| dc.description.abstract | This study evaluates different solutions for a LNG facility, partially placed on shore and partially placed on a floater, hereby referred to as at-shore FLNG. There are several advantages with this solution where reduced cost, shorter development time and potential for standardization is highlighted as the greatest.
To illustrate the challenges for an at-shore FLNG project, two main scenarios linked to a potential location for LNG production have been identified. The chosen locations are the Gulf of Mexico and Northern Norway. An initial configuration for liquefaction, refrigerant compressor driver, NGL extraction, heat generation and cooling has been established based on weather data, governmental restrictions and local conditions at the locations. The result of this configuration has been calculated using HYSYS and reference data. Next, the process or utility systems have been swapped with other configuration alternatives. This is done one alternative at the time, and the result has been measured against the initial result to identify and quantify the consequence of other process or utility systems. The desired production rate is approximately 4 MTPA, but this varies at the different configuration alternatives. Additionally, all configurations are simulated with average and high temperature to identify and quantify the consequences this have for the plant efficiency and capacity.
Next, three subcases, each linked to other potential locations for LNG production, has been identified to evaluate the consequences of combining more than one alternative at the time. The alternative system combination is considered the most likely combination at the given location. The potential locations for the subcases are the west coast of Canada, the Northwest coast of Russia and the Northwest coast of Australia. As for the scenarios, the subcases are simulated with average and high temperature.
Complete process models of the different scenarios and subcases have been made in HYSYS and the simulation results forms the main basis for comparison for the different configuration alternatives. The PRICO and Niche liquefaction process have been optimized with the optimization function Hyprotech SQP in HYSYS. The reason for using the optimizer is to achieve a good basis of comparison between the different process configurations and utility systems. The optimizer is configured to obtain a liquefaction process as efficient as possible based on the available compressor power different scenarios and subcases, which in this case means a specific power as low as possible. Based on the simulation results for power demand and heating duties, fuel gas consumption and CO2 emissions are calculated for the different scenarios.
The simulation and optimization results for the liquefaction processes show that PRICO has both higher production rate and lower specific power than Niche. Next, the simulation results clearly underline the superiority of seawater cooling combined with electrical drive compared to gas turbines and any air based cooling system in terms of specific power, production stability and CO2-emmissions. The combination of a warm climate, air cooling and gas turbine driven compressors proves to be the least efficient combination. The results imply that specific power increase with increasing cooling water temperature, thus the production rate decrease. If gas turbine compressor drivers are used, the power output drops with increasing air temperature, thus reducing the production even further.
In addition to the simulations, issues that cannot be quantified by HYSYS have been addressed. This mainly regards the complexity each system entails and the reliability of the alternatives. The reliability evaluation focuses mostly on the liquefaction trains and NGL extraction as these are regarded to have the greatest impact on the overall plant reliability. As expected, electrical drive results in more operating days with 100% production when compared to gas turbine drive. However, each train operates independently of each other, meaning that if one driver fails, the rest of the liquefaction trains can still maintain their production. This indicates that the increased reliability of electrical drive has a smaller impact on the multiple liquefaction trains in this study than it would in a single DMR train where the drivers are configured in series.
It has also been a great focus on the potential for standardization of systems placed on the FLSO, regardless of feed gas composition, local conditions and climate. The study shows that the liquefaction process, storage tanks, offloading and flare system can be standardized to a certain point and placed on the FLSO. However, the production capacity and efficiency of the facility largely depends on the type of driver and available cooling water temperature. Whereas electrical motor has a constant power output despite temperature, gas turbine performance drops rapidly when the ambient temperature increase. This results in large variation in the gas turbine output at the evaluated locations, which further results in large variation in LNG production. These variations also indicate that the plant will be more challenging in operation and may be hard to operate at the optimal specifications.
The results imply that to fully exploit the potential for high and efficient production, part of the process systems should be selected after the location and feed gas composition is known. This mainly regards driver, cooling and gas processing systems required upstream of the liquefaction process. A general solution is proposed, where the deck of the FLSO may have a standardized section and a field specific section that can be fitted to the given location and feed gas composition.
Next, the results show that relocation of the floater is possible but not favourable, especially if the variations in ambient air temperature and feed gas composition are large. If gas turbine compressor drivers are used, they will be either be undersized or oversized depending on whether the floater is moved from cold to hot climate or opposite. Based on this, electrical compressor drive combined with a seawater based cooling system is regarded to be the only favourable option for relocation without a major loss in production or efficiency. This configuration is also the only one regarded to be favourable for standardization before location and feed gas composition are known. However, this is also the most expensive configuration, especially if power must be generated locally, but a detailed cost analysis for the different alternatives has not been performed in this study. | |
