1、 ABSGUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR VESSELS .2004 1 GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR VESSELS (FOR THE SFA OF SFA (years) CLASSIFICATION NOTATION) JANUARY 2004 NOTICE NO. 1 August 2006 The following Changes become EFFECTIVE AS OF 15 AUGUST 2006. (See http:/w
2、ww.eagle.org/rules/downloads.html for the consolidated version of the Guidance Notes on Spectral-based Fatigue Analysis for Vessels 2004, updated in August 2006, with all Notices and Corrigenda incorporated.) Notes - The date in the parentheses means the date that the Rule becomes effective for new
3、construction based on the contract date for construction, unless otherwise noted. SECTION 2 ESTABLISHING FATIGUE DEMAND 7 Combined Fatigue from Multiple Base Vessel Loading Conditions (15 August 2006) (Revise and expand Note in Subparagraph 2/7, as follows.) Because of the variability in Base Vessel
4、 Loading Conditions and its effects on the fatigue strength predictions, it is necessary to consider more than one base case in the fatigue analysis. As a minimum, two cases should be modeled and used in the Spectral-based Fatigue Analysis process. The two cases are ones resulting from, and represen
5、ting, the probable deepest and shallowest drafts, respectively, that the vessel is expected to experience during its service life. Note: Suggested Approach: In some (so-called “Closed Form”) formulations to calculate fatigue demand, the fraction of the total time for each Base Vessel Loading Conditi
6、on is used directly. In this case, potentially useful information about the separate fatigue damage from each vessel loading condition is not obtained. Therefore, it is suggested that the fatigue damage from each vessel loading condition be calculated separately. The combined fatigue life is then ca
7、lculated as a weighted average of the reciprocals of the lives resulting from considering each case separately. For example, if two base loading conditions are employed, and the calculated fatigue life for a structural location due to the respective base vessel loading conditions are denoted L1and L
8、2, and it is assumed that each case is experienced for one-half of the sailing time during the vessels service life, then the combined fatigue life, LC, is: LC= 1/0.850.5(1/L1)+ 0.5(1/L2). As a further example, if there were three base vessel loading conditions L1, L2, and L3with exposure time facto
9、rs of 40, 40, and 20 percent, respectively; then the combined fatigue life, LC, is: LC= 1/0.850.4(1/L1) + 0.4(1/L2) + 0.2(1/L3). The factor of 0.85 takes into account non-sailing time for operations such as loading and unloading, repairs, etc. Notice No. 1 August 2006 2 ABSGUIDANCE NOTES ON SPECTRAL
10、-BASED FATIGUE ANALYSIS FOR VESSELS .2004 SECTION 5 WAVE-INDUCED LOAD COMPONENTS 3 External Pressure Component (Add new Note in Paragraph 5/3.3 and number last paragraph as 5/3.5, as follows.) 3.3 Intermittent Wetting (15 August 2006) Ship motion analysis based on linear theory will not predict the
11、non-linear effects near the mean waterline due to intermittent wetting. In actual service, this phenomenon is manifested by a reduction in the number of fatigue cracks at side shell plating stiffeners located near the waterline compared to those about four (4) or five (5) bays below. To take into ac
12、count the pressure reduction near the mean waterline due to this non-linearity, the following reduction factor can be used: RF = 0.51.0 + tanh(0.35d) where d is depth, in meters, of the field point below the still-water waterline. Note: In order to correctly implement the intermittent wetting effect
13、s, the size of hydrodynamic panel of side shell near waterline should be appropriately modeled with consideration of longitudinal spacing. It is recommended that the size of panel be no greater than two times of side longitudinal spacing in the vertical direction. 3.5 Pressure Distribution on Finite
14、 Element Models The pressure distribution over a hydrodynamic panel model may be too coarse to be used directly in the structural FEM analysis. Therefore, as needed, the pressure distribution is to be interpolated (3-D linear interpolation) over the finer structural mesh. SECTION 8 FATIGUE STRENGTH
15、(Revise third paragraph of Subsection 8/3, as follows.) 3 S-N Data (15 August 2006) To provide a ready reference, the S-N Data recommended by ABS are given in Appendix 2 of this Guide. (Note: source United Kingdoms Dept of Energy (HSE) Guidance Notes, 4thEdition.) There are various adjustments (redu
16、ctions in capacity) that may be required to account for factors such as a lack of corrosion protection (coating) of structural steel and relatively large plate thickness. The imposition of these adjustments on fatigue capacity will be in accordance with ABS practice for vessels. There are other adju
17、stments that could be considered to increase fatigue capacity above that portrayed by the cited S-N data. These include adjustments for compressive “mean stress” effects, a high compressive portion of the acting variable stress range and the use of “weld improvement” techniques. The use of a weld im
18、provement technique, such as weld toe grinding or peening to relieve ambient residual stress, can be effective in increasing fatigue life. However, credit should not be taken of such a weld improvement in the design phase of the structure. Consideration for granting credit for the use of weld improv
19、ement techniques should be reserved for situations arising during construction, operation or future reconditioning of the structure. Notice No. 1 August 2006 ABSGUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR VESSELS .2004 3 APPENDIX 3 OUTLINE OF A CLOSED FORM SPECTRAL-BASED FATIGUE ANALYSIS P
20、ROCEDURE 3 Key Steps in Closed Form Damage Calculation (Revise Item 5, as follows.) 5. (15 August 2006) Calculate cumulative fatigue damage based on Palmgren-Miners rule, which assumes that the cumulative fatigue damage (D) inflicted by a group of variable amplitude stress cycles is the sum of the d
21、amage inflicted by each stress range (di), independent of the sequence in which the stress cycles occur: =JiiiJiiNndD11(7) where ni= number of stress cycles of a particular stress range Ni= average number of loading cycles to failure under constant amplitude loading at that stress range according to
22、 the relevant S-N curve J = number of considered stress range intervals Failure is predicted to occur when the cumulative damage (D) over J exceeds a critical value equal to unity. The short term damage incurred in the i-th sea-state, assuming a S-N curve of the form N = AS-m, is given by: =00dsgpfs
23、ATDiiimi(8) where Di= damage incurred in the i-th sea-state m, A = physical parameters describing the S-N curve T = design life, in seconds f0i= zero-up-crossing frequency of the stress response, Hz pi= joint probability of Hsand Tzgi= probability density function governing s in the i-th sea state s
24、 = specific value of stress range Summing Diover all of the sea-states in the wave scatter diagram leads to the total cumulative damage, D. Therefore: =01000/ dsfgpfsATfDMiiiimmms (9) Notice No. 1 August 2006 4 ABSGUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR VESSELS .2004 where D = total cu
25、mulative damage If it can be conclusively established that the detail under consideration is always subject to a mean stress of ms, D is to be adjusted by a factor msms= a factor for mean stress effect, which is = 1.0 for ms s4/2 = 0.85 + 0.3 ms/s4for -s4/2 ms s4/2 = 0.7 for ms -s4 /2 ms= mean stres
26、s s4= long-term stress range corresponding to the representative probability level of 10-4f0= “average” frequency of s over the lifetime = ipif0i (where the summation is done from i = 1 to M, the number of considered sea-states) Introducing long-term probability density function, g(s) of the stress range as: =iiiiiiipfgpfsg00)( .(10) and NT= total number of cycles in design life = f0T the expression for total cumulative damage, D, can be rewritten as: =0)( dssgsANDmTmms (11)
