Defects and Weaknesses

As part of the estimation of overall Resistance, the stress-carrying capacity must be understood. This in turn requires understanding of loads and stresses acting on a component as well as possible weaknesses in the component. As a modeling convenience, an effective wall thickness concept can be used to integrate all of these concepts in a simpler, albeit less accurate way.

Implicit in this analysis is the critical aspect of largest remaining defect. This is critical in that it defines the quality of inspections and integrity assessments and is an essential input into TTF (remaining life) estimations which in turn feed directly into setting re-assessment intervals (scheduling).

The estimation of weaknesses begins with identifying potential anomalies. The first idea that comes to mind when hearing ‘anomaly’ may involve ‘defect’.

All defects are anomalies but not all anomalies are defects.

An anomaly is a deviation in some property of the manufactured product. A defect is considered to be any anomaly, such as a crack, gouge, dent, or metal loss, which reduces the component’s capacity to carry a load. Some anomalies—shallow dents, smooth, shallow gouges, minor metal loss, and even some cracks—will not affect the strength or service life of a pipeline. Hence the statement ‘not all anomalies are defects’. An anomaly becomes a defect when it introduces a weakness.

As used here, ‘defects’ include flaws or damages to components from original manufacture, construction, or time-independent mechanisms (not degradations). Examples include dents, gouges, girth weld defects, lack of fusion in welded seams, and others as detailed later. Time-dependent failure mechanisms (corrosion and cracking) also introduce weaknesses, but those are more efficiently measured independently from defects. At any location along a pipeline, there may be defects or degradation weaknesses or both, interacting with each other. This ‘layering’ of risk issues underpins the risk assessment methodology recommended here.

Besides defects, there are other types of location specific weaknesses, many of which arise through stress concentrators or due to components with inherently less strength than neighboring pipe, such as:

Wrinkle bends
Acetylene welds
Mechanical couplings
Substandard repairs
Older and currently-avoided appurtenances

There are also deficiencies in material properties that are efficiently modeled as weaknesses. These deficiencies can be created from inferior or incorrect construction practice. Examples include introduction of hard spots (potential crack initiation sites) and residual stresses. Deficiencies may also be present from undetected errors in original manufacture or from unrecognized issues at the time of manufacture (for example, LF ERW).

The potential for weaknesses introduced in manufacturing and construction is discussed in later in this chapter as well as in .

Finally, weaknesses occur through degradation mechanisms. This aspect is most efficiently captured as part of the degradation mechanism assessment. As corrosion metal loss potential and cracking phenomena are assessed, a degradation rate (or rates) naturally emerges. The rate multiplied by the amount of time the rate could have been active yields a remaining wall thickness. This value is adjusted by the non-degradation potential weaknesses assessed as discussed here.

Weakness Identification/Characterization

Defect Types

As noted, some anomalies originate from manufacturing processes, such as laminations, hard spots, inclusions, and seam weaknesses associated with low-frequency ERW and electric flash welded pipe. Others such as girth weld defects, dents, and arc burns occur during installation or repair. Finally, anomalies arise during operations: dents or gouges from excavation damage or other external forces, corrosion wall losses, and cracks. Anomalies introduced during repair/replacement operations are also possible.

API 579 provides a more extensive listing of causes of types and origins of manufacturing and construction defects in structures. Such listings serve as checklists for designers of risk assessments, helping to ensure that all plausible defects are considered in the assessment.

Anomaly prioritization is often governed by industry standards if not regulations, as described in PRMM.

Probability of original defects

The types of pre-service deficiencies that can be present before components enter service are:

Material Production Flaws – Flaws which occur during production including laminations and laps in wrought products, and voids, segregation, shrinks, cracks, and bursts in cast products.
Welding Related Flaws – Flaws which occur as a result of the welding process including lack of penetration, lack of fusion, delayed hydrogen cracking, porosity, slag, undercut, weld cracking, and hot shortness.
Fabrication Related Flaws – Imperfections associated with fabrication including out-of-roundness, forming cracks, grinding cracks and marks, dents, gouges, dent-gouge combinations, and lamellar tearing.
Heat Treatment Related Flaws or Embrittlement – Flaws associated with heat treatment including reheat cracking, quench cracking, sensitization, and embrittlement. Similar flaws are also associated with in-service elevated temperature exposure.
Wrong Material of Construction – Due to either faulty materials selection, poor choice of a specification break (i.e. a location in a component where a change in material specification is designated), or due to the inadvertent substitution of a different alloy or heat treatment condition due to a lack of positive material identification, the installed component does not have the expected resistance or needed properties to the service or loading.

In most instances, one or more of these pre-service deficiencies do not lead to an immediate failure. Usually, only gross errors cause a failure, normally identified during a pre-service pressure test.

A probability can be assigned based on detailed manufacture/construction documents when available and on era of manufacture/construction when details are unknown. The probability is efficiently expressed as a frequency per mile.

Residual stress

A well-known static surface stress may be generated from in-service conditions, such as sustained internal pressures. An acting stress may also be residual in nature, introduced during bending and welding in manufacturing or construction, or it may arise from external soil pressure and differential settlement. At sites of surface damage, such as dents and corrosion pits, stress levels in the circumferential and axial directions are higher than on undamaged portions of the pipe surface. The same locations on the pipe that concentrate cyclic stresses, such as gouges, surface discontinuities, and appurtenances, can concentrate static stresses. In many cases, the stress will be virtually undetectable. Furthermore, breaks in the surface film may occur at these discontinuities to make the area more prone to electrochemical corrosion.

Manufacturing/Construction Weaknesses

A list of common manufacturing and construction weaknesses found in onshore steel pipelines over many decades has been compiled in several references (including [1020, 1022, 1035]). The following information is extracted from such references:

Feature	comments on source	impact on resistance
hook crack	older ERW, both LF and HF	fatigue cracking
cold weld; pinhole	inadequate bonding in LF and DC-welded ERW	small leaks
Penetrator	inadequate bonding in flash-welded or HF ERW	small leaks
mismatched skelp edges	DSAW, ERW, flash-welds	fatigue cracking
off seam weld; incomplete penetration; incomplete fusion; centerline crack; toe crack	DSAW and/or SSAW	fatigue cracking
excessively hard HAZ	late 40’s early 50’s X-grade Youngstown pipe mill	increased crack probability, especially with H exposure
unbonded or partially bonded seam	lap-weld pipe	fatigue cracking
burned metal	crack-like voids in lap-welded pipe	loss of effective wall
Lamination	common in pre 1980 seamless pipe	blister formation if H exposure
hard spot	arc burns are one cause of hard spots	increased crack probability, especially with H exposure
defective weld		leaks; rupture when external forces applied
acetylene girth weld	pre WW II	little strain resistance; rupture when external force applied
mechanical coupling	pre WW II	low resistance to axial and lateral forces
wrinkle bends	pre WW II	cold-working reduces toughness; increased crack potential
transportation fatigue cracks	cracks produced during transportation, more common on pipe with D/t>70 produced prior to 1970 and shipped by rail	fatigue cracking
high levels of impurities and non-metallic inclusions	increased crack probability, especially with H exposure
Toughness		fatigue cracking

Some source references cite incident statistics linked to these features, sometimes tracing back to specific steel mills and dates. This information can be very useful in assigning probabilities of defects to pipeline segments. It can also provide inferential information on strength-reduction magnitudes of certain defects. However, without a full understanding of the incidents underlying these statistics, caution in their use is recommended. Recall that these defects, normally having survived pressure tests, inspections, and on-going service loads for many years, fail only when additional loads are introduced or after degradation has occurred. Without knowledge of the degradation and/or additional loads, the knowledge provided by the statistics alone is incomplete.

Manufacturing issues

It is commonly accepted that older manufacturing and construction methods do not match today’s standards for rigor of specifications nor quality control. Nonetheless, many very old systems have successfully and admirably withstood the test of time—decades of service in sometimes challenging environments, with no reduction in strength.

All other things equal however, it is reasonable to assume superior product quality in modern manufacturing. Technological and quality-control advances have improved quality and consistency of both manufactured components and construction techniques. These improvements have varying degrees of importance in a risk assessment. In a more extreme case, depending on the method and age of manufacture, the assumption of uniform material may not be valid. If this is the case, the maximum allowable strength value should reflect the true strength of the material.

Purchasing specifications now cover strength properties such as minimum yield strength (SMYS) and toughness, all of which are certified by the manufacturer. The risk assessment should consider the probabilities that the specifications were correct, were followed, and were applicable to the pipe or component in question.

A pattern of failures connected to a particular manufacturer or process should lead the risk evaluator to question the strength of any components produced in that way. Materials from steel mills whose pipe has been known to have higher rates of weakness should be penalized in the risk assessment where appropriate.

Some weaknesses are actually an increased susceptibility to later damages such as from corrosion and cracking. Preferential corrosion (selective corrosion, seam corrosion, etc) is a possibility for several types of steel pipe. It is commonly associated with variable quality LF ERW or flash weld seams or non-heat treated HF ERW seams. Certain steel pipe manufacture dates and locations (pipe mill) can be correlated with increased occurrence rates. This information can be efficiently modeled as reduced wall thickness in the resistance estimation.

Hard spots created during pipe manufacture or construction (for example, arc burns, girth weld HAZ) can support cracking, especially in the presence of hydrogen. Hard spots can be large—covering the full circumference of the pipe over several inches of length. H₂ stress cracking (HSC) occurs at a hard spot when sources of hydrogen are present and sufficient stress exists. H₂ sources include sour service (H₂S), higher CP (cathodic protection applied for external corrosion control) levels, and in association with higher microbiological activity (swamps, MIC, etc). Susceptibility factors include sufficient hardness, hydrogen availability, and sufficient stress level.

Lack of toughness normally arises during manufacture and should also be considered in the resistance assessment. Lower toughness makes crack initiation, activation, and propagation more probable and rupture more likely. At higher stress levels, more toughness is required to arrest a running brittle fracture. Larger diameter or thinner wall pipes require proportionally higher toughness to prevent running brittle fracture. Hole size, also a function of toughness, is discussed in CoF.

Increasing crack susceptibility can be assumed when:

toughness is low
H₂ charging of steel could have occurred
There may be or have been temperature effects on toughness
hard spot, arc burns could be present.

When no inspection information is available, increasing susceptibility to cracking can be modeled to occur in pipe manufactured before 1960 and/or with higher CP levels (perhaps a threshold of -1.2volts pipe-to-soil, CuCuSO₄ reference electrode) and with increasing stress and with higher potential H₂ availability. [1020]

Construction issues

Similar to the evolution of pipe manufacturing techniques, the methods for construction practices such as welding pipe joints have improved over the years. See PRMM for a relevant discussion on girth weld defects.

A wrinkle bend is a type of buckle, often an artifact of an intentional bending process used in early pipeline installations. Wrinkle bends are known locations of stress concentrations, with the severity of the effect increasing with decreasing D/t and severity of the wrinkle (height and width of wrinkle). Axial stress cycles, combined with the stress concentration effect, reduces the fatigue life of a component with a wrinkle bend. Depending on material properties, a doubling of stress due to a stress concentrator can shorten life by a factor of 16 or more. [1023]

As an artifact of a past, discontinued practice, a wrinkle bend is today considered by most to be an anomaly, sometimes requiring repair or replacement. In the risk assessment, the anomaly could be modeled as a resistance vulnerability.

Date of construction provides evidence of the existence of older features of concern, when inspection data is not available. For example, mechanical couplings were used from 1890’s until about 1940, acetylene welding was employed from about 1915 to 1940, miter bends are found in pipelines built prior to 1940, and wrinkle bends pre-1955 [1020]. Prior to the introduction and adoption of engineering standards and regulations, all repair practices may be suspect.

All such features should be considered in evaluating the strength of the system. Buried bends, girth welds, substandard repairs, and couplings are not normally highly loaded during normal service [1020] and hence, these features may enjoy long service lives. However, when abnormal loadings—including external forces and pressure or thermal cycling—occur, they will often be the points of failure.

With all this in mind, the fact of a pipeline’s long-term reliable operation can to some extent offset these concerns and be a “plus” in the overall evaluation. This is the “withstood the test of time” argument for evidence of low probability of failure. See discussion in .

Damages during operations is the final opportunity for weaknesses to be introduced. PRMM describes common mechanical damages to pipelines. The Pipeline Research Council International & Institute (PRCI) provides useful insights into these mechanical damages on pipelines [1036]:

Mechanical damage can cause changes to:

The shape of the pipeline’s cross section, as for example where the line sits on a rock ledge, and
The wall thickness or its properties, as for example where earthmoving equipment scrapes along the pipeline displacing, or cold-working, and/or tearing the wall as it passes.

Mechanical damage also can involve combinations of (1) and (2).

The consequences of mechanical damage fall into one of four categories, depending on the nature of the outside force, the pipeline’s design and operating conditions, and the line-pipe properties. These consequence categories are:

immediate failure due to plastic collapse or cracking on the inside diameter (ID) during contact
immediate failure due to plastic collapse or OD cracking during re-rounding in the wake of the contact
delayed failure due to in-service cracking, and
no threat for failure for the current service or possible upset conditions.

Other important observations are that re-rounding of dents has been shown to cause crack initiation and that damages incurred prior to pressurization are more benign than those post pressurization. Damage inflicted at zero pressure is not as severe as that inflicted under pressure, all else being equal. This occurs because the unpressurized pipe changes shape over much of its cross section and consequently avoids the localized deformation that leads to puncture or cracking. In contrast, pressure in the pipeline keeps the pipe round except where outside forces contact, which leads to localized deformation and possibly severe damage. Thus, while a severity criterion for damage done at zero pressure could prove useful, such a criterion would be nonconservative for applications involving damage done at pressure.

Although the pipeline is subjected to a pre-service pressure test, it is unlikely that existing damage would be detected, except for areas pierced as a result of the damage incident. Data in published literature indicate that very severe damage involving gouges in dents with depths greater than 15 percent of the diameter seldom leads to failure in full-scale testing after just one major pressure cycle. For this reason, line pipe damaged at zero pressure probably survives the pre-service pressure and thus may exist in operating pipelines, or possibly lead to delayed failure.

Repairs and Reinforcements

As with general construction practice, repair practice has evolved over the years. Some previously acceptable repair methods would no longer be considered by most modern operators. Examples include deposition of weld metal to fill in corrosion damages; use of metal patches or complex shaped shells installed over leaks; converting temporary clamps to permanent installations; and even the use of wooden plugs driven into holes in low pressure steel and cast iron pipelines. Repairs that, by today’s standards, are judged to be inferior, may contribute weaknesses. Their likelihood of existence must be estimated, especially when inspection cannot reliably provide identification and characterization. This is discussed in a later section.

A full encirclement sleeve serves to carry some of the stresses otherwise carried by the pipeline, thereby providing increased resistance to new loads. It also provides increased impact resistance and, especially when made from a composite material, corrosion protection equivalent or superior to a corrosion control coating system. If pressure-containing, it increases TTF from degradation mechanisms by effectively increasing the amount of material that must be degraded before leak or rupture. The sleeve also provides benefit as a crack arrestor, potentially reducing consequence potential by limiting hole size.

Composite sleeve materials are popular repair choices. The underlying concept of composite materials is very old. Straw and mud bricks and concrete (cement and aggregate) take advantage of the best properties of multiple materials to provide a stronger final product. Modern pipeline repair wraps or sleeves are layered systems of solid fibers, such as carbon or fiberglass, and a bonding resin such as urethane or epoxy, installed around a short section of pipeline containing a defect. The characteristics of the applied and cured repair wrap, such as flexibility, yield strength, UV resistance, and others, will determine the ability of these types of repairs to not only restore the component’s strength, but also provide additional resistance, perhaps beyond original capabilities. [1036, 1037]

Modeling of Repairs in Risk Assessment

Modern repairs will reduce risk, sometimes far beyond their role in offsetting weakness caused by a defect. Repairs often act as reinforcement, mitigation, and consequence reduction in addition to restoration of desired strength. These normally cover a small portion of a system, but a detailed risk assessment can recognize that risk is significantly reduced at these short locations. This is often in stark contrast to the risk immediately prior to the repair.

Repairs, especially when made with full encirclement sleeves, can be modeled as providing a general increased resistance, perhaps using a simple factor to increase effective wall thickness by some amount. Alternatively, a repair’s role in specific risk reduction can be modeled in a detailed way, quantifying its independent contributions to:

Increased impact resistance
Increased stress carrying capability
Corrosion mitigation (when non-corrosive sleeve material is used)
Increased effective wall thickness for TTF estimates
Crack arresting is modeled as consequence hole size reduction
Clamps and non-pressure-containing repairs often provide less resistance. Given the typically short length of repairs, detailed modeling may not be warranted and a simple factor, scaling up resistance at the repair location, will be sufficient.

Older repair techniques, no longer allowed in current industry recommended practice, may cause unintended weaknesses such as stress concentration points and brittleness at welds. Even acceptable repairs may have unintended consequences as was noted in the example of hydrogen permeation into the annular space between a repair sleeve and the carrier pipe, eventually causing buckling of the carrier pipe [1001]. In some of these cases, the repair actually causes a new exposure to be included in the risk assessment.

The evaluation of resistance will also include non-pipe components since they will typically be included in the risk assessment. These include flanges, valve bodies, fittings, filters, pumps, compressors, flow measurement devices, pressure vessels, and others. Each will be acted upon by various exposures, have mitigations to protect it, and will have varying amounts of resistance to failure.

Emerging Weaknesses

In addition to original weaknesses arising from manufacture and construction, weaknesses can also be introduced during a component’s service life. This can be estimated in units of frequency per mile per year and is logically a function of other threats such as geohazards, excavator damage, and others. PoD rates from these failure mechanisms can be used to predict future damage rates.

Characterizing Potential Weaknesses

A risk assessment that examines available pipe strength should probably treat anomalies (identified defects whose severity has not yet been evaluated) as evidence of reduced strength and possible active failure mechanisms.

A complete assessment of remaining pipe strength in consideration of an anomaly requires accurate characterization of the anomaly—its dimensions and shape. In the absence of detailed remaining strength calculations, the evaluator can reduce pipe strength by a percentage based on the severity of the anomaly.

Increased crack susceptibility is a common concern for all of these features. This is efficiently modeled as reduced wall thickness and/or increased probability of crack initiation/activation/propagation, both used in the cracking PoF estimation. Some features may also impact the ability to resist other loadings including internal pressure and external forces.