Background and History
Uncoated steel natural gas and hazardous liquids pipelines are also known as bare steel pipelines. While many of these pipelines have been taken out of service, and no longer transport these commodities to customers, some of them continue to operate today. The typical age and the lack of a protective outer coating have to be considered by the pipeline operators, and this may lead to accelerated replacement or rehabilitation of bare steel pipelines.
Bare steel pipe was used extensively in natural gas and hazardous liquids pipelines until the 1960’s, when the use of plastic pipe expanded for natural gas distribution systems and Federal regulations came to fruition. Until pipeline coatings were required with the establishment of federal mandates in 1971, some natural gas transmission and distribution operators continued to install pipelines without coatings, particularly in dry areas of the country.
The lack of an outer coating, which helps to protect the steel from the environment, makes a high level of protection from corrosion, and careful assessment, necessary. This typical protection is referred to as cathodic protection. Methods used to determine the effectiveness of cathodic protection on bare steel pipelines focus on identifying larger corrosion cells, called “hot spots”. However, small, localized corrosion areas, receiving insufficient cathodic protection, are difficult to identify and can lead to integrity issues.
Integrity Management Programs
Pipeline integrity, refers to a system that is in sound, unimpaired condition that can safely carry out its function under the conditions and parameters for which it was designed. An integrity management program is a documented set of policies, processes, and procedures that are implemented to ensure the integrity of a pipeline, and these plans are required within the Federal regulations for natural gas distribution and transmission pipelines, as well as for hazardous liquids pipelines.
Assessing the integrity of bare steel pipelines can be more difficult than for other coated pipelines and the assessment methodology employed must be carefully selected. Magnetic flux leakage is most commonly used to assess pipelines, in order to identify areas of metal loss. In the case of bare steel pipelines, external corrosion can mask some of the magnetic leakage, which is critical for this technology, and cause measurement inaccuracy for the depth of metal loss. Bare steel pipelines are also difficult to inspect with external corrosion direct assessment (ECDA) because many ECDA tools rely on indications of coating faults and coating conditions which are not applicable to uncoated pipelines. Inspection tools that make direct measurements, such as ultrasonic metal loss tools, can yield better assessment results on bare steel lines.
To assess the level of protection from corrosion imposed on the pipe, cathodic protection surveys can be used to identify areas of active corrosion and interference currents. Additionally, leak detection surveys may be performed at frequencies beyond the minimum federal requirements, and can help to identify leaks expeditiously.
Often as a result of an operator’s integrity management programs, bare steel pipelines are a focus for many pipeline replacement and rehabilitation programs across the country. In areas where pipe replacement by excavation is not feasible, such as certain urban downtown areas, bare steel pipe may be inserted with new pipe, often plastic, instead of being replaced outright.