Effective Wall Thickness

As part of the stress-carrying capacity concept underlying Resistance estimations, the use of ‘effective wall thickness’ is introduced here as a modeling convenience.

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Effective Wall Thickness Concept

With an understanding of loads, stresses, and potential weaknesses, the next step in estimating resistance is the bridge between this information and a resistance value to be applied to each component in the risk assessment. This too can be a complex step unless some simplifying assumptions are made. An effective wall thickness can be an efficient intermediate step or at least a conceptual framework for this final assignment.

Next to internal pressure capacity, a component’s wall thickness is probably the most referenced characteristic used in strength and safety margin determinations of pipeline components. Minimum required wall thicknesses are determined from material properties and the amount of stress that the component must withstand. As a pressure containment system, the importance of wall thickness is intuitive. The role of increased wall thickness in risk reduction is also intuitive and verified by experimental work. Component wall thickness, above what is needed for internal pressure and known loadings, provides a margin of safety against unanticipated loads as well as an increased survival time when corrosion or cracking mechanisms are active. Increased wall thicknesses are also known to substantially reduce the chances of failure from external forces such as from excavating equipment. Some wall thickness–internal pressure combinations provide enough strength (safety margin) that most conventional excavating equipment cannot puncture them. Of course, material type must be considered along with the component dimensions. Even among steels, the material strength, often reported as SMYS, can vary greatly.^[3]

However, experience also indicates that increased wall thickness is not a cure-all. Increased brittleness, greater difficulties in detecting material defects, and installation challenges are cited as factors that might partially offset the desired increase in damage resistance [58].

Furthermore, avoidance of immediate failure is only part of the threat reduction—nonlethal damages can still precipitate future failures through fatigue and/or corrosion mechanisms. Nonetheless, increased wall thickness provides failure protection in most failure scenarios.

Defects can also be modeled as equivalent reductions in wall thickness. An effective wall thickness—actual thickness less some amount of wall loss to account for defects—can be estimated. Effective wall thickness then is an efficient basis for modeling pipe resistance to loads. As wall thickness is reduced, implications for component strength include:

• Less capacity for pressure containment
• Faster TTF for degradation mechanisms
• Higher D/t leading to reduced buckling capacity
• Lowered resistance to external forces including localized (puncture) and uniform (subsea hydrostatic pressure).

With a modeling assumption that all potential weaknesses can be effectively treated as reductions in pipe wall thickness, an ‘effective’ or ‘equivalent’ wall thickness can be used to represent resistance. The term ‘effective’ is added to the wall thickness label to capture the idea of equivalencies. It provides a common denominator by which all stress-carrying capacity reductions can be captured in similar units. When evaluating a variety of pipe materials, distinctions in material strengths and toughness will be needed when assessing the role of component wall thickness. With respect to resisting many types of loadings, a tenth of an inch of steel offers more than does a tenth of an inch of fiberglass. When evaluating defects, some will have a more profound effect on strength than others.

As a measure of strength, or stress-carrying capacity, wall thickness is a useful surrogate for the whole suite of factors to be considered in a full strength assessment. The evaluation of stress levels in the component will focus on wall thickness, enabling a risk assessment methodology to similarly focus on ‘effective’ wall thickness as the modeled resistance. The concept of effective wall thickness is therefore efficiently used in risk assessment.

Nominal Wall Thickness

Effective wall thickness estimation begins with actual wall thickness. In the absence of recent measurements of wall thickness, the actual wall thickness may need to be derived from the originally specified or nominal wall thickness.

General stress calculations assume a uniform pipe wall, free from any defect that might reduce the material strength. It discounts possible reductions in actual or effective wall thickness caused by defects such as cracks, laminations, hard spots, gouges, etc. A specific stress calculation on a component requires consideration of such features. Finding or positing all differences between specified and actual and effective wall thickness is essential to risk assessment. Pipeline integrity assessments are designed to identify areas of weaknesses, in the form of wall thinning or in-wall defects, which might have originated from any of several causes. Other inspection may also reveal areas of actual or a high-probability of wall loss, pinhole corrosion, graphitization (in the case of cast iron), and leaks.

Most pipeline systems have incorporated some “extra” wall thickness—beyond that required for anticipated loads, and hence have extra strength. This is often because of the availability of standard manufactured pipe and appurtenance wall thicknesses. Such “off-the-shelf” purchases are normally more economical than special designs even though they may involve more material than may be required for the intended service. This extra thickness will provide some additional protection against corrosion, external damage, and most other failure mechanisms.

When actual wall thickness and wall condition measurements are not available, the nominal wall thickness can be the starting point for estimating current wall thickness. The difference between nominal or “specified” wall thickness and actual wall thickness is a key aspect of resistance determination in this risk assessment. Especially in a conservative risk assessment, the nominal value as a estimate of current, must be adjusted for all variances pertinent to the estimation of the strength provided by likely (or worst case) actual wall thickness.

Differences between nominal and effective wall thickness include:

• Allowable manufacturing tolerances—the actual wall thickness can be some percentage thicker or thinner than specified and still be within acceptable specification.
• Manufacturing defects including material inclusions, voids, and laminations.
• Installation/construction damages or errors such as during joining (welding, fusion, coupling, etc.) processes
• Damages suffered since manufacture: ie, during transportation, installation, and operation, including corrosion and cracking.

Some of these adjustments are actual reductions in thickness while others are reductions in effective strength, ie, features such as cracks, girth weld defects, hard spots, etc are not measured in terms of thinning but rather by some other loss of stress-carrying capacity.

Current Wall Thickness

As used here, the current wall thickness is not always a direct measurement of the component’s wall by UT, caliper, or other means. It also includes inferential indications of current wall thickness that often must be made in the absence of the direct measurement. Actual or current wall thickness values emerge from whichever of the following provides the strongest evidence:

• Direct measurements, with considerations for age and accuracies of all readings, as well as other uncertainties, such as if a measurement at one location is to be extrapolated to another location. These measurements include those taken by NDE examinations including direct-measurement ILI, UT, etc.
• Thickness inferred by ILI techniques designed to find changes in wall rather than measure thickness, external only indications such as visual and pit depth gauge
• Thickness inferred by pressure test
• Thickness inferred by normal or recent high pressure levels (see NOP as Pressure Test discussion)
• Specified or nominal thickness, with previously described adjustments (manufacturing tolerances and error rates, damage rates during and since installation, etc.)

All of these possible information sources will grow more uncertain over time except for wall thickness implied by a current operating pressure (which carries its own significant uncertainties).

It is not unusual to have data from several or all of these information types available at the same location but with widely varying accuracies and age. For instance, one or more ILI’s, multiple excavations, and at least a post-installation pressure test, will each offer one or more pieces of information in each category, for an operating pipeline. The risk assessment will need to efficiently filter through the disparate information to determine the best indicator of today’s thickness. This mirrors the process the SME would also have to use when faced with the same information set and the need to determine the single best estimate.

With a consistent application of conservatism in uncertainty estimates, the more optimistic value—the information suggesting the best wall thickness after adjustments for age and accuracy—will usually govern, as discussed early in this text. Refer to early discussion of measurements versus estimates—the general approach for efficiently integrating many disparate pieces of evidence into the risk assessment. See .

In summary, the evidence of current wall thickness will often arise from multiple sources and may be conflicting. The most compelling evidence will be the information with the highest accuracy and timeliness. Each piece of evidence–from inspections, tests, etc–must be adjusted for age and accuracy before comparing to other evidence. Then, the more recent and more accurate information should govern the value used in subsequent risk assessment calculations.

Effective Wall Thickness Estimation

Beginning with the best available estimate of current wall thickness, we now assess for any weaknesses that may detract from the strength implied by this wall thickness. A weakness will reduce the current, actual wall thickness into an ‘effective wall thicknesses’.

Since resistance, as it is being modeled here, is proportional to available stress-carrying capacity it is also generally proportional to material thickness. Wall thickness is often the single most important component characteristic in most loadings of components. Use of wall thickness to represent resistance is intuitive for degradation from corrosion and withstanding internal pressure, longitudinal loads, and puncture. It is less intuitive, for example, in assessing cracking.

Nonetheless, it is still efficient, as previously discussed. Cracking can be modeled as follows: defects that increase crack initiation potential and/or stress intensifications and/or lower toughness, are modeled as either reductions in pipe wall or increases in cracking rates. Technically, lower toughness does not directly cause faster cracking but rather allows smaller defects to initiate/activate a crack.

Either results in increased failure probability as the probability or severity of defects increases. General insights from structural theory can be incorporated into a risk analysis. Component wall thickness is usually proportional to structural strength—greater wall thickness leads to greater structural strength (not always linearly)—with the accompanying assumption of uniform material properties and absence of defects.

Defects modeled as reductions in effective pipe wall thickness is a simplification of the complex analysis that would require consideration for each possible anomaly under every possible loading scenario. In a robust solution, for each anomaly’s characteristics such as:

• length, width, depth;
• location in wall;
• clock position on circumference; and
• orientation relative to axes (axial, radial, circumferential)

loads would be applied, stresses calculated, and ability to survive under various scenarios assessed. The simplification is intended to represent this spectrum of scenarios with an equivalent wall thickness: defect X causes an equivalent loss of strength as does a reduction of wall thickness by Y%.

Knowledge or suspicion of potential weaknesses arises from:

• discovery via NDE
• era of manufacture including manufacture specifications used
• construction practices including construction specifications used
• experience on current component or with similar (relevant) collections of components
• defect-introduction mechanisms possibly active
• includes benefits from sleeves and other repairs

The ratio of effective pipe wall thickness to required wall thickness is another way to view the resistance concept. A ratio greater than one means that extra wall thickness (above design requirements) exists. For instance, a ratio of 1.1 means that there is 10% more pipe wall material than is required by design and 1.25 means 25% more material. If this ratio of effective wall thickness to required wall thickness is less than one, the pipe does not meet the design criteria—there is less actual wall thickness than is required by design calculations. The pipeline system has not failed either because it has not yet been exposed to the maximum design conditions, there is excess conservatism in the calculation, or some error in the calculations or associated assumptions has been made.

This ratio concept is used in some inspections. Certain NDE, especially ILI, often reports wall loss not only in terms of length, width, and depth, but also as implications in pressure containing capacity. Estimated Repair Factor (ERF) and Rupture Pressure Ratio (RPR) are common types of ratios reported by ILI. These reported ratios based on theoretical rupture pressure versus MAOP are readily converted into equivalent wall thicknesses.

Resistance and Effective Wall Thickness

All resistance estimates can use ‘effective wall thickness’ as an efficient foundation. While applied loads produce stress in different ways, wall thickness is a key strength determinant in most loading scenarios of thin-walled structures (most pipeline components are modeled as shell type structures). Degradation resistance considers potential wall loss by corrosion as well as fatigue life reduction—‘wall loss’ by cracking
^[4]. Reduced wall thickness leads to reduced load carrying capacity. So, wall thickness as a measure of load-carrying capacity, when coupled with degradation rate (mpy or mm per year), leads to an estimate of time before degradation advances to point of containment loss (or yield).