API 579: a comprehensive fitness-for-service guide

This article presents an overview of the recently published American Petroleum Institute (API) Recommended Practice 579, which covers fitness-for-service assessment of pressure equipment in petrochemical and other industries. Although API 579 covers a wide range of flaws and damage mechanisms, including local metal loss, pitting corrosion, blisters, weld misalignment, and fire damage, the emphasis of the present article is on the assessment of crack-like flaws. The API 579 procedure for evaluating cracks incorporates a failure assessment diagram (FAD) methodology very similar to that in other documents, such as the British Energy R6 approach and the BS 7910 method. The API document contains an extensive compendium of K solutions, including a number of new cases generated specifically for API 579. In the initial release of the document, API has adopted existing reference stress solutions for the calculation of L_r in the FAD procedure. In a future release, however, API plans to replace these solutions with values based on a more rational definition of reference stress. These revised reference stress solutions will incorporate the effect of weld mismatch. In addition to the Appendices of K and reference stress solutions, API 579 includes appendices that provide guidance on estimating fracture toughness and weld residual stress distributions. Over the next few years these appendices will be enhanced with advances in technology. Recently, API has entered into discussions with the American Society of Mechanical Engineers (ASME) to convert API 579 into a joint API/ASME fitness-for-service guide.     

Fitness-for-service assessment of spherical pressure vessels with hot spots

Spherical shapes are used in industry as hemispherical vessel heads or high-pressure storage vessels due to the inherent strength associated with the shape. Structural integrity of such components needs to be evaluated periodically to prevent failure of the vessels under operating conditions. The paper develops a method for Level 2 (as defined by API 579 [(2000). Fitness-for-service, API 579. Washington, DC: American Petroleum Institute]) fitness-for-service estimation of spherical shapes subject to local hot spots where the temperatures are elevated due to local damage. The decay length for spherical shells is determined, and the size of hot spot to be identified as local is proposed. A lower bound “remaining strength factor” (RSF) for spherical pressure vessels containing hot spots is formulated by the application of Mura's variational formulation and the m-multiplier method. The effectiveness of the proposed Level 2 method is evaluated and demonstrated through an example.     

A criterion-selection diagram for the thermal shock resistance of a hollow circular cylinder

Abstract

The characteristics of thermally induced failure are investigated for a hollow circular cylinder exposed to a convective cooling at the inner surface. The transient fields of both temperature and thermal stresses are given in closed forms over the full range of Biot number. The thermal shock resistance (TSR) is analyzed based on two distinct failure criteria. Namely, the strength-based failure criterion is adopted for a nearly flaw-free cylinder while the fracture toughness-based one is used for the counterpart with an inner crack-like flaw. From each criterion, the admissible maximum temperature drop is obtained assuring that a hollow cylinder can tolerate without failure. The influence of flaw size on the TSR is quantified and a criterion-selection diagram is proposed for the TSR characterization. These results are deemed to be of importance from the perspective of estimating the TSR of a hollow circular cylinder.     

Structural integrity evaluation of X52 gas pipes subjected to external corrosion defects using the SINTAP procedure

In the present study, the SINTAP procedure has been proposed as a general structural integrity tool for semi-spherical, semi-elliptical and long blunt notch defects. The notch stress intensity factor concept and SINTAP structural integrity procedure are employed to assess gas pipelines integrity. The external longitudinal defects have been investigated via elastic–plastic finite element method results. The notch stress intensity concept is implemented into SINTAP procedure. The safety factor is calculated via SINTAP procedure levels 0B and 1B. The extracted evaluations are compared with the limit load analysis based on ASME B31G, modified ASME B31G, DNV RP-F101 and recent proposed formulation [Choi JB, Goo BK, Kim JC, Kim YJ, Kim WS. Development of limit load solutions for corroded gas pipelines. Int J Pressure Vessel Piping 2003;80(2):121–128]. The comparison among extracted safety factors exhibits that SINTAP predictions are located between lower and upper safety factor bounds. The SINTAP procedure including notch-based assessment diagram or so-called ‘NFAD’ involves wide range of defect geometries with low, moderate and high stress concentrations and relative stress gradients. Finally, some inspired and advanced viewpoints have been investigated.     

On relating the reference stress, limit load and the ASME stress classification concepts

This paper discusses the relationship between the concepts of reference stress, limit load and the ASME stress classification framework, as it applies to common mechanical components and structures. The relationship is based on the underlying notion of load-control.

The reference stress method, which has been used mainly in the UK, attempts to correlate creep deformations in a structure with the results of a uniaxial creep test. It has been observed that the reference stress is relatively insensitive to material parameters characterizing creep behaviour. The method has been widely used in the design and life assessment of nuclear as well as conventional components. Specifically, it has been applied to problems pertaining to creep growth, rupture damage, creep buckling, and more recently, elastic-plastic fracture toughness.

Determination of the reference stress is not always a simple task. An approximate method of its determination relies on prior knowledge of limit loads for various configurations and loadings. The drawback, of course, is that there is a reliance on the available catalogue of limit loads. Rather than attempting to ‘fit someone into a garment’, this paper discusses a new and a direct procedure for reference stress determination using the GLOSS R-Node method. The reference stress is determined for some pressure component configurations. The ideas are extended to the ASME stress classification framework, and the various stress categories are discussed.     

Limit load and J estimates of a centre cracked plate with an asymmetric crack in a mismatched weld

The limit load and J estimates of a centre cracked plate with an asymmetric crack in the tensile properties mismatched weld were investigated. A limit load expression was derived on the basis of a simplified slip-line field. A good agreement between the predictions of the expression and finite element (FE) results was found for ratios of half-weld width to the crack ligament, H/l, of less than 0.5. The equivalent stress–strain relationship method (ESSRM) was used to predict elastic–plastic J values. Results from FE analyses show that the ESSRM is accurate for the crack with asymmetry in the mismatched weld provided an accurate theoretical or numerical value of the limit load of the same specimen is available. Defect assessment methods are discussed, and it is found that the failure assessment diagram (FAD) of an asymmetrically cracked mismatched weld can be constructed from the equivalent stress–strain relationship for the same mismatched geometry with a symmetric crack. The effect of an asymmetric crack on the FAD may then be covered by the limit load solution for the asymmetrically cracked mismatched weld.     

Simplified fitness-for-service assessment of pressure vessels and piping systems containing thermal hot spots and corrosion damage

Fitness-for-service (FFS) assessment is a quantitative engineering evaluation of operational components. In the context of pressure vessels and piping systems FFS assessment is performed periodically to ensure the operational safety and structural integrity. In this paper, a simplified method is developed for Level 2 FFS assessment (as described in API 579) of pressure vessels and piping systems containing thermal hot spots or corrosion damage. The method is based upon variational principles in plasticity, the m_α-tangent method (an extension of the m_α method), the concept of decay length and reference volume. The use of the m_α-tangent method extends the range of applicability to components and structures experiencing significant stress gradients in and around the damaged spot. The method is shown to provide a reasonably accurate estimate of the remaining strength of ageing pressure components. The method is demonstrated through an example, and the results are compared with Level 3 inelastic finite element analyses.     

The use of failure assessment diagrams to describe delayed hydride cracking initiation at a blunt flaw

Delayed hydride cracking (DHC) in Zr–2.5Nb alloy material is of interest to the CANadian Deuterium Uranium (CANDU)⁽¹⁾ industry in the context of the potential to initiate DHC at a blunt flaw in a CANDU reactor pressure tube. The existing CANDU blunt flaw DHC evaluation procedure is based on a threshold peak flaw-tip stress for DHC initiation that is independent of flaw geometry. Work is underway to improve the existing blunt flaw DHC evaluation procedure by developing a methodology that takes into account the effect of flaw geometry parameters. The methodology is based on representing the stress relaxation due to hydride formation and crack initiation by a process zone. A process-zone model was used to develop failure assessment diagrams in the conventional format of R6 as described in this paper, although the coordinates of the diagram have a different physical meaning. The failure assessment diagrams were shown to be independent of the scale of the configuration as well as specific values of material properties. The failure assessment curves as derived in the conventional R6 format exhibit a strong dependence on the geometry of the flaw due to the stress concentration and associated stress gradient. By redefining the ordinate of the failure assessment diagram in terms of peak flaw-tip stress the geometry dependence of the failure assessment curves was reduced significantly. This minimal geometry dependence of the failure assessment curves is valuable with regards to practical engineering flaw evaluations. Agreement between the failure assessment diagram predictions and experimental results is reasonable.     

Performance study of Ke factors in simplified elastic plastic fatigue analyses with emphasis on thermal cyclic loading

As code-based fully elastic plastic code conforming fatigue analyses are still time consuming, simplified elastic plastic analysis is often applied. This procedure is known to be overly conservative for some conditions due to the applied plastification (penalty) factor K_e. As a consequence, less conservative fully elastic plastic fatigue analyses based on non-linear finite element analyses (FEA) or simplified elastic plastic analysis based on more realistic K_e factors have to be used for fatigue design. The demand for more realistic K_e factors is covered as a requirement of practical fatigue analysis.Different code-based K_e procedures are reviewed in this paper with special regard to performance under thermal cyclic loading conditions. Other approximation formulae such as those by Neuber, Seeger/Beste or Kuehnapfel are not evaluated in this context because of their applicability to mechanical loading excluding thermal cyclic loading conditions typical for power plant operation. Besides the current code-based K_e corrections, the ASME Code Case N-779 (e.g. Adam’s proposal) and its modification in ASME Section VIII is considered.Comparison of elastic plastic results and results from the Rules for Nuclear Facility Components and Rules for Pressure Vessels reveals a considerable overestimation of usage factor in the case of ASME III and KTA 3201.2 for the examined examples. Usage factors according to RCC-M, Adams (ASME Code Case N-779), ASME VIII (alternative) and EN 13445-3 are essentially comparable and less conservative for these examples. The K_v correction as well as the applied yield criterion (Tresca or von Mises) essentially influence the quality of the more advanced plasticity corrections (e.g. ASME Code Case N-779 and RCC-M). Hence, new proposals are based on a refined K_v correction.     

Weight functions for the determination of stress intensity factor and T-stress for edge-cracked plates with built-in ends

This paper presents the weight functions for the determination of the stress intensity factor and T-stress solutions for edge-cracked plates with built-in ends under complex stress distributions. First, a compliance analysis approach is used to calculate stress intensity factor and T-stress for edge cracks in finite width plates with built-in ends with uniform or linear stress distributions acting on the crack face. The results serve as the reference solutions for the next step in which the approaches of deriving weight functions from reference stress intensity factor and T-stress solutions developed for stress boundary conditions are extended to obtain the corresponding weight functions for edge-cracked plates with built-in ends. Finite element analysis is conducted to validate the derived solutions. The weight functions derived are suitable for obtaining stress intensity factors and T-stress solutions under any complex stress field.     

Determination of the elastic-plastic fracture mechanics Z-factor for alloy 182 weld metal flaws

One of the ways that the ASME Section XI code incorporates elastic-plastic fracture mechanics (EPFM) in the Section XI Appendix C flaw evaluation procedures for circumferential cracks is through a parameter called Z-factor. This parameter allows the simpler limit-load (or Net-Section-Collapse) solutions to be used with a multiplier from EPFM analyses. This paper shows how 3-D finite element (FE) analyses were employed to investigate the sensitivity of the crack-driving force as a function of crack location (i.e., crack in the center of weld, or closer to the stainless or low alloy steel sides) in an Alloy 182 dissimilar metal weld (DMW), and how an appropriate (or equivalent) stress-strain curve was determined for use in the J-estimation schemes. The J-estimation schemes are then used to cover a wider range of variables, i.e., pipe diameters, cracks lengths, and also incorporate crack growth by ductile tearing. The Z-factor equations as a function of pipe diameter were calculated using the LBB.ENG2 J-estimation scheme along with the most conservative equivalent stress-strain curve from the FE analyses. The proposed Z-factor approach was then validated against an Alloy 182 DMW full-scale pipe test that had a circumferential through-wall crack in the fusion line. The predicted EPFM maximum load showed excellent agreement with the experimental result. Furthermore, it was shown that the proposed Z-factor equation is not sensitive to the location of the crack.     

Use of the cylindrical instability stress for blunt metal loss defects in linepipe

Assessment of metal loss defects in gas pipelines can be analysed by a number of methods. In analyses with finite element methods a failure criterion is required. A material property is introduced, the cylindrical instability stress, which determines the plastic collapse of cylindrical pressure containing vessels. The use of this is extended to cover blunt metal loss defects. Some published finite element studies of defected vessels are re-analysed using this failure criterion.

The cylindrical instability stress is a more accurate failure criterion for plastic collapse in pipelines and pressure vessels than commonly used measures such as flow stress, Specified Minimum Yield Stress plus 10 ksi or multiplied by 1.1. It can be used in determining burst pressure of defected and un-defected pressure vessels and piping.     

Weight function method for transient thermomechanical fracture analysis of a functionally graded hollow cylinder possessing a circumferential crack

This article introduces a weight function method for fracture analysis of a circumferentially cracked functionally graded hollow cylinder subjected to transient thermomechanical loading. Analytical solutions for transient temperature and stress distributions in the uncracked cylinder are derived by applying finite Hankel transformation. These solutions are utilized to determine stress acting on the faces of the circumferential crack in the local perturbation problem. Thermomechanical material properties are assumed to be power functions of the radial coordinate in the derivations. Coefficients of the weight function are found using reference stress intensity factors computed through the finite element method. Domain form of the J-integral is used in the finite element calculations. Comparisons of the numerical results calculated by the proposed weight function method to those generated by finite element analysis demonstrate the high level of accuracy attained by the application of the developed procedures. Further parametric analyses are presented to illustrate the influences of dimensionless time, crack depth to thickness ratio, power law index, and convection coefficient upon transient mode I thermomechanical stress intensity factors.     

Evaluation of J-integral for surface cracked plates under biaxial loading using extended reference stress method

In this paper, reference stress solutions for plates with semi-elliptical surface cracks were firstly reviewed, and the applicability of the solutions was examined through the comparison with finite element analysis results under uniaxial loading. Next, an extended reference stress method was newly developed to evaluate J-integral for cracked plates under biaxial loading. The predictive accuracy of the method was validated through the comparison with finite element analysis results under biaxial loading. As a result, it was ascertained that the proposed method together with Lei's reference stress solution predicts J-integral with acceptable accuracy.     

Natural convection of non-Newtonian fluids through permeable axisymmetric and two-dimensional bodies in a porous medium

The present work concerns the natural convection of non-Newtonian power-law fluids with or without yield stress over the permeable two-dimensional or axisymmetric bodies of arbitrary shape in a fluid-saturated porous medium. Using the fourth-order Runge-Kutta scheme method and shooting method we obtain the local non-similarity solutions. The parameters that include the dimensionless yield stress Ω, permeable constant c and power index n are studied, and the heat flux and the wall temperature are taken into consideration as variables. The local non-similarity solutions are found to be in excellent agreement with the exact solution. It is found that the results depend strongly on the values of the yield stress parameter, the wall temperature distributions, the lateral mass flux rate, and the heat flux at the boundary.     

J-integral and local damage fracture analyses for a pump casing containing large weld repairs

Three-dimensional J-integral and two-dimensional Local Approach finite element studies are described for postulated crack-like defects in a large repair weld to the casing of a light water reactor circulation pump. The repair weld residual stress field is simulated and plant operating pressure and thermal transient loads are applied. Crack tip constraint effects are quantified through detailed analysis of the cracked structure and compact tension fracture toughness specimens. Fracture initiation crack sizes are shown to be larger than conceivable fabrication defects that are detectable using modern ultrasonic inspection techniques. The Local Approach study demonstrates the benefits of quantifying crack tip constraint conditions, compared with conventional J-estimation schemes and cracked body J-integral analysis. The method of introducing the crack into the finite element model is shown to have a large effect on calculated crack tip fracture parameters; a slowly developing crack in the residual stress field being more benign.     

A phenomenological theory of subcritical creep crack growth in a cylindrical vessel weakened by an axial part-through crack

A phenomenological theory of subcritical creep crack growth is formulated for materials with large creep exponent. The subcritical crack growth is shown to be mainly controlled by the average net section stress and the plastic deformations in the vicinity of the crack tip. Plastic zone size is evaluated by considering the effects of relaxation on the plastic stress singularity at the crack tip for a power hardening material. The theory has been applied to a cylindrical vessel weakened by an axial part-through crack. The predicted rupture behaviour compares favourably with published creep rupture tests on 9Cr 1Mo steel tubing pressurised at 550°C. The concept of flaw size stress is introduced and is used in failure mode prediction. Failure will be by breakage or leakage, depending on the relative values of flaw size stress and material yield stress.     

Fitness for service of welded components under creep and creep–fatigue loading

The welded materials are prone to damage particularly in heat-affected zones in base metal as well as in fusion line and weld metal. Therefore the treatment of welded components operating in the creep and creep–fatigue regime is of immediate industrial interest. It needs to be addressed by a methodology that will be developed and incorporated into design codes such as ASME, as well as in Fitness-for-Service assessment procedures. Assessment of welded components for Fitness-for-Service is addressed in fragments in various procedures. This is due to the complex microstructures of weldments that include base metal, heat-affected zone and weld metal. The R5 high temperature assessment procedure, produced by BE in the UK, formalized the methods for base metals loaded in static and creep–fatigue with sections describing the treatment of welds. Subsequently, procedures for assessing defects at high temperature, similar to those in R5, have been included in British Standards, the French A16 procedure and in API 579. Section 8 of the recent European thematic network FITNET Fitness-for Service procedure which is incorporated into R5, specifies methods for assessing defects in structures operating at high temperatures and subject to creep and creep–fatigue loading conditions. The reliability of the structural assessments following the codes depends strongly on availability of reliable data required as input data. Most of the data obtained to date on high temperature materials has been reported on creep crack growth (CCG), ignoring the creep crack initiation (CCI) where as creep–fatigue data is scarce. Recent European and international collaborative effort included EC projects CRETE and ECCC, and ESIS TC11, WG on High Temperature Testing of Welds concentrated on CCI as well as CCG testing and analysis of industrial specimens and assessment. This has led to development of a Code of Practice for High Temperature Testing of Weldments via Int. Institute of Welding (IIW) to be presented to ISO for standardization.     

Evaluation of leak-before-break assessment methodology for pipes with a circumferential through-wall crack. Part II: J-integral estimation

Evaluation of the J-integral plays a central part in evaluation of the critical crack length for unstable fracture for piping systems. Simplified evaluation methods for the J-integral for a circumferential through-wall crack in pipes subjected to axial and bending loading or their combination is reviewed in this paper. Use of the LBB.ENG2 method and a similar approach based on the η-factor concept were found to result in significant underestimation of the J-integral for small and medium crack angles. On the other hand, the reference stress method based on the solutions for stress intensity factor and limit load recommended in the companion paper (Part I) provides solutions which agree well with the available non-linear finite-element solutions and can be utilized as a powerful tool for J-integral evaluation for arbitrary materials, not restricted to simple power-law hardening.     

Validation of BS7910:2005 failure assessment diagrams for cracked square hollow section T-, Y- and K-joints

This paper describes the usage of finite element (FE) analyses results to validate the standard BS7910 assessment procedure for the safe design of cracked square hollow section (SHS) T-, Y- and K-joints. In the study, the actual 3D surface cracks obtained from previous fatigue tests have been included in the FE models. An automatic mesh generation program is then developed and used to produce the failure assessment diagram (FAD) through the J-integral method. The ultimate strength of uncracked SHS joints with reduced load bearing areas have been referenced to derive the plastic collapse loads of cracked SHS joints for the development of FAD. These loads have been validated against the previous experimental results. In comparison with the existing standard BS7910 Level 2A/3A FAD curve and the proposed assessment procedure for circular hollow section joints, it is found that a plastic collapse load with a penalty factor of 1.05 will be sufficient for the safe assessment of cracked SHS T, Y, and K-joints under brace end axial loading.