Originally written by: J. S. Mandke, Southwest Research Institute, San Antonio.
Corrosion is the leading cause of failures of subsea pipelines in the U.S. Gulf of Mexico. Third-party incidents, storms, and mud slides are additional principal causes of offshore pipeline failures. These are among the major conclusions of an analysis of 20-year pipeline-failure data compiled by the U.S. Minerals Management Service. For small size lines, additionally, failures due to external corrosion were more frequent during the period studied than internal corrosion. In medium and large-size lines, failures due to internal corrosion were more frequent than those due to external corrosion. Also, the majority of corrosion failures occurred on or near the platform and among the small-size pipelines. The motivation for the study described here was to perform a more in-depth evaluation of the pipeline failure data for the Gulf of Mexico than reported earlier, using an extended data base for the period 1967-87, and to compare the results with those reported earlier. The study results presented here provide an improved basis for assessment of safety of pipelines and for further improvements to current pipeline design, inspection, maintenance, and construction procedures.
Failure Data Analysis
The significant components of a typical offshore pipeline system transporting hydrocarbons are: Platform risers, expansion loops or thermal offsets, subsea valves and fittings, tie-in spools, and the main trunk line or the infield flow line. An understanding of the varying risks of damage and their consequences associated with these components can be developed from an evaluation of the historical data on the reported pipeline failures.
Failure data on offshore pipelines are not readily available for all regions of the world. Most of the reported information is on the pipelines in the Gulf of Mexico and the North Sea. In the U.S., the Department of Interior's MMS has kept a record of offshore pipeline failures since 1967. No other data source with comparable details is available in the public domain on failures of offshore pipelines. Failure data published by the MMS' for about 690 failures that occurred during 1967-87 was compiled into a personal-computer data base.
Although the MMS data on pipeline failures are the most comprehensive source of information available, the information for some of the failures reported is either insufficient or unclear. In those instances, some judgment and assumptions had to be exercised during compilation of these data. This did not affect the actual results, however, because the emphasis of this study has been on detecting the overall failure trends for offshore pipelines rather than the absolute numbers on failures.
Pipeline Failure Causes in GoM
- Material failures. Material failures include instances where the pipe material ruptured or the weld cracked and failed. Equipment failures were primarily due to leakages or malfunctioning of fittings such as flanges, clamps, valves, etc. Out of the 60 total failures that were grouped under this category, about 23% were attributed to material failure, and the remaining 77% were attributed to equipment failure.
- Operational problems. Only seven failures were attributed to operational problems. These were mostly the result of lines being overpressured either during the normal operation or the pigging operation.
- Corrosion failures. Three subcategories comprise corrosion failures. In the first two cases, the failure was clearly identified as the result of either internal or external corrosion. In the third case, the origin of the corrosion was not clearly identified. We will refer to this as general corrosion. Out of the 343 total cases of corrosion failures, 15% resulted from internal corrosion, 46% from external corrosion, and 39% from general corrosion. Further evaluation of these data showed that for the smaller-sized pipe, external corrosion failures were more common, whereas for medium and larger-sized pipe internal corrosion was more common. This latter observation is consistent with the observation made by Andersen and Misund. About 78% of the total corrosion failures occurred on the platform, in the riser section or its vicinity on the seabed, and 20% occurred on pipelines on the seabed away from the platform.
- Storms, mud slides.
The analysis of the failure data presented here has indicated significant trends in pipeline failures. It is customary to convert the failure data to probability of failure or the failure rate per km-year or mile-year of the pipeline. Because the appropriate actuarial details on these failures were not available, probabilistic analysis of the failure data could not be performed. Corrosion is the leading cause of pipeline failures. It is followed by third-party incidents and storms and mud slides as the other principal causes of offshore pipeline failures in the Gulf of Mexico.
References: “Corrosion Causes Most Pipeline Failures in Gulf of Mexico”. http://www.ogj.com/articles/print/volume-88/issue-44/in-this-issue/pipeline/corrosion-causes-most-pipeline-failures-in-gulf-of-mexico.html. January 2014.