Availability formulae for standby systems of similar units that are preventively maintained

Some simple approximation formulae for the availability of standby redundant systems comprising similar units that are preventively maintained are presented. The formulae are established using standard Markov theory. A number of simulations have been performed in order to evaluate the formulae. The simulations show that the formulae give very good approximations for various preventive maintenance regimes. These formulae have been used to model the performance of standby systems on offshore platforms