Risers design

[FX103]

I'm doing clamps spacing calculations for a 6" risers to be installed on a jacket.

I'm using criteria of fatigue analysis due to VIV and direct wave action (DNV RP F105). Are they applicable out of the submerged zone but inside the splash zone? If not, which kind of criteria should be used? Only static?

Thanks for any help

 

[narenr]

Splash zone above the submerged area will be designed for direct wave impact loads and operational loads. Normally additional external corrosion allowance is provided in the splash zone.


Narendranath R
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[Deepray]

Personally I disagree the use of F105 for riser span checks as this code is formulated for pipelines.

Pipeliners

 

[engrom]

I agree with Deepray. I do remember checking this point with Principals whether F101 applicable to riser assessment in cyclic load conditions and the reply was negative. OS-101 normally covers for submarine pipelines which take into consideration VIV, wave loading, wind, tide impact etc. However, I believe there is quite a confusion in this thoery as I've understood certain Opco's use F101 for Riser Defect assessment taking into account environment load impacts.

DNV-RP-F105 covers 'Free spanning of Pipelines' and hence shall not be considered for vertical risers.

DNV-RP-F101 covers 'Corroded pipelines' with assessment mainly based on internal pressure loading only and internal pressure loading combined with longitudinal compressive stresses. One of the exclusions in this RP is that it does not cover 'cyclic loading'.

I think, for 6" dia riser, clamp spacing should be approx 16ft (std weight AISI) or 20ft (Extra strong). Probably consider using Stoke's Fifth Order Wave calculation in combination with API-2A WSD & API-RP1111. If strong wind or hurricane expected geographical locations, try to make it more stringent!