ssc-367 - Ship Structure Committee

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esults of work covering assessment of 634 structural configurations (fromReferences4.4 and 4.5). It establishes the basis for selecting and evaluating ship details and developing a ship details design procedure. This method accounts for three of the most important parameters that affect fatigue life of a ship detail: ● Mean fatigue resistance of local fatigue details (S-N curve) ● Applicationof a ’’reliability”factorto accountforS-N data scatter and slope ● Applicationofa “randomload” factorto account for the projected stress history Munse’s design method can also be used to estimate fatigue life based on actual or assumed stress history and a reliability factor. A study carried out at the American Bureau of <strong>Ship</strong>ping (ABS) (Reference4.6) to evaluate fatigue life predictionsutilizedseveralmethods, includingMunse’s. The study, based on stress histories derived from strain measurements of containership hatch-corners, provided good comparative results. Although Munse’s method neglects the effect of mean stress, the fatigue lives computed compared well with lives that are computed using other methods. Munse’s design method is an acceptable fatigue design procedure for all vessels. This design method allows proper selectionof design details and providesfor design of a costeffective vessel appropriate for the long term environmental loadings. Vessels that are considered non-standard due to their configuration and/or function (such as a tanker with internal turret mooring or a drillship) should be further analyzed, including a thorough spectral fatigue analysis. Munse’s design procedureis suimnarizedin the block diagramon Figure 4-2. 4-5
Offshore structures such as a semisubmersibledrilling vessel is a continuous system,typicallyhaving orthogonallystiffenedmembers. While a simplified method, such as Munse’s, may be an applicable screening method, such structures have very specialized configurations, response characteristics and structural details. Thus, each structure should be considered unique, requiring a detailed fatigue analyses. An offshore platform is made up discrete members and joints. Since each structure is unique, a detailed fatigue analysis is recommended. However, asimplified method may beappl icable if such a method can be developed based on a large number of similar structures in a given geographic region. Such a method was developed for the Gulf of Mexico by American Petroleum Institute (Reference 1.5) and discussed further. The simplifiedAPI method (Section5.1.1 of Reference 1.5) is based on defining the allowable peak stresses as a function of water depth, design fatigue life, member location and the applicable S-N curve. Although the approach can be modified to apply to other geographic areas, it was developed by calibrating previously completed fatigue analyses of fixed offshore platforms. The maximum allowable stress method is applicable to typical Gulf of Mexico platforms with structural redundancy, natural periods less than three seconds, and the water depths of 400 feet or less. ThisAPI allowablestressmethod i$ intendedfor use as a simplified fatigue assessmentprocedurefor Gulf of Mexico platforms subjected to long-term cyclic stresses considered small relative to the extreme environment stresses. The method attempts to predict fatigue behavior as a function of the design wave event for a generalized platform. It should be noted that the applied force levels can vary substantiallywith platform geometry. The relative importanceof extremedesignwaves and operatingenvironment fatigue waves changes with both the water depth and the actual member/joint 4-6
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SSC-367 - FATIGUE TECHNOLOGY ASSESS
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SHIP STRUCTURF COMMllTFF The SHIP S
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—.--— 1. Report No. 2. Gowotnmr
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FATIGUE TECHNOLOGY Assessment AND D
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6. FATIGUE STRESS HISTORY MODELS 6,
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APPENDICES A. REVIEW OF OCEAN ENVIR
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D. VORTEX SHEDDING AVOIDANCE AND FA
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LIST OF FIGURES (cent.) FIGURE TITL
Page 18 and 19: HOT-SPOT STRESS . The hot-spot stre
Page 20 and 21: QA/Qc : Quality Assurance/QualityCo
Page 22 and 23: 1. INTRODUCTION 1.1 BACKGROUND The
Page 24 and 25: general assessmentof fatiguewhile t
Page 26 and 27: 2. OVERVIEW OF FATIGUE 2.1 FATIGUE
Page 28 and 29: 2.2 FATIGUE ANALYSIS 2.2.1 Anal.vsi
Page 30 and 31: ● Loads generated as affected by
Page 32 and 33: A deterministic method is sometimes
Page 34 and 35: 1 APPLICATION OF NUMEROUS CYCLIC ST
Page 36 and 37: I MOBiLEANDSTATIONARY MARWE STRUCTU
Page 38 and 39: 3. FATIGUE DESIGN AND ANALYSIS PARA
Page 40 and 41: and in-plane/out-of-planeangles,and
Page 42 and 43: the general quality of fabrication,
Page 44 and 45: 3.2 REVIEW OF FATIGUE ANALYSIS PARA
Page 46 and 47: frequencies of interest, requiring
Page 48 and 49: ● Reqular Waves in FrecjuencvDoma
Page 50 and 51: The finite element models of increa
Page 52 and 53: The stressmodel parametersdiscussed
Page 54 and 55: While API S-N curves are applicable
Page 56 and 57: DESIGN Osn40H rAnNsirsR8 FAMcA~ M P
Page 58 and 59: Primarily Affect .--------+’ & In
Page 60 and 61: MOTIONS MODEL — .—. ● ;LOADS
Page 62 and 63: loa 1 10 -— — — -t-i-nT 1[ -t
Page 64 and 65: 4. GLOBAL REVIEW OF FATIGUE 4.1 APP
Page 66 and 67: An allowable stress method, also co
Page 70 and 71: location. Thus, the method should b
Page 72 and 73: 1. SDectral FaticlueAnalvsis Althou
Page 74 and 75: 2. Weibull AtIDroach The Weibull sh
Page 76 and 77: energy about the central direction
Page 78 and 79: Although considered to be an emergi
Page 80 and 81: many design rules implement this ap
Page 82 and 83: Detailed Anal.vsisMethods The detai
Page 84 and 85: The DnV X-curve and the DEn Guidanc
Page 86 and 87: ending restrictions. Cruciform and
Page 88 and 89: period, are also designed tomeet th
Page 90 and 91: incorporates inspection-strategy(Re
Page 92 and 93: ‘Thereare numerous finite element
Page 94 and 95: LOCATIONs oF FATIGUE CMCKS Figure 4
Page 96 and 97: TW=’T ——— ———___ __ r
Page 98 and 99: 1 —. — — — — -.. - .. - .
Page 100 and 101: TOP[C U.K.DEPARTMENTOF ENERGY(OEn)
Page 102 and 103: :1 U.K.DEPARTMENTOF ENERGY(DEn) AME
Page 104 and 105: TOPIC U.K.DEPARTMENJOFEIWRGV(oEn) O
Page 106 and 107: 5. FATIGUE STRESS MODELS 5.1 REVIEW
Page 108 and 109: ● The water particle kinematicsar
Page 110 and 111: “radiation pressure” of waves,
Page 112 and 113: influence on the applied loading. H
Page 114 and 115: their function, selecting appropria
Page 116 and 117: hydrostatic stiffness is introduced
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structure is unique and an allowabl
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loads directly. Since the diffracti
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typically from 0.6 to 0.8 and 1.5 t
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therefore be defined uniformly alon
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load analysisof adetailed three-dim
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esponsesare required , or where con
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either multiple stick elements (for
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Braces may have stubs or cones, whi
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Whatever the basis for an empirical
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The SCF equations currently in use
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life. During the comprehensive desi
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?.s I z - ]LEvE~ ~ 1,5 \ i i+ 11 Is
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-. Figure 5-5 Comparison of Detaile
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( -t ) ( T-JOINT Y-JOINT ( \ I / K-
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SMEDLEY-WORDSWORTH 0 d =— = D 600
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Wave Records Older wave and wind in
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In many cases wind informationmay b
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q = 3.3 s = 0.07, for f< fm s = 0.0
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Seasonal Variation The annual wave
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4. ~arpkaya spreading. (Reference 6
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~. \“ life of a structure,cannot
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6.3 TIME-DOMAIN ANALYSES Nonlinear
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The wave environmentdefinitionsbase
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.,..,, ....- ., ;., .. .... .- .. .
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● Fatiguetest data should be care
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assessmentis typically identifiedas
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damage accumulation similar to that
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“FabricationRestrictions Fabricat
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Fatigue strength of a tubular joint
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Current recommendations,rules and s
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n NB=T 1 [p=+pi Ei . ‘c where: NB
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(2) Damage computation does not acc
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DNB = (f. T/K) (2/2 U)m r (f+ 1) wh
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interactionof multiple fatigue crac
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amplitude loading and loading seque
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n“ CAP PASSES ROOT = * { A A 4 I
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It should be pointed out that the e
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v= flow velocity normal to the cyli
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8.4 METHODS OF MINIMIZING VORTEX SH
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Ra < S REGIME OF UNSE?ARATE~ FLOW 5
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ages. The designer has no control o
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failure data on various structures,
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connections, knife edge crossings,
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9.3.1 Fabrication Effects The fatig
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● Controlled Erosion An alternate
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after a given number of stress cycl
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A second pass with polishingdisc is
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others. Extensive analytical and ex
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d) Stress SDectrum Hot-spot stresse
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Cumulativefatiguedamagecomputations
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as a parametric study intended to i
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Additional areas requiring further
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GROUP Modification of Weld Profile
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400 ‘,, . . 350 - I I 300r [ ~ ,0
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TOPIC SUBJECT INVESTIGATOR COMPLETI
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3.1 Soyak, J. F., Caldwell, J. W.,
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4.12 Daidola, J.C., and Basar, N.S.
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5.4 Papanikolaou,A. and Zaraphoniti
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5.21 -”’ ” ”---”---”
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“7.2 Jordan, C.R., and Cochran, C
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7.19 Niemi, E.J., “FatigueTests o
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9.1 Skaar, K.T., “ContributingFac
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COMMllTEE ON MARINE STRUCTURES Comm
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U.S.Department of Transportation Un
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Member Agencies: United States Coas
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‘ METRIC CONVERSION FACTORS Appro
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APPENDIX A REVIEW OF OCEAN ENVIRONM
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The most distinctive feature-of a r
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s(w) = 2*(4*112) - For a “height
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common probability distributions, t
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T: v. Visually Observed Period, or
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The characteristic wave heights of
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H~ is the significantwave height, a
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a = 0.09, for UJ>wm The U value of
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H~ is the significant height, ‘m
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If the wind duration is less than t
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Sample Wave scatter Diagram s 12 +.
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Nm is the total number of waves in
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turbulence largely a function of te
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Applied Wind Speed Vz(t) ft/s (m/s)
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A.1O REFERENCES A*I Mechanics of Wa
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APPENDIX B REVIEW OF LINEAR SYSTEM
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Awhite noise function-may be used t
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Multiplication of the wave spectrum
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B.2.2.2 Response Amplitude Operator
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0.2.3 Wave Spectra The wave spectru
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9 is the acceleration of gravity in
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The following is..a. sample...of..a
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is the damping ratio,..the ratio of
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the set of extreme conditions, and
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A response scatter diagram could be
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2.4 2.2 2 1 ‘Sen &-”Swull$p~ctr
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L ‘2
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o Kuang (ReferenceCl) “ o Wordswo
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C.2 STRESS CONCENTRATION FACTOR EQU
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C.3 PARAMETRIC STUDY RESULTS . C.3.
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30. 2s , 20. 15. 10. 5. & wORDSWORT
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ChordSide Brace Side K-Joints SCFU
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Definition of Parameters, Validitv
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-, l— I + ?1 -— I . 4> a ,- I I
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Kuang SCF Computdion 1 L u UI \ f-
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., I , 1- out— FIano SCF m tg n I
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— 6 Smedley-Wordswoti SCF Computa
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T ln— Plan- SCF CrOwn POsltlon o
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C.3.l(C) Kuang Chord SCF’S for T-
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5 Kuang SCF Computation 4 IL u M 3
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C.3.l(d) Smedley-WordsworthChord SC
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. ,i 5 Smedley-Wordsworth SCF Compu
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I I K Out— Plan= SCF —— Saddl
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- 10 Smedley-Wordsworth SCFComputat
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‘\, ?v x out— Plan- SCF ——
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C.3.2(a) Kuang Chord SCF’S for T-
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KuwqSE _im T-joint In%neW UrrdSicko
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C.3.2(b) Smedley-HordsworthChord SC
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T-joint Axial SF Saddle Posiiim
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-— T-joint CtkuHlane ELFSaddlePci
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I 1 1 1 1 ~. l~o ~ ha= 1s0 : ~. ~J
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1 ! ! O.fi ;0.762 %841 0,769 O,= !0
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Ikrdwth-kdlw SF l%quiaiifm K-joint
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hdsmthqwdhy W Camputatim l-joint Ax
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Ejrnnt flkf+lane SF SaddlePmitim ~,
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MAXIMUM PRINCIPAL STRESS SCF = ----
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‘-W, .\ I Ill I : II m -,, !) .
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’ \./ ““’ w~”mm m ““-
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. . I Column L’ Longitudinal Gird
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C.8 Underwater Engineering Group,
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APPENDIX D VORTEX SHEDDING AVOIDANC
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a an b d f“ f ar f~ ‘bmax f~ fn
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D. VORTEX SHEDDING 0.1. INTRODUCTIO
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Vortex Shedding Frequency (f”) fv
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waves. When only isolated members u
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member’s nominal diameter and thi
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\Lf-! of cycles. Thus, the reduced
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s ; J steady current, by substituti
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Step 3: Obtain a new valueof 1/10 b
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vortex shedding will occur, i.e., V
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h. The fatigue life may -be modifie
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NOTE: The relationship given is bas
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CELL C7: KINEMATIC VISCOSITY = U CE
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COLUMN N: THE STRESS AMPLITUDE IS C
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D.8.3 Devices and Spoilers Devices
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D*9 REFERENCES D.1 King, R., “A R
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Fint Insmbility Rtgion I !%mmd Inat
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.—____ *U.S. E.P.O. :1993-343-273
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COMMllTEE ON MARINE STRUCTURES Comm
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4!! . ,—.-.,
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