| dc.description.abstract | With the environmental awareness increasing in recent decades and the imminent prospect of carbon neutrality more attention has been paid to the ways in which the greenhouse gas emissions and the energy demand of industrial activities can be reduced, especially in the oil and gas field which occupies a prominent place in the global energy consumption.
Energy- and exergy-based thermodynamic performance indicators constitute a useful tool for the evaluation of oil and gas processes that can motivate optimal operation of offshore platforms. The objective of this thesis is to perform an energy and exergy analysis of a typical North Sea offshore processing plant, consider the power and heat demands of different process design configurations and frame conditions, as well as calculate and evaluate different thermodynamic performance indicators introduced in literature. The energy and exergy efficiencies are then assessed in order to identify improvement potentials and a new idea is proposed for more efficient performance of the oil and gas processes. Finally, a simplified method for the evaluation of the indicators examined in this thesis is presented.
In this work the simulation of a typical offshore platform (Base Case scenario) is considered based on realistic data provided from the oil and gas company Equinor, Norway and it is simulated in ASPEN HYSYS ®. An energy and exergy analysis is carried out and the thermodynamic performance indicators are calculated. The indicators presented are the following: Specific CO2 emissions, Specific energy and exergy use, Specific power consumption, Specific exergy destruction, Total, Task and Component-by-Component exergy efficiency, Exergy destruction ratio, Exergy loss ratio and Efficiency defect.
The results of the conducted calculations indicate a power consumption of the platform around 23.1 MW mainly detected in the gas compression train (20.5 MW). Heat demands are approximately 11.2 MW, while energy of cooling reaches the number of 42.8 MW. The total exergy destruction rate is around 19.1 MW and it is mostly due to throttling in the production manifold. Exergy losses which range around 5.2 MW result mainly from cooling, which accounts for 65.2% of the total exergy lost.
In order to get a more complete view of the performance of the indicators examined in this work different case studies are set that consider changes in various frame conditions and process configurations for the case at issue, while the effect of the component chemical exergy and the oil production lifetime of the field are also investigated. The parameters under discussion are the following: reservoir fluid composition, Cricondenbar pressure (CDB) and True vapor pressure (TVP) specifications, pressure of export gas, efficiency of the rotating equipment, temperature of cooling and temperature and pressure levels of the separation train.
These case studies show that overall energy-based indicators are easier and quicker to use. They change according to the variations in heat and power demands when a specific platform with small deviations in the products is examined. However, some of them (Specific power consumption, CO2 intensity) may not reflect important changes in heating duties giving incomplete information regarding the performance of the process. When different platforms are considered they focus on the reservoir fluid treated in the process without promoting the most efficient utilization of the resources. ExU and ExD indicators seem to perform similarly, but when they are expressed per product exergy (ExUexergy, ExDexergy) they show not only the effect of heat and power demands, but also the effect of the different export conditions.
Total exergy efficiency is insensitive to any type of changes in the process, due to the high exergy of the hydrocarbons passing through the system producing misleading results and conclusions over the performance of the platform. On the other hand, Task and Component-by-Component exergy efficiency focus on the optimal utilization of the exergy resources of a processing plant and not the type of field examined. Task exergy efficiency εΙΙ-3 is heavily influenced by variations in the conditions of the inlet well stream and the export products even leading to negative results that make the evaluation process more difficult. Task exergy efficiency εΙΙ-4 responds to changes in both heat and power demands and outlet conditions of the platform or the distribution of the components in the two product streams. Task exergy efficiency εΙΙ-5 and Component-by-Component exergy efficiency εΙΙI take into account the allocation of the components in the two product streams, but the latter shows a higher sensitivity to the inlet and outlet conditions, as well as the chemical exergy increases of the inlet and outlet fluid streams.
The Exergy destruction ratio and the Efficiency defect give inspection of the distribution of exergy in the subsystems of the process, with the former giving more accurate results even without the calculation of the component chemical exergy term. Exergy loss ratio can pinpoint where in the process exergy is mainly lost to the environment indicating and it is more useful when the utilities are also included in the system being studied.
The exergy analysis performed for the Base Case highlights that inefficiencies are mainly detected in the production manifold due to the exergy destruction associated with choking. In this work a new idea is proposed that aims in saving a part of exergy lost due to throttling and transform it to useful energy, work. This approach is based on a combined separation-multiphase flow expansion system for the substitution of choke valves in the production manifold and it is applied and simulated for the Base Case scenario of this work. The energy and exergy analysis conducted underlines that the implementation of such a combined system could results in a rise in efficiency of up to 34% and a 27% reduction in work demands and CO2 emissions.
The analyses conducted in this thesis show that choosing between the different indicators at issue for the description of the performance of a platform is a complicated process that depends on multiple parameters. On that account an evaluation procedure is proposed based on the Multicriteria analysis that aims to reveal the most appropriate indicator or combination of indicators according to the desired use through a scoring process. For that reason, a set of six criteria is established that aim to cover all the characteristics an indicator is desired to attain and scores are assigned to each indicator against each criterion. The weighting factors defined from the user derive from the expected application and determine the priority of the criteria when calculating the overall score of the indicators. According to an example presented in this work the combination of the Task exergy efficiency εΙΙ-4 and the Exergy destruction ratio are considered to be the best for the investigation of an oil and gas processing plant, like the one investigated in this work. | |
