API 6HP Example Analysis Project

1. API 6HP Example Analysis Project API E&P Standards Conference Applications of Standards Research, 24 June 2008 173454-06 API 6HP Process

2. API 6HP Example Project • Objective • Meet the ECS Oversight Committee request • Document a process, not validate a product • Scope • Relatively simple HPHT model similar to a C&K valve body • The API 6HP design committee defined the input parameters • Model configuration • Service conditions • Material properties • Document the process in a technical report 173454-06 API 6HP Process

3. User’s Functional Design Spec Mfg Design Specification Design Equipment Design Verification Analysis Design Validation Design Meets Spec No Yes Manufacture Equipment 173454-06 API 6HP Process

4. Design Verification Analysis Plastic Collapse Analysis ASME VIII-2 Par 5.2.4 Local Strain Limit Analysis ASME VIII-2 Par 5.3.3 Ratcheting Analysis ASME VIII-2 Par 5.5.7 Leak Before Burst Yes S-N Fatigue Analysis ASME VIII-3 KD-3 No LEFM Fatigue Analysis ASME VIII-3 KD-4 Design Verification Analysis • Design Verification Analysis • Applies to pressure containing parts • Does not apply to pressure retaining parts • Does not apply to closure bolting • Does not apply to ring gaskets 173454-06 API 6HP Process

5. LEFM Fatigue Analysis Initial crack size based upon NDE or incremental crack size Calculate stress intensity factor based upon crack depth, a Calculate crack growth for service life requirements New crack size < allowable Yes Redesign No Design meets spec No LEFM Fatigue Analysis • Analysis required for each critical section • Assume initial crack size based upon NDE capability • Crack aspect ratio should be updated as crack grows • Use appropriate material crack growth rate data for environment and loading • Allowable crack size based upon ASME Div 3 KD-412 173454-06 API 6HP Process

6. Example Model M P = 20 ksi T = 105,000 lb M = 10,000 ft-lb Temp = 350°F int, 35°F ext T 173454-06 API 6HP Process

7. Process – Plastic Collapse • Process per ASME Sect VIII, Div 2, paragraph 5.2.4 • FEA Model • Geometry • Generate FEA model accurately representing the component geometry, boundary conditions, and applied loads for the pressure containing component • Refinement of the model around areas of stress and strain concentration shall be provided appropriate to good engineering practices • The effects of non-linear geometry shall be considered in the model • Material • Use elastic-plastic material model in accordance with ASME Div 2 Annex 3.D • Use SMYS, SMUTS, and Modulus at max rated temperature • Boundary Conditions • Apply all relevant loads and all applicable load cases per ASME VIII-2 Table 5.5 173454-06 API 6HP Process

8. Process – Plastic Collapse • Load Cases • Run all relevant load case combinations per ASME VIII-2 Table 5.5 • Analysis • Perform an analysis for each load resistance factor (LRF) case • Evaluation • If analysis converges, the component is stable under the applied loads for each load case and meets the Plastic Collapse criteria • If analysis does not converge, either • Reduce load rating • Increase structural design • Increase material strength properties 173454-06 API 6HP Process

9. Process – Localized Failure • Process per ASME Sect. VIII, Div. 2, paragraph 5.3.3 • FEA Model • Use Plastic Collapse model for geometry, material, boundary conditions, and load cases • Analysis • Equivalent plastic strain shall be less than triaxial strain limits at each location as per ASME VIII-2 paragraph 5.3.3 • Applies to all load cases defined for plastic collapse analysis • Evaluation • If analysis meets the triaxial strain limits for all load cases, the component meets the local failure criteria • If analysis does not meet the strain limit criteria, either • Reduce load rating • Increase structural design • Increase material strength properties 173454-06 API 6HP Process

10. Process – Ratcheting Analysis • Process per ASME Sect. VIII, Div. 2, paragraph 5.5.7 • FEA Model • Geometry • Generate FEA model accurately representing the component geometry, boundary conditions and applied loads for the pressure containing component • Refinement of the model around areas of stress and strain concentrations shall be provided appropriate to good engineering practices • The effects of non-linear geometry shall be considered in the model • Material • Use elastic-perfectly plastic material model with kinematic strain hardening • Use SMYS, SMUTS, and Modulus at room temperature for hydro test cycles and at max rated temperature for working pressure cycles • Boundary Conditions • Run using all relevant loads and all applicable load cases 173454-06 API 6HP Process

11. Process – Ratcheting Analysis • Analysis • Perform preload of mating flange bolting as necessary • Perform hydro pressure test cycles at room temp as required (normally 2 cycles) • Perform 3 working cycles at max rated temperature • Evaluation • After 3 working cycle loads • No plastic action in component is permissible • Must have an elastic core in primary load bearing boundary • No permanent change in overall dimensions between last and next to last cycle is permissible 173454-06 API 6HP Process

12. Input Conditions – Structural Analysis • Hydrostatic pressure test – 27.5 ksi (2 cycles) • Based upon ASME Div 3 requirements of 1.25 x rated pressure x material derating factor for 350°F (1 / 91%) • Service conditions • Pressure • 12 pressure cycles at 20 ksi every two weeks based upon bi-weekly BOP pressure testing • Loads • Pressure end load, plus • Constant external applied tension of 105,000 lb applied along axis of flange neck, plus • Bending moment of 10,000 ft-lb applied to axis of flange neck alternating on a period of 10 sec • Temperature • Material strength reduced to 91% for 350°F service • Environment • Assume air for model 173454-06 API 6HP Process

13. Process – LEFM Analysis • Process per ASME Sect. VIII, Div. 3, KD-4 • FEA Model • Geometry • Generate FEA model accurately representing the component geometry, boundary conditions, and applied loads • Refinement of the model around areas of stress and strain concentrations shall be provided appropriate to good engineering practices • Material • Use linear elastic material model • Boundary Conditions • Apply all relevant working loads • Internal and external pressure • External applied loads • Thermal gradients 173454-06 API 6HP Process

14. Process – LEFM Analysis • Analysis • Superimpose thermal stress with applied loads • Calculate the max principal stresses through the wall at all critical sections (worst case section may not be obvious) • Define the initial crack size based upon NDE criteria • Surface cracks should assume an initial aspect ratio of 1:3 (KD-410) • A surface crack in a stress concentration area, such as cross-bores, can be assumed to have an initial aspect ratio of 1:1 • Calculate the stress intensity factor at the crack tip • Apply crack face opening pressure as appropriate • Calculate incremental crack growth with incremental working cycles based upon material properties (da/dN vs. ΔK) • Reference MMS report www.mms.gov/tarprojects/583.htm • Repeat crack growth cycles until crack depth meets the final allowable crack depth 173454-06 API 6HP Process

15. Process – LEFM Analysis • Final allowable crack depth • The final allowable crack depth shall be the lesser of: • Half the number of cycle required to grow the crack from initial depth to the depth where the crack stress intensity factor exceeds the material toughness, K1C, or • Number of cycles required to grow the crack from initial depth to 25% of the section thickness, or • Number of cycles required to grow the crack for initial crack depth to 25% of the critical crack depth • Repeat fatigue calculation for each critical section • Evaluation • If fatigue life meets criteria, component is acceptable • If fatigue life does not meet criteria • Change inspection intervals • Redesign • Reduce loads 173454-06 API 6HP Process

16. Analysis Summary • Materials • Material properties are attainable by testing • Some data is available in existing standards • Structural analysis • Max equivalent plastic strain ≈ 0.5% at working loads • Shakedown occurs within 3 working cycles • Fatigue analysis • Design life exceeds goal • Consideration of stresses from thermal gradient is important • Thermal stresses may change high stress point and crack initiation from ID surface to OD surface 173454-06 API 6HP Process