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APPENDIX W

HIGH CYCLE FATIGUE ASSESMENT OF PIPING SYSTEMS (Draft 3/25/09)

W300 Application

This appendix addresses the fatigue evaluation of B31.3 piping subjected to cyclic

loadings when the total number of significant stress cycles due to all causes exceeds

100,000. A significant stress cycle is defined as a cycle with a computed stress rangegreater than 41.4 Mpa (6.0 ksi). The allowable displacement stress range requirements of

302, using the computed stress range in accordance with 319, provides an acceptable

method of evaluating piping systems for fatigue when the number of significant stress

cycles is less than 100,000. The piping cyclic loadings may be due to thermal expansion,anchor motion, vibration, inertial loads, wave motion or other sources.

Fatigue due to pressure cycling is not addressed in this appendix but must be considered

in design. The methodology in ASME BPV Section VIII Division 2 may be applied toaddress pressure cycling.

The design, fabrication, examination and testing requirements of this Appendix are in

addition to the requirements of Chapters I through IX.

W301 Nomenclature

CF = Welded Joint Fatigue Curve Coefficient, SI (US)

CME = Conversion factor, = 1.0 for stress in MPa and = 6.8948 for stress in Ksi

dt = Fatigue damage due to thermal stress with constant amplitude

dw = Fatigue damage due to wave stress with variable amplitude

E = Modulus of elasticity for the displacement cycle under consideration MPa

(Ksi)

Ea = Modulus of elasticity at ambient temperature MPa (Ksi)

fI = Fatigue Improvement Factor from ASME BPV Section VIII Division 2

fE = Environmental Correction Factor (See Table W302.2)

fMT = Material Temperature Correction Factor

= Gamma Function


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h = Weibull stress range shape distribution parameter

k = fatigue strength thickness exponent

Ld = Piping Cyclic Design Life (Years)

Lw = Design Storm Period of Occurrence (Years)

m = Welded Joint Fatigue Curve Exponent

Ni = cycles for condition i

q = Weibull stress range scale distribution parameter which can be expressedin terms of stress range

Saw = Allowable maximum probable stress range during Nw wave cycles MPa

(ksi)

SEi = computed displacement stress range for condition i corresponding tocycles Ni, MPa (ksi)

TE = Effective component thickness at weld joint, mm (in)

_

T = component nominal thickness at weld joint, mm (in)

SEW = Computed maximum stress range due to wave motion MPa (ksi)

Nw = Design Storm Wave height associated cycles

Nd = Design number of pipe stress cycles

Ni = number of cycles for loading condition, i.

Nti = allowable number of cycles for loading condition, i.

Vo = average zero-crossing frequency (hz)

W302 Design for Fatigue

The fatigue design procedure in this appendix addresses two types of cyclic loading:

Fatigue loading where the loading spectrum may be reduced to a series of stress range cycle pairs and fatigue loading where the loading spectrum may be represented by a two

parameter Weibull distribution. Fatigue damage is the summation based on the linear


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damage rule. The fatigue design analysis methodology in this appendix is based on the

following general requirements:

a) In the absence of more directly applicable data the stress intensification

factors shown in Appendix D for elbows, bends and B16.9 tees may be used.

The stress intensification factors for other components are the responsibility ofthe designer and their validity documented in the engineering design.

b) Integral construction is recommended. Fabricated components such as

unreinforced branch connections, pad reinforced branch connections, andmiter elbows are not recommended.

c) The maximum stress range from all sources of loadings shall not exceed the

displacement stress range requirements of 302.3.5 with f = 1.0.

d) The maximum longitudinal stress due to pressure, weight and inertial forcesdue to wave loading shall meet the requirements of 302.3.6.

W302.1Fatigue damage due to Cyclic Stress Range from other than Wave Motion

The maximum stress range SE shall be computed in accordance with 319 and meet the

allowable displacement stress range requirements of 302.3.5 with f=1.0. The stress range cycle pairs (SEi, Ni) shall be established from a stress cycle histogram by the Rainflow

method of ASME BPV Section VIII Division 2 Annex 5.B. Fatigue damage shall be

computed as follows.

Allowable Fatigue Cycles for load case i:

Nti =

m

k

EEi

MT

E

I

TS

CFf

f

f

(W-1)

Where,

fMT=a

E

E

fI = 1.0 unless otherwise documented in the engineering design.

TE = 16 mm (0.625 in.) for _

T 16 mm (0.625 in.)

TE =_

T for 16 mm (0.625 in.) .8 Sa or SEW > .8 Saw the requirements for severe cyclic service in Chapters I

through IX apply, in addition to the requirements of this Appendix, except that pipedesigned in accordance with this Appendix shall meet the requirements of 305.2.3 and

lap joint flanges are not permitted.

W323 Materials

The impact test exemptions of Fig. 323.2.2B are not permitted.

W328 Fabrication

All circumferential girth welds in piping greater than 1 1/2 NPS shall be full penetration

butt welds. If the offset at the circumferential butt weld exceeds the lesser of .1_

T or 1/8

in (3mm) an additional stress concentration factor shall be applied.

W341 Examination

All longitudinal welds shall be full penetration butt welds and subjected to 100%

radiography per para. 344.5.1 and Table 341.3.2.

The extent of radiographic examination of circumferential butt welds shall be as follows:


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1. ASME Class 150 through Class 600: 10% of the welds shall be subjected 100%

radiography and 20% of the welds shall be subjected to Liquid Penetrant (LP) or

Magnetic Particle (MP) examination.

2. ASME Class 900 through Class 1500: 20% of the welds shall be subjected 100%

radiography and 100% LP or MP

3. ASME Class 2500 through API 10K: 100% radiography and 100% LP or MP

4. The requirements of W305 apply when applicable.

In addition to the above requirements, 100% radiographic examination is required for the

following conditions, regardless of pressure rating:

1. All circumferential butt welds, in other than Category D Fluid Service, made in

the field.

2. All circumferential butt welds within 10 meters from personnel accommodations.3. All circumferential butt welds requiring Post Weld Heat Treatment.

4. All circumferential butt welds, in other than Category D Fluid Service, notaccessible for in service inspection and examination.

W341 Pressure Testing

Pressure testing of the system shall be in accordance with the requirements of para. 347except the test duration for hydrostatic testing, which shall be a minimum of one hour

after the test pressure has been adequately stabilized.


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BACKGROUND

High cycle fatigue evaluation of offshore piping systems has been performed by the DNV

methodology. The stress cycle S-N curve for welded construction used in the DNV

method has a slope of 1/3 and agrees with approaches used in Europe. The slope of thewelded component fatigue curve also agrees with the Structural Stress Approach in

paragraph 5.5.5 of the 2007 Edition of the ASME BPV Code Section VIII Division 2.

However the Markl correlation used by the B31 Codes has a slope of 1/5 and may benon-conservative relative to the other approaches at high cycles. The problem is to bring

high cycle fatigue into B31.3, recognizing the long successful history of the Markl

approach for displacement stress analysis. The stress intensification factors in AppendixD were also determined using Markl. Comparing fatigue curves shows that the Markl

correlation is conservative at low cycles, where the typical piping system operates.

The approach used in this appendix is to defer to the traditional B31.3 approach for the

conventional displacement stress analysis and the Structural Stress approach for highcycle fatigue.

The coefficients and factors agree with the Div. 2 Structural Stress approach based on the

following definition of terms:

CF = I1/mss C

I1/mss = 1.221 (If the stress range is all membrane, Rb,k= 0, the most conservativeassumption. If it is all bending, Rb,k= 1 and I

1/mss = 1.3308)

m = 1/h

Using the -3 correlation for coefficient C

C (SI) (US) CF (SI) (US)

Ferritic and StainlessSteels

11578 (818.3) 14137 (999.1)

Aluminum 1886 (133.3) 2303 (162.8)

SI units include SEi (MPa) and TE (mm)

US units include SEi (Ksi) and TE (in)

The environmental factors, fE, are based on the DNV document.

The 100,000 cycle limit for the base code was based on discussion on the PaulinResearch report at the spring 2008 B31 MDC meeting. The concerns with the base code

are the construction details allowed and the known inaccuracies in the SIFs.


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A comparison of C.S. design curves DNV F3, Div. 2 Structural Stress (-3), and B31.3for two assumptions on SL using SH = SC = 20 Ksi is shown in the figure below.

Design Curves: Div 2 Structural Stress, DNV, B31.3 Air Data, t = 32mm (1.26")

1

10

100

1000

1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10

Cycles, N

StressAmplitude(ksi)

Div 2

DNV F

B31.3

B31.3

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