1. On an existing mud reciprocating pump piping, the current stress analysis shows the block type Tico pads (at all supports/restraints) have NOT been modeled and the calculated natural frequencies and stresses reflect that.

The system is now undergoing an upgrade due to a higher pressure requirement, thus pipe wall thickness, valve weights, pump operating cycles, pulsating pressure, pulsating forces, etc. are all changed.
2. To account for a more realistic stress model, a decision was made to include the Tico pads in the new stress analysis work, so that their stiffness (radial & axial) could be accounted for in the line 'behavior'.
To comply with B31.3 stress requirements, some restraints had to be be released. This, together with the heavier piping, resulted in pipe natural frequencies being as or too near the combined pumps operating cycles, hence unacceptable.

Could anyone with an adequate experience in such installation kindly advise which of the above approaches is in principle the one to follow when performing the stress analysis and why:
1 - Tico properties ignored, Fn's derived, and Tico separately selected to suit the loads or,
2 - Tico properties included?
Many thanks in advance.

It depends ......

The system mass compared to the Tico stiffness will determine whether there is any appreciable effect. Big pipes, problematic, smaller, probably not.

I would recommend:

Do a sensitivity check with and without spring stiffness of Tico blocks.

If that demonstrates a significant shift downward in Fn's with Tico active ...

Look where greatest compression on Tico occurs and observe envelope movements.

Where largest dynamic amplitudes are found at Tico restraints, replace with a harder, but resilient material such as Tufnol.

Thanks for your input, MoverZ.
In many companies, engineers separate the stress analysis from the Tico pad selection and this is not necessarily a recommended practice. There are two important parameters related to the ‘frequency ratio’ Fn,pipe/Fn,Tico:
-The degree of vibration isolation
-The degree of transmissibility.
So far, in the few jobs I have been involved, no corporate specifications were provided regarding the degree of vibration transmissibility required on reciprocating pressure lines.
Common practice and Tico suppliers call for ‘frequency ratio’ below 0.5 or above 2 in order keep away from the high oscillation zone. Similarly, the ratio of piping natural frequency to the forcing frequency needs to be outside this range too.
Fn,Tico depends upon the pad properties, compression stress and its thickness
Fn,pipe is derived by modal analysis (one needs to decide with or without Tico stiffness).

However, two problems when it comes to clamped Tico pads:
1. The info re Fn,Tico is based on flat pad, not encircling pad;
2. The clamped pipe could be assumed as being compressed by two springs (one for each side clamp's Tico), ie two springs in series. Without an initial compression, applying an external lateral force on the pipe, would cause a compression stress on one half clamp only (load over 2/3 of bearing area), and that is what most engineer assume.
However, when the two clamps are tightened to achieve a specific compressions rate (per vendor recommendation), radial pipe movement under an external load produces an increase of force on one side of the movement and an ‘equal’ force reduction on the other (assume tico pad behaves like a spring), so in effect, if side shear is ignored, then pipe moves with no tico resistance, at least till the movement exceeds the initial compression. Obviously when shear is also included, then this pure case falls.
From what I saw so far, this latter case has generally been ignored by the engineering firms and the Tico suppliers. Since this is the tricky part of the clamped tico assembly, it would be quite interesting to share thoughts and experience by anyone with good exposure to this issue, be it academical or practical. Relevant to reciprocating compressor/pump lines, riser lines, etc.

Correction to the conclusion arrived at the second last paragraph, that now should read:
However, when the two clamps are each tightened by an amount X to achieve a specific compressions rate (per vendor recommendation), an external radial pipe movement X1 produces an increase of force (F1) on one side of the pipe and an equal force reduction (F2) on the other (assume tico pad behaves like a spring), so in effect,
deltaF = F1-F2= K(X+X1)- K(X-X1) = K(X1+X1)=2KX1 (valid till the movement exceeds the initial compression).
In other words, if side shear is ignored, then the pipe movements is resisted by double the tico K spring rate, ie,
the spring constant of the system is twice the spring constant of the individual springs.
Obviously when shear is also included, then this pure case falls.