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Pipe properties: yield-to-tensile ratio (Y/T)
EPRG has been concerned over many years to ensure that appropriate measures for Y/T are incorporated in specifications. Recent studies have focused on the use of probabilistic structural reliability analysis methods to quantify the influence of Y/T on safety margins with respect to various most-probable failure modes; corrosion, equipment impact or ground movement. Based on this work, a Y/T limit of 0.95 appears feasible for all design situations except where high longitudinal strains can be expected.

Effect of Y/T ration on safety factor

Pipe properties: influence of specimen type on tensile test results
EPRG has reviewed a substantial body of results comparing tensile tests performed on flattened rectangular specimens with those performed using round bar specimens. Proof strengths measured using transversely-oriented round bar specimens were generally higher than those measured using transversely-oriented flattened rectangular specimens, the difference becoming more pronounced as the strength increased. The round bar specimens generally represented the yielding behaviour in full-scale pipe tests more closely than the flattened rectangular specimens.

Comparision of yield strengths measured using flattened rectangular anf round bar specimens, with those measured in pipe samples

Pipe properties: strength de-rating factors at elevated temperatures
EPRG commissioned a review of the strength de-rating factors used for line pipe and fittings located in the vicinity of compressor discharges, where the operating temperatures can be above ambient. For most of the steels studied there was a sharp reduction in proof strength at 60°C compared with the room temperature value, but little further change as the temperature increased to 150°C.

Effect of test temperature on proof strength

Drop Weight Tear Testing Procedures
Instrumented Drop Weight Tear (DWT) tests can be used to measure crack initiation and crack propagation energy, but they are not standardised. A round-robin test programme was initiated by EPRG to investigate the reproducibility of test set-up and measurement procedures. Ongoing research focuses on defining standardised test and evaluation procedures for instrumented DWT tests, particularly for smaller diameter pipes.

Round robin test results: Total DWT energy vs. shear area

Methods for evaluating the fracture resistance of high grade steel pipelines
Design to resist ductile fracture propagation has traditionally utilised semi-empirical methods based on Charpy V-notch impact energy. However, recent full-scale fracture propagation tests have shown these methods begin to fall short in characterising the fracture behaviour of higher strength steels such as Grade X80 and higher. Recent EPRG studies have concluded that, while the applicability of the Battelle Two-Curve Method is very questionable for the highest steel grades, no unambiguous alternative fracture parameters are available. The DWT energy is one of the most promising parameters for properly taking into account the fracture event, but it does not establish a precise value for a reliable design. Ongoing work on this subject is focused on the combined effects of geometry, constraint and loading rate on the inherent ductility and capacity for energy absorption in the material close to the fracture path.

Comparision between Charpy V-notch and DWTT energy for high strength steels

Comparison of European (EN) and International (ISO) linepipe standards
The European standard for steel pipes carrying combustible fluids is EN 10208. At the time that EN 10208-2 was issued in 1996, the International Standards Organisation also issued an equivalent standard, ISO 3183:1996; both were subsequently revised. EPRG has examined the outstanding differences between the current EN and API/ISO versions, identifying a number of important differences. These findings will be used to inform the ongoing international discussions regarding harmonisation and improvement of linepipe standards.

Assessment of time-delayed failure
It has long been recognised that a defect in a pipeline, such as a gouge or a dent and gouge, can fail at a constant pressure after some period of time has elapsed. Commonly referred to as time-delayed failure, this behaviour has been observed both in the laboratory and in the field. Modern line pipe steels are more defect-tolerant than older line pipe steels, but are also more susceptible to time-delayed failures. To better understand the implications of time-dependent growth under constant pressure, EPRG commissioned a study to identify the conditions when time-delayed failure should be considered and provide a basis for avoiding the problem.

Results from the study have confirmed that higher toughness steels can sustain longer and deeper defects that are more prone to bulging than the shallower or shorter defects in the low to moderate toughness steels. Significant local wall thinning occurs in the net ligament in higher toughness steels; bulging and wall thinning introduce significant non-linearity that are not inherent in lower toughness steels. A model has been developed that gives reasonable predictions of crack growth and leak versus rupture and shows no bias in the predictions with respect to toughness. A test programme is being initiated in order to confirm the model.

The integrity of high-frequency welded pipe seam welds with low toughness
EPRG member companies have reported problems with electric resistance welded and high-frequency welded pipe where the bond line has shown low Charpy impact energies. In some cases the issue has emerged during pipe manufacture or pipeline construction. In other cases the problem has been discovered when pipe removed from service, sometimes after many years of problem-free operation has been found to have low bond-line toughness.

EPRG commissioned a review of industry experience. The main conclusions were that, to detect low toughness bond lines, the notch of the Charpy specimen must be located within 0.2 mm of the bond line. Fitness for purpose methods can be used to assess low bond line toughness, but the analysis should be supplemented by testing.

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