IDEAS home Printed from https://ideas.repec.org/a/eee/rensus/v144y2021ics1364032121001805.html
   My bibliography  Save this article

Offshore wind turbine operations and maintenance: A state-of-the-art review

Author

Listed:
  • Ren, Zhengru
  • Verma, Amrit Shankar
  • Li, Ye
  • Teuwen, Julie J.E.
  • Jiang, Zhiyu

Abstract

Operations and maintenance of offshore wind turbines (OWTs) play an important role in the development of offshore wind farms. Compared with operations, maintenance is a critical element in the levelized cost of energy, given the practical constraints imposed by offshore operations and the relatively high costs. The effects of maintenance on the life cycle of an offshore wind farm are highly complex and uncertain. The selection of maintenance strategies influences the overall efficiency, profit margin, safety, and sustainability of offshore wind farms. For an offshore wind project, after a maintenance strategy is selected, schedule planning will be considered, which is an optimization problem. Onsite maintenance will involve complex marine operations whose efficiency and safety depend on practical factors. Moreover, negative environmental impacts due to offshore maintenance deserve attention. To address these issues, this paper reviews the state-of-the-art research on OWT maintenance, covering strategy selection, schedule optimization, onsite operations, repair, assessment criteria, recycling, and environmental concerns. Many methods are summarized and compared. Limitations in the research and shortcomings in industrial development of OWT operations and maintenance are described. Finally, promising areas are identified with regard to future studies of maintenance strategies.

Suggested Citation

  • Ren, Zhengru & Verma, Amrit Shankar & Li, Ye & Teuwen, Julie J.E. & Jiang, Zhiyu, 2021. "Offshore wind turbine operations and maintenance: A state-of-the-art review," Renewable and Sustainable Energy Reviews, Elsevier, vol. 144(C).
  • Handle: RePEc:eee:rensus:v:144:y:2021:i:c:s1364032121001805
    DOI: 10.1016/j.rser.2021.110886
    as

    Download full text from publisher

    File URL: http://www.sciencedirect.com/science/article/pii/S1364032121001805
    Download Restriction: Full text for ScienceDirect subscribers only

    File URL: https://libkey.io/10.1016/j.rser.2021.110886?utm_source=ideas
    LibKey link: if access is restricted and if your library uses this service, LibKey will redirect you to where you can use your library subscription to access this item
    ---><---

    As the access to this document is restricted, you may want to search for a different version of it.

    References listed on IDEAS

    as
    1. Zhang, Chen & Gao, Wei & Yang, Tao & Guo, Sheng, 2019. "Opportunistic maintenance strategy for wind turbines considering weather conditions and spare parts inventory management," Renewable Energy, Elsevier, vol. 133(C), pages 703-711.
    2. Amrit Shankar Verma & Zhiyu Jiang & Zhengru Ren & Zhen Gao & Nils Petter Vedvik, 2019. "Response-Based Assessment of Operational Limits for Mating Blades on Monopile-Type Offshore Wind Turbines," Energies, MDPI, vol. 12(10), pages 1-26, May.
    3. Stålhane, Magnus & Halvorsen-Weare, Elin E. & Nonås, Lars Magne & Pantuso, Giovanni, 2019. "Optimizing vessel fleet size and mix to support maintenance operations at offshore wind farms," European Journal of Operational Research, Elsevier, vol. 276(2), pages 495-509.
    4. Dui, Hongyan & Si, Shubin & Yam, Richard C.M., 2017. "A cost-based integrated importance measure of system components for preventive maintenance," Reliability Engineering and System Safety, Elsevier, vol. 168(C), pages 98-104.
    5. Arcos Jiménez, Alfredo & Zhang, Long & Gómez Muñoz, Carlos Quiterio & García Márquez, Fausto Pedro, 2020. "Maintenance management based on Machine Learning and nonlinear features in wind turbines," Renewable Energy, Elsevier, vol. 146(C), pages 316-328.
    6. Jijian Lian & Ou Cai & Xiaofeng Dong & Qi Jiang & Yue Zhao, 2019. "Health Monitoring and Safety Evaluation of the Offshore Wind Turbine Structure: A Review and Discussion of Future Development," Sustainability, MDPI, vol. 11(2), pages 1-29, January.
    7. Lubing Xie & Xiaoming Rui & Shuai Li & Xin Hu, 2019. "Maintenance Optimization of Offshore Wind Turbines Based on an Opportunistic Maintenance Strategy," Energies, MDPI, vol. 12(14), pages 1-26, July.
    8. Zhou, P. & Yin, P.T., 2019. "An opportunistic condition-based maintenance strategy for offshore wind farm based on predictive analytics," Renewable and Sustainable Energy Reviews, Elsevier, vol. 109(C), pages 1-9.
    9. Jiang, Zhiyu, 2021. "Installation of offshore wind turbines: A technical review," Renewable and Sustainable Energy Reviews, Elsevier, vol. 139(C).
    10. Jensen, J.P. & Skelton, K., 2018. "Wind turbine blade recycling: Experiences, challenges and possibilities in a circular economy," Renewable and Sustainable Energy Reviews, Elsevier, vol. 97(C), pages 165-176.
    11. Perez-Canto, Salvador & Rubio-Romero, Juan Carlos, 2013. "A model for the preventive maintenance scheduling of power plants including wind farms," Reliability Engineering and System Safety, Elsevier, vol. 119(C), pages 67-75.
    12. Nielsen, Jannie Jessen & Sørensen, John Dalsgaard, 2011. "On risk-based operation and maintenance of offshore wind turbine components," Reliability Engineering and System Safety, Elsevier, vol. 96(1), pages 218-229.
    13. Jenny Niebsch & Ronny Ramlau & Thien T. Nguyen, 2010. "Mass and Aerodynamic Imbalance Estimates of Wind Turbines," Energies, MDPI, vol. 3(4), pages 1-15, April.
    14. Sun, Wei & Lin, Wei-Cheng & You, Fei & Shu, Chi-Min & Qin, Sheng-Hui, 2019. "Prevention of green energy loss: Estimation of fire hazard potential in wind turbines," Renewable Energy, Elsevier, vol. 140(C), pages 62-69.
    15. Reimers, Britta & Özdirik, Burcu & Kaltschmitt, Martin, 2014. "Greenhouse gas emissions from electricity generated by offshore wind farms," Renewable Energy, Elsevier, vol. 72(C), pages 428-438.
    16. Yang, Wenxian & Little, Christian & Court, Richard, 2014. "S-Transform and its contribution to wind turbine condition monitoring," Renewable Energy, Elsevier, vol. 62(C), pages 137-146.
    17. Chandra Ade Irawan & Majid Eskandarpour & Djamila Ouelhadj & Dylan Jones, 2019. "Simulation-based optimisation for stochastic maintenance routing in an offshore wind farm," Post-Print hal-02509382, HAL.
    18. Wu, Xiaoni & Hu, Yu & Li, Ye & Yang, Jian & Duan, Lei & Wang, Tongguang & Adcock, Thomas & Jiang, Zhiyu & Gao, Zhen & Lin, Zhiliang & Borthwick, Alistair & Liao, Shijun, 2019. "Foundations of offshore wind turbines: A review," Renewable and Sustainable Energy Reviews, Elsevier, vol. 104(C), pages 379-393.
    19. Zhu, Wenjin & Castanier, Bruno & Bettayeb, Belgacem, 2019. "A dynamic programming-based maintenance model of offshore wind turbine considering logistic delay and weather condition," Reliability Engineering and System Safety, Elsevier, vol. 190(C), pages 1-1.
    20. Shafiee, Mahmood & Finkelstein, Maxim & Bérenguer, Christophe, 2015. "An opportunistic condition-based maintenance policy for offshore wind turbine blades subjected to degradation and environmental shocks," Reliability Engineering and System Safety, Elsevier, vol. 142(C), pages 463-471.
    21. Abdollahzadeh, Hadi & Atashgar, Karim & Abbasi, Morteza, 2016. "Multi-objective opportunistic maintenance optimization of a wind farm considering limited number of maintenance groups," Renewable Energy, Elsevier, vol. 88(C), pages 247-261.
    22. Krokoszinski, H.-J., 2003. "Efficiency and effectiveness of wind farms—keys to cost optimized operation and maintenance," Renewable Energy, Elsevier, vol. 28(14), pages 2165-2178.
    23. Xue, Bing & Ma, Zhixiao & Geng, Yong & Heck, Peter & Ren, Wanxia & Tobias, Mario & Maas, Achim & Jiang, Ping & Puppim de Oliveira, Jose A. & Fujita, Tsuyoshi, 2015. "A life cycle co-benefits assessment of wind power in China," Renewable and Sustainable Energy Reviews, Elsevier, vol. 41(C), pages 338-346.
    24. Xin Wu & Hong Wang & Guoqian Jiang & Ping Xie & Xiaoli Li, 2019. "Monitoring Wind Turbine Gearbox with Echo State Network Modeling and Dynamic Threshold Using SCADA Vibration Data," Energies, MDPI, vol. 12(6), pages 1-19, March.
    25. Sarker, Bhaba R. & Faiz, Tasnim Ibn, 2016. "Minimizing maintenance cost for offshore wind turbines following multi-level opportunistic preventive strategy," Renewable Energy, Elsevier, vol. 85(C), pages 104-113.
    26. Klinge Jacobsen, Henrik & Hevia-Koch, Pablo & Wolter, Christoph, 2018. "Costs and competitive advantage of nearshore wind energy," MPRA Paper 92869, University Library of Munich, Germany.
    27. uit het Broek, Michiel A.J. & Veldman, Jasper & Fazi, Stefano & Greijdanus, Roy, 2019. "Evaluating resource sharing for offshore wind farm maintenance: The case of jack-up vessels," Renewable and Sustainable Energy Reviews, Elsevier, vol. 109(C), pages 619-632.
    28. Schleisner, L, 2000. "Life cycle assessment of a wind farm and related externalities," Renewable Energy, Elsevier, vol. 20(3), pages 279-288.
    29. Gutierrez-Alcoba, A. & Hendrix, E.M.T. & Ortega, G. & Halvorsen-Weare, E.E. & Haugland, D., 2019. "On offshore wind farm maintenance scheduling for decision support on vessel fleet composition," European Journal of Operational Research, Elsevier, vol. 279(1), pages 124-131.
    30. Roy Radner & Dale W. Jorgenson, 1963. "Opportunistic Replacement of a Single Part in the Presence of Several Monitored Parts," Management Science, INFORMS, vol. 10(1), pages 70-84, October.
    31. Shafiee, Mahmood, 2015. "Maintenance logistics organization for offshore wind energy: Current progress and future perspectives," Renewable Energy, Elsevier, vol. 77(C), pages 182-193.
    32. Nguyen, Thi Anh Tuyet & Chou, Shuo-Yan, 2019. "Improved maintenance optimization of offshore wind systems considering effects of government subsidies, lost production and discounted cost model," Energy, Elsevier, vol. 187(C).
    33. Soares-Ramos, Emanuel P.P. & de Oliveira-Assis, Lais & Sarrias-Mena, Raúl & Fernández-Ramírez, Luis M., 2020. "Current status and future trends of offshore wind power in Europe," Energy, Elsevier, vol. 202(C).
    34. Taylor, James W. & Jeon, Jooyoung, 2018. "Probabilistic forecasting of wave height for offshore wind turbine maintenance," European Journal of Operational Research, Elsevier, vol. 267(3), pages 877-890.
    35. Arvesen, Anders & Hertwich, Edgar G., 2012. "Assessing the life cycle environmental impacts of wind power: A review of present knowledge and research needs," Renewable and Sustainable Energy Reviews, Elsevier, vol. 16(8), pages 5994-6006.
    36. Maienza, C. & Avossa, A.M. & Ricciardelli, F. & Coiro, D. & Troise, G. & Georgakis, C.T., 2020. "A life cycle cost model for floating offshore wind farms," Applied Energy, Elsevier, vol. 266(C).
    37. Bai, X. & Avital, E.J. & Munjiza, A. & Williams, J.J.R., 2014. "Numerical simulation of a marine current turbine in free surface flow," Renewable Energy, Elsevier, vol. 63(C), pages 715-723.
    38. James, Eric P. & Benjamin, Stanley G. & Marquis, Melinda, 2017. "A unified high-resolution wind and solar dataset from a rapidly updating numerical weather prediction model," Renewable Energy, Elsevier, vol. 102(PB), pages 390-405.
    39. Ding, Fangfang & Tian, Zhigang, 2012. "Opportunistic maintenance for wind farms considering multi-level imperfect maintenance thresholds," Renewable Energy, Elsevier, vol. 45(C), pages 175-182.
    40. Hameed, Z. & Vatn, J. & Heggset, J., 2011. "Challenges in the reliability and maintainability data collection for offshore wind turbines," Renewable Energy, Elsevier, vol. 36(8), pages 2154-2165.
    41. Yürüşen, Nurseda Y. & Rowley, Paul N. & Watson, Simon J. & Melero, Julio J., 2020. "Automated wind turbine maintenance scheduling," Reliability Engineering and System Safety, Elsevier, vol. 200(C).
    42. Bhardwaj, U. & Teixeira, A.P. & Soares, C. Guedes, 2019. "Reliability prediction of an offshore wind turbine gearbox," Renewable Energy, Elsevier, vol. 141(C), pages 693-706.
    43. de Azevedo, Henrique Dias Machado & Araújo, Alex Maurício & Bouchonneau, Nadège, 2016. "A review of wind turbine bearing condition monitoring: State of the art and challenges," Renewable and Sustainable Energy Reviews, Elsevier, vol. 56(C), pages 368-379.
    44. Maria Martinez Luengo & Athanasios Kolios, 2015. "Failure Mode Identification and End of Life Scenarios of Offshore Wind Turbines: A Review," Energies, MDPI, vol. 8(8), pages 1-16, August.
    45. Pierre Tchakoua & René Wamkeue & Mohand Ouhrouche & Fouad Slaoui-Hasnaoui & Tommy Andy Tameghe & Gabriel Ekemb, 2014. "Wind Turbine Condition Monitoring: State-of-the-Art Review, New Trends, and Future Challenges," Energies, MDPI, vol. 7(4), pages 1-36, April.
    46. Zhang, Chen & Gao, Wei & Guo, Sheng & Li, Youliang & Yang, Tao, 2017. "Opportunistic maintenance for wind turbines considering imperfect, reliability-based maintenance," Renewable Energy, Elsevier, vol. 103(C), pages 606-612.
    47. Dai, Kaoshan & Bergot, Anthony & Liang, Chao & Xiang, Wei-Ning & Huang, Zhenhua, 2015. "Environmental issues associated with wind energy – A review," Renewable Energy, Elsevier, vol. 75(C), pages 911-921.
    48. Zhao, Xin-gang & Ren, Ling-zhi, 2015. "Focus on the development of offshore wind power in China: Has the golden period come?," Renewable Energy, Elsevier, vol. 81(C), pages 644-657.
    49. Dong, Wenbin & Moan, Torgeir & Gao, Zhen, 2012. "Fatigue reliability analysis of the jacket support structure for offshore wind turbine considering the effect of corrosion and inspection," Reliability Engineering and System Safety, Elsevier, vol. 106(C), pages 11-27.
    50. Dai, Lijuan & Ehlers, Sören & Rausand, Marvin & Utne, Ingrid Bouwer, 2013. "Risk of collision between service vessels and offshore wind turbines," Reliability Engineering and System Safety, Elsevier, vol. 109(C), pages 18-31.
    51. Da, Zhang & Xiliang, Zhang & Jiankun, He & Qimin, Chai, 2011. "Offshore wind energy development in China: Current status and future perspective," Renewable and Sustainable Energy Reviews, Elsevier, vol. 15(9), pages 4673-4684.
    52. M’hammed Sahnoun & David Baudry & Navonil Mustafee & Anne Louis & Philip Andi Smart & Phil Godsiff & Belahcene Mazari, 2019. "Modelling and simulation of operation and maintenance strategy for offshore wind farms based on multi-agent system," Journal of Intelligent Manufacturing, Springer, vol. 30(8), pages 2981-2997, December.
    53. Li, Xiaodong & Ouelhadj, Djamila & Song, Xiang & Jones, Dylan & Wall, Graham & Howell, Kerry E. & Igwe, Paul & Martin, Simon & Song, Dongping & Pertin, Emmanuel, 2016. "A decision support system for strategic maintenance planning in offshore wind farms," Renewable Energy, Elsevier, vol. 99(C), pages 784-799.
    54. Wang, Yuxuan & Sun, Tianye, 2012. "Life cycle assessment of CO2 emissions from wind power plants: Methodology and case studies," Renewable Energy, Elsevier, vol. 43(C), pages 30-36.
    55. Sun, Xiaojing & Huang, Diangui & Wu, Guoqing, 2012. "The current state of offshore wind energy technology development," Energy, Elsevier, vol. 41(1), pages 298-312.
    56. Walgern, Julia & Peters, Lennart & Madlener, Reinhard, 2017. "Economic Evaluation of Maintenance Strategies for Offshore Wind Turbines Based on Condition Monitoring Systems," FCN Working Papers 8/2017, E.ON Energy Research Center, Future Energy Consumer Needs and Behavior (FCN).
    57. Laura, Castro-Santos & Vicente, Diaz-Casas, 2014. "Life-cycle cost analysis of floating offshore wind farms," Renewable Energy, Elsevier, vol. 66(C), pages 41-48.
    58. Esther Dornhelm & Helene Seyr & Michael Muskulus, 2019. "Vindby—A Serious Offshore Wind Farm Design Game," Energies, MDPI, vol. 12(8), pages 1-22, April.
    59. Staffell, Iain & Green, Richard, 2014. "How does wind farm performance decline with age?," Renewable Energy, Elsevier, vol. 66(C), pages 775-786.
    60. Liu, W.Y. & Tang, B.P. & Han, J.G. & Lu, X.N. & Hu, N.N. & He, Z.Z., 2015. "The structure healthy condition monitoring and fault diagnosis methods in wind turbines: A review," Renewable and Sustainable Energy Reviews, Elsevier, vol. 44(C), pages 466-472.
    61. Stock-Williams, Clym & Swamy, Siddharth Krishna, 2019. "Automated daily maintenance planning for offshore wind farms," Renewable Energy, Elsevier, vol. 133(C), pages 1393-1403.
    Full references (including those not matched with items on IDEAS)

    Most related items

    These are the items that most often cite the same works as this one and are cited by the same works as this one.
    1. Nguyen, Thi-Anh-Tuyet & Chou, Shuo-Yan & Yu, Tiffany Hui-Kuang, 2022. "Developing an exhaustive optimal maintenance schedule for offshore wind turbines based on risk-assessment, technical factors and cost-effective evaluation," Energy, Elsevier, vol. 249(C).
    2. McMorland, J. & Collu, M. & McMillan, D. & Carroll, J. & Coraddu, A., 2023. "Opportunistic maintenance for offshore wind: A review and proposal of future framework," Renewable and Sustainable Energy Reviews, Elsevier, vol. 184(C).
    3. Shafiee, Mahmood & Sørensen, John Dalsgaard, 2019. "Maintenance optimization and inspection planning of wind energy assets: Models, methods and strategies," Reliability Engineering and System Safety, Elsevier, vol. 192(C).
    4. Shuo-Yan Chou & Xuan Loc Pham & Thi Anh Tuyet Nguyen & Tiffany Hui-Kuang Yu, 2023. "Optimal maintenance planning with special emphasis on deterioration process and vessel routing for offshore wind systems," Energy & Environment, , vol. 34(4), pages 739-763, June.
    5. Li, Mingxin & Jiang, Xiaoli & Carroll, James & Negenborn, Rudy R., 2022. "A multi-objective maintenance strategy optimization framework for offshore wind farms considering uncertainty," Applied Energy, Elsevier, vol. 321(C).
    6. Vu, Hai Canh & Do, Phuc & Fouladirad, Mitra & Grall, Antoine, 2020. "Dynamic opportunistic maintenance planning for multi-component redundant systems with various types of opportunities," Reliability Engineering and System Safety, Elsevier, vol. 198(C).
    7. Zhang, Chen & Gao, Wei & Yang, Tao & Guo, Sheng, 2019. "Opportunistic maintenance strategy for wind turbines considering weather conditions and spare parts inventory management," Renewable Energy, Elsevier, vol. 133(C), pages 703-711.
    8. Pliego Marugán, Alberto & García Márquez, Fausto Pedro & Pinar Pérez, Jesús María, 2022. "A techno-economic model for avoiding conflicts of interest between owners of offshore wind farms and maintenance suppliers," Renewable and Sustainable Energy Reviews, Elsevier, vol. 168(C).
    9. Leimeister, Mareike & Kolios, Athanasios, 2018. "A review of reliability-based methods for risk analysis and their application in the offshore wind industry," Renewable and Sustainable Energy Reviews, Elsevier, vol. 91(C), pages 1065-1076.
    10. Shafiee, Mahmood, 2015. "Maintenance logistics organization for offshore wind energy: Current progress and future perspectives," Renewable Energy, Elsevier, vol. 77(C), pages 182-193.
    11. Li, Mingxin & Jiang, Xiaoli & Carroll, James & Negenborn, Rudy R., 2023. "A closed-loop maintenance strategy for offshore wind farms: Incorporating dynamic wind farm states and uncertainty-awareness in decision-making," Renewable and Sustainable Energy Reviews, Elsevier, vol. 184(C).
    12. Li, He & Teixeira, Angelo P. & Guedes Soares, C., 2020. "A two-stage Failure Mode and Effect Analysis of offshore wind turbines," Renewable Energy, Elsevier, vol. 162(C), pages 1438-1461.
    13. Zhou, Yifan & Miao, Jindan & Yan, Bin & Zhang, Zhisheng, 2020. "Bio-objective long-term maintenance scheduling for wind turbines in multiple wind farms," Renewable Energy, Elsevier, vol. 160(C), pages 1136-1147.
    14. Erguido, A. & Crespo Márquez, A. & Castellano, E. & Gómez Fernández, J.F., 2017. "A dynamic opportunistic maintenance model to maximize energy-based availability while reducing the life cycle cost of wind farms," Renewable Energy, Elsevier, vol. 114(PB), pages 843-856.
    15. McMorland, Jade & Flannigan, Callum & Carroll, James & Collu, Maurizio & McMillan, David & Leithead, William & Coraddu, Andrea, 2022. "A review of operations and maintenance modelling with considerations for novel wind turbine concepts," Renewable and Sustainable Energy Reviews, Elsevier, vol. 165(C).
    16. Izquierdo, J. & Márquez, A. Crespo & Uribetxebarria, J. & Erguido, A., 2020. "On the importance of assessing the operational context impact on maintenance management for life cycle cost of wind energy projects," Renewable Energy, Elsevier, vol. 153(C), pages 1100-1110.
    17. Niklas Andersen & Ola Eriksson & Karl Hillman & Marita Wallhagen, 2016. "Wind Turbines’ End-of-Life: Quantification and Characterisation of Future Waste Materials on a National Level," Energies, MDPI, vol. 9(12), pages 1-24, November.
    18. Lombardi, Lidia & Mendecka, Barbara & Carnevale, Ennio & Stanek, Wojciech, 2018. "Environmental impacts of electricity production of micro wind turbines with vertical axis," Renewable Energy, Elsevier, vol. 128(PB), pages 553-564.
    19. Juan Izquierdo & Adolfo Crespo Márquez & Jone Uribetxebarria & Asier Erguido, 2019. "Framework for Managing Maintenance of Wind Farms Based on a Clustering Approach and Dynamic Opportunistic Maintenance," Energies, MDPI, vol. 12(11), pages 1-17, May.
    20. Anne P. M. Velenturf, 2021. "A Framework and Baseline for the Integration of a Sustainable Circular Economy in Offshore Wind," Energies, MDPI, vol. 14(17), pages 1-41, September.

    Corrections

    All material on this site has been provided by the respective publishers and authors. You can help correct errors and omissions. When requesting a correction, please mention this item's handle: RePEc:eee:rensus:v:144:y:2021:i:c:s1364032121001805. See general information about how to correct material in RePEc.

    If you have authored this item and are not yet registered with RePEc, we encourage you to do it here. This allows to link your profile to this item. It also allows you to accept potential citations to this item that we are uncertain about.

    If CitEc recognized a bibliographic reference but did not link an item in RePEc to it, you can help with this form .

    If you know of missing items citing this one, you can help us creating those links by adding the relevant references in the same way as above, for each refering item. If you are a registered author of this item, you may also want to check the "citations" tab in your RePEc Author Service profile, as there may be some citations waiting for confirmation.

    For technical questions regarding this item, or to correct its authors, title, abstract, bibliographic or download information, contact: Catherine Liu (email available below). General contact details of provider: http://www.elsevier.com/wps/find/journaldescription.cws_home/600126/description#description .

    Please note that corrections may take a couple of weeks to filter through the various RePEc services.

    IDEAS is a RePEc service. RePEc uses bibliographic data supplied by the respective publishers.