Feasibility WT in WIS in Offshore Facilities JS-Vf

7/26/2019 Feasibility WT in WIS in Offshore Facilities JS-Vf

http://slidepdf.com/reader/full/feasibility-wt-in-wis-in-offshore-facilities-js-vf 1/5

Feasibility of Wind Integration to Supply Water Injection Systems in Offshore Facilities

Jesús Silva Norwegian University of Science and Technology

Trondheim, Norway

Abstract — CO2 emissions of gas turbines on oil and gasplatforms can be reduced by local electrification using offshorewind turbines. Local generation avoids investments on expensivetransmission systems connected to shore; similarly, improvedactive-reactive support can be obtained from wind turbines onexisting platforms. This is important especially now that matureoil fields require more energy to keep up production levels. Theavailability of large offshore turbines ranging up to 6-8MWallows the integration of significant renewable power to the O&Ginstallations were wind conditions are typically excellent.However, having a large wind power generation share, putchallenges on maintaining the secure and stable operation of the

overall power system. Challenges on low inertia response, lowshort circuit capacity, harmonics, reactive power support, andwind intermittency become matters of main concern. Two studycases are investigated: the first case referred to integration of alarge wind turbine to an O&G platform and second case analyzesthe power-frequency stability of a stand-alone water injectionsystem that feed a local load using a novel control techniqueknown as virtual synchronous machine. The second case havebeen a concept proposed by DNV-gl for supplying power to waterpressure support systems located far from the host platform,where usually there is a long step-out distance and cables costsmakes floating wind turbine an alternative solution.

Keywords—Large-scale integration; off-shore wind turbines;O&G platforms; isolated system; virtual synchronous machine;water injection facilities.

I. I NTRODUCTION

Most the O&G installations are supplied by gas turbines (GT)in open cycled mode that have even lower efficiency(e.g.30%-35%) compared to the combined cycle gas turbines(CCGT) on shore, these provide the required energy supplyfor heat, electrical and mechanical operations. Electrificationfrom shore have been realized by HVDC transmission links infew cases, as Valhall and Troll [1]. In 2014, the offshore O&Goperations emitted 14% of the greenhouse gas emissions in Norway [2] of which, 80% were generated on the gas turbinedriven compressors. The Norwegian government has madeefforts in order to reduce the emissions, the electrification of

O&G offshore facilities on the Norwegian Continental shelf(NCS) can contribute with a reduction of 4.6 million ton CO2[1]. Cost savings on greenhouse gases taxation, sharedinvestment between petroleum and wind power plantoperators, the existence of wind resources on NCS, andtechnology developments are main drivers for electrification.

The O&G facilities are located kilometers from the coast.Typically, at water depths above 100m, under thiscircumstance, only floating structures are technically feasible.There have been some effort using floating wind powergeneration showing positive results, Statoil Hywind Demo [3]concept was installed in June 2009 on the west coast of Norway with a 2.3MW Siemens turbine. The capacity factorhas been beyond 60%, compared to 30-47% of the bestoffshore generation sites [1].

Some studies related to wind integration in O&G facilities has been performed for off-grid or islanded operation in terms of power stability and control [4] [5], fuel savings and C02emissions [3] and HVDC-link connections [6][7] , which haveshown promising results in terms of power system stability.

Large power wind share on these installations should beevaluated individually, since each the wind resources andvariability are location dependent. Load types and operationare also particular for each case.

This paper deals with some of the challenges of large wind power integration. Two test cases are developed inMATLAB/Simulink environment. The first case consists of oiland gas platform supplied by synchronous generator driven bya gas turbine, the loads include a water injection motor and anaccumulated PQ load, an AC connection to an aggregatedwind turbine, represented by DC source. In the second case,the wind turbine is in stand-alone condition supplying a passive load; the grid side converter of the wind turbine usesvector control in the synchronous frame. Different controlloops are developed, including: inner current control,frequency droop, and virtual synchronous machine controlloop. This last case, evaluates critical conditions for stand-alone solutions as black-start capability and power-frequencystability under wind loss condition.

II. STATE OF THE ART

An O&G offshore facility is typically an islanded powersystem, consisting of many energy consumption unitsincluding drilling, accommodation, processing, exporting andwater/gas injection [3]



Declining production in the NCS, requires pressure support bymeans of water or gas injection, these are processes energeticintensive. The industry will need more energy in the nearfuture, but at the same time, it needs to find alternative waysto reduce gas emissions and maintain economic viableoperations. This leads to the incremental participation of windrenewable sources.

Clearly, an O&G facility has critical services that cannot relycompletely on wind power due to the intermittency of thisresource. Some core processes cannot be disturbed by powerinterruptions. Therefore, a controllable generation should beguaranteed, provided by gas turbines. However, large parts ofthe loads are flexible; for example, water injection pumps andthermal loads, this specific attribute is essential and it can beeventually used when shedding non-critical loads to keep the power system instability, in the occurrence of a temporaryevent or counteracting the wind power deficits.

Furthermore, load adjustment can plays an interesting rolewhen generation is lost due to faulty conditions or windturbine power fluctuations or downtime. As said before, acapacity factor of 60% from Hywind prototype will result inturbines operating more than half at rated power. Provisioncan be made for the rest 40% needed.

When large wind power integration is considered, it must beon the early stage of the planning phase, the power stabilityshould be ensured by fast actuators and appropriate controlmechanisms, and proper measures should be taken to addressthe challenges it involves.

III. LARGE SCALE WIND INTEGRATION CHALLENGES INSTAND-ALONE SYSTEMS

Gas turbines are responsible for main system regulation, buttheir control capabilities are reduced as more wind power isintegrated. Consequently, wind turbines must also offerauxiliaries services supporting the frequency balance andvoltage. Simultaneously, load management should be applied,as counteracting effect during low wind regimes or turbinedowntime. The next paragraphs summarized importantchallenges when large wind power integration is intended.

A. Inertia ResponseSynchronous machines driven by gas turbines and theirinertial response is the main source of control during an event.However, when wind turbines are added the inertial responseis lost, due to the decoupling effect of power electronicconverters. This leads to limited control by the gas turbinesduring transient events, in part due to the time responseconstants. On the other hand, wind turbines are not designedoriginally to offer ancillary services supporting frequency andvoltage stability. Nevertheless, a limited degree of support can be implemented by using adequate wind turbine controlsystems and concepts like virtual synchronous machine inertia[8].

B. Black-star capabilityBlack star is an ancillary service procured for systemrestoration after a complete or partial outage. These BSresources must be able to energize buses and have on site-diesel or batteries, in order to provide power for the auxiliaryservices used to star the unit. Communication infrastructureand smart switches are vital as well as auxiliary power forrestoration scheme success [9]. The control algorithm should be able to build up the low voltage network, controllingvoltage and frequency, connecting the controllable loads andfrequency synchronization with the distribution grid[10].Commercial solution for BS in islanded turbines are notavailable, but some efforts are being made based on dieselunits.

C. Variable Speed Drives on Large MotorsUsually large motors are driven by variable speed drives(VSD). The VSD has the advantage of regulating speed andtorque independently. This capability is interesting in case ofwind intermittency. Even though, the technology commonlyused for large motors compressors for VSD are current sourceinverters (CSI). The tendency is to be replaced by modernvoltage source converters (VSC); this technology offers a power factor above 0.95 [1], which translates to lower demandfor reactive support and a more feasible implementation forstand-alone wind turbines, that have a limited reactive provision. D. Limited short-circuit capacity and reactive controlWind turbines are interfaced by power electronic converters;thus, their short circuit contribution is limited to their rating,and hence restricted, compared to synchronous generators.Similarly, motors driven by VSD cannot provide short circuitcapability. A detailed protection system scheme should becarefully considered to mitigate the occurrence of destructivelarge currents. However, old gas turbines can be refurbished assynchronous condensers and proposed in [11], to provide

reactive support, short-circuit capacity and inertia. D. Load Control and Wind IntermittencyThe operation and performance of an isolated system,generally depends on the characteristics of the load. The loadis the sum of many individual loads and can represent different quality and response requirements. Loads alsoinherently change over time, which is something that must beincorporated during initial system design. For instance, someloads schemes can be implemented in order to control theirdemand. This characteristic gives the system’s flexibility asthe case for O&G facilities, this helps to keep the power balance and likewise saving the system from having to start an additional generator. This is often referred to as demand-side

management (DSM) ; the use of this scheme allows a greatdeal of flexibility in the control of the power system [12]. Iftrusted wind data is available, low wind regime periods can be predicted and load could be adapted accordingly. This role ofload contribution and management should be illustrated in thefirst study case, when a wind generation lost is counteracted by a load reduction on a 5MW water injection pump.



E. Power Quality:Power electronics used in wind turbines converters deterioratethe voltage and current signals. Thus, filtering will be requiredin order to reduce the harmonics produced by these devices.The tuning of filtering components might require an importanteffort and substantial investments. On the other hand, becauseof the limited wind turbine reactive support; compensatorsmight be required to be installed.

F. Gas turbine efficiency and maintenance:Gas turbine efficiency is affected by wind power contribution.The average efficiency of the gas turbine is reduced byroughly 5% [1] .In the case of platforms equipped with twoor more gas turbines, it is possible to shut one down duringmaximum power delivery from the wind turbine. On the otherhand, considering a high wind variability this might let toturning on and off gas turbines, which results in highermaintenance costs and reduced components lifetime if it is nothandled appropriately.

IV. THE STUDYCASES:

A.

Case 1: Wind Power and Gas Turbine IntegrationThis case will investigate the control and system stability of a power system, see Fig. 1, that consist of an offshore wind farmconnected to an oil and gas platform through a 5 km ACsubmarine cable AC. The case study is based on an aggregated8MW wind turbine connected to a 25MW gas turbine. TheO&G platform has the following electrical loads:

25+2j MVA, process loads (heating, lighting, processes)

5 MW water injection pump/motor

Fig. 1, System 1 diagram. Process loads are lumped in a fixedthree phase parallel load.

The total system loading is changed dynamically during 20s.After the steady state is stablished, a 5MW water injectioninduction motor is started-up at second 5, later during 8-10s asudden lost of wind generation is simulated and then motorde-loaded during this event.

Fig. 2, a. Voltage-current, b. Active-reactive powerc. Frequency, synchronous machine

As seen from fig. 2, the active and reactive power of thesynchronous machine driven by the gas turbine changes overtime depending on the loading. The first seconds, the gasturbine condition is adapted by means of the governor andfrequency droop, affecting the machine speed. As 8MW areimported from the wind turbine through the 5km AC cable, thegas turbine closes fuel valves

Motor Start-upThe 5MW motor is started-up unloaded, afterwards at 5s a 1pu

torque is applied, the governor actuates to supply the newenergy required fig 2, and it reaches a stable conditions asseen in fig 2 and 3, at11s .

Fig. 3, a. Rotor speed, b. Electromagnetic torquec. Stator current, induction motor

Sudden wind power lossAfter the system is fully loaded, a sudden interruption of windgeneration is simulated fig 4, 8WM generation is lost. The gasturbine responds effectively supplying a peak of 1.2 itscapacity, as seen from fig 2.

After the event is cleared at 11s, the synchronous machinereached its previous state fig 3, and the wind turbine startsinjecting power back to the network.

The frequency experienced small disturbances but within60Hz valid range. The disturbance was counteracted byeffective control of the motor torque; during the event, themotor was de-loaded. It can be interpreted that by adequateload management it is possible to counteract wind variationsor power outages. If the wind energy is constantly monitored,it is possible to control the water pumps flow rate in order to



account for wind variations and decrease pump mechanical pulsations.

Fig. 4, a. Voltage-current, b. Active-reactive powerc. Frequency

B. Case 2: Stand-alone water injection system

DNV GL has conducted a high-level study on the potential useof floating wind turbines for powering water injectionsystems. Indicators of economic and technical performanceshow an interesting window of opportunity for marginal fieldsor platforms constrained by available power and space forwater injection systems[13], as well as cost associated withtransmission infrastructure.

Many water injection installations should be located atspecific points far from the host platform, even at step-updistance of 30km as the case from Tyrihans Field [14] . Thesystem, see fig 8, consist of a 8MW passive load and a 8MWwind turbine, the objective of the system is to inject raw saltwater to help pressurized the well and increase the oilrecovery rate using energy produced locally by the windturbine.

Fig 5, Stand-alone wind turbine and load

In this case, the load is feed with 100% wind power. The wind

turbine is set in order to extract the maximum power possibleand feed the pump/load. The load actually is considering onlythe active part, vector control allows a decoupling betweenactive and reactive power, here the interest is on the active part. During times of low wind regimes, the pump must work

at a low flow rates or be disconnected in high wind regimes,for example in speeds exceeding 25 m/s.

Due to the stand-alone nature of the system, there is notvoltage and frequency reference provided by either a stiff-gridor gas turbine as the previous case. The approach tested here isthe emulation of a synthetic frequency signal, this is done bythe implementation of virtual synchronous machine (VSM)concept [8]. The control implemented here are described indetail in [15].This systems is inherently stable, but the VSM ismore general and can run for cases where multiple turbines areconnected in parallel and system synchronization is needed.

The control loops considered here are:

Fig 6, Current controller[15]

Fig. 7, Virtual Synchronous Machine Inertia with powerfrequency droop [15]

The current PI controllers are tuned by Modulus OptimumCriterion. The voltage loop *

,d qADv are not implemented and

the feed-forward terms vkff are disable or set to zero. The parameters are taken from [15].

In order to test the system, a sudden wind loss is simulatedduring the interval 2-3s as seen from figure 8. When the eventis cleared at 3s, the system begins to recover showing someoscillations until regaining steady state at full power, similarto a second order swing equation. A point worth mentioning isthe inertia emulation, during the event the frequency starts toaccelerate reaching 62Hz, this is consistent with thesynchronous machine behavior since it is injectingaccumulated kinetic energy. Once the power is reestablished,the frequency decreases from 62Hz to 58Hz, showing thesame behavior that a real synchronous machine would. The



control system offers a fast response and a black-startcapability, recovering 2 seconds after the power, whichcoincides with the parameter aT that emulates the syntheticinertia.

Fig 8, a. Voltage-current, b. Active-reactive powerc. Frequency

V. CONCLUSIONS

The increasing need of energy at the O&G installations, CO2 restrictions, weight and space limitations on the platformsalong with the existence of large turbine technology, has brought attention to large wind power integration. Large-scalewind power has challenges in terms of inertia loss, reducedshort circuit capacity, power quality, power reactive control, black-start capability and gas turbine efficiency. In general,60% capacity factor in wind turbines found in many locationsin NCS, along with the existence of large motor with flexibleloads, as it is the case for water injection system can help tocounteract the wind variability.

This work have developed two test cases for the evaluation ofislanded wind farms supplying local power, using wind

powered water injection as a specific reference application.The first case, investigates the load flexibility on a generic platform installation as a counteracting strategy to theintermittent nature of wind, allowing large integration of wind power to the platform. Successful 5MW motor start-up isaccomplished and 8MW wind loss is effectively testedshowing a stable power-frequency condition after thecontingency is cleared.

The second case, have evaluated a 100% passive load fed by awind turbine, the virtual synchronous machine controltechnique in islanded mode is tested by implementing vectorcontrol, three control loops: current, frequency andsynchronous machine. A wind loss event is simulated and

inertial response, black-start capability and frequency stabilityis demonstrated.

REFERENCES

[1] A. R. Ardal, K. Sharifabadi, O. Bergvoll, and V. Berge, “Challengeswith integration and operation of offshore oil & gas platforms

connected to an offshore wind power plant,”2014 Pet. Chem. Ind.Conf. Eur. , pp. 1–9, 2014.

[2] P. H. Hansen, CO 2 - Emission effect on electrification.Commissioned by Statoil ASA R-2011-041.

[3] W. He, G. Jacobsen, T. Anderson, F. Olsen, T. D. Hanson, M.Korpås, T. Toftevaag, J. Eek, K. Uhlen, and E. Johansson, “ThePotential of Integrating Wind Power with Offshore Oil and GasPlatforms,”Wind Engineering . 2010.

[4] A. R. Årdal, S. D. Arco, R. E. Torres-olguin, T. Undeland, O. S.Bragstads, and S. Asa, “Parametric sensitivity of Transients in anIslanded System with an Offshore Wind Farm Connected to an Oil

Platform and Technology ( NTNU ) Keywords.”[5] H. Svendsen and M. Hadiya, “Integration of offshore wind farmwith multiple oil and gas platforms,”2011 IEEE Trondheim , pp. 1– 3, 2011.

[6] J. I. Marvik, E. V. Øyslebø, and M. Korpås, “Electrification ofoffshore petroleum installations with offshore wind integration,”

Renew. Energy , vol. 50, pp. 558–564, 2013.[7] G. Shi, S. Peng, X. Cai, and Z. Chen, “Grid Integration of Offshore

Wind Farms and Offshore Oil / Gas Platforms,” pp. 1301–1305,2012.

[8] S. D’Arco, J. A. Suul, and O. B. Fosso, “Small-signal modeling and parametric sensitivity of a virtual synchronous machine in islandedoperation,” Int. J. Electr. Power Energy Syst. , vol. 72, pp. 3–15,2015.

[9] J. Li, J. Su, X. Yang, and T. Zhao, “Study on microgrid operationcontrol and black start,” DRPT 2011 - 2011 4th Int. Conf. Electr.Util. Deregul. Restruct. Power Technol. , pp. 1652–1655, 2011.

[10] W. Sun, C.-C. Liu, and S. Liu, “Black start capability assessment in power system restoration,”2011 IEEE Power Energy Soc. Gen. Meet. , pp. 1–7, 2011.

[11] Z. H. Rather, Z. Chen, and P. Thøgersen, “Challenge of primaryvoltage control in large scale wind integrated Power System: ADanish power system case study,”2013 4th IEEE/PES Innov. SmartGrid Technol. Eur. ISGT Eur. 2013 , pp. 1–5, 2013.

[12] D. Nikolic, M. Negnevitsky, M. de Groot, S. Gamble, J. Forbes, andM. Ross, “Fast demand response as an enabling technology for highrenewable energy penetration in isolated power systems,”2014

IEEE PES Gen. Meet. | Conf. Expo. , pp. 1–5, 2014.[13] J. Slätte, D. N. V Gl, J. Sandberg, T. Flach, G. Dekker, and C.

Sixtensson, “OTC 25284-MS Wind-powered Subsea WaterInjection Pumping : Technical and Economic Feasibility,” 2014.

[14] A. Grynning, S. V. Larsen, and I. Skaale, “Tyrihans Raw SeawaterInjection,” Proc. Offshore Technol. Conf. , no. May, pp. 4–7, 2009.

[15] S. D’Arco, J. A. Suul, and O. B. Fosso, “A Virtual SynchronousMachine implementation for distributed control of power convertersin SmartGrids,” Electr. Power Syst. Res. , vol. 122, pp. 180–197,2015.

Documents

Feasibility WT in WIS in Offshore Facilities JS-Vf