We want to use the below shown type 3

[Anandhanachu]

We want to use the below shown type of stiffener ring in staorage tank.
How could I provide rat hole in this stiffener.
If so what happens at the intersection of stiffener and shell vertical joint?

 

[HTURKAK]

Anandhanachu (Mechanical);

I draw a free hand sketch which explains the rat hole...

 

[IFRs]

What do you do with the vertical leg at the tank shell at the rat hole?

 

[Jay_]

Hello,
I generally don’t like this detail, it needs a lot of precision during fabrication and assembly, and it’s not easy to bring the two horizontal legs together during assembly due to some fabrication errors.
Why do you want to use this detail?
Can you please advise the tank size and tank design vacuum and the required inertia for the stiffener?

Detailing is a hobby,

 

[HTURKAK]

What do you do with the vertical leg at the tank shell at the rat hole?

I think this question is for the detail that i posted yesterday.

Stiffener composed of two Angles is one of the acceptable detail as per API 650. The picture posted by Anandhanachu (Mechanical)(OP) is one of typical stiffening-Ring Sections for Tank Shells .(API 650, 11 th ed. Figure 5-24 )
and API 650 5.9.3.4 states ' Welds joining stiffening rings to the tank shell may cross vertical tank seam welds. Any splice weld in the ring shall be located a minimum of 150 mm (6 in.) from any vertical shell weld. Stiffening rings may also cross vertical tank seam welds with the use of coping (rat hole) of the stiffening ring at the vertical tank seam.....)

The vertical leg at the tank shell shall be cut (50 mm ) interfacing with rat hole.

In past ,( at least) i do not remember that we used two angles stiffener. However, as per API 650, the rat hole detail should be similar to the sketch that i posted.

 

[IFRs]

And the cut edges of the vertical legs would be seal welded to the tank shell?

 

[JStephen]

The way I read that requirement in 5.9.3.3, is that stiffeners MAY use a rat-hole at a vert, but aren't obligated. It says "may also" use the rathole, and "WHEN the coping method is used", etc.
That said, why not grind the vert flush and weld right over it?

 

[HTURKAK]

............... It says "may also" use the rathole, and "WHEN the coping method is used", etc.
That said, why not grind the vert flush and weld right over it?

I also think coping (providing rat hole ) is not a mandatory requirement ..One of the reason the "rat hole" is , the wind girder will not be pushed away from the tank outer surface due to the vertical weld seam. If rat hole is not used, the vert seam shall be plain grinded. However, weld right over is not reasonable...50 mm at both sides of vert should not be fillet welded..Another reason drain holes..

If coping is not provided, adequate drain holes shall be provided as per 5.9.3.3.

 

[Anandhanachu]

The vertical leg at the tank shell shall be cut (50 mm ) interfacing with rat hole.

Please tell me ,where this 50mm is specified.(API650 ?)

Please confirm my hand sketch is correct or not?
Note; Only half portion

of the stiffener is shown in the sketch for easiness.

 

[HTURKAK]

The vertical leg at the tank shell shall be cut (50 mm ) interfacing with rat hole.

Please tell me ,where this 50mm is specified.(API650 ?)

BS EN 14015 9.3.3.2....Whether or not adjacent sections are butt welded, the welding of such a joint shall be such that it produces fusion between the adjacent sections only and not fusion to the shell plate surface.
Mouse holes (approximately 20 mm radius) shall be provided for drainage purposes.

API 650 5.9.3.3 ... If drain holes are provided, they shall be at least 25 mm (1 in.) diameter (or slot width) on 2400 mm (8 ft) centers or less...

The common practice for rathole of minimum radius 0.75" . ..and in general 1.0" is provided.

Regarding your detail, for single angle stiffener pls look API 650 Figure 5-24 Detail C . If it is for half of the double angle, the vertical leg shpuld be cut 50 mm not 100 mm. Pls look to detail below:

 

[Anandhanachu]

Sir, why this much portion(Red marked) is left not welded.
On which code this one is specified?

 

[HTURKAK]

why this much portion(Red marked) is left not welded.
On which code this one is specified?

Because the shell vertical and horizontal butt welds are fusion weld and with full penetration and the stiffeners are fillet welded to shell for joining to shell. The fillet weld SHALL NOT produce fusion with
shell. Remember also HAZ.

 

[IFRs]

API 650:

5.9.3.3 Welds joining stiffening rings to the tank shell may cross vertical tank seam welds. Any splice weld in the ring shall be located a minimum of 150 mm (6 in.) from any vertical shell weld. Stiffening rings may also cross vertical tank seam welds with the use of coping (rat hole) of the stiffening ring at the vertical tank seam. Where the coping method is used, the required section modulus of the stiffening ring and weld spacing must be maintained.

5.1.3.7 Attachments to tank exterior surfaces shall be as follows.
a) Except as provided in item b. below, all attachments to tank exterior surfaces shall be completely seal welded (no intermittent welding) to minimize rust streaking.
b) If specified on the Data Sheet, intermittent welding is permitted for:
1) wind girders as described in 5.1.5.8;
2) attachments to surfaces that will be covered by insulation;

5.1.5.8 Wind Girder Joints
a) Full-penetration butt-welds shall be used for joining ring sections.
b) Continuous welds shall be used for all horizontal top-side joints and for all vertical joints. Horizontal bottom-side joints shall be seal-welded unless specified otherwise by the Purchaser.

Seems to me that the strength of the stiffener can not be reduced (as it would be [slightly] if the vertical leg were to be cut off) and unless the purchaser specified that intermittent welds can be used, the intent is that the shell stiffeners be fully seal welded to the tank shell.

I'd either grind flat the shell butt weld and do additional NDE there as well as above and below the stiffener or choose a different style of stiffener that does not present these issues. I'm also a fan of having the stiffener slope away from the tank shell, which is difficult for this style.

My opinions only, remember - you read it on the world wild web !

 

[HTURKAK]

5.1.5.8 Wind Girder Joints
a) Full-penetration butt-welds shall be used for joining ring sections.
b) Continuous welds shall be used for all horizontal top-side joints and for all vertical joints. Horizontal bottom-side joints shall be seal-welded unless specified otherwise by the Purchaser.

Just curious, how did you understand the subject tank is API 650 TANK? Anandhanachu (Mechanical) did not mention at his thread..

I think your concern is corrosion from entrapped moisture rather than strength requirements.

However API 620 approach is somehow different..

API 620 R.3.5.4 All fillet welds shall consist of a minimum of two passes. The ends of the fillet welds shall be 2 in. from the rat hole (see Figure R-1), and these welds shall be deposited by starting 2 in. from the rat hole and welding away from the rat hole....

 

[IFRs]

HTURKAK - The very first post of the OP included an image that looks exactly like a snip out of API 650 Figure 5.24 that includes five details of acceptable shell stiffeners. The OP showed us Detail D.