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The effect of mean stress on the fatigue behaviour of pipes

Fatigue test data on flaw-free pipes (corrected where necessary according to mean stress) compared to DIN 2413

Design codes do not generally address the high mean stress loading seen by pipelines. To address this concern, EPRG has undertaken a programme of full-scale fatigue tests on welded and seamless pipes. The results were compared initially with DIN 2413. Seam weld type and manufacturing route had as much influence on fatigue life as mean stress, resulting in substantial scatter; however, a lower bound to overall behaviour could be discerned. The design approach adopted in IGE/TD/1, based on growth of a defect surviving the hydrostatic test, appeared quite conservative when applied to the nominally defect-free pipes tested.

Update of the EPRG Tier 2 guidelines on girth weld defect assessment
The current EPRG Tier 2 guidelines for girth weld size limits were published in 1996. They ensure that girth welds containing the maximum allowable defect size will survive a remote axial strain of at least 0.5%, provided the material’s yield-to-tensile strength ratio is smaller than or equal to 0.9, the weld metal is matching or over-matching and there is sufficient Charpy toughness in the region containing the defect. However, the guidelines contain several restrictions on their application, particularly concerning the through-wall height of the defect, the material grade and the pipe wall thickness.

EPRG commissioned a study reviewing the technical basis for the original development and validation of the 1996 Tier 2 guidelines, in the light of the additional experimental data and analyses now available. Proposed extensions to the guidelines now allow consideration of through-wall defect heights up to 5 mm and pipe grades up to L555 (X80).

Dependence of remote strain on defect area ratio, strength mismatch and Y/T ratio — collated wide plate test results

The Tier 2 limits can be used when defects exceeding the workmanship standards are discovered during a post-construction inspection or audit, but are not intended to allow poor quality workmanship; for new construction, defect acceptance should be based on welding workmanship standards such as EPRG Tier 1 or API 1104.

The burst strength of low-toughness pipes containing gouge damage
The EPRG guidelines for assessing gouge-like mechanical damage to are based on an experimental database that contains comparatively few results relevant to older pipelines, many of which are known to have low toughness. To address this issue, EPRG conducted a programme of analytical and experimental work with the aim of determining a toughness threshold above which behaviour could be described using the toughness-independent criterion. However, it appears that Charpy energy alone may not be the most appropriate discriminator, and a criterion based on the constraint level has been proposed.

Hooking of pipelines by excavating machines
In an extension of earlier work concerned with the penetration of pipelines by excavators, EPRG commissioned a short study into the affect of 'hooking', whereby an excavator bucket becomes stuck under the pipeline and the machine pulls the pipeline upwards out of the ground. A simple model was developed; this showed that puncture was the more likely limiting event in most instances.

Seamless line pipe for offshore applications
As part of a longer-term project to establish the properties of line pipe for offshore applications, EPRG commissioned a study to determine the influence of reeling and straightening on seamless pipe properties. Reeling increased the pipe ovality, but there was no discernable variation in compressive strength around the pipe circumference, and no significant change in collapse pressure. Ageing at 250°C, in line with DNV rules, was effective in restoring and improving the collapse resistance.

Calculating the yield and burst pressure of flaw-free pipes
To make maximum use of the properties of highly-rated pipe steels, it is essential to model precisely the ultimate limit states and thereby to calculate accurately their yield and burst pressure. The available calculation methods for yield and burst pressure were compared with experimental data from defect-free pipes. For burst pressure, models incorporating descriptions of strain hardening and uniform elongation were more accurate than those based solely on yield and tensile strength. The von Mises criterion proved very good for determining yield behaviour.

Crack arrestor design for high-strength and low-toughness pipelines
A recent short study has reviewed existing approaches for crack propagation control based on crack arrestors, both in older low-toughness pipelines and in recently developed very-high-strength steels, for which the methods developed in the 1980s may not be conservative. The review highlighted the current gaps in knowledge about fracture propagation and crack arrestor design, and the benefits of using numerical simulation tools for crack propagation and crack arrestor behaviour.

Predicting the burst strength of pipes containing dent + gouge damage

Measured circumferential failure strain compared to predictions, in pressurised flaw-free pipe

The method currently recommended by EPRG for assessing the remaining strength of pipes with combined dent and gouge damage is based on the ‘British Gas Model’ developed in the early 1980s. A recent study for the United Kingdom Onshore Pipeline Association (UKOPA) developed an improved limit state function to address some of the perceived shortcomings of the British Gas Model. EPRG commissioned a study applying the UKOPA model to the wider set of test results now available in the public domain, enabling recalibration and further refinement of the factors determining failure behaviour. A subsequent EPRG study examined in more detail the factors identified during the recalibration study, showing that while a significant reduction in scatter and bias can be achieved for individual datasets by refining the definitions of toughness and micro-crack depth, the improvements can still be masked by the inherent variations between different datasets.

Collapse behaviour of seamless pipe for offshore applications

Collapse test results for seamless pipe

The design of deep-water pipelines requires additional data on pipe mechanical properties and geometrical uniformity. Furthermore, deformation introduced during reel-lay pipe-laying operations induces changes in the material properties. Collapse tests performed on seamless pipe in its as-received, reeled+straightened and subsequently aged heat treatment conditions have confirmed that ageing heat treatment, in line with DNV rules, is effective in restoring and improving the collapse resistance of seamless pipe.

Modelling limit states

Burst pressure predictions for flaw-free pipes with different Y/T values

The challenges of higher design pressures and higher grades of pipe steel can be met by reliable limit state design, for which an accurate prediction of the limit states is essential. EPRG has recently reviewed the methods for limit state calculation, focusing particularly on the yield and burst behaviour of flaw-free high strength pipes subjected to high usage factors. The study compared several published formulae of each class with results from 38 burst tests on flaw-free pipes. For burst pressure, models incorporating descriptions of strain hardening and uniform elongation were more accurate than those based solely on yield and tensile strength. The von Mises criterion proved very good for determining yield behaviour.

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