Hi,
In one of our projects, there is requirement of addition of one ESD valve on an oil line on an offshore platform. The client guidelines ask to 'perform stress analysis of the line for a change in weight because of extra valves, flanges, etc.'

Is it essential to do the analysis, as the additional valve is adequately supported so that the flexibility of the exisiting system is not disturbed, and the support details & types of the particular long line is very difficult to trace.

How can we convince the client that a comprehensive stress analysis is not required and the new valve is properly supported? Details of any stress analysis performed for the existing line is not available.

Hello,

Some thoughts:

".....perform a stress analysis...."

The question is what does this phrase really mean and to whom. If the letter of the scope of work does not say that "...the piping stress analysis must demonstrate that the design of the piping system complies with all the requirements of ANSI/ASME B31.3, Process Piping..." then what is it asking for?

What I mean is, I assume that this system has been in service for some time before the valve modification was made so what is the issue? Well, since the flexibility of the sysytem has apparently been demonstrated by successful service (well, not really, not yet) the issue is the sustained loading of weight and internal longitudinal pressure (and B31.3 doesn't even give us an equation for this - but I have become distracted). This (sustained loading) is the loading that was affected by the modification. You would really only have to model a portion of the system (how much depends upon the system's geometry and it support design) to get a good handle on the stresses local to the new valve. Adding more of the system to the model (away from the new valve) would have increasingly less effect upon the calculated pipe stresses local to the modification. Also, be sure to "follow the load to the ground" - look at the new hanger loads and be sure that none of the existing support structure is not overloaded due to the new load path.

Now there is some possibility that the opening and closing of this new valve may result in changing the operating temperature of portions of the system (i.e., create additional "modes of operation") resulting in expasion/contraction cases that have not been evaluated in the previous analyses..... You better check out this possibility.

Having said all of that consider the possibility that the system has never had a complete Code stress compliance analysis. This might just be the owner's way of finally getting a good analysis performed. Take it as a compliment.

Also, it sounds like it might be easier in the long run just to do the analysis rather than try to fight the battle of wills.

Good luck my friend.

Best regards, John.

Dear friend,

When we add a new valve in to the oil line, the flexibility is already effected.I will tell you how?.All valves generally we consider(even any program considers three times the normal pipe thickness) the thickness factor to be 3 time the pipe thickness.When thickness is increased the stiffness is automatically changed.
So the pipe behaviour changes.Now you are saying that pipe is adequately supported.This will create artificial stiffness in the analysis computations.
Honestly I feel that you conduct thermal; analysis
and check the compliance to the code.

c srinivas

Use caution with this assumption. Most programs that I have tested use "3 * I" (where "I" is the moment of inertial of standard pipe of the same diameter) as the stiffness parameter for rigid elements. I also encountered one program that assumed 12 inch STD pipe to compute "I", but then increased "E" by a factor of 10.

CAESAR II on the other hand uses "10 * thickness" to determine the cross section properties of rigids.