Hi all,

I am working on pipe stress analysis for offshore platforms and FPSO projects.
In onshore, the pipe supports (L post / T post) are directly welded to structure. But in offshore the support specification strictly asks to weld a base plate above deck plate and then weld support steel on this base plate. It is only when we take support above platform decks.
For supports taken below platform deck (i.e, inverted L post or inverted T post and Goal post types), it is directly welded below structural beam.
I don't understand this requirement. One thing I know is if we cut deck plate and weld support stanchion over the structure, the water accumulation in the cut is possible and thus corrosion. But in this case we can weld the stanchion over deck plate without cutting it. so, why we need an additional base plate above deck plate?
Please give your views if anyone has worked on these sort of support detailing.

thanks in advance.
-shankar.

I have never worked on the design of either of these two types of projects so, I have a question.
What are the decks of an "Offshore Platforms" and "FPSO" projects made of?

A- "Solid Plate" (which does not drain so good)?
or
B- "Open Grating" (which will drain free real easy)?

I would think that since these types of installations are subject to high waves and the potential heavy rain due to Hurricanes (Cyclones or Typhoons) then "Open Grating" would offer a grater degree of safety (no standing puddles) and reduced corrosion.

Hi Jop,

Thanks for your reply.
The decks are made of either beams or pipes above which a deck plate or grating is placed.
we prefer grating for exactly the same reason you mentioned. but in places where handling or any safety measure is required, they provide deck plate of usually 10mm thk. it is placed all over the structural steel or pipe and welded completely.

wherever the grating is provided, we can cut grating and place support directly over the structural steel or structural pipe. But wherever the deck plate is provided over structure, it is not advised to cut.
i can think of cutting the deck plate to a size wider than beam flange. in this way, we create a gap on both side of the beam flange, so it facilitates draining.
but i am not sure if it is acceptable. that's why i am looking forward to some expert's view on this who has done similar jobs before.

Though I dint worked in offshore platform Stress jobs, what comes to my mind is that, if u weld a inverted T-post or L-post on deck grating plate the welding will be interminent & weld joint strength would get compromised & if we weld directing on Deck plate then in future if the support is to removed it will leave a metal beeds on deck plate, if deck plate gets grinded it may effect the deck metal plate strength , hence a wear pad like base pad is advised to be used to defeat these problems.
Hope it added some value to the answer.

I have no experience of off shore, but I do have experience of supporing off grating/decking.
Grating/decking will have a design load ie. XkN/Msq. So if your support load is 0.5X then your base plate will have to have an area of 0.5Msq to effectively spread the load accross the decking/grating so as to not comprimise the design load of the decking. HTH.