Issue 37

P.S. van Lieshout et al., Frattura ed Integrità Strutturale, 37 (2016) 173-192; DOI: 10.3221/IGF-ESIS.37.24 183 multiaxial fatigue methods only the results from MCSC and MWCM were considered because they use the same reference SN-curve. Including the results obtained with the EESH would not be just because this method is based on a local approach. The results were normalized with the lowest fatigue damage and are listed in Tab. 3b. Variable amplitude loading – Case study In various previous studies on the applicability and validity of multiaxial fatigue methods conceptual load histories have been used [49-51] . However, difficulties start to arise when it is intended to execute a multiaxial fatigue analysis on a structure under OP VA loading, which is representative for the actual day-to-day loading on marine structures. For this purpose a case study was developed by the authors and was then used to investigate the effect of stress amplitude ratio on fatigue damage using PDMR based multiaxial cycle counting [52]. Table 3a : Normalized effect of stress multiaxiallity on fatigue damage predicted using selected codes – Comparison between the different codes. Critical plane method LC 1 LC 2 LC 3 LC 4 LC 5 MCSC 1.0 1.0 2.3 1.7 2.0 MWCM 1.0 1.0 1.0 1.0 1.0 Table 3b : Normalized effect of stress multiaxiallity on fatigue damage predicted using selected multiaxial fatigue methods – Comparison between the different methods. Marine structures are in reality subjected to a combination of stationary sea states whereby each sea state consists of one, two or several wave systems [53]. Each wave system is then defined by a spectral density function. Simultaneous wind seas and swells generally dominate the wave spectrum of floating marine structures. However, as a simplified approach, Zou & Kaminski (2016) suggest to use one expression to represent all swells by a particular spectrum with a mean wave direction. This suggestion is based on wave energy conservation The same assumption can be made for wind seas. In this case study this simplified approach has been followed. Wind driven seas were described by the mean JONSWAP spectrum as advised by the 17 th ITTC in 1984 [54] and swell sea by a Gaussian swell spectrum (see Eqs. 8 and 9).                        2 , 5 4 JONSWAP 4 4 , , 320 1950 S ω s wind A p wind p wind H exp T T (8)                                               2 ,  , 1 0.07   2 ; ;  0.09   ,  2 p p wind p wind p p if whereby A exp if T wind                            2 2 ,  2 ,  ( / 4) 2 2 p s swell Gaussian swell H S exp (9) 2 Considering an in air environment Code LC 1 LC 2 LC 3 LC 4 LC 5 LC 5.1 LC 5.2 Eurocode 3 1.0 3.0 1.3 1.4 2.7 1.7 IIW 1.0 3.0 2.4 9.8 19 3.3 DNV-GL-RP-0005 2 1.0 1.0 1.0 1.0 1.0