Southwest Research Institute (SwRI) News
Printer Friendly VersionA longer life for the trans-Alaska pipeline
Pipeline integrity critical to environment, economy
San Antonio -- November 25, 1996 -- Southwest Research Institute™ (SwRI®) engineers are helping extend the service life of the trans-Alaska pipeline. The structural integrity of the pipeline is critical for both environmental and economic reasons. Traversing hundreds of miles of pristine Alaskan wilderness, the pipeline supplies roughly 40 percent of the nation's oil needs at a rate of up to two million barrels of crude oil a day.
Placed into service in the late 1970s, the 800-mile long, 4-foot diameter pipeline has experienced anticipated and normal levels of settlement and corrosion. The freeze-thaw action of the permafrost subjects buried sections of the pipeline to significant axial bending, and oil temperature fluctuations averaging ± 50 degrees F introduce high thermal stresses in the pipe wall.
The Alyeska Pipeline Service Company, which operates and maintains the Trans Alaska Pipeline System, recently conducted experimental and analytical studies at the Institute that led to an improved understanding of the combined effects of loading and corrosion on pipeline integrity. One result was the SwRI-developed Shell Analysis Failure Envelope (SAFE) computer program, which allows an analytical assessment of the safety of corroded pipe and a more accurate, cost-effective indicator of when a specific corroded area should be excavated for repair.
SAFE is designed to predict pipe rupture and wrinkling. Engineering calculations embedded in SAFE 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 ABAQUS computer program. However, the wrinkling limit state has not been comprehensively studied. Therefore, current and proposed work for Alyeska will produce 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.
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. Once the finite element procedure has been validated against detailed experimental results, it will be used to build the rest of the database.
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 in a 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 area size will be derived from a representative case. At program completion, wrinkling capacity will thus be defined over a range of conditions that reproduce anticipated upper to lower limits of loading and corrosion.
An improved analytical procedure has been developed for use in
the finite element test simulations. This unique procedure employs finite strain shell
elements in a three-dimensional model of the pipe, as well as an advanced plasticity
material model, to simulate wrinkling of the pipe wall in the thinned region. The
analytical procedure has been proven experimentally, thus ensuring the validity of the
approach and its applicability to the Alyeska test program, as well as to other pipeline
safety investigations.
For more information about the SwRI Alyeska program, contact Deborah Deffenbaugh, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510, Phone (210) 522-2046, Fax (210) 522-3547.
