Reducing conservatism in fatigue crack growth assessments through testing

Pipelines may be topic to varied varieties of repeat loading, for instance from visitors loading, or vibration because of water movement. Variation in inner strain is the most typical concern for operators, and has been linked to numerous failures. This cyclic loading results in progressive, collected injury within the metal, often known as fatigue, which if of enough magnitude can result in the initiation and propagation of cracks that may trigger failure. Usually, pipeline fatigue is extra of a priority for liquid pipelines the place a lot bigger magnitude strain swings, and therefore stress variations, are noticed in comparison with these carrying a compressible fuel product.

There have been numerous high-profile pipeline failures attributable to fatigue cracking; for instance a 40-inch diameter crude oil pipeline in France in 2009[ ]. A majority of these failure are uncommon , however it’s cheap to count on that the fatigue drawback will get tougher to handle as pipelines age.

Evaluation strategies for quantifying fatigue injury in pipelines are properly established within the business (e.g., API 579 or BS 7910). Fatigue life is decreased by orders-of-magnitude within the presence of a stress focus; e.g., a geometrical deformation akin to a dent, or an anomaly akin to a weld flaw. Methodologies akin to API 579 or BS 7910 can explicitly take into account the presence of an anomaly or the magnitude of a localised stress focus.

A less complicated ‘S-N’ strategy is used to find out the fatigue lifetime of a pipeline, normally on the pipeline’s design stage, which is predicated on empirical curves which hyperlink stress vary (S) to predicted variety of cycles to failure (N). The SD-N strategy is restricted to predicting the fatigue lifetime of an anomaly-free seam weld or pipe physique. As a rule, throughout an evaluation of remaining fatigue life for an current pipeline, the presence of an anomaly must be thought of explicitly. This may very well be an precise anomaly detected by ILI, or one which may very well be theoretically current within the pipeline, within the type of one which survived the pre-service strain take a look at, one sized beneath the ILI detection thresholds, or one that may have handed the pipe mill high quality management necessities.

When calculating the remaining lifetime of a crack-like anomaly within the presence of cyclic loading, the fracture mechanics approaches in API 579 and BS 7910 are required. These strategies have been efficiently utilized for figuring out remaining fatigue life, in a number of industries, together with the pipeline business; nonetheless, the strategies are identified to be conservative. This text outlines an strategy for lowering the conservatism in these assessments, permitting operators to securely lengthen anomaly remediation dates and reassessment intervals, supporting economical operation by lowering pointless digs.

Fracture Mechanics Strategies

Fatigue crack development charge may be described because the crack development per stress cycle. The connection between crack development and utilized stress is represented by a sigmoidal curve with three distinct areas, Determine 1, which is typical of crack development in-air. The primary area, on the left-hand facet of the Determine, is the place crack development charges are anticipated to be extraordinarily low, or there isn’t a development in any respect. The second area is characterised by a linear relationship between the crack’s stress depth issue (stress depth is said to the utilized stress and the scale of the crack) and the crack development charge. The ultimate area describes the fast crack development resulting in failure.

The central linear portion of the curve may be described by the Paris-Erdogan equation, which is a straightforward single slope relationship, and the most typical method to mannequin crack development inside business. It’s given by:  

           =A.∆Okm

The place da/dN is the crack development charge and ∆Ok the utilized stress depth issue vary. A and m are empirical constants that relate to the actual materials or setting which the crack is in. Throughout the pipeline business, this equation is broadly used to find out the speed of fatigue crack development and for estimating when a crack would attain a important dimension. Dissecting this equation offers a fundamental understanding of the place the conservatism within the assessments come from.

First, take into account ∆Ok, which describes the variation in stress depth on the tip of the crack in query. The variation in stress depth is calculated based mostly on the cyclic stress, and that is typically decided by simplifying cyclic strain information (retrieved from a pipeline’s SCADA system) into an equal variety of stress cycles at a single strain utilizing Miner’s rule, a form issue, and the crack depth. An possibility for lowering ∆Ok and fatigue crack development charge is by lowering the strain biking regime, particularly a smaller vary between the utmost and minimal strain; for instance, by controlling pumping station sequences, valve configurations, and many others. Lowering strain vary can produce dramatic will increase in fatigue life, as crack development charge is said to the exponent ‘m’ within the Paris-Erdogan equation. The worth of ‘m’ is usually 3: which means that a halving of the strain vary produces a discount in crack development charge of (0.5)3 or a rise in fatigue lifetime of 8. Equally, if strain ranges are decreased by an element of three, the rise in fatigue life is 3x3x3=27.

If altering the operation of the pipeline is not possible then we should take a look at how ∆Ok is said to the crack development charge. That is the place the main focus strikes to the 2 constants throughout the equation, A and m, which outline the magnitude of fatigue crack development versus ∆Ok. These are obtained experimentally by curve becoming fatigue crack development information. Numerous these constants are given by requirements akin to BS 7910 for various supplies and environments, akin to dry air or a marine setting. The impression of those empirically derived constants is subsequently important on figuring out the fatigue crack development charge (FCGR). Current take a look at work carried out by the Pipeline Analysis Council Worldwide (PRCI) derived new constants to outline fatigue crack development in pipelines. 72 fatigue crack development assessments had been carried out on 12 pipeline supplies (together with one ERW weld materials) (ref. [ ]). The take a look at work noticed that basically development charges in air had been 2 to three occasions decrease than the expansion charges derived when the generic cross business constants really useful in BS 7910 are thought of.

What work may be accomplished to scale back FCGR conservatisms

As mentioned, lowering the extent of strain biking is usually not an possibility because of throughput necessities or operational constraints (e.g., line-packing, batch operation). One of many remaining choices is to make use of much less conservative crack development constants. For conditions the place line pipe materials is accessible, a sequence of assessments may be accomplished to acquire pipeline particular fatigue crack development constants, which may be anticipated to be much less conservative than the generic constants. A sequence of three take a look at sorts gives a complete methodology for acquiring validated FCGRs.

Take a look at 1 – lab take a look at

Lab assessments may be competed to find out pipeline particular constants. One fashionable method is single edge notch bend (SENB) assessments, the place a pattern of pipe is taken and flattened, a single notch added within the centre of the pattern and clips added both facet to supply the repeated hundreds. Crack development is measured from the notch by direct present potential drop and correlated with the stress depth issue to find out constants A and m.

Throughout earlier work, ROSEN accomplished SENB assessments for an Australian shopper to find out FCGRs in-air. A comparability was made to the PRCI outcomes, see Determine 2. The PRCI outcomes proven are solely these for mum or dad materials; different leads to the programme for ERW bond strains haven’t been proven. The PRCI work fitted a extra conservative single slope Paris regulation, reasonably than a much less conservative two slope regulation. Nonetheless, it may be seen that the imply of the ROSEN outcomes could be very near the imply of the PRCI mum or dad information. The higher certain of the ROSEN outcomes is barely above the corresponding higher certain of the PRCI information. Total, it’s thought of that the ROSEN outcomes and the PRCI outcomes are constant and help the view that the BS 7910 really useful higher certain development charge is conservative for pipeline purposes. The measured FCGRs allowed the shopper to make extra knowledgeable future integrity choices.

It’s thought of prudent to validate the derived charges with additional testing if pipeline-specific laboratory derived constants are utilized in actual life purposes.

Take a look at 2 – ring testing

For many small-scale laboratory assessments akin to SENB, the curved pipe segments are flattened, introducing tensile and compressive strains that change the behaviour of the fabric: these modifications must be understood earlier than utilizing laboratory-derived FCGRs for discipline purposes.

‘Ring’ testing may be described as a 2-dimensional mannequin of the (third-dimensional) pipeline construction, the place a brief ‘ring’ of metal is reduce from the pipeline and positioned inside a take a look at rigidity rig. This ring may be strain cycled to research the results of fatigue loading.

Introducing electro discharge machining (EDM) slits into the ring permits for the modelling of FCGRs as fatigue cracks can nucleate and develop from the sting of the slit. The speed at which these cracks develop may be measured utilizing the direct present potential drop method; the FCGR can then be in comparison with what can be predicted by the expansion charge constants derived in Take a look at 1. The testing additionally permits for the presence of different radial stress concentrations anomalies to be understood; e.g., pipe roof topping and ovalities.

The primary advantage of ring testing is that it permits a number of mannequin fatigue assessments to be accomplished in a time efficient method and at a decrease price. Permitting for the FCGRs to be validated contemplating distinction EDM slit sizes, strain ranges and variety of cycles. Additionally, because the anomaly modelled is taken into account infinitely lengthy, it provides a layer of conservatism to what can be anticipated in a real-life state of affairs.

Validating the predictions from the FCGRs decided within the small scale SENB lab assessments with ring enlargement assessments permit for the total pipe wall to be examined beneath sensible hoop stress situations, and with out deformation within the easy SENB specimens in Take a look at 1.

Take a look at 3 – full scale take a look at

The ultimate step to confirming that FCGRs are appropriate to be used in important resolution making is by finishing a full-scale fatigue take a look at. This requires a pattern pipe spool that may be changed into a strain vessel by welding on two ends. EDM slits can be machined to supply initiation websites for fatigue crack development. The vessel is then stuffed with a take a look at liquid and an appropriate pump used to use repeated strain hundreds, simulating operational strain swings. Once more, the speed of fatigue crack development is measured and in comparison with that discovered within the SENB and ring enlargement assessments. 

ROSEN accomplished a full-scale fatigue take a look at for a shopper who wished to know whether or not the operational regime was enough to trigger fatigue crack development. The arrange for the take a look at may be seen in Determine 3. For this setup, fatigue crack development was measured utilizing the alternating present potential drop measurement method. For this shopper, it was demonstrated that the operational regime was not enough to advertise fatigue crack development.

The complete-scale take a look at is the ultimate stage in validating FCGRs, and enhances Exams 1 and a pair of. FCGRs may be confirmed in a full-scale setup, and extra sensible security margins used. This permits an operator a greater understanding and prediction of when and the way cracks might develop by fatigue to an unacceptable dimension. One other benefit of a full-scale take a look at is it permits for actual anomalies taken from the sector to be investigated for potential development because of fatigue cracking beneath completely different strain biking regimes. Utilizing actual anomalies additionally leaves the choice of burst testing on the finish of any take a look at to validate fashions used to foretell failure strain. 

Abstract

FCGRs revealed in requirements like API 579 and BS 7910 are essentially conservative to cowl the requirements’ various use. This conservatism has been quantified by the PRCI within the USA, and corroborated by the work accomplished by ROSEN within the UK. Extra sensible, and fewer conservative FCGRs may be obtained from laboratory assessments. Easy bend assessments will scale back conservatism, and ring testing (2-dimensional mannequin of a pipeline) and full-scale assessments with additional scale back conservatism, offering a pipeline operator with strong and validated FCGRs which may be confidently used for price efficient future integrity planning.

REFERENCES
i  Dafea, et. al., “Investigation into the Failure of a 40” Diameter Crude Oil Pipeline”, IPC2021-90597, Proceedings of the ninth Worldwide Pipeline Convention, Calgary, September 2012.

ii  PRCI – “Improved Strategies for Estimating Remaining Fatigue Lifetime of ERW Pipelines”, Mission BMT 20147, closing Report, 27 October.

For extra info go to: https://www.rosen-group.com/