A Risk-Based Methodology to Estimate Shutdown Interval Considering System Availability Abdul Hameed, Faisal Khan, and Salim Ahmed Safety and Risk Engineering Group, Faculty of Engineering & Applied Science, Memorial University, St John’s, NL, Canada A1B 3X5; fikhan@mun.ca (for correspondence) Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/prs.11722 This article presents a risk-based methodology to estimate shutdown inspection and maintenance interval considering system availability. Most inspection and maintenance activ- ities are performed when the plant/unit is in the operational state. However, some inspection and maintenance activities require the plant to be in a nonoperational or shutdown state. In most cases, operating companies adopt a shutdown schedule based on the original equipment manufacturer’s (OEM) suggested recommended periods. However, this may not be the best strategy as OEM recommended duration is general and may not reflect the current state of operation. The proposed methodology is unique in the sense that it iden- tifies a shutdown interval by identifying the critical equip- ment in terms of risk considering availability and safety of the operating unit. It optimizes process plant shutdown inter- val to minimize the risk (in dollar terms). The Markov pro- cess is used to establish the state diagram to calculate system availability. The proposed methodology is comprised of three steps namely, risk-based equipment selection, shutdown availability modeling of a complex system using the Markov process, and risk-based shutdown inspection and mainte- nance interval modeling. It can be applied to process plants such as those for liquefied natural gas processing, petrochem- icals, and refineries. The key elements for the success of the proposed methodology are the plant-specific data and identi- fication of critical equipment. V C 2014 American Institute of Chemical Engineers Process Saf Prog 000: 000–000, 2014 Keywords: plant availability; risk-based inspection; risk- based maintenance; shutdown interval estimation; opera- tional safety INTRODUCTION Inspection and maintenance have evolved from a nonissue into a strategic concern in a short span of time. Mourbray [1] reported that over the last few decades, mainte- nance has changed more so than any other management dis- cipline. The changes are due to a huge increase in the number and the variety of physical assets (plant, equipment, and buildings), more complex design, new maintenance techniques, and changing views on maintenance organiza- tion and responsibilities. In general, a complex system consists of a large number of interacting components or equipment that performs the system’s required functions. The system is subject to periodi- cally or nonperiodically planned inspection and maintenance during its life cycle. The inspection and maintenance actions are generally taken to fix a piece of equipment if it is found defective and having the potential to fail or to perform pre- ventive maintenance to avoid any possible failure. In certain cases, these equipment or system cannot be isolated to per- form inspection and maintenance, and this requires the plant to be taken out of service, which is known as shutdown or turnaround. Shutdowns are generally classified as sudden or emergency and nonemergency shutdowns as shown in Fig- ure 1. The sudden or emergency shutdown is an unsched- uled event which is initiated in the event of a failure or breach of containment (such as fire, major spill, instrument failure, power failure, or total loss of control of chemical or physical processes). Nonemergency shutdowns can be fur- ther classified as planned or unplanned shutdowns. Accord- ing to [2,3] a planned, periodic shut down (total or partial) of a processing unit or facility is carried out to perform main- tenance, overhaul, and repair operations and to inspect, test, and replace process materials and equipment. Generally, planning for total shutdown begins well in advance and involves the departments of procurement, engineering, main- tenance, operations, quality assurance, HSE, security, and administration. An unplanned shutdown is initiated when a possible failure scenario seems to exist but does not require immediate stoppage of the operation, and can be delayed for a few weeks. Both planned and unplanned shutdowns can be total or partial in nature. In extended shutdown, a facility is put in a condition of preservation to prevent degra- dation over time for future usage. The focus of this article is on planned shutdown, both total and partial. Shutdown inspection and maintenance management is one of the maintenance management strategies used in con- tinuously operating plants to improve plant reliability, avail- ability and integrity. Extensive literature is available on inspection and mainte- nance interval modeling considering cost, reliability, avail- ability, and risk. Inspection and maintenance intervals can be estimated on equipment by equipment basis. However, in a process plant, a unit or system consists of hundreds of pieces V C 2014 American Institute of Chemical Engineers Process Safety Progress (Vol.00, No.00) Month 2014 1