173454-06 API 6HP Process1 API 6HP Example Analysis Project API E&P Standards Conference Applications of Standards Research, 24 June 2008

173454-06 API 6HP Process1

API 6HP Example Analysis Project

API E&P Standards ConferenceApplications of Standards Research, 24 June 2008


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


User’s Functional

Design Spec

Design Equipment

Design Meets Spec

Design Verification

Analysis

Design Validation

Manufacture Equipment

No

Yes

Mfg Design Specification


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

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

LEFM Fatigue AnalysisASME VIII-3 KD-4

Leak Before Burst

S-N Fatigue AnalysisASME VIII-3 KD-3

No

Yes


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

Calculate crack growth for service life requirements

LEFM Fatigue Analysis

Initial crack size based upon NDE or incremental

crack size

Calculate stress intensity factor based upon crack

depth, a

New crack size <

allowable

No

Yes

Design meets spec

Redesign

No

mKKRfC

dN

da


Example Model

T

M P = 20 ksi

T = 105,000 lb

M = 10,000 ft-lb

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


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


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


Process – Ratcheting Analysis

• Process per ASME Sect. VIII, Div. 2, paragraph 5.5.7


– 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


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


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


Process – LEFM Analysis

• Process per ASME Sect. VIII, Div. 3, KD-4


– 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



• 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

http://www.mms.gov/tarprojects/583.htm



• 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


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

Documents

173454-06 API 6HP Process1 API 6HP Example Analysis Project API E&P Standards Conference Applications of Standards Research, 24 June 2008