Bottom Plate Replacement for FR tanks 1

[rakucheleri]

Dear Experts,
This is my first time repairing a open top Floating roof ,Double Deck storage tank designed according to API 650 for Oil Storage., date of fabrication is 1999, this tank suffers from heavily corrosion in Entire bottom plates (according to the inspection report, Entire bottom plates are set to be changed , No changes for annular plates). in order to do that , i am going to use gas cutting technique to remove the plates. Once these plates are removed and before welding the new ones, the tank shell may suffer buckling due to its weight.ck
Tank dimensions : 65 Mtr Dia x 20 Mtr Ht x Double deck floating roof tank
First Shell course Thk : 32 mm
Material of construction : IS 2062
Bottom Plate Thk : 8 mm
Annular Plate Thk : 14 mm
Deck Weight : 400 Ton
Shell Weight : 1000 Ton approx
Roof Legs : placed 5 mtrs apart from each other x 151 Nos x 3" sch 80

looking for recommendations and comments for
1) how to change entire bottom plate except annular plate
2) How to control Buckling/mechanical distortions due to shell weight
3) how to remove plate when deck is in Top leg position
4) how many area can be removed at a time
5) How & Where to do cutting on shell for removal /entry for bottom plates
6) is PWHT is required for shell openings.

all feedback is always welcome

thank you in advance

 

[IFRs]

Why would the tank care that you are removing the bottom plate inside the annular ring? It gets no support from these plates. Since you are leaving the annular plates intact, I would think you could remove the entire bottom without issues. Does the annular plate meet API 650? Does API 650 require an annular plate?

PWHT is not required for a door sheet. You will need to cut a door sheet large enough for a tugger or Bobcat, also use for material, equipment and personnel entry. Follow API 653 for door sheet details.

If you are adding shell nozzles or manways, follow shop PWHT requirements in API 650 for the fitting size and your material grade, thickness and design metal temperature.

With the legs there, you will have to crib around each and remove it to take old plate out and slid a new bottom plate under the leg area. This will slow you down but it is not rocket science.

Cribbing is needed in any case, for safety while working under the deck. The deck can spiral down and crush what's under it. Ventilation will be an issue, put fans in all the deck manways to exhaust fumes and draw fresh air in.

Best to have an experienced tank crew do this. Where is the tank located?

 

[bimr]

A tank fabricator will be able to answer your qestions.

http://www.eng-tips.com/viewthread.cfm?qid=392099

 

[rakucheleri]

Dear IFRs / Bimar
thank you for your reply, The tank is situated in india & am not getting a proper fabricator for the work who having wast experience in this field.So i have to take a decision regards the issue.

Now we decided to
1) Replace the bottom part by part ie Replacing 25% of bottom plate at a time and repeat the sequence.Put the plates on tack position and do the welding only after the completion of entire bottom plate change.

2) Providing cross bracing between Tank shell to annular plate in a span of 2 meters to avoid any mechanical distortions due to dead load of tank shell

3) Placing of additional support to deck where ever found necessary ,By using a 3" pipe having length of 1.8 Mtrs (High Leg Hight) with plates welded on both sides.

4) Put bitumen pad where ever is required and keep the slope as 1:120 As a minimum.

i expect no issues or bulging will occur due to load.

please advice