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Develop and demonstrate new non-destructive evaluation methods to quantify remaining strength of line pipe steel and or pipeline fittings

Description:

The U.S. Code of Federal Regulations (CFR) Title 49, Parts 192 and 195 stipulates that ASME B31G or RSTRENG be used to assess the remaining strength of corroded pipe. A review of existing burst test data raised some concerns that use of these methods can, in some instances, result in predicted failure pressures that are greater than the recorded burst pressures from actual tests. No burst testing data exist on steel pipeline fittings.

 

Industry has also researched methods for assessing the remaining strength of corroded pipelines. This has led to the development of new criteria and has extended the range of assessment methods to include numerical analysis. While there has been substantial progress, there are areas where the existing criteria require improvements, including steel pipeline fittings. Issues identified include limitations on the interaction of closely spaced defects, the effects of external loading, and cyclic pressure loading. Furthermore, as operators start to use higher strength materials, there will be an increasing need to assess the integrity of high strength steel pipeline fittings that have been corroded while further validating the application of existing criteria and models for these materials.

 

Past work by industry and the U.S. Department of Transportation’s Pipeline and Hazardous Safety Administration (PHMSA) has funded research to address these issues in recent years on pipeline steels. The work has included a program of materials testing, finite element (FE) analyses, and full scale burst testing to develop methods for assessing corrosion damage in pipelines of strength grade up to X100. Reports from this work are available at: http://primis.phmsa.dot.gov/matrix/PrjHome.rdm?prj=171

Background:

Corrosion metal loss is one of the major damage mechanisms to transmission pipelines worldwide. A corrosion metal-loss defect further reduces the strength of the damaged pipeline sections while introducing localized stress and strain concentrations. Several methods have been developed for assessing the remaining strength of corroded pipelines, such as the ASME B31G and RSTRENG models. These models were derived from experimental tests and theoretical/numerical studies of the failure behavior of corroded pipelines. The test pipes contained either corrosion metal-loss defects or simulated metal-loss defects and featured materials with relatively high toughness properties for X65 and above and lower toughness properties for X60 and below. The early burst tests used vintage pipe with low toughness properties. Plastic deformation and collapse of the ligament or surrounding material determines the failure behavior of the corroded pipe. In principle, the existing assessment methods are only applicable to pipelines with toughness levels that are sufficient to prevent a toughness-dependent failure.

 

The research completed did not include analysis of burst test data on steel line pipe with real corrosion defects in strength grades above X65, as the data were not available.  To address this gap, a focused program of full-scale tests is recommended on higher strength line pipe of strength grades above X65 with electro-chemically induced, simulated corrosion defects. These defects can be produced using electrochemical means to approximate real corrosion in the field, as opposed to flat-bottomed rectangular machined patches. Failure pressure predictions using ASME B31G, Modified ASME B31G, and RSTRENG should then be compared to the recorded burst test pressures to confirm that these methods are applicable for higher strength pipelines.

 

Mechanical properties of pipe metal help define the principal characteristics of its technical state. These properties can change (degrade) during long-term operation not as a result of an aging process but rather from exposure to cyclic pressures, extreme temperatures, excessive forces or detrimental environmental conditions.  Heat input during the coating process may change these properties on the pipe surface but not necessarily throughout the thickness of the pipe wall. Developing new methods for pipeline technical diagnosis and evaluating a line pipe’s actual technical state will help ensure the pipe's safe lifetime operation.

 

Sub-topic challenge– Proposals are being sought for the development of future guidance and consideration of the background factors described above. The descriptive physical model of impact strength change effect on the pipeline’s actual technical state needs to be investigated. The objective of this sub-topic is to determine the next steps after an operator determines the mechanical properties of the steel line pipe and or pipeline fittings are insufficient. Issues to specifically be considered when developing and demonstrating new non-destructive evaluation methods can/should include:

  • Is hardness (other method) a good indicator for remaining strength of steel line pipe and or pipeline fittings?
  • How are variable steel properties in thickness of material and at different surface locations taken into account in determining strength?
  • Are some example cut-out calibration material samples required for determining uncertainties and if so at what frequency?
  • What are the recommended procedures to be used and uncertainties?
  • Will hardness testing be an iterative process to be conducted at various time or distance intervals?
  • How does the intended methodology assess and evaluate the threat?

 

Proposals may consider the following attributes:

1.  The variation of mechanical properties resulting from changes in the operational parameters. Long-term operating conditions in corroded pipe may lead to the degradation of stress and strain resistance capacity of the material and an increasing sensitivity to stress concentrators and defects.

2.  The material steel rolling/manufacturing processes, chemical composition, any heat treatment for fittings, and strength.

3. The magnitude of critical brittle temperature, which is the temperature where the nature of a material’s fracture changes from ductile to brittle.  This temperature is determined by fracture energy. It is determined by the energy used for fracture. Impact strength value is the figure of this energy. The reduction of impact strength could cause an increase of cold shortness temperature to the range of operation temperature of pipeline steels.

 

Expected Phase I Outcomes:

A successful Phase I will demonstrate, through mathematical models and scientific analysis, a determination as to whether hardness is a valid indicator of remaining strength for pipe and or pipeline fittings.

Expected Phase II Outcomes:

Phase II will include the validation and testing of potential models that predict the remaining strength of pipe and or pipeline fittings based on hardness or other properties.

 

 

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