API 6X Design Calculations

API Standard 6X: API/ASME Design Calculations New Draft Standard under Subcommittee 6

Task Group Membership • • • • • • • • • • • • • •

John Fowler – On-Line Resources - Chairman Jim Britton – Cameron – 17D and 16C Paul Bunch – Cameron – 16A and 17D Jean Brunjes – GE – 6A Bill Carbaugh – GE – 16A Maynard Chance – West Engineering – All specs Chris Johnson - NOV – 16A Jim Kaculi – DrilQuip – 17D Chris Kocurek – Cameron – 17D John McCaskill - Expro – 16C Nigel McKie – FMC – 17D Mark Miser – DNV – All specs Ramon Sanpredro – Stress Engineering – all specs Alireza Shirani – GE – All specs

History and Objective • API 6A, 16A, and 16C referenced the ASME Code Section VIII Division 2, 2004 Edition, Appendix 4. • Since referencing an obsolete spec is awkward, an Annex (Annex I) was prepared for 16A capturing the method from that ASME Appendix. • Subcommittee 6 proposed that the method be put in a separate document to assure uniformity.

Task Group Charge • Create a new API standard documenting the rules of the ASME Code, based on Annex I. • Problem: 16A and 6A used different allowable stresses. Stress value

Spec 6A requirement

Spec 16A requirement

Max SI at test pressure

5/6 of Sy

90% of Sy

Sm for standard materials

2/3 of Sy

2/3 of Sy

Sm for non-standard materials

Lower of 2/3 Sy or ½ Su

2/3 of Sy

Resolution • The task group consensus was to use the rules of 16A, since going to the slightly more conservative 6A rules would penalize 16A/16C users who had designed to higher allowable stresses. • The 90% of Sy used in 16A and 16C is the same as the limit in the 2004 ASME Code Section VIII Division 2 • Using only the yield strength as a basis follows Section VIII Division 3 practice.

Changes to Annex I • Several revisions were made to the Annex I document to clarify the requirements, including several changes to the wording.. • Information of elastic-plastic collapse was added to the document. ASME Appendix 4 had referred to a method in Appendix 6 on experimental stress analysis. • The uses of limit analysis and elastic/plastic analysis were clarified. • The requirement on the sum of the three principal stresses was clarified as to the calculation basis.

Follow-on actions after publication of the new Standard • Specifications 6A, 16A and 16C should revise their design section to direct the designer to this new standard instead of the 2004 Code. • This may require several changes in the body of the specifications. • Since 17D refers to 6A for design requirements, no change is needed. • The1004 ASME Code Section VIII Division 2 can be removed as a referenced standard, and Standard 6X added.

Status of the draft Standard • The TG has finalized the draft and agreed it is ready for ballot. • API will edit the document into standard API format and return it to the TG for review. • After review the draft standard will be sent out for ballot by SC6. • SC 16 and SC 17 will solicit comments from their member companies, which will be considered along with those from SC6 members.

API Std 6X Draft

November 20, 2012

API Standard 6X API/ASME Design Calculations 1

General

This Standard describes the design analysis methodology used in the ASME Boiler and Pressure Vessel Code, 2004 with 2005 and 2006 addenda, Section VIII, Pressure Vessels, Division 2, Alternative Methods, Appendix 4, Methods are included for both elastic and elastic-plastic analysis, and for closed-form as well as finiteelement analysis methods of calculation, in accordance with the rules of Appendix 4 of the 2004 Code, Section VIII Division 2. API has adopted different stress limits from the 2004 ASME Code. The criteria used assume defect-free, tough, and ductile material behavior. For the purpose of this international standard, the basic stress limits are based on Sm and St, which are defined as follows:

1.1 Sm, the design stress intensity The design stress intensity is 2/3 of the minimum specified yield strength S y. For materials with high ratios of yield strength to tensile strength, reduction of the design stress intensity should be considered.

1.2 St, the maximum allowable general primary membrane stress intensity at test pressure API limits this stress to 90% of the yield strength for all materials.

1.3 Temperature effects The effect of temperature on the mechanical properties of the material shall be considered.

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Elastic Analysis

For elastic analysis stress components are calculated, combined, and compared to limits for each category of stress based on multiples of the Design Stress Intensity, S m, for the material in use and for the category of stress. Stress components are combined to find the stress intensity, which is defined as twice the maximum shear stress. This can be calculated as the difference between the largest and smallest of the three principal stresses.

2.1

Stress Categories

The following categories are used to classify stresses based on the consequences of exceeding the yield strength in various manners:

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API Std 6X Draft

2.1.1

November 20, 2012

Primary Stress

The basic characteristic of primary stress is that it is not self-limiting, and failure, or at least gross distortion, can occur from one application of the loading. Primary stress is stress caused by the application of mechanical pressure, forces and moments. Primary stress includes both membrane and bending stress and is linearly distributed across the wall section. Local primary stress can redistribute, like in a threaded connector. Thermal stresses are not primary stresses. 2.1.1.1

Primary Membrane Stress Intensity

Primary membrane stress intensity is calculated from the average values of the stress components through the wall of the vessel. Depending on the extent of the stress, it can be classified as either General or Local.  

 2.1.1.2

General Primary Membrane Stress Intensity, Pm: Membrane stress distributed in a way such that load redistribution cannot occur, and loading beyond the yield strength can proceed to failure. P m is limited to Sm. Local primary Membrane Stress Intensity, PL: The following is a direct quote from ASME Section VIII Division 2 Appendix 4: “Cases arise in which a membrane stress produced by pressure or other mechanical loading and associated with a primary and/or a discontinuity effect would, if not limited, produce excessive distortion in the transfer of load to other portions of the structure. Conservatism requires that such a stress be classified as a local primary membrane stress even though it has some characteristics of a secondary stress. A stressed region may be considered as local if the distance over which the stress intensity exceeds 1.1 S m does not extend in the meridional 1/2 direction more than 1.0(Rt) , where R is the midsurface radius of curvature measured normal to the surface from the axis of rotation and t is the minimum thickness in the region considered. Regions of local primary membrane stress which exceed 1.1 Sm shall not be closer in the 1/2 meridional direction than 2.5(Rt) where R is defined as (R1 + R2)/2, and t is defined as (t1+ t2)/2, where t1 and t2 are the minimum thicknesses at each of the regions considered, and R1 and R2 are the midsurface radii of curvature measured normal to the surface from the axis of rotation at these regions where the membrane stress exceeds 1.1 Sm. Discrete regions of local primary membrane stress, such as those resulting from concentrated loads acting on brackets, where the membrane stress exceeds 1.1 Sm shall be spaced so that there is no overlapping of the areas in which the membrane stress exceeds 1.1 Sm. An example of a local primary membrane stress is the membrane stress in a shell produced by external load and moment at a permanent support or at a nozzle connection.” Local primary stress intensity PL is limited to 1.5 Sm. Primary Bending Stress Intensity

The components of primary bending stress intensity Pb are calculated from the linear primary stress component distributions that have the same net bending moment as the actual stress component distribution. Bending stress components are defined as being proportional to the distance from the centroid of a solid section. When the bending stress components are combined with the membrane stress components at each surface, the resulting stress intensities Pm+Pb are limited to 1.5 Sm. 2.1.2

Secondary Stress

Secondary stress Q is caused by the constraint of adjacent parts or by self-constraint of the structure, and yielding can cause the magnitude of the stress to be reduced. One load cycle can cause local yielding and stress redistribution but cannot result in failure or gross distortion. 2

API Std 6X Draft

November 20, 2012

Secondary stresses are membrane plus bending stresses that can occur at gross structural discontinuities, from general thermal stress, from mechanical preload conditions, or from combinations of these sources. The secondary stress variation, for any sequence of test or operating conditions, is limited to 3 S m. 2.1.3

Peak Stress

Peak stress is the increment of stress added by a stress concentration or other source that does not cause noticeable distortion. Such sources include thermal stress in a cladding material with a different coefficient of expansion from the base material; transient thermal stress, or the non-linear portion of a thermal stress distribution. The only concern with peak stress is that it may cause the initiation of a fatigue crack or brittle fracture. The total stress, including peak stress, may be used in fatigue analysis, which is beyond the scope of this standard.

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Special stress limits Bearing Stress

Bearing stress is allowed to exceed the yield strength of the material provided that the other stresses in the vicinity of the bearing load are within acceptable limits. When bearing loads are applied to parts having free edges, the possibility of a shear failure shall be considered.

3.2

Pure Shear

The average primary shear stress across a section loaded under design conditions in pure shear (for example, keys, shear rings, or screw threads) shall be limited to 0.6 Sm. The maximum primary shear under design conditions, exclusive of stress concentration at the periphery of a solid section in torsion, shall be limited to 0.8 Sm. For hydrostatic test conditions shear stress is limited to 0.6 St.

3.3

Progressive distortion of non-integral connections

Screwed-on caps, screwed-in plugs, shear ring closures, breech lock closures, clamps and unions are examples of non-integral connections which are subject to failure by bell-mouthing or other types of progressive deformation. If any combination of loading produces yielding, such joints are subject to ratcheting because the mating members may slip at the end of each complete cycle, and start the next cycle in a new relationship with one another. Additional distortion may occur at each subsequent cycle so that interlocking parts like threads may lose engagement. Therefore, primary plus secondary stress intensities which could produce slippage shall be limited to Sy.

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API Std 6X Draft

November 20, 2012

3.4 Triaxial Stresses The algebraic sum of the three primary principal stresses (σ1+σ2+σ3) shall not exceed four times the design stress intensity Sm. The sum of the local primary membrane plus bending principal stresses shall be used for checking this criterion.

3.5 Stress Linearization When it is necessary to extract the membrane and bending stresses from finite-element analyses, a numerical technique called “linearization” is used. This procedure involves numerical integration of the stress components to separate the membrane and bending portion of the stress from the total stress. The total stress includes the non-linear peak stress. Guidance on linearization can be found in the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, Annex 5A.

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Non-linear analysis

4.1

General

The limits on primary and secondary stresses need not be satisfied if thorough non-linear finite element analyses are performed. Limit Analysis can be used for determining the actual rated load capacity but not assessing local strain, ratcheting or shakedown. Plastic Analysis can be used for assessing local strain, ratcheting and shakedown but not determining the actual rated load capacity.

4.2 Limit analysis Limit analysis assumes elastic-perfectly plastic material properties, and may be based on smalldisplacement analysis. The stress-strain curve that is used has a bi-linear representation. This curve, for stress less than the yield strength has a slope equal to the elastic modulus of the material. Above yield, the slope is as near zero as practical. A zero slope can cause numerical problems in most finite-element programs when yield is exceeded. The yield strength to be used is 1.5 Sm, which for non-standard materials may be less than the actual specified minimum yield strength. Loading is incrementally increased until the model diverges, which is the collapse load. Actual rated load capacity can be no more than 2/3 of the limit analysis collapse loading. Limit analysis may be used to justify high primary stresses but not secondary stresses. In addition, limit analysis cannot be used to justify a wall thickness thinner than that calculated on an elastic membrane stress basis.

4.3 Plastic Analysis Plastic analysis is a method of structural analysis by which the structural behaviour under given loads is computed by considering the actual material stress-strain curve and may assume small or large deformation theory as required. This method is more accurate than limit analysis because strain hardening effects are included. The material stress-strain curve may be obtained by either actual material test data or approximated via analytical methods using minimum specified yield and ultimate tensile strength values (for example, ASME, Sect. VIII Div.2, Annex 3D).

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API Std 6X Draft

November 20, 2012

If a stress-strain curve from actual testing is used, appropriate corrections may be needed to ensure that the data used in the analysis is representative of the minimum specified yield strength of the material. The effect of temperature on material properties shall be considered. The design is satisfactory if the design loads do not exceed two-thirds of the elastic-plastic collapse load as defined below and the limit analysis is satisfactory. The elastic-plastic collapse criterion is defined as follows: Plot one or more curves of deformation vs. loading. Deflection may be the actual deflection at a point or it may be the strain at a highly-stressed location. The loading should be applied in steps and should include all loads on the product. Select a point Y in the elastic (linear) portion of the curve. Call the deformation at that point “x”. Plot another point at a distance of 2x from the vertical axis. Now extend a line from the origin through the new point until it reached the actual load-deformation curve. The load at that point is the elastic-plastic collapse load.

4.4

Shakedown Analysis

Shakedown analysis can be used to justify high local primary and secondary stresses. Actual true-stress and true-strain curves are to be used as they are used for plastic analysis in section 4.3.

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API Std 6X Draft

November 20, 2012

The design is acceptable if shakedown occurs. That is, after successive applications of the design loading, there is no progressive distortion or stress ratcheting. In addition the deformations which occur prior to shakedown shall not exceed specified functional limits of the design. It is acceptable to include the effect of hydrostatic testing as well as operational loading.

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