Debottlenecking Existing Offshore Production Facilities To Safely Extend Their Operation Capacities
For most early oil-production facilities, higher oil production is expected in the initial days of field life because of the high reservoir pressure. In this specific case study, oil production has reached the floating production, storage, and offloading (FPSO) unit topside-design limit of 100,000 BOPD (662 m3/h) with only five wells connected. Most of these wells were choked to between 30 and 50% to limit the oil production within the installed nameplate capacity of the FPSO unit. Therefore, to take advantage of the low gas/oil ratio (GOR) and the high dry-oil production during early field life, the client has requested this study be undertaken to identify the maximum oil production without compromising process safety and while avoiding major modifications.
The main bottlenecks to increasing crude-oil production to greater than the design capacity are identified as
- Increased temperature resulting in nonstabilized crude: On the basis of the oil composition, stabilization of the crude before leaving the topside is an issue. With an increase in production and the limitation on utility systems, there is not enough heat available to flash off light components and/or cool down the crude before it enters the cargo tank to prevent any further flashing. These flashed-off gases will be vented through the cargo-vent system, along with displaced gases (which are significantly higher than flash gases). The cargo-vent system is designed for vent load during cargo-terminal loading, and, therefore, crude-flash rates will not be anywhere near the design capacity of the vent system.
- Limitations of utility systems: As expected, all utility systems are designed for 100,000-BLPD (662-m3/h) liquid production, and any increase in production will impact the utility balance. To accommodate for additional flow, either modifications are required to the existing system (e.g., additional duty, exchanger modifications) or optimization of the existing system is required.
- Separator capacity: As long as water cut is negligible and a small quantity of water can be allowed to settle in the cargo tank, current crude-oil production has the potential to be increased with the separator operating as a two-phase separator.
A potential crude-oil-production rate of 120,000 BOPD (795 m3/h) can be achieved immediately, without any modifications, if slightly-higher-temperature crude oil is allowed, along with some additional hydrocarbon venting of the gases, from the cargo vent to the atmosphere. Changes in process parameters (pressure and temperature) will allow optimization of the utilities and will allow the crude-storage specifications to be met without any physical modifications.
The actual field trial did match with the study results, and production was increased to greater than the design capacity without many process-stability issues. Therefore, this type of study provides a quick but thorough method of investigating the way forward to improve production without compromising safety limits, allowing the operator to take full advantage of favorable reservoir performance to optimize field economics.
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18 June 2019
19 June 2019