Analyse av direkte- og indirektekjølesystemer på offshore prosess plattformer

Abstract

In oil and gas processing industry, cooling of the oil and gas is necessary to run processes at optimal temperatures. Cooling the process flow requires large amount of big expensive equipment. The traditional way of cooling the process flow has been to use a cooling medium in a closed loop to do this; the cooling medium is a mixture of TEG and water. Seawater is then again used to cool the cooling medium. In recent years a new way doing it as developed. This method cuts out the closed cooling loop, and uses the seawater directly to cool the process flow, thus the name direct cooling system. The data used in this thesis is form a recent offshore production platform project. The two systems are designed by the use of Aspen s Exchanger Design & Rating and Aspen HYSYS. Exchanger Design & Rating is used to design the plate heat exchangers and the shell and tube heat exchangers, HYSYS is used to create the intermediate cooling medium loop. Three systems have been modeled to meet the process specifications of the said project, these under systems are: 24-HA-001 A/B/C Wet Gas Cooler 26-HA-001 Injection Compression Suction Cooler 27-HA-001 A/B Export Compression Cooler By comparing weight, capital expenses, operational expenses and footprints each system has its advantages and shortcomings. A direct system weighs only 55% of the total weight of the indirect system, the equipment used costs 31% more to buy than the equipment used in a indirect system. On offshore platforms there is a shortage on free space to put either equipment or house working staff. Therefore the cooling system that takes up the least space might have an advantage. The indirect system s equipment total footprint is 38% larger that the footprint of the direct system. The indirect system also utilizes additional cooling medium pumps to keep the pressure up in the closed loop. Pumps running on electricity produced on the platform increase the yearly operational cost with $45 150. An indirect system reduces risk for fouling and hydrate-formation inside the heat exchangers and is a well proven system. The direct system reduces the use of toxic substances on the platform, by not using triethylene glycol.