Int. J Sup. Chain. Mgt Vol. 6 No. 3, February 2017 1 An Investigation of Optimum Safety Stock Level for Maintenance, Reliability and Operation Materials based on Criticality of Material and Equipment Bambang Sudjatmiko 1 and Taufik Roni Sahroni 2 1,2 Industrial Engineering Department, Binus Graduate Programs, Bina Nusantara University Jl KH Syahdan 9, Jakarta 11480, Indonesia 1 Bambang.sudjatmiko@binus.ac.id; 2 taufik@binus.edu Abstract— This paper presents a study to determine a safety stock level calculation for Maintenance, Reliability and Operation (MRO) materials in oil and gas industries. The challenge to set a safety stock for MRO materials is uncertainty of demand and lead time especially in industry where the operation plant is far away from materials manufacturer. Many models currently used to calculate the safety stock is independent between inventory control as service provider and maintenance as service receiver. Current models of safety stock are referred to equipment reliability or stock out situation to measure the effectiveness of the models. In this paper, studied the correlation between maintenance activities and inventory control strategy to provide acceptable service level thru combined equipment and material criticality with optimum inventory safety stock level by linked the acceptable downtime of equipment with lead time of ordering materials. Based on this study and comparison of the proposal with available models related to equipment and stock out focus, the proposal result is between current models of safety stock level calculation result. Keywords— Safety Stock Level; Criticality of Equipment and Materials; Lead Time; Downtime; Mean Time between Failure 1. Introduction Material or spare parts are very significant to ensure the reliability and production process in production plant. To ensure the material availability to serve the plant requirement when required is required to have stock near the production plant. The concerns for stocking materials are uncertainty of requirement in terms of quantity or time and other concern is the uncertainty of order lead time because of many considerations during order and delivery process from internal organization or external organization. There are different focus area between service provider and service receiver related to stocking purpose. Maintenance as service receiver calculate the safety stock level with main concern on the equipment reliability where the purpose of safety stock is to ensure material is available at any time and any quantity when required. In other side, inventory controller as service provider is required to have stock level as minimum as possible. Those contradictive of aim to have stock in inventory continuously become a topic of discussion for both parties. Safety stock model refer to maintenance approach usually proposed high safety stock and with stock out avoidance model rever to deviation of lead time used by inventory controller usually proposed loh safety stock level. Those conditions are generated other issues, for maintenance model intention is the stock will high and potential create over stock in inventory. From inventory control models, the proposed of safety stock level is very less compare with maintenance model due to inventory control only capture the safety stock for deviation of lead time and unexpected requirement. Lead time is a main factor to divined the level of safety stock, also downtime of equipment need to consider on the calculation of safety stock because of long lead time compare with acceptable downtime is raise the uncertainty to production process and with information of acceptable downtime is help inventory control to manage the delivery process either deliver the spares by faster delivery with higher cost or inventory control could choose the lowest cost with longer delivery time. These considerations are support inventory management to manage more affective of inventory and ordering cost.