KNSwing—On the Mooring Loads of a Ship-Like Wave Energy Converter

Round  1

Reviewer 1 Report

The paper is very well written and presented. The experimental and numerical procedures seem sound and the results are convincing. I recommend the following minor corrections and clarifications:

1) Excitation and Horizontal are mis-spelled in figure 4

2) Would full scale mooring lines likely have similar properties to the model scale rubber moorings? Might they be more non-linear due to different materials or construction? The moorings are quite horizontal because of the relatively shallow water so could they behave like a  catenary morring? These points could be discussed briefly.

3) Equation 1 - the added mass in surge is not included - is there a reason for this? Is the added mass insignificant compared to the mass of the vessel?

4) Equation 4 assumes a linear stiffness value, but the mooring stiffness is dependent on the angle of the lines. For large motions the stiffness could change significantly - this should be discussed.

5) Referring to figure 15. After adjustment of the numerical damping the match to experimental results is much improved. However the period seems (very slightly) different. This is related to point 4). I assume the orcaflex simulations were time-domain and could accommodate the change in stiffness of the mooring system as the position of the vessel changes? If frequency domain simulations were conducted then this will not be the case and we have the same assumption of linearity. This should be clarified and discussed.

Author Response

Dear Reviewer 1 and 2

Thank you for your valuable comments - Please find enclosed our response to both of you 

Author Response File: Author Response.docx

Reviewer 2 Report

Review Comments

This paper reports on a series of tests performed on a specific type of WEC, to investigate the mooring systems in extreme seas, which is crucial for device survivability. The test results are also used to validate a numerical model developed in a previous publication, which is seen to perform well and can be used with confidence in future studies. The paper is based on solid research and would be of interest to readers  in the Marine Science and Engineering field.

However, a number of comments are detailed below which should be addressed before the paper can be considered for publication.

General

In some parts you use Fig. x and in other parts you use Figure x: Choose one and be consistent throughout the document.

Please use a decimal point rather than a comma to indicate decimals.

Abstract:

Line 9: First sentence: “stationary” is probably the wrong word here. Most floating WECs need to oscillate in order ot absorb power. Maybe you need ‘on station’/’in position’.

Line 20:  English: The second ‘even’ in this sentence is probably not needed.

1. Introduction:

Line 27-28: First sentence: Does this mean that all renewable energy sources have the potential to contribute 10-25% or that ocean wave energy has the potential to contribute 10-25%? It is a bit ambiguous the way it is written.

Line 31: You use acronym ‘OWC’ but dont define until line 39

2. Preliminary design of the KNSWING

Line 61: You have already defined ‘OWC’

3. The Experimental Model KNSWING

Line 80 and 82: ‘wave energy converter’ = ‘WEC’

4. The Numerical Model of the Structure and Mooring Solution

Line 111: ‘build’ = ‘built’

Line 120: It doesn’t make sense to include ‘hydrodynamic coefficients’ in the brackets after ‘hydrodynamic coefficients’. Perhaps write ‘hydrodynamic radiation and exctiation’ instead

Line 125: Which coefficients?

Please provide a brief description of the mooring model used within Orcaflex

5. The Wave Conditions in The North Sea

Line 152: Caption – Figure 5 : What is ‘Gorm’?

6. The Experimental Set-up in the basin

Is there a difference between ‘wave gauges’ and ‘wave probes’? If not, choose one name and be consistent

Line 164-165 : Caption-Figure 6: ‘under QUEENS’ ? Please re-write this as it is not clear what is meant by this. Also you have previously referred to the university as QUB, which is probably less ambiguous than QUEENS.

Line 169: ‘measures’ = ‘measured’

Line 178: ‘pretention’ = ‘pretension’

Line 179: ‘uncertainty with the experiments’ = ‘uncertainty of the experiments’

Line 184 and 213: Please reference the table number, rather than state ‘in the table below’

Figure 7: What is the dashed blue horizontal line in the top diagram?

Line 196: Caption-Figure 7: What is meant by ‘previously tested’?

Line 223: ‘line’=’lines’

Figure 9: Please provide more detail in this diamgram. What does SG mean?

Line 233: ‘QEB’ = ‘QUB’?

Line 236-237: Please rewrite this sentence, it is unclear what you mean.

Table 3: Use decimal points rather than commas

7. Results Comparing Experiments to Numerical Simulations

Figure 10: What is the difference between the dashed and solid blue horizontal lines? (see comment 6.7)

Lines 241-242: Please rewrite this sentence. (1)  ‘because of’ = ‘due to’ (2) Is it due to a static ‘external load’  on the WEC rather than due to the ‘moorinig load’? Based on the figure, it would be appear the mooring load would be in the opposite direction to the indicated load arrow.

Figure 11 : Please provide a comment/description of  the results in this graph.

Lines 253-254 : Please re-write “The results illustrate an agreement between the experimental and mooring system the numerical”.

Equation (1) – Please define the parameters in this equation.

Line 269: Please use decimal points rather than commas

Line 270: Please reference the figure number rather than ‘figure below’

Line 272: Please comment/discuss the discrepancies in value for the natural period found by the two different methods

Table 5: Please use decimal points rather than commas

Line 290: ‘Brecht Schneider’ = ‘Bretschneider’

Line 297: Capitalise ‘experimental’ in the heading

Line 298: ‘measures’ = measured’

Line 300: ‘tree’ = three??

Section 7.5 – This seems to be direct repeat of Section 6.1

Line 316: Please reference the figure number rather than ‘figure below’

Line 319: ‘to expect’ = ‘to be expected’

Figure 17: Please comment/discuss the spread of values for each wave period/height. Are these the repeat tests? If so, why are the compliant line tests more repeatable than the stiff line tests?

Lines 339-340: Why are only the results for the compliant line chosen?

Conclusion

Please provide a conclusion to the paper

Acknowledgement:

Line 381 : ‘Portafery’ = ‘Portaferry’

References:

Reference [17] has extra information in it. Please remove:

Author 1, A.; Author 2, B. Title of the chapter. In Book Title, 2nd ed.; Editor 1, A., Editor 2, B., Eds.; Publisher: Publisher Location, Country, 2007; Volume 3, pp. 154–196.

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Author Response

Dear Reviewer 2

Thank you for your valuable comments - to help improve the paper - our response to you and reviewer 1 is enclosed.

Author Response File: Author Response.docx