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Doubler plate connections

We had an interesting discussion way back in April 2002 about the doubler plate connections in offshore

structures. These pictures and following discussion provide some interesting insights. (I had this discussion

and the pictures still in my email archive and thought it worth posting.)

Many thanks to our senior members agaraju, HV Shankar and Partha Sarathi, for insights on doubler

plates, and to P Mahesh, for sharing these pictures.

Doubler plate

Doubler plate

The use of doubler plate to be limited to secondary attachments where there is a possibility that

the attachment will be removed or replaced during the operating period or where the attachment

is for temporary connection viz., sea-fastenings, etc. In this case you can remove the attachment

easily at the surface of the doubler plate without damaging the parent material.

The allowable punching shear stress for the tubular joints are based on shell behavior of the

entire chord tubular. Please refer to API-RP-2A commentary and the references included there).

As such the doubler plate will not change the allowable stress (Vpa) in the tubular connection

much. In reality what doubler plate does is that it spreads the brace load over a larger area

(similar to our spread footing but the SBC remains same analogy and hence anodes has doubler

plate due to this analogy). A slight reduction may be seen on the acting punching stress (Vp)

depending on the doubler plate thickness. (note- the tau in API formula depends on thickness of

brace to chord ratio. Hence if the doubler plate thickness is more than the brace then we can use

doubler plate thickness instead of the brace in tau calculation. Alternatively the actual stresses

are checked considering the doubler plate dia and weld thickness at the doubler end instead of

the brace dia and thickness which is a bit ambiguous. These tau modification is to be limited to

actual punching stress Vp in API). The saving is not much in the refined analysis and hence donut

take the trouble of including the doubler plate in punching stress calculation.

Please note for the fatigue analysis, the usual tubular SCF formulas are not applicable for doubler

plates, since the SCF for these are to be derived considering the weld details at the doubler end

and other geometric properties.

agaraju

On a request for: any reference material to furnish S curve and SCF evaluation techniques for estimating

fatigue life of fillet welded doubler plate connection joint in jacket:

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Use DOE F2 curve and the SCF calculation is based on:

SCF = 1+a(square)/(2*r square)+(3*a(power 4))+/(2*r(power 4)).

This is at doubler plate edge. The formulae is based on JBEC design brief. The details are given

in Thoery of elasticity by Timoshenko and Goodier. If anybody has the book, they can tell me

what is a and r.

HV Shankar

The equation you have mentioned in fact is the equation for stress concentration for stress

normal to cross section of a plate with a large hole. This equation is both in Theory of Elasticity

and Strength of materials book by Timoshenko (SOM book has one full chapter on SCF -mainly

openings). I assume, use of this in a doubler plate situation supposes that member material

directly under the doubler plate is assumed to be a hole (or not effective) and all stress is

delivered only at the edges of the doubler plate.

In the equation, r is the radial distance from the centre of the plate (i.e., the center of the hole as

the formulation assumes that hole is in the middle of plate of width b) at which stress is being

evaluated and a is the radius of the hole. Maximum SCF occurs when r = a and will be equal

when r = b the width of the plate. Incidentally the max SCF that can be realised by the use of

this formulation is equal to 3.0 (i.e., when r = a).

Partha Sarathi

Good reminder to take care of the pal in the middle. We hardly put effort / time to design the

weld at the doubler plate ends. The picture shows the doubler plate thickness requires to be

designed for transferring the tension/moments from the brace (using Roark formula for flat plate

with ends fixed subjected to circular distribution of force/moment??)

Surprisingly in ADMA we have offshore structures standing from last 40+years and quite a few

of the older platforms were designed using hand calculations. Most of the problems we are facing

are related to attachments and secondary connections with very few cracks or corrosion issues or

any mamber failures related to primary members/joints. May it is time we should spend more

time on these poor chaps as well (may eat into the main if left??)

agaraju

Last updated on: May 22, 2009

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