The water injection platform, Eldfisk E has four 13.8 MW gas turbines dedicated for injection and one 22.7 MW for compression. The waste heat from three of these turbines (1x22.7 MW and 2x13.8 MW) is utilized by a bottom cycle steam turbine, with a maximum power output of 10.3 MW. The steam turbine is the main supplier of power for the entire field, in addition to the unmanned platform on Embla [72]. The steam turbine was installed in 1998 and upgraded in 2013 due to unstable operation and frequent use of the backup generator. The upgrade was done on the waste heat recovery system, to be able to cover exhaust heat from all four injection turbines (only two at a time) in addition to the compression turbine [73].
Figure 5-5 shows the flow diagram of Eldfisk steam power cycle.
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Figure 5-5: Eldfisk Steam Power Cycle [72]
The HRSG has three inlets for the three turbines and consists of four heat transfer zones. The economizer, evaporator and two superheating zones. Because of the varying power demand of the field, the HRSG produces 10% more steam than required from the steam turbine, to ensure control possibilities at load changes. The surplus steam is routed directly to the condenser via a bypass valve. To save energy, the condenser uses injection water instead of seawater as a cooling medium, eliminating the need for additional seawater lift [72].
5.3.1 Energy Calculation
Reduced fuel consumption due to steam bottoming cycle on Eldfisk E, compared to the simple cycle gas turbine solution, amounted to 23 million Sm3 per year before upgrading the system [72], and 28.4 million Sm3 per year after the upgrade [73].
To calculate the energy requirements of the combined cycle, the gas consumption, energy production and CO2-emission table from 2018 is used. A scenario without the combined cycle in place is used for comparison, where 28.4 million Sm3 is added to the total gas consumption, power generation stays constant. The results can be seen in Table 5-1 below.
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Table 5-1: Gas Consumption, Energy Production and CO2-emissions for GEA with and without combined cycle installed
Combined Cycle 357 453 347 1 336 876 789 972
Reduction 28 400 000 62 764 7 %
With a CO2-emission factor of 2.21, reduced CO2-emission is 62 764 tons per year. The steam turbine began operation in the last quarter of 1999, since then the CO2-emission reduction has accumulated to 1.1 million tons. With a life expectancy of the field to 2049, the steam turbine will contribute to a total of 2.9 million tons of CO2 reduced
5.3.2 TRL
Combined cycle in commercial power plants is well known technology. It is not that common on offshore installations due to weight limitations. Studies for design optimization for
combined cycles on offshore installations are being done, to make the alternative more attractive. As of today, combined cycles are installed on three different platforms on the NCS, proving the technology to be possible. Offshore combined cycles are therefore rated with TRL 9 from Table 2-3.
5.3.3 CO2-Emission Reduction
The CO2-emission factor for GEA with combined cycle installed is 0.59 ton/MWh. Had only simple cycled gas turbines been used on the fields, the CO2-emission factor would have been:
πΆπ2πΊπ = 852 736 π‘ππ
1 336 876 ππβ = 0.64 π‘ππ ππββ
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Calculated from Eq. (2.1) with numbers from Table 5-1.
The CO2-emission reduction for the installed combined cycle can then be calculated from Eq.
(2.2), and gives:
% πΆπ2 ππππ π πππ ππππ’ππ‘πππ = β0.59
0.64β 100% + 100% = 7.81%
5.3.4 Efficiency
The efficiency of the combined cycle on Eldfisk is taken as the average efficiency of the 5 gas turbines supplying the steam turbine with heat, listed in NPDs scheme of NOx-taxable equipment [10]. The efficiencies for the turbines are given with the WHRU included, and net efficiency for combined cycle is calculated to be 40.53%. Table 5-2 gives the efficiencies and average of the five turbines.
Table 5-2: Efficiencies including WHRU for top cycle turbines on Eldfisk
Turbine Operation Efficiency [%]
LM β 1600 Injection 39.61
LM β 1600 Injection 39.61
LM β 1600 Injection 39.61
LM β 1600 Injection 39.61
LM β 2500 GJ Compression 44.21
Average 40.53 %
Efficiency improvement is calculated from Eq. (2.5):
πΈπππππππππ¦ ππππππ£πππππ‘ =40.53% β 34.7%
34.7% β 100% = 16.8%
63 5.3.5 Cost
No cost estimation for offshore combined cycles has been found online or in literatures.
However, the average construction cost for combined cycle in US in 2017, was set to be 7 400 NOK/kW [74]. This cost is largely based on big industrial power plants, therefore the cost of a smaller offshore system, is assumed to be higher. With too high uncertainties and limited resources, cost estimations for offshore combined cycle would be purely speculative and are therefore chosen not to be studied any further.
5.3.6 Rating Table
Alternative TRL
CO2 -Emission Reduction
Efficiency Improvement
Abatement
Cost Comments
Gas
Turbines 9 0% 34.7% 765 NOK/ton Base case
Combined
Cycle 9 7.8% 16.8% -
Cost estimate for offshore
combined cycle has not
been found
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6 Evaporative Cycle
Evaporative cycles, or Humid Air Turbine (HAT) cycles, is an advanced gas turbine cycle with potential to reach similar efficiency figures as combined cycle plants. In the HAT cycle, water is evaporated into the compressed air before entering the combustion chamber, thereby reducing combustion temperature and the formation of NOx. In addition, the increased mass flow expanded in the turbine has a positive effect on the thermal efficiency of the cycle [75].
Compared to the combined cycle, investment and operational costs is lower, due to the avoidance of the bottoming cycle. In addition, the fact that water is evaporated into the air stream (rather than boiling steam), lower water qualities can be used and the control of the process is much easier compared to combined cycle, since the humidification is
self-controlled.