Water Depth Variation Influence on the Mooring Line Design for FOWT within Shallow Water Region
Round 1
Reviewer 1 Report
The paper focuses on the optimization of mooring systems for floating offshore wind turbines located in Taiwan Strait. The mooring system consists of 3 mooring lines made of chain connected to three offsets columns connected to the main column which is the foundation of the wind turbine. The optimization is made considering five water depths and three nominal diameters. The mooring design procedure follows the standards of DNV GL and API, and is based on the “safety class” approach formulated in terms of three different load case for metocean conditions: Ultimate Limit State, Fatigue Limit State and Maximum Operating Sea State. An analysis of costs is performed and the optimal mooring system is identified with length and nominal diameter.
Comments:
Some figures like Fig 1, Fig 4 , Fig 5, have very low resolution and are not clear.
Not all figures, equations and captions are centred, for example Figure 1, caption of figure 6, 7, 8, Eq 1, 6, 7-12.
Some numbers are not aligned with the corresponding equation.
In Figure 5 the water depth seems indicated with z while in the text it is defined with h.
The text and the equations should be aligned in sections 1.3.1 and 1.3.2, it is not always clear when reading equation or variable definitions, maybe a nomenclature could help to order things.
In Table 2 the red square seems in the wrong position and a description of why it is set there could be given.
In Figure 6 some latitudes and parallels lines and coordinates could be represented to better define the area.
Pag 7 line 235 there is a double “the”.
Paragraph on page 7 from line 235 to 238 is not clearly written.
Table 4 of the wave scatter diagram has an inconsistency between the single row and columns sum value, expressed to achieve a total equal to 100, and the total sum which is defined by 1.
In section 2.2 point 3) and Table 6 would deserve a description.
Figure 8 could be bigger and with better contrast between orange background and white text.
Table 8 has a “\” at the end that needs to be erased.
On page Lines 311-312 state that the fatigue damage prediction suggests much more structure margin than the ultimate tension prediction, but it does not seem the case for mooring line 1.
Figures 9 to 12 are very small and not all data can be clearly read.
In Figure 10 the fatigue damage on the y axis is what has been defined by Dc in equation (9), maybe could be reported with symbol Dc.
In figure 12 maybe the same range for x and y axis would be better to sea difference between offsets for the different water depths. In the caption of the figure after wind the “w” is missing from the word wave
Author Response
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Reviewer 1 |
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Comments |
Response |
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1. |
Some figures like Fig 1, Fig 4, Fig 5, have very low resolution and are not clear. |
The image resolution has been adjusted as much as possible. |
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2. |
Not all figures, equations and captions are centred, for example Figure 1, caption of figure 6, 7, 8, Eq 1, 6, 7-12. |
The mentioned figures, equations and captions have been centred. |
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3. |
Some numbers are not aligned with the corresponding equation |
The positions of equation numbers have been corrected. |
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4. |
In Figure 5 the water depth seems indicated with z while in the text it is defined with h. |
The water depth z in figure 5 has been revised to "h". |
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5. |
The text and the equations should be aligned in sections 1.3.1 and 1.3.2, it is not always clear when reading equation or variable definitions, maybe a nomenclature could help to order things. |
The text and the equations in sections 1.3.1 and 1.3.2 have been aligned. The definition and description of each parameter have been added below respective formula. |
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6. |
In Table 2 the red square seems in the wrong position and a description of why it is set there could be given. |
The description of the Table 2 has been added. |
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7. |
In Figure 6 some latitudes and parallels lines and coordinates could be represented to better define the area. |
Follow instructions. |
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8. |
Page 7 line 235 there is a double “the”. |
One of them has been deleted. |
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9. |
Paragraph on page 7 from line 235 to 238 is not clearly written |
The mentioned paragraph has been adjusted. |
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10. |
Table 4 of the wave scatter diagram has an inconsistency between the single row and columns sum value, expressed to achieve a total equal to 100, and the total sum which is defined by 1. |
The total sum in Table 4 has been corrected to 100 (percent). |
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11. |
In section 2.2 point 3) and Table 6 would deserve a description. |
The mentioned paragraph has been added. |
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12. |
Figure 8 could be bigger and with better contrast between orange background and white text. |
Follow instructions. |
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13. |
Table 8 has a “\” at the end that needs to be erased. |
“\” has been erased. |
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14. |
On page Lines 311-312 state that the fatigue damage prediction suggests much more structure margin than the ultimate tension prediction, but it does not seem the case for mooring line 1. |
The sentence mentioned has been adjusted. |
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15. |
Figures 9 to 12 are very small and not all data can be clearly read. |
The size of figure 9 to 12 has been enlarged. |
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16. |
In Figure 10 the fatigue damage on the y axis is what has been defined by Dc in equation (9), maybe could be reported with symbol Dc. |
Follow instructions. |
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17. |
In figure 12 maybe the same range for x and y axis would be better to sea difference between offsets for the different water depths. In the caption of the figure after wind the “w” is missing from the word wave |
Figure 12 has been drawn on the same scale The word wave has been corrected. |
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper is very interesting and gives the influence of the mooring line design on the depth of installation. The authors proposed and calculated the Performance index (PI) that includes ultimate and fatigue criteria for the final choice of mooring lines. It is recommended the usage the PI with some other criteria, e.g. price of the installation and less weight. Some clarifications are needed and some recommendations for improvements are given below.
Line 17-23 Too long sentence, consider revising it. DNV GL is one company, the entity GL was incorporated in the new name i.e. DNV GL 2013. After 1.3.2021 only the name DNV exists.
Line 49 Abbreviation missing, FOWT
Figure 1. blur, not visible
Word “thesis” should be changed to word “paper”.
Line 81, consider revising like, The considered FOWT model in this pape is based on…
Figure 4, To small, not visible
Line 123-127 Too long sentence, consider revising it, maybe with bullets.
Some formula numbers are not next to formulas.
Line140, consider using the word safety instead of security
Line 153 and 154 change TMPM to Tmpm
Line 257, it seems that Table 5 should be mentioned instead Table 6, and Table 6 should be given reference in the text.
Line 266, is some text missing after 3)?
Line 270 and Figure 8. It is not quite clear the optimization process. It seems that the mentioned optimization consists of only changing chain length until ultimate tension does not exceed the MBL. Some additional sentences will clarify this.
Line 281 „Morring line design“, in Figure 8 there is an expression „Mooring design“
Line 294 it is difficult to state exponential increase according to the two last points. It follows for 5 points more line polynomial of the 2nd order.s
Figures 9, 10, and 11 too small, not clear and visible
Figure 11, it is recommended to draw on the same scale.
Figure 12, To small font, not visible
Line 327, consider word “better” instead of word “good”
Line 365, missing letter W.
Final mooring system design Table 8 and consequently results in Table 9 and Figure 13 should be explained additionally. Namely, the choice of chain diameter according to the performance index value (PI).
For the mooring line 1 water depth 50 m the D95 is chosen and the PI is between 0.05 and 0.1 while for the Mooring line 2 PI for the D95 is between 0.02 and 0.03 that is lower than for the Mooring line 1. The PI for D135 is indeed lower but what is the sense to use a heavier chain. For Mooring line 3 the PI is in the same interval as it is for Mooring Line 1 so, again the D95 could be used instead D135. Again, PI for D135 is lower but when it is compared with PI for deeper depths mooring lines that have much higher PI values the engineering reasoning should be lower chain dimension. Another issue is that the choice of the mooring system design is based on one combined parameter. The conclusion using the installation cost on deeper depths would be interesting and probably different.
Line 403, in the paper nothing has been said deeply about different line materials and the cons for using the chain lines. Some pros (line 112) are mentioned but the comparison between materials is missing. So, it is suggested to soften this statement or just deleted it.
Author Response
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Reviewer 2 |
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1. |
Line 17-23 Too long sentence, consider revising it. DNV GL is one company, the entity GL was incorporated in the new name i.e. DNV GL 2013. After 1.3.2021 only the name DNV exists. |
The mentioned paragraph has been adjusted. The company name, DNV has been revised. |
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2. |
Line 49 Abbreviation missing, FOWT |
The abbreviation of floating offshore wind turbine (FOWT) has been defined in Line 9. |
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3. |
Figure 1. blur, not visible |
The image resolution has been adjusted as much as possible. |
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4. |
Word “thesis” should be changed to word “paper”. |
Follow instructions. |
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5. |
Line 81, consider revising like, The considered FOWT model in this pape is based on… |
The sentence has been revised. |
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6. |
Figure 4, To small, not visible Line 123-127 Too long sentence, consider revising it, maybe with bullets. Some formula numbers are not next to formulas. |
The size of Figure 4 has been enlarged The sentence mentioned has been adjusted. The positions of equation numbers have been corrected. |
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7. |
Line140, consider using the word safety instead of security |
Follow instructions. |
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8. |
Line 153 and 154 change TMPM to Tmpm |
The sentence mentioning Tmpm has been deleted |
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9. |
Line 257, it seems that Table 5 should be mentioned instead Table 6, and Table 6 should be given reference in the text. |
Line 257 has been corrected. The description of Table 6 has been added. |
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10. |
Line 266, is some text missing after 3)? |
The mentioned paragraph has been added. |
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11. |
Line 270 and Figure 8. It is not quite clear the optimization process. It seems that the mentioned optimization consists of only changing chain length until ultimate tension does not exceed the MBL. Some additional sentences will clarify this. |
The sentence mentioned has been added. |
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12. |
Line 281 „Morring line design“, in Figure 8 there is an expression „Mooring design“ |
The sentence mentioned has been revised to „Mooring design“. |
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13. |
Line 294 it is difficult to state exponential increase according to the two last points. It follows for 5 points more line polynomial of the 2nd order. |
The word exponentially has been deleted and revised to sharply. |
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14. |
Figures 9, 10, and 11 too small, not clear and visible |
The size of figure 9 to 12 has been enlarged. |
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15. |
Figure 11, it is recommended to draw on the same scale. |
Figure 11 has ben adjusted to same range for x and y axis. |
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16. |
Figure 12, To small font, not visible |
The font of figure 12 has been enlarged. |
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17. |
Line 327, consider word “better” instead of word “good” |
Follow instructions. |
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18. |
Line 365, missing letter W. |
The word wave has been corrected. |
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19. |
Final mooring system design Table 8 and consequently results in Table 9 and Figure 13 should be explained additionally. Namely, the choice of chain diameter according to the performance index value (PI). |
The detail descriptions of Table 8, Table 9 and Figure 13 have been added. |
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20. |
For the mooring line 1 water depth 50 m the D95 is chosen and the PI is between 0.05 and 0.1 while for the Mooring line 2 PI for the D95 is between 0.02 and 0.03 that is lower than for the Mooring line 1. The PI for D135 is indeed lower but what is the sense to use a heavier chain. For Mooring line 3 the PI is in the same interval as it is for Mooring Line 1 so, again the D95 could be used instead D135. Again, PI for D135 is lower but when it is compared with PI for deeper depths mooring lines that have much higher PI values the engineering reasoning should be lower chain dimension. Another issue is that the choice of the mooring system design is based on one combined parameter. The conclusion using the installation cost on deeper depths would be interesting and probably different. |
In this study, final selection of mooring system is depend on the performance index, rather than the cost index (chain weight). In addition, for the five studied water cases, each mooring line is independently designed. Therefore, the best option of the mooring design for the case of 50m water depth is D95 in mooring line 1, and D135 in mooring line2 and 3. The cost analysis of the study is only focused on the hardware costs of the mooring lines, not including the installation costs. |
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21. |
Line 403, in the paper nothing has been said deeply about different line materials and the cons for using the chain lines. Some pros (line 112) are mentioned but the comparison between materials is missing. So, it is suggested to soften this statement or just deleted it. |
The sentence mentioned has been deleted. |
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Author Response File: Author Response.pdf
