Ultra-Reliable Low Latency Communications (URLLC) and especially those related to the Industrial Internet of Things (IIoT) are characterized by a large number of users transmitting sporadically information to a central controller. We consider in this paper scenarios where transmitted packets have to be conveyed within a very short time so that it is not possible to make per-packet resource reservation, i.e. contention-based access is needed. Moreover, in case of loss, there is no room for waiting for acknowledgement before retransmissions so that blind replication is needed for reaching the ultra high reliability targets. Knowing the limited, but large, number of potential users in the system, we propose a semi-centralized resource allocations scheme where each user is pre-allocated positions for its replicas in case he has a packet to convey. We show, using coding theory, how to design sequences for users so that the number of collisions is minimized. We further exploit our pre-allocation scheme to develop a successive interference cancellation method where the base station tries to decode a packet based on the knowledge of the already decoded colliding packet. We show that the proposed schemes succeed to attain very low loss rates with low resource reservation.