same pressure. In the event of a pipe failure, the liquid pipeline will release its energy during depressurization from operating pressure to atmospheric pressure. Since the liquid is non-compressible, there is no decompression of the product. The opposite is true of compressed gas, which will violently decompress prior to depressurization, in the event of a pipe failure.
When contemplating conversion of an older pre-70s pipeline from liquid to gas service, an intensive due diligence should be performed considering leak history, seam failure, material test data, construction methods, and corrosion history.
For pre-1963 pipe, metallurgical testing of several sections of pipe is recommended. Specifically, longitudinal and traverse charpy tests, metalographic examination on the parent metal, seam welds and girth welds and chemical analysis of the parent metal to determine ductility, base metal composition and grain structure. Early pipeline weld zones typically may exhibit transition temperatures (ductile to brittle) at ambient temperatures. Under these conditions, a critical flaw could become an uncontrolled long running brittle fracture under normal operating conditions. A fracture would be pushed by the gas decompression wave until absorbed by a tougher section of steel, fitting or valve causing violent energy release and property damage. This type of pipe should not be considered for compressed gas service unless major sections of the line are replaced.
Testing and Inspection:
Assuming the weld zones and parent metal are satisfactory, several methods are available for determination of the pipeline's fitness for service.
Hydrostatic testing is a proven method for determining the pipeline integrity. However, traditional hydrotesting to 1.25X or 1.5X operating pressure of early steel pipelines may result in enlarging flaws not exposed by the test. Such flaws can grow over time resulting in subsequent failures before the next assessment period. A spike test may be performed where the test pressure is brought up to 100% of pipe yield strength and held for 5 to 10 minutes then reduced to the required pressure for test duration. The spike test, in many cases, will expose critical flaws and provide a larger margin of safety and is accepted by DOT to confirm the integrity of seams.
Modern internal inspection tools are capable of detecting crack-like defects in pipe such as SCC, ERW defects, and hook cracks. Ultrasonic crack detection, Traverse Field Inspection (TFI) and Electro Magnetic Acoustic Transducer (EMAT) tools are now able to detect and locate flaws in pipe that would not be exposed under traditional hydrostatic testing. However, if the pipeline is not piggable, which will be the case with many older pipelines, then the only alternative is a hydrotest.
General Manager--Pipeline Services
Praxair Services, Inc., Houston, TX
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|Title Annotation:||2004 Capabilities Guide|
|Publication:||Pipeline & Gas Journal|
|Date:||Jul 1, 2004|
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