Can superbubbles accelerate ultrahigh energy protons?

We critically assess limits on the maximum energy of protons accelerated within superbubbles around massive stellar clusters, considering a number different scenarios. In particular, we derive under which circumstances acceleration above peta-electronvolt (PeV) energies can be expected. While external forward shock superbubble may account for particles up to 100 TeV, internal primary shocks such as supernova remnants expanding in low density medium or collective wind termination forms young compact cluster provide more favourable channels accelerate 1 PeV, and possibly beyond. Under reasonable conditions, clustered supernovae launching powerful into magnetised star are found most promising systems 10 PeV. On other hand, stochastic re-acceleration strongly turbulent plasma is much less effective than claimed previous works, with proton at few hundred TeV.