GL Noble Denton has over 30 years experience of ring tension testing and has tested a wide range ofmaterial grades, pipe sizes and defect combinations. GL Noble Denton's ring tension rigs cover the full range of pipe sizes from
273 to 1,422 mm. Rings may be free from defects or, alternatively, a wide range of defects can be introduced in order to simulate the types of damage found in pipelines, namely:
- Reduced ligament thickness - metal loss due to general corrosion
- Machined slits (parent metal, seam weld, heat affected zone) - cracks, gouges
- Fatigue cracks - cracks
- Dents - mechanical damage
- Dent/slit combinations - mechanical damage
Pipe Yield and Deformation Behaviour
Ring expansion tests allow an accurate measure of the yield and deformation behaviour of the full pipe wall in the "hoop" direction. Flattening of curved pipe segments to produce test specimens introduces tensile and compressive strains that change the subsequent behaviour of the sample.
Round bar specimens machined from the weld or pipe segment avoid the issues associated with flattening, but do not sample the full wall thickness. The database of ring expansion results, originally developed in the 1960s, pro-vides the basis for interpreting small-scale specimen data both for design purposes and for setting purchase specifications. New pipe manufacturing and processing methods and higher pipeline grades have introduced uncertainties in the previous relationships.
New ring expansion data provide the essential confi rmation of both material purchasing specifications and pipeline designs based on conventional tensile specimen data.
The Bauschinger effect is more pronounced in X100-grade pipe steel than lower strength grades, giving rise to bigger differences between flattened strip and round bar tensile properties. Several leading manufacturers and operators collaborated in a joint industry project to establish relationships between pipe section behaviour and small-scale specimen behaviour, to underpin pipeline designs and material specifications utilizing X100-grade pipe.
Significance of In-Service Damage
Ring expansion tests provide a cost-effective means of establishing the significance of any in-service damage occurring in pressurised pipelines, for example: metal-loss corrosion, mechanical damage and crack-like defects.
Damage assessment methods have traditionally utilised small-scale tensile or bend-loaded test specimens to develop analytical methods of assessing the structural significance of sharp and blunt in-service damage, and set criteria for safe operation or remediation. However, confirmation of the analyses has required expensive full-scale burst testing of pipes containing representative damage. Ring expansion tests are a low-cost method of developing and confirming the suitability of the assessment methods for a wide range of component and defect geometries. Testing has been used to develop general guidance and also to assess the significance of specific defect types for individual customers.
Results from ring expansion tests undertaken over many years have been used to establish the guidelines for assessing mechanical damage due to external interference for the UK pipeline system. In anticipation of the first use of X 80-grade steel, a systematic study of the damage tolerance of 48"-diameter X 80 was undertaken, extending the range of applicability of the generic guidance.
At a late stage in the construction of a major offshore pipeline, a client discovered instances of low seam weld toughness. A programme of ring expansion tests established the potential for failure in the event of mechanical interface damage in the vicinity of the seam weld. The results defined the most critical conditions, which were subsequently evaluated using full-scale tests. The exercise enabled the client to accept the pipe and avoid lengthy project construction delays.
