Optical networks carry an increasing amount of traffic due to the rapid growth of IP and multimedia services. Therefore the impact of network failures on this traffic and thus user services is growing fast. In addition some network services are heavily dependent upon the availability of communication resources. Consequently, network operators have to deal with the steadily growing availability requirement. Thus, survivability mechanisms need to be provided in the networks to meet the connection availability requirement. There are several protection schemes that can be applied to improve connection availability in the network. Shared Path Protection (SPP) scheme is selected in this paper due to its efficiency in resource utilization. In many works the SPP scheme has been studied to solve a single failure problem. However, in large networks the occurrence of multiple concurrent failures cannot be neglected -. In this respect we studied the SPP scheme for mesh optical transport networks with multiple failure scenario and developed connection availability model with multiple failure assumption instead of traditional single failure assumption. In this paper we present the analytical model for connection availability in SPP mesh network with multiple failure assumption. Our model is more general than the traditional model adopted for single failure assumption and also is more general than the work for two protection paths. We also developed a simulator to evaluate our analytical model. The results show that the connection unavailability obtained by simulations is very close to the theoretical value, which approve that our algorithm is accurate.