## 1 Introduction  
The supply chain of petroleum products faces major challenges, i.e., demand growth and the complexity of fluid transportation. The petroleum supply chain contains multiple stations extending from oil wells in exploration and production areas to the final destination. Each station has its own difficulties and challenges in the contribution towards the success of the safe and continuous supply. Undoubtedly, meeting a regional demand of petroleum products requires an uninterrupted safe operation. In the last station, the petroleum products pass through a process called Custody Transfer in which the ownership of the products transfers from one party to another. These massive quantities need to be measured accurately when their possession is being transferred to reserve the rights of each party. Nevertheless, fluid measurements are complex due to multiple factors which, when they deviate, may have a significance impact of the genuineness of the measurements. For instance, if the fluid volume is measured directly in a Custody Transfer process, then fluid pressure and temperature have to be maintained at certain levels to have an accurate volume measurement. In bulk plants and distribution points, the petroleum products are transferred from truck tanks to storage tanks through the truck unloading system, in which the fluid passes through multiple mechanical components and instruments, among which is the flow meter where the quantity of the fluid is being measured in a process called truck unloading. The truck unloading system is a vital part of any supply chain and can be available in many applications such as airports, petroleum distribution plants and some other stations, where the high traffic requires large quantities of fuel. Apparently, any failure in these systems leads to a cut in the petroleum supply chain. Therefore, it is important to enhance the reliability of unloading systems by performing a deeper study of the reliability of their components to find out how each component contributes to the system reliability. The truck unloading system is an integration of a large number of different mechanisms working as one unit to simultaneously unload the petroleum products and genuinely measure its quantity with high measurement accuracy and high reliability to have an uninterrupted operation. It contains a variety of interacting mechanical, piping, electrical, and instrumentation components. The sequence of operation starts when trucks arrive to unload the petroleum product to storage tanks. The unloading arms are then connected to the bottom of the truck and the vapor arm is connected to the top of the truck to prevent a vacuum, which might cause the truck to collapse. Through the effect of the gravitational force and the difference in elevation, the fluid will start flowing slowly from the truck tank into the unloading arms until it reaches the pump, and then fluid will be pumped in the desired predefined rate in the pre-set controller, which is considered the brain of the system. Passing through the system piping, the fluid eventually reaches the outlet with the same quantity measured by the flow meter. Because petroleum products are dangerous and flammable, they require special attention from their extraction until they reach the final customer. The unloading system is an important part of the long petroleum supply chain, which plays a connecting role between different modes of transportation. The unloading system contains many linked, sophisticated and complex components. Any failures in one of its components may lead to a cut in the whole petroleum supply chain. Therefore, it is important to evaluate the reliability of the unloading system by performing a deep study of the reliability of its components to find out how each component contributes to the system reliability. In this context, this paper presents the operation philosophy of the truck unloading system, failure modes of the components within the system, and a bottom-up approach to analyze the reliability of the system.