Tow Fatigue - How to count for structural natural frequencies?

Dear Sirs,

I'm doing spectral fatigue for Topside modules of a floating structure using Tow Fatigue. I have tried someway to include the effects of natrual frequencies of the structures into their responses but it seems that SACS does not support such dynamic effects. Please advise the current calculation procedure of TOW FATIGUE? Is there any way to count for such thing?

Thanks & Regards,

HoanVV.

  • Hello Hoan V. V.

    Your question can be read several ways. Let me see if I understand.
    You are looking for a way to include the dynamic effects of the barge in the fatigue calculation. By natural frequencies are you thinking about the deformation of the barge?
    MOSES has the capabilities to do transportation fatigue in what is commonly known as stochastic analysis. This topic is also covered in OTC 4161, "A quasi-static approach for transportation analysis of offshore platforms".

    If you could clarify your request we could better offer a suggestion.

    Thank you,
  • Hello Georgina Maldonado-Aguirre,

    It's about TOW FATIGUE in SACS. My model contains the TLP Topside only, no TLWP hull because I just care about the fatigue of the TS in the drilling & operating phases. The program already calls for RAO inputs (motion of platform, TLP in my case), Wind Spec and Weibull Distribution of the wind speed. I'm just wonderring that it seems that SACS does not consider the dynamic effects of the structure itself.
    Please advise!

    Thanks & regards,
    Hoan V.V.

    Thanks & Regards,

    Hoan V.V.

  • Hello Hoan V.V.
    Please tell us more about what you mean with "consider the dynamic effects". RAOs are transfer functions that represent the structural dynamic motion. If this is not they dynamic effect you are concerned with, please provide further details.
  • Dear Georgina Maldonado-Aguirre,
    Sorry for the limitation of my English. I hope belows may help you get my point.

    - To investigate fatigue life of a TLP topside, I'm using SACS, TOW FATIGUE function.
    - The motion RAO input resulted by geometry of floating body and total weight/COG of the platform only.
    - My concern herein is about the dynamic effects of the Topside structure itself (i.e. resonance at natural frequencies of Topside structure in absorbing the inerial loads caused by hull motions).

    My understanding of TOW FATIGUE in SACS:
    1.Environmental dynamic: already considered by Wave Satter Diagram;
    2.Floating body dynamic: already considered by Motion RAO input;
    3. Topside structure dynamic: not yet considered (i.e. Topside nat frequencies do not result any effects on TS Stress Spectrum).

    If item 3 is significant, it should be accounted for. Otherwise, could you please show supporting references?

    Hope this is clear enough.

    Thanks & Regards,

    Hoan V.V.

  • Hi, O.K. You are looking for how the topsides deforms due to the motion of the TLP? The deformation is what is being labeled as dynamic effects. The deformation (dynamic) natural frequencies can also be thought of modes of vibration. Please confirm.