API RP2201 - Hot tape minimum metal thickness 1

[aldocigna]

The API RP2201 reccomended a minimum metal thickness like a function of the thickness required for strenght plus a safety factor (usually 2.4 mm) to prevente burn through.
I don't understand very well what temperature i've to use to calculate the thickness for strenght. If this temperature is the temperature of pipe during the welding operation i don't know calculate it.
Could you help me about this?

thanks
Aldo Cigna

 

[GregLamberson]

aldocigna

You use the design temperature (and design pressure) for your calculations.

Additionally you will also need to consider the loads applied by the tapping equipment.


Greg Lamberson
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[aldocigna]

Tx Mr Lamberson

Your answer is very usefull for me.
If possible, I would know if your answer is a your personal opinion or is an official interpretation by API organizzation

Thanks again

 

[GregLamberson]

aldocigna

Very astute question - not an "official" interpretation. It is the interpretation of ExxonMobil when I have done work for them, as well as others such as Shell.

To go a little deeper though, API RP 2201 refers to ASME B31.3 for on-plot piping and either ASME B31.4 (oil lines), or ANSI/ASME B31.8 (gas lines) for off-plot.

All of these codes use design temperature for calculating wall thickness.

One caveat how3ever, the design methods in these codes are based on 'area replacement'. For pipleines with large branch diameter/pipeline diameter ratios in highly stressed critical situations, you may need to do a a finite element analysis to confirm that the stresses in the connection are acceptable.

Greg Lamberson
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[aldocigna]

Sorry Mr Lamberson

I found the italian Exxon hot tape procedure.
It has a different approach to the problem considering the temperature during the welding operation

It permit hot tape with 5 mm minimum tickness

It give at first the welding penetration percent expected (%p) based on the electrode diameter and residual thikness (tr)

Then calculate the usefull tickness
tu= tr*(1-%p)

Then calculate the average temperature in tickness tu like Ta=T2-(T2-T1)*(1/ln(De/Di)-Di/(De-Di))
where
Ta= average temperature in Tu
T1= operating temperature
T2= crystallization temperature (it suppose 750°C)
De= tu external diameter recalculated considering %p
Di= tu internal diameter recalculated considering tr

then it calculate the max pressure Pmax based on code Asme B31.3 at temperature Ta

if Pmax is greater then P preview during the welding operation the hot tapping is possible

Is it correct for you? Or you know an other operative procedure by shell and Exxon

Thanks
Aldo Cigna

 

[GregLamberson]

ExxonMobil's Global Practices for Hot Taps titled, "Hot Tapping", GP 03-01-04, July 2001 states what I have indicated above. I don't have the Shell DEP with me, but it is consistent.

There may be some differences in regional documents, I know Chevron have stark differences in sp[ecifications and practices from W. Africa and GOM.

I would defer to the codes, I have always used design temperature (as long as the service parameters have not changed drastically), and if in any doubt, ask the service provider, TD Williamson or whoever is going to perform the hot tap.


Greg Lamberson
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[SJones]

From the 'Shell point of view outside of the US', the minimum wall thickness is 4.8 mm for carbon steel operating up to 350 deg C and not exceeding 72% of SMYS: a rule of thumb probably based on the old British Gas specifications and BS 6990.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/