Somnath,

Read ASME B31.3 Para. 301.8 and 319.2.1(c).
It is clearly stated that settlement represents a displacement induced stress - quite a "single cycle effect" displacement stress, so that, based on engineering judgement, an increased allowable stress range might considered. In this regard, ASME B31.1 indicates 3 times allowable "hot" stress as stress range limit.

Athough your inquiry includes some logical judgement (e.g. there is no "un-loading - return" cycle), B31 Codes approach is clearly stated and documented, and has been used long time for storage tank piping design purpose.

Of course, as stress engineer, it's up you to employ a more conservative approach than provided by design Code and consider Settlement together with dead weight and pressure as Sustained Load Case - however, be aware to consider SIFs instead of SSIs for this load case, since settlement induces significant bending bending stresses that should be treated properly.

My opinion is in such case, you need to document this conservative approach by a Design Basis document that should be approved by Owner/Client, since the design solution might be more sophisticated and expensive than expected...and we are leaving now bad times, isn't it?, when industry is lead by accountants and not any more by real engineers...and reducing costs (i.e. his highness "the profit") represents the main target for any Project, sometimes despite quality/safety are affected...

Best regards,