API 6 HP Example Analysis Project API EP
- Slides: 16
API 6 HP Example Analysis Project API E&P Standards Conference Applications of Standards Research, 24 June 2008 1 173454 -06 API 6 HP Process
API 6 HP 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 6 HP design committee defined the input parameters – Model configuration – Service conditions – Material properties § Document the process in a technical report 2 173454 -06 API 6 HP Process
User’s Functional Design Spec Mfg Design Specification Design Equipment Design Verification Analysis Design Validation Design Meets Spec Yes Manufacture Equipment 3 173454 -06 API 6 HP Process No
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 Design Verification Analysis Plastic Collapse Analysis ASME VIII-2 Par 5. 2. 4 Local Strain Limit Analysis ASME VIII-2 Par 5. 3. 3 Leak Before Burst Ratcheting Analysis ASME VIII-2 Par 5. 5. 7 Yes No LEFM Fatigue Analysis ASME VIII-3 KD-4 4 173454 -06 API 6 HP Process S-N Fatigue Analysis ASME VIII-3 KD-3
LEFM Fatigue Analysis Initial crack size based upon NDE or incremental crack size Calculate stress intensity factor based upon crack depth, a • 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 • Calculate crack growth for service life requirements New crack size < allowable • Yes Redesign No Design meets spec 5 • No 173454 -06 API 6 HP Process Allowable crack size based upon ASME Div 3 KD-412
Example Model M P = 20 ksi T = 105, 000 lb M = 10, 000 ft-lb T 6 173454 -06 API 6 HP Process Temp = 350°F int, 35°F ext
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 7 173454 -06 API 6 HP Process
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 8 173454 -06 API 6 HP Process
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 9 173454 -06 API 6 HP Process
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 10 173454 -06 API 6 HP Process
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 11 173454 -06 API 6 HP Process
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 12 173454 -06 API 6 HP Process
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 13 173454 -06 API 6 HP Process
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/d. N vs. ΔK) – Reference MMS report www. mms. gov/tarprojects/583. htm § Repeat crack growth cycles until crack depth meets the final allowable crack depth 14 173454 -06 API 6 HP Process
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, K 1 C, 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 15 173454 -06 API 6 HP Process
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 16 173454 -06 API 6 HP Process
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