MODELLING THE CRUDE OIL STORAGE MANAGEMENT SOR 2015 Lorenzo Ros McDonnell María Victoria de la Fuente Aragón

Frame of the work presented The supply of raw materials needed to develop the refining activity is performed by tankers or pipelines. These are the two basic means by which Spain get crude oil from producing countries. Crude oil storage is done in specially designed tanks. The importance of this operation is critical for refining, it must properly operate with the storage capacity of these facilities, in order to enable supply of charge to distillation units, to fulfill the safety stock, and to optimize the storage capacity, ensuring the raw material supply and a proper stock rotation, to avoid the freezing of oil for long periods time. Therefore, the refinery tries to meet the market needs thorough a comprehensive planning and a proper stock management. The operator of the system does this process manually, with possible errors associated to the human factor.

Diagram of the crude oil storage system in a Spanish refinery

Storage elements involved Storage Tanks (Ti): there are 20 floating roof tanks with a usable storage capacity of 92,477 m3/tank (total usable volume of 1,849,540 m3) Pipeline: allows pumping crude to facilities located within the country. It has a large capacity (12,000-25,000 ton/day). In this study it has been considered a variable pumping stream S= [500 - 1000] m3/h. Distillation unit (CDU): where the first stage of refining crude oil occurs. In the present work, the distillation unit loads crude from the storage tanks fed from the oil terminal, which receives the discharge of the tanker. Usually it works with a variable load stream S = [500-750] m3/h.

Element & Constraint Strategic storage - Ci - (CORES): Compulsory volume of crude oil, as strategic stock. The strategic quantity of oil, according to Spanish Law 34/1998 and European Directive 2009/119/EC, will be at least equal to 90 days of the country consumption. This situation forces the refinery having immobilized 5 tanks from the set of 20 tanks, with a approximate capacity of 500,000 m3. The short-term oil refinery-scheduling problem is one of the most challenging problems in operational research due to the complexity of the refinery operations and the corresponding process models (Pinto et al, 2000).

Issues to be considered: Safety Stock: Volume which keeps the distillation unit operative for almost 45 days. (approx. 810.000m3), reaching a volume level near this amount, the refinery should be planned future purchases of crude oil. Product mix: The refinery has established a group of crudes for purchase, so that there are no problems regarding their possible blending All Ti tanks are eligible for loading and supplying to CDU and Pipeline, and the movement of crude among them is allowed. Ci tanks are immobilized (strategic stock) and will not participate in the loading/unloading operations. Only once a year, the movement of crude among Ci and Ti is made for maintaining them and to avoid problems with raw material in Ci.

input data for the study of the storage Ti tanks levels

Simulation results of crude oil storage until the arrival In the presented study case in the paper, it is scheduled a tanker unloading: Tanker GENMAR SPYRIDON, ready to download 94,000 m3 of crude oil type "Castilla". Then, the initial conditions of the storage system is fixed as shown in Table 1. After that, we have study the evolution of the stock in the set of Ti and Ci tanks, results obtained from the simulation of the modelled system. We have established appropriate time sequences, in this case has set a time-window of 24h (that includes the full unloading of the tanker - approx. 12h).

Simulated evolution of crude oil stocks in the Spanish refinery

Conclusions As a real improvement, and in the way of optimizing the loading/ unloading process of tanks, was proposed a division of the set of 20 tanks into three groups: Group 1: CORES (Ci) tanks for the storage of strategic reserves. Group 2: storage tanks for pumping to pipeline, P1- P7 (seven tanks) Group 3: storage tanks for loading the distillation unit (CDU), T1-T8 (eight tanks). “All storage tanks can receive load from tankers, and crude oil movements are permitted among all storage tanks”. This proposal seems to reduce the operational flexibility of the system (facilities), because each tank will se assigned to exclusive uses, but the benefits, which will be achieved in the short-medium term of the system operation, are: Simplicity in facility circuits, with greater clarity in the system operation. Reduction of set-up times, operation and maintenance. Higher control and safer facilities (system).