Fatigue calculations - Offshore fixed platform

While performing fatigue calculations of fixed offshore platforms

Typically, I chose the maximum shear option ("MS") in the WAVE card to generate the transfer function in the Seastate module (static or dynamic analysis); subsequently the fatigue module is invoked to calculate damage (csf input for fatigue damage using the "SEAS" card).

While performing some sensitivity analyses, I noticed the following (note wave frequency is about 7-8 sec; structure nat freq is 1.8 sec so negligible dynamic amplification)

a) that the transfer function generated using the MS option in WAVE (and subsequently stress range for fatigue) by static analysis is much lower than that generated using the MS option in WAVE by dynamic analysis. The stress range calculated using the static analysis is about half of that from the dynamic analysis.

b) Further the transfer function generated using the AL option in WAVE using static analysis is lower than that generated using AL option in WAVE using dynamic analysis.

c) I notice the following note in Section 2.2.1 of the wave response manual. "Note: Although a load case is to be created for every wave position, the ‘AL’ wave position option should not be used on the WAVE line. The number of positions to be saved as load cases is input in the Wave Response input file. See the ensuing section ‘Load Options’ for details on selecting wave positions for loading"

Please comment on the significant difference in the computed stress range using the static and dynamic analysis given that the nat. frequency is much lower than the wave excitation frequency. Also please comment on the AL and MS options as well as the note highlighted in item c) above.

Thanks

  • A transfer function at a given point on the structure for given period (or frequency) represents the maximum stress range when a wave of the that period (or frequency) is stepped through the structure. The 'MS' option on the WAVE line for a static analysis will only produce a single load case corresponding to the maximum base shear and therefore does not represent a stress range. Please explain how you are using the 'MS' option in wave response - the equivalent of 'MS' for a static analysis is 'MAXS' in wave response.
    Regarding the 'AL' options, does the dynamic analysis yield more fatigue?


    This is a test