  We discuss the effect of compression on Urca shells in the ocean and crust of
accreting neutron stars, especially in superbursting sources. We find that Urca
shells may be deviated from chemical equilibrium in neutron stars which accrete
at several tenths of the local Eddington accretion rate. The deviation depends
on the energy threshold of the parent and daughter nuclei, the transition
strength, the temperature, and the local accretion rate. In a typical crust
model of accreting neutron stars, the chemical departures range from a few
tenths of kBT to tens of kBT for various Urca pairs. If the Urca shell can
exist in crusts of accreting neutron stars, compression may enhance the net
neutrino cooling rate by a factor of about 1 ? 2 relative to the neutrino
emissivity in chemical equilibrium. For some cases, such as Urca pairs with
small energy thresholds and/or weak transition strength, the large chemical
departure may result in net heating rather than cooling, although the released
heat can be small. Strong Urca pairs in the deep crust are hard to be deviated
even in neutron stars accreting at the local Eddington accretion rate.
