Olefin plants employ multi-type cracking furnaces in parallel to convert various hydrocarbon feeds into products such as ethylene and propylene. The cracking process of each furnace is a performance-decaying batch operation, which needs periodic shutdowns for decoking and thus involves inherent and frequent upsets to the downstream process. Thus, the furnace feed allocation, batch processing time, and decoking sequence of the entire furnace system must be optimally scheduled to maximize the plant profitability as well as minimize the process upset induced by furnace decoking for the sake of the plant operability. In this paper, a novel MIQCP (mixed-integer quadratically constrained programming) model has been developed for the optimal scheduling of such parallel and performance-decaying unit system with consideration of inherent upset reduction. The idea is to group decoking operations of different furnaces together, so that the downstream has less frequency to suffer the product throughput upsets from the furnace system. The MIQCP model also addresses major scheduling issues of furnace systems, such as semi-continuous operation, non-simultaneous shutdowns, and secondary ethane cracking. The MIQCP model can be further converted to the MILP model by piece-wise approximation to speed the solving time. Case studies have demonstrated the efficacy of the developed methodology with significant economic potential and operational benefits. (C) 2019 Elsevier Ltd. All rights reserved.