Where the Pipeline Meets the Permafrost
Improved corrosion assessment techniques promise to reduce costly excavations.
by Marina Q. Smith
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Senior Research Engineer Marina Smith of the Materials and Structures Division was part of an Institute team present at three trans-Alaska pipeline corrosion-assessment digs earlier this year. All three digs were located on the Sagavanirktok River, between 75 and 80 miles from Prudhoe Bay. Most excavations to assess damage to the pipeline are conducted during the winter season, when river bottoms through which the pipe runs are frozen. Observations made during the trip are aiding development of improved methods of modeling complex failure modes in large pipelines. |
After nearly 20 years of operation, the trans-Alaska pipeline has experienced anticipated and normal levels of settlement and/or corrosion. The freeze-thaw action of arctic permafrost subjects buried sections of the pipeline to significant axial bending, and oil temperature fluctuations introduce high thermal stresses in the pipe wall. Settled areas are identified reliably using an internal measurement device called a "smart pig," which defines pipeline curvature based on changes in pig orientation. A smart pig may also be used to measure changes in pipe wall thickness, thereby identifying areas of external corrosion in the pipeline. Though these measurements are highly accurate, they do not necessarily indicate how the settled or corroded areas affect pipe strength. Settled portions of pipe may need to be excavated and leveled to guard against wrinkling of the pipe wall, and corroded areas may need to be excavated and sleeved to guard against rupture.
Because substantial cost and environmental risk are involved, quantitative rather than just experimental procedures are needed for determining whether or not to excavate. For the last six years, Southwest Research Institute has examined the effects of combined loading and corrosion on pipeline integrity for the Alyeska Pipeline Service Company, which operates and maintains the Trans Alaska Pipeline system. Currently, Institute engineers are developing analytical techniques and experimental tests to help extend the service life of the pipeline.
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The trans-Alaska pipeline stretches from the northern settlement of Prudhoe Bay on the Arctic Ocean, across the Brooks, Alaska, and Chugach mountain ranges, to Port Valdez on the state's southern coast. Half of the 799-mile pipeline is buried and half rests on supports aboveground, to avoid permafrost melting that can lead to structural and environmental damage. Up to two million barrels of crude oil are pumped through the four-foot diameter pipeline daily, cooling from an average temperature of 49 degrees Celsius at Prudhoe Bay to about 21 degrees Celsius at Port Valdez. |
Alyeska Pipeline recently conducted experimental and analytical studies at the Institute that led to a greatly improved understanding of pipeline behavior. One result of these studies is the Shell Analysis Failure Envelope (SAFE) computer program developed by SwRI. Using this program, the safety of corroded pipe can be assessed and a decision made regarding whether to excavate a specific corroded area and whether to add a sleeve to the exposed area.
SAFE considers conditions to limit states, namely pipe rupture and wrinkling. Engineering calculations embedded in the SAFE program to predict the rupture capacity of a given corroded section have been calibrated and validated against full-scale experiments and finite element analyses developed using the ABAQUSTM computer program. However, the wrinkling limit state has not been comprehensively studied. Therefore, current and proposed work for Alyeska Pipeline consists of developing a suitable experimental and analytical database for the definition and calibration of SAFE engineering calculations to predict the wrinkling capacity of a corroded and settled pipeline section.
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Protected from subzero temperatures by a heated tent-like structure, workers take detailed measurements of corroded sections of pipe using grids that allow a map of the area to be developed for integrity assessment analyses. The measurements are compared with those estimated by a smart pig and then analyzed to determine the remaining strength of the corroded region. |
Institute researchers have formulated a combined experimental and analytical approach to provide a comprehensive wrinkling calibration database. Full-scale experiments will be conducted to provide a failure point for the database and to allow the qualitative comparison of experimental and finite element simulations of the pipe tested.
A total of nine data points, in addition to data from a representative test case, are required to provide a comprehensive database for calibration of engineering calculations. In addition to the recently completed representative test case, a minimum of three additional full-scale experiments have been proposed for 1997. Using the SwRI pipe test facility, sections of 48-inch diameter pipe will be evaluated under combined pressure, axial, and lateral loading conditions representative of those in arctic regions. These conditions will concentrate bending deformations within the thinned region at the test specimen center. The thinned region is machined to a reduced wall thickness that represents a general corrosion defect. For each experiment, a single test parameter such as applied load or thinned region size will be perturbed from the representative case. At program completion, wrinkling capacity will thus be defined over a range of conditions representing anticipated upper to lower bound loading and corrosion.
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A full-scale representative test at SwRI served to define baseline pipe wrinkling capacity for typical loading and corrosion flaw sizes observed in the field. During the test, an internal pressure of 500 psi and an axial load equivalent to a 24 degrees Celsius temperature change was applied to a 15-inch long, 15-inch wide, and 15-percent deep (based on a wall thickness of 0.496 inches) general corrosion flaw. The 48-inch diameter pipe section was subsequently loaded in four-point bending until wrinkling of the pipe wall occurred in the thinned region. This level of bending and axial loading was held constant as the internal pressure was increased until rupture of the pipe in the thinned region was produced at 1,654 psi. [Photograph courtesy of J.A. Maple and Associates] |
An SwRI internal research project has produced an improved analytical procedure that will be used for the SAFE finite element test simulations. This procedure, which differs from corroded pipe simulations performed previously at SwRI and elsewhere, employs finite strain shell elements in a three-dimensional model of the pipe, as well as an advanced plasticity material model, to achieve wrinkling of the pipe wall in the thinned region. Initial phases of the project focused on implementing the material model in ABAQUS and validating the procedure through a comparison with tension and compression mechanical tests of pipe material samples. The analytical procedure has been employed successfully to simulate the representative test case, thus ensuring the validity of the approach and its applicability to the Alyeska Pipeline test program, as well as to other pipeline safety investigations.
Published in the Fall 1996 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.
