API 6HP Example Analysis Project

1
API 6HP Example Analysis Project

• API EP Standards Conference
• Applications of Standards Research, 24 June 2008

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 CK
  valve body
• The API 6HP design committee defined the input
  parameters
• Model configuration
• Service conditions
• Material properties
• Document the process in a technical report

3
Users Functional Design Spec
Mfg Design Specification
Design Equipment
Design Verification Analysis
Design Validation
Design Meets Spec
No
Yes
Manufacture Equipment
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

5
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

6
Example Model
M
P 20 ksi T 105,000 lb M 10,000 ft-lb Temp
350F int, 35F ext
T
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

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

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

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

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

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
  350F (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 350F
  service
• Environment
• Assume air for model

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

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 13 (KD-410)
• A surface crack in a stress concentration area,
  such as cross-bores, can be assumed to have an
  initial aspect ratio of 11
• 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. ?K)
• Reference MMS report www.mms.gov/tarprojects/583.
  htm
• Repeat crack growth cycles until crack depth
  meets the final allowable crack depth

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

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