Unified System Analysis for Time-Variant Reliability of a Floating Offshore Substation
, Mestapha Oumouni 1,†, Morteza Ahmadivala 2,†, Neil Luxcey 2, Florian Dupriez-Robin 2 and Patrick Guerin 1
Round 1
Reviewer 1 Report
The paper deals with the reliability of a floating Offshore Sub-Station, which is a component of a wind farm that has not been much studied. Therefore, a description if the physical aspects of such platform is required for the readers to understand what is at stake. Chapter 3 mentions some equipment but this is not a sufficient description.
The authors look at the reliability of the mooring system. This is a topic very much studied on offshore engineering and several papers are published in the reliability assessment of such systems. It would be appropriate that the authors refer to some of those approaches and indicate if they are following established procedures or whether they are proposing a completely new approach, in which case more detailed justification needs to be supplied.
The authors mention that there is not enough data on failure rates of equipment to be used in the FOSS and even on the mooring for these platforms. Maybe they can consider how to use and adapt the failure data from other sectors as proposed recently in https://doi.org/10.3390/jmse10121965
In the assessment of the moorings it is not clear if the authors are considering catenary chain moorings of the types of moorings more appropriate to this type of platforms in the range of water depth being considered? In this case elastic moorings in a tauter configuration is used as described in several relatively recent papers published for example in Ocean Engineering or Applied Ocean Research. Different formulations are probably required.
Maybe a bit more background should be given about the Deep Lines software.
The reference list is very poor. Few references with several from web pages, others from conferences and very few from Established journals. This should be improved as this is not a paper for a small conference publication.
On more detailed aspects:
Refs 1, 2, 4, 6 need a URL
Refs 7, 8 is missing the authors and the doi needs correction
Figures 3 and 4 do not have the appropriate resolution
Author Response
The authors would like to thank the reviewer for the very constructive comments.
1. The paper deals with the reliability of a floating Offshore Sub-Station, which is a component of a wind farm that has not been much studied. Therefore, a description if the physical aspects of such platform is required for the readers to understand what is at stake. Chapter 3 mentions some equipment but this is not a sufficient description.
The reviewer is right, we should define more precisely what is an OSS. This description has been added after the 1st section of the introduction: The usual way to connect commercial fixed farms is to use regular topside design inspired by Oil & Gas offshore platforms and called (Static) Offshore Sub-Stations. Floating offshore sub-stations are key components due to their central role and their novelty – e.g. electrical material subjected to permanent inertial loads – bringing high expectations on the object. The specificity of floating or subsea substation, as compared to a fixed substation, is therefore in the use of components deployed between the seabed and the floating wind or tidal and the floating substation. These mechanical or electrical elements are solicited by the movements of the floating support, the swell, the marine current and are moreover sensitive to bio-fouling.
These equipment, used in onshore wind farm or in offshore Oil & Gas must be redesigned for an optimal fit to actual marine renewable energies environment. They must be designed specifically to withstand stresses of harsh environment during a period of 25 years. Internal components of the substation are therefore likely to be different from the ones used in conventional static substation.
2. The authors look at the reliability of the mooring system. This is a topic very much studied on offshore engineering and several papers are published in the reliability assessment of such systems. It would be appropriate that the authors refer to some of those approaches and indicate if they are following established procedures or whether they are proposing a completely new approach, in which case more detailed justification needs to be supplied.
In the paper, we propose a simple way for this reliability based on forensic engineering for consistency with other components of the system. That is less sophisticated than other approaches available in the literature. This approach allows to get a global failure rate without investigating the failure modes and the combination of theses failures modes. The authors mention that there is not enough data on failure rates of equipment to be used in the FOSS and even on the mooring for these platforms. Maybe they can consider how to use and adapt the failure data from other sectors as proposed recently in https://doi.org/10.3390/jmse10121965. That is why a sentence and references have been added in section 3.2.
This approach is consistent with the estimation of failure rates of other components of the FOSS. It accounts for all failure modes and avoid discussing on the competition between failure modes and the occurence of the demand: corrosion, fatigue, corrosion under stress, extreme events. Such a approaches available in [12], [14] and [15] for OWT but the failure rates cannot be extrapolated to FOWT since the movement of a FOWT is much less as well as the effect of wind on the loading. Other studies have been carried out for Floating Production Storage and Offloading vessels (FPSOs) [16]; they cannot be used as well, because their hydrodynamic behavior differs significantly due to the difference of size (FPOSO is much larger) and due to the difference of mooring system (close to a sigle point mooring for a FPSO).
[14] Guanhua Zhao, Yuliang Zhao, Sheng Dong, System reliability analysis of mooring system for floating offshore wind turbine based on environmental contour approach, Ocean Engineering, Volume 285, Part 2, 2023, 115157,
ISSN 0029-8018, https://doi.org/10.1016/j.oceaneng.2023.115157.
[15] Li H, Peng W, Huang C-G, Guedes Soares C. Failure Rate Assessment for Onshore and Floating Offshore Wind Turbines. Journal of Marine Science and Engineering. 2022; 10(12):1965. https://doi.org/10.3390/jmse10121965
[16] Yuanhui Wang, Chuntai Zou, Fuguang Ding, Xianghui Dou, Yanqin Ma, Yanwen Liu, "Structural Reliability Based Dynamic Positioning of Turret-Moored FPSOs in Extreme Seas", Mathematical Problems in Engineering, vol. 2014, Article ID 302481, 6 pages, 2014. https://doi.org/10.1155/2014/302481
3. In the assessment of the moorings it is not clear if the authors are considering catenary chain moorings of the types of moorings more appropriate to this type of platforms in the range of water depth being considered? In this case elastic moorings in a tauter configuration is used as described in several relatively recent papers published for example in Ocean Engineering or Applied Ocean Research. Different formulations are probably required.
The reviewer is right. What is interesting with the suggested approach we use is that we consider a forensic engineering of the failures of mooring lines without discussion the detailed design as it is done for the electrotechnical part. That is consistent and help to see which component further studies should focussed on.
4. Maybe a bit more background should be given about the Deep Lines software.
The reviewer is right. Thanks for this comment. reference has been added:
[18] "IFPEN and Principia release DeepLines Wind FEA software", Renewable Energy Magazine, Thursday, 12 November 2015. https://www.renewableenergymagazine.com/wind/ifpen-and-principia-release-deeplines-wind-fea-20151112
5. The reference list is very poor. Few references with several from web pages, others from conferences and very few from Established journals. This should be improved as this is not a paper for a small conference publication.
The reviewers agree but this paper being a prospective study, it was difficult to get references. We added 4 references and check the format of the existing references (Refs 1, 2, 4, 6, 7, 8).
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Very interesting paper, quite good.
Recomendations:
1. There are 2 cases in table 3. Are they the same cases shown on for example fig. 6? If so I would name them in the same manner.
2. Quality of some figures can be improved.
3. Number of cited references can be higher.
Author Response
The authors thank the reviewer for the positive comments. Here are our answers:
1. There are 2 cases in table 3. Are they the same cases shown on for example fig. 6? If so I would name them in the same manner.
The reviewer is right. It has been changed.
2. Quality of some figures can be improved.
Figures 3 and 4 were improved.
3. Number of cited references can be higher.
This is a prospective study about a new concept and it was hard to find related papers but the reviewer is rights there are some papers that highlights the role of OFF in a floating wind farm. 12 references have been added.
Author Response File: Author Response.pdf
Reviewer 3 Report
In this nicely written manuscript, the authors present a time variant reliability analysis of a floating offshore substation. The proposed work fits within the aim of the journal and presents an interesting topic. However, there are some errors that should be addressed before its publication. Hence, I recommend the publication of the manuscript once the following points have been addressed:
1. Acronyms should be avoided in the abstract.
2. The introduction section should be further improved:
· An overarching review of offshore wind substations previous research is missing. There are plenty of references covering this topic that have not been considered.
· The novelty of the proposed reliability system analysis should be better justified.
3. Manuscript should be structured as. (i) introduction; (ii) materials and methods; (iii) results; (iv) discussion, and (v) conclusions.
4. Table 1 data sources are not credibly justified.
5. The installation site is not well characterised. Further details defining the wind and wave resources should be included.
6. Figures 6 to 11. Symbols should be included when plotting two graphs together, as at the moment, when seeing it in black and white colour it is not possible to differentiate them.
The quality of the English and the fluency of the text is excellent, and I do not have further comments.
Author Response
In this nicely written manuscript, the authors present a time variant reliability analysis of a floating offshore substation. The proposed work fits within the aim of the journal and presents an interesting topic. However, there are some errors that should be addressed before its publication. Hence, I recommend the publication of the manuscript once the following points have been addressed:
The authors thank the reviewer for this positive review.
- Acronyms should be avoided in the abstract.
Thank you. It has been changed.
2. The introduction section should be further improved:
- An overarching review of offshore wind substations previous research is missing. There are plenty of references covering this topic that have not been considered.
This is a prospective study about a new concept and it was hard to find related papers but the reviewer is rights there are some papers that highlights the role of OFF in a floating wind farm. 12 references have been added.
- The novelty of the proposed reliability system analysis should be better justified.
Introduction has been completed.
he usual way to connect commercial fixed farms is to use regular topside design inspired by Oil & Gas offshore pla=orms and called (Sta@c) Offshore Sub- Sta@ons. Floa@ng offshore sub-sta@ons are key components due to their central role and their novelty – e.g. electrical material subjected to permanent iner@al loads – bringing high expecta@ons on the object. The specificity of floa@ng or subsea substa@on, as compared to a fixed substa@on, is therefore in the use of components deployed between the seabed and the floa@ng wind or @dal and the floa@ng substa@on. These mechanical or electrical elements are solicited by the movements of the floa@ng support, the swell, the marine current and are moreover sensi@ve to bio-fouling.
These equipment, used in onshore wind farm or in offshore Oil & Gas must be redesigned for an op@mal fit to actual marine renewable energies environment. They must be designed specifically to withstand stresses of harsh environment during a period of 25 years. Internal components of the substation are therefore likely to be different from the ones used in conventional static substation.
3. Manuscript should be structured as. (i) introduction; (ii) materials and methods; (iii) results; (iv) discussion, and (v) conclusions.
It has been changed. For this reason. First section of section 3 was moved as section 2.5.
4. Table 1 data sources are not credibly justified.
They come from expert judgement and there is no source for this new type of concept on a floating structure. But the way for providing this data was carefully processed with an interview of experts in the top management of Siemens and RTE.
5. The installation site is not well characterized. Further details defining the wind and wave resources should be included.
The reviewer is right. A table has been added: Table 2 now.
6. Figures 6 to 11. Symbols should be included when plotting two graphs together, as at the moment, when seeing it in black and white colour it is not possible to differentiate them.
In fact there is no difference between curves but we added a symbol.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
I would like to congratulate the authors as this new version has improved substantially. Therefore, I am happy to accept for publication the manuscript.
