A Stabilizing Design: Changing Design Considerations for Condensate Stabilization on the Delta House FPS in the Gulf of Mexico

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In order to achieve this milestone and keep pace with the highly aggressive production schedule, topsides engineering con- tractor Audubon Engineering Solutions had to begin front-end engineering design (FEED) efforts before reservoir characteristics were defined. Assuming a reservoir composition similar to a nearby analo- gous field (GOR of ~2000 scf/bbl and 28-32°API), the initial design included routing liquids from compressor scrubbers to production separators with lower operating pressures – a method commonly used on offshore facilities to reduce liquid recycle and minimize compression requirements. During this initial design process, however, a well sample phase behavior (pressure, volume, temperature or PVT) analysis conducted by a third-party laboratory revealed that the reser- voir contained a stock GOR of approximately 2300 scf/bbl and a crude oil gravity of 38°API – a substantial deviation from the previously assumed composition. The analysis also indicated that reservoir fluid was rich in condensates, including propane (C3), butane (C4), and pentane (C5), resulting in a much larger liquid recycle and higher compression requirements in order to meet oil pipeline Reid vapor pressure (RVP) specifications. Although Audubon Engineering Solutions and LLOG were suspicious whether this data was entirely accurate, given the unusual numbers, they proceeded under that assumption that if it were even remotely indicative of the reservoir's composition, a conventional liquids handling philosophy would be impracti- cal and other options would need to be explored. Liquid handling options When considering a new liquid handling design approach, two options were examined. The first option was to retain much of the equipment used in the original design and add a conden- sate processing train in order to dehydrate the condensate from the scrubbers before pumping it into the gas export pipeline. This approach would effectively eliminate the recycle loop and reduce compression requirements, but a lower volume of sales oil would be produced. The second option included a complete overhaul of the original design by replacing much of the topsides equipment with a crude stabilizer. This would enhance separation of the condensates, reduce compression, and allow RVP speci- fications to be met. The process itself, however, was overly complex and required a great deal of heat in order to operate. The necessary equipment presented structural concern due to its weight as well. During the process of evaluating these options, efforts were made to replicate the data from the initial PVT analysis in a process simulator. When it could not be done, a second analysis was ordered to confirm the suspicion that the data was incorrect. As expected, the second analysis revealed that the reservoir com- position was in fact much closer to what was originally assumed (stock GOR and specific gravity were calculated to be ~2100 scf/ bbl and 37°, respectively). This data was more favorable from a processing standpoint, but with relatively high amounts of con- densate still detected, the large recycle loop remained. After the new PVT data was verified in a process simula- tor, the total overhaul of topsides equipment and addition of a crude stabilizer was taken off the table. The option of adding a condensate processing train by itself was also reconsidered due to its inability to provide adequate separation. After examin- ing different seasonal and environmental conditions in order to better understand how the facility would operate throughout the year, it was determined by LLOG and Audubon Engineering Solutions that a hybrid of these two options would be most appropriate given the unique composition of the field. The solution: condensate stabilization The new hybrid design for liquid handling at Delta House included keeping most of the equipment from the initial FEED phase – along with the condensate processing train – and adding a stabilizer with overhead cooling and two-phase separation of the overhead product. After receiving condensate from a free- water knockout and coalescing filters, bottoms product from the stabilizer would feed into the low-pressure separator. This design would ultimately improve compression capabilities and increase the volume of sales oil that could be recovered. A flow diagram of the condensate stabilization system used at Delta House is outlined in Figure 1. From FGC scrubbers To compression To compression To sales NGL To LP SEP To LP SEP Stabilized condensate Condensate Freewater knockout Coalescing filters Stabilizer Overhead condenser Overhead separator Reboiler Bottom Cooler Charge pumps & pipeline pumps Fig. 1: Simplified flow diagram of the condensate stabilization system

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