1/-Is tank settlement a primary or secondary stress or load ?
It is a stress given by strain.
We are more familiar with stress given by force. I don't know for which reason engineers forget there are two distinct lessons in Strength Mechanics.
And it is true that in real life it's easy to get confused with such terminology: when you bend a paper clip back and forth until it breaks, you are applying strain... next question would be "but I need force to do it!".
Strain gives secondary stress. For this reason, strange enough for a normal engineer, "settlement is an applied displacement and should be considered together with thermal loads".
2/-Most of the settlement occurs during tank hydrotest , when the piping is not yet connected to tank nozzles
Yes, in practice the remaining settlement ( residual) is small, let's say one-third of total.
The only problem might be the EPC company that wants (wishes!) to have hydrotest with all piping connected to tank.
So just obtain from them (impose to them!) the normal procedure- hydrotest should be carried out without piping connected. Connect piping after hydrotest, when the major part of settlement is consumed.
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