Wind-storage-turbine Bundled Technology for the Power Supply of Offshore Oil and Gas Platforms

The offshore oil and gas industry is embracing renewable energy such as wind power to reduce carbon emissions. However, the intermittent characteristics of renewable power generation bring new challenges to the operation of existing offshore oil and gas platforms featured in minor load variation. To address the concern, this paper proposes a coordinative control strategy for offshore oil and gas platforms with floating wind power integrated. A saturated filter controller is applied to decompose the wind power fluctuation and smooth it with energy storage and a gas/oil turbine generator. A feedforward control unit is embedded into the controller to compensate for the wind power variation caused by the floating foundation. The proposed strategy is validated using a model extracted from a real offshore oil and gas platform. The result shows that the proposed strategy can validly eliminate the impact of wind power volatility.


Introduction
In recent years, the offshore oil and gas industry develops fast owing to the increasing energy demand along with social development.For offshore oil and gas platforms (OOGPs) in deep and remote seas, it is not technically and economically feasible to connect onshore power due to expensive submarine cables [1] .Currently, the power demand of offshore platforms is mainly met by gas/oil turbine generators driven by self-produced oil or gas, leading to a significant amount of carbon emission [2] .To reduce energy consumption as well as carbon emission, offshore platforms are seeking new solutions in their energy supply system.
The rapid development of offshore wind power provides a good opportunity for the low-carbon transformation of OOGPs [3][4] [5] .On the one hand, there is a good spatial matching between the distribution of offshore wind resources and fossil resources.On the other hand, offshore wind turbine installation can be combined with abandoned OOGPs.
Although offshore wind power brings many benefits to OOGPs, the randomness and volatility of wind power pose challenges to the safe and stable operation of OOGPs [6] .The key question lies in how to increase the penetration level of wind power while ensuring the safety as well as stability of the platforms' electrical power systems [7] .The feasibility of combining offshore wind power and onshore power to power OOGPs is validated [8] .Transient analysis of an offshore oil and gas platform with wind power integrated is conducted [9] .The results indicate that gas turbine generators need to have stronger adjustability to cope with the integration of wind power.A hierarchical optimization model is established to coordinate equipment with various response characteristics [10] .An energy storage system (ESS) is applied to cooperatively work with turbine generators and support the penetration of offshore wind power [11][12] .An energy management strategy is designed to improve the transient stability of offshore platforms with wind power [13] .
Even though the involvement of a new element could provide significant support to the power integration, it usually needs to update the platforms' whole energy management system (EMS) which prevents facility owners from embracing it, as the platforms have already been equipped with a comprehensive EMS.In addition, OOGPs that are located in deep water are mainly suitable for floating wind turbines [14] .Its uncertainty is affected not only by the wind speed, but also waves, and ocean currents, which leads to the movement of a floating foundation in six degrees of freedom [15] .Operators of the OOGPs are looking for a solution that could support the integration of offshore wind power without completely replacing existing EMS.
To achieve the above goal, a coordinative controller is proposed to support the integration of offshore wind turbines and ESS to an OOGP.The controller combines the operation of the newly installed equipment with one existing gas/oil turbine generator as one equivalent generator.Considering the special feature of floating wind turbines, both feedforward control and feedback control are employed.The feedforward control is used to compensate for the periodic wind power output variation caused by the wave and ocean currents.The feedback control is used to decompose the variation of wind power into high-frequency and low-frequency signals and smooth them with ESS and turbine generators.The effectiveness of the proposed method is validated using a simplified model of a real offshore oil and gas platform.

System structure of OOGPs
Figure 1 shows the structure of OOGPs with floating wind power.Induction motors are the main loads used for drilling rig, oil transmission, gas compression, and water injection.With the expansion of offshore oil and gas exploration, the electricity demand keeps increasing.To improve the safety and economy of offshore power supply, OOGPs are interconnected via submarine cables.The platforms can provide mutual support, so as to reduce backup capacity requirements, improve operational efficiency and enhance operational security level.

Bundled Structure
The structure of the proposed bundled structure including wind turbine, ESS, and gas/oil turbine generator is displayed in Figure 2. The power exchange between the bundled equipment and the platforms is regulated by a coordinative controller.
tur,min tur tur,max where P tur (t) and Q tur (t) are the active power and reactive power of the bundled gas/oil turbine at time t; P tur,min and P tur,max are the lower and upper bounds of P tur (t); Q tur,min and Q tur,max are the lower and upper bounds of Q tur (t); S tur,max is the maximum turbine apparent power; R tur (t) is the ramp rate of the turbine generator; R tur,min and R tur,max are the lower and upper bounds of R tur (t).
The operation of floating wind turbine satisfies wind,min wind wind,max ( ) wind,min wind wind,max ( ) where P wind (t) and Q wind (t) are the active power and reactive power output of the wind turbine at time t; P wind,min and P wind,max are the lower and upper bounds of P wind (t); Q wind,min and Q wind,max are the lower and upper bounds of Q wind (t).The operation of ESS satisfies bat,min bat bat,max ( ) SOC,min SOC SOC,max ( ) where P bat (t) is the charging or discharging power of ESS at time t; P bat,min and P bat,max are the maximum charging and discharging power of the ESS; E SOC (t) is the state of charge (SOC) of ESS at time t; E SOC,min and E SOC,max are the lower and upper bounds of the SOC.

Coordinative control strategy
Figure 3 shows the process of the proposed control strategy, composed of feedforward and feedback control.The goal is to achieve fast power distribution between ESS and the gas/oil turbine generator under various wind power fluctuation scenarios.The feedforward control is used to smooth the wind  To ensure the accuracy of the control action under equipment operational constraints, the decomposed signal satisfies the following relationship in the frequency domain.The time constant T can be evaluated as follow [16] cap tur,min tur,max min{| |, } where Pcap is the total power capacity of the bundled gas/oil turbine.

Case Studies
In this section, a model extracted from a real offshore oil and gas platform is used to test the proposed technology.Figure 4 shows the equivalent generator, including a 4 MW floating wind turbine, a 5 MW gas turbine, and a 1 MW / 1 MWh battery storage.Three scenarios are studied: 1) A floating wind turbine is connected to the platforms without ESS; 2) Battery storage is configurated without feedforward control; 3) Battery storage and feedforward control are configurated.

Case 1
In this case, a signal with periodic and nonperiodic behaviors is generated to mimic the power output variation of a floating wind turbine caused by floating foundation motion and wind speed.As the wind turbine power fluctuates in the above form, the corresponding power output of the equivalent generator in different scenarios is shown in Figure 5.The variation of gas turbine power is shown in Figure 6.In the initial state, the power of the floating wind turbine fluctuates periodically.At 10 s, the step increase in wind speed causes a surge in wind turbine power and equivalent generator power output in each scenario.In an effort to stabilize the power output of the equivalent generator, the control system will reduce the power of the gas turbine.At about 12 s, the control system together with periodic fluctuation leads to a decrease in the power output of the equivalent generator.During the whole process, slow power regulation of the gas turbine limited by ramp constraint results in the largest power fluctuation of the equivalent generator in Scenario 1.In Scenario 2, battery storage shares a portion of the burden on the gas turbine and makes up for the insufficient power regulation capability of the gas turbine, thus reducing the power fluctuation of the equivalent generator.Based on Scenario 2, the feedforward control in Scenario 3 predicts the periodic fluctuation of wind turbine power and separates it into battery storage for smoothing, improving the stability of the output power of the equivalent generator.It's verified that the power output variation of the equivalent generator in Scenario 3 decreases by about 80.0% compared to Scenario 1 and by about 44.6% compared to Scenario 2 after the wind speed steps up, which confirms the effectiveness of the proposed method in eliminating wind power fluctuation.Besides, the power variation of the gas turbine in Scenario 3 is the smoothest, which is beneficial for the gas turbine's life.

Case 2
In this case, a signal is generated to mimic the power output step-down of the wind turbine, in order to further test the effectiveness of the proposed strategy.The results are shown in Figure 7 and Figure 8.The conclusions in Case 2 are similar to those in Case 1.Therefore, the proposed control strategy is effective to eliminate the floating wind power fluctuation.

Conclusion
This paper proposes a collaborative control strategy to support the integration of wind power and ESS into OOGPs.The new strategy enables the platforms to integrate wind power and ESS without upgrading the existing EMS completely.The results show that the proposed control technology based on feedforward control and saturation filter control can effectively smooth the power fluctuation of the equivalent generator, as well as the power fluctuation of the turbine generator selected for bundling.In the future, the optimal configuration of ESS, as well as controller parameters under more complex wind conditions will be further studied.

Figure 2 .
Figure 2. Structure of the bundled wind turbine, ESS, and gas/oil turbine.The gas/oil turbines in OOGPs satisfy the following constraints.
power output variation caused by the floating basis.The feedback control is a saturation filter control.The wind power fluctuation component ΔP is decomposed into fast and slow signals.The highfrequency fluctuation component along with the feedforward signal is sent to the ESS, and the remaining low-frequency fluctuation component is sent to the gas/oil turbine generator.

Figure 3 .
Figure 3. Diagram of the proposed collaborative control strategy.

Figure 4 .
Figure 4. Structure of the bundled wind turbine, battery storage, and gas turbine.

Figure 5 .
Figure 5. Power output of the wind turbine and the equivalent generator (P base = 5 MW).

Figure 7 .
Figure 7. Power output of the wind turbine and the equivalent generator (P base = 5 MW).