S-N curve above water level

Dear SACS expert,

Need your help,

How to describe S-N curve above water level (for example at jacket walkway level).

Any specific option at SACS fatigue input?

Parents
  • S-N curve in SACS fatigue can be chosen from code library or defined by user self in stress-cycles curve format in FTOPT command line.

    • If you choose S-N curve from code library, then SACS will directly use S-N curve from that code. Some code has described different S-N curves for members above & below waterline, then SACS will identify member location and apply different S-N curve accordingly. However, some code has only one curve for all members. So you need to select which code you would differ to.
    • If you want to define S-N curve by specifying your own stress versus cycles relationship, then you can define multi curves and apply different curves to different members manually.
  • Hi Kevin,

    Just to confirm, which S-N curve between 'air' and 'sea water' used by SACs for element located in 'Splash Zone' ?

    Regards,

    Kasiphon

  • Hi Kasiphon,

    Are you asking which option of S-N curve source in SACS  has a separate curve for element located in splash zone?

  • I mean when some member located in splash zone, SACs will consider as they locate in air or in water.

    i.e. Assume water line is EL+0m and splash zone = EL-5 to EL+5.

    If we have member \ joint located in this range. SACs will use SN in air or in water to calculate fatigue.

    Or SACS do not care about splash zone and use water line instead i.e. below EL+0m will be used SN in water and above EL+0 will be used SN in air.

    Btw, SACS also require user to define splash zone in ftginp. What is the purpose for splash zone elevation in ftginp ?

  • Except you define S-N curve manually, SACS strictly abide for the code your selected. For example

    • If you select WJT or IJT option for S-N source(API RP2A WSD 2007 and ISO 19902). There are only two curves for members in air and in water with cathodic protection correspondingly. So all members are divided in two categories: in water or in air, no splash zone is considered.
    • For some S-N source code such as AWS, there is only one S-N curve for all members. All members will use 1 S-N curve no matter they are in water, in air or splash zone.
    • If you choose S-N source from API thickness dependent curve, there would be 3 separate S-N curves for member in, above and below splash zone. SACS will identify all members locations( 3 categories) and apply different S-N curve accordingly.
  • Just to confirm,

    Fex, I select SN curve from SACS library i.e. NT (NORSOK T curve).

    In FTOPT2 Col. 37-50, we can also define Splash zone which I understand this zone will be actively used for AXX SN curve only (API thickness dependent curve), please confirm.

    However, refer to DNV-RP-C203, it is recommend to use 'SN in Seawater' for joint located in Splash zone.

    Then, if splash zone is -5 to +5, the joint located at EL+2 is already above water line (Assume water line is EL+0). SACS will pick SN in Air to check this joint fatigue. As SACS use water line to classify the SN curve, please confirm.

    If I would like to use SN in Seawater for joints locate from EL+5 to mudline , How can I do in SACS ?

    Regards,

    Kasiphon

    • Yes, splash zone elevation defined in FTOPT2 is only valid for API thickness dependent curve(AXX, AXP).
    • Yes, for other S-N curves options which differentiating members above/below waterline, SACS use waterline to classify members. Splash zone elevation is not considered.
    • As SACS use water line to classify members by default, you need to use joint override feature if you don't want to do like this. Using user-defined S-N curves to describe the curves above/below waterline, then using joint override to apply these S-N curves to joints as you want.
Reply
    • Yes, splash zone elevation defined in FTOPT2 is only valid for API thickness dependent curve(AXX, AXP).
    • Yes, for other S-N curves options which differentiating members above/below waterline, SACS use waterline to classify members. Splash zone elevation is not considered.
    • As SACS use water line to classify members by default, you need to use joint override feature if you don't want to do like this. Using user-defined S-N curves to describe the curves above/below waterline, then using joint override to apply these S-N curves to joints as you want.
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