Mechanism of Phase-Locked Ice Crushing against Offshore Structures
Round 1
Reviewer 1 Report
Regarding to the paper entitled “Mechanism of phase locked ice crushing against offshore structures” that has been submitted to Journal of Marine Science and Engineering, here in follow some explanations in this matter are presented.
• Article innovation should be more explained clearly in comparison with other research works in this field.
• Explain more about application, importance and results of this research in the engineering.
• Explain more about details of Figures 2 and 4.
• Explain more about the load panels used on the lighthouse in Figure 6.
• Explain more interpretation and conclusion of results.
• Provide more references from recent years in relation to this topic.
• The English language of this manuscript should be improved.
Author Response
Response to Reviewer 1 Comments
Point 1: Article innovation should be more explained clearly in comparison with other research works in this field.
Response 1: Several theoretical models have been developed to analyze the dynamic ice action effect in PLC. These models are based on different explanations of the ice failure process. Some models assume that the ice failure is inherently periodic and that the lock-in between the frequency of ice failure and the natural frequency of structure causes the frequency lock-in between the ice load and structure response . Other models consider that the intermittent crushing is due to the descending de-pendence of ice strength on the indentation rate in a certain interval of indentation rates. The ice failure transitions from ductile to brittle crushing mode and causes fluctuations of ice forces and self-excited vibration. For wide structures, Gagnon presents a system reso-nant capped spalling and erratic spalling model and illustrates the process as resonance of the platform and ice sheet system.
ISO 19906 adopted a simplified method that uses a saw teeth load function to analyze the response. This method neglects the fact that ice load is coupled with structure response. Moreover, it is difficult to define the ice load for wide structures because the ice load frequency does not lock-in with the natural frequency of structure.
This paper adopts ductile damage-collapse (DDC) as the ice failure mechanism for PLC based on a summary of field measurement results The DDC process is considered as an inherent failure mechanism of ice under a low loading rate. The failure is a discrete process coupled with structure displacement and in each failure cycle the ice sheet fails in a limited depth. The ice failure length is the key parameter in this process and it is used as one of the occurrence conditions of PLC along with natural period of structure.
Point 2: Explain more about application, importance and results of this research in the engineering
Response 2: This paper defines the ice failure that occurs during the PLC process as DDC failure mode by studying the field measurement data of frequency lock-in vibration of the jacket and light house structures and the phase locked loading on the caisson structure. The DDC failure mechanism provide a different explanation for the PLC process. Key feature of the DDC mechanism is the failure is a discrete process and each time the ice break with a limited length. The failure length is an important parameter for under-standing and analyzing the PLC process., the ice failure length has not been intensively investigated in the past because the failure process has not been clearly recognized. The field measurement indicates that the failure length is less than 5 cm and does not de-pend on the ice thickness. Supposing the failure length is an inherent feature of PLC process, the PLC will occur when ice speed close to the ratio between structure natural period and ice failure length. That will give a much narrow ice condition window for PLC occurrence for wind turbine structures compare with results obtained from other mechanisms.
Point 3:Explain more about details of Figures 2 and 4
Response 3: For figure2: Thirty-one MEDOF panels were installed on the north, northeast and east faces of the caisson to measure ice load. The load panel had a width of 1.135 m and a height of 2.715 m. The panels were arranged in seven clusters of four or five panels each. The center-to-center spacing between different clusters was 19.5 m or 17 m. The Molikpaq encountered severe ice conditions in the winter of 1985/1986 and four events were identified as PLC based on the dynamic response and the phase-locked signatures of the platform-ice interaction.
For figure4: PLC ice action has also been observed on most jacket offshore oil & gas infra-structures in Bohai Bay China. It has occurred on jacket platforms with three or four legs or mono leg mooring piles. It has caused strong lock-in vibration with a magnitude of up to 2 g/s2 and a duration of more than 20 minutes costly. Ice breaking cones were added on most of the platforms after the frequency lock-in vibration damaged the gas pipe on the topside. The ice load during PLC process was measured by load panels that covered about 170 degrees of the jacket leg surface with a diameter of 1.5 to 2 meters. The di-mensions of each load panel were 62 cm in height and 27 cm in width. Along with the load data, structure response was measured by accelerometers. The ice failure process against the structure was recorded by video cameras and in-situ observations. Figure 3 and Figure 4 show the structures and measurement system on the mooring pile of JZ9-3 oil field. The analysis of the PLC events from Bohai Sea indicates that the slender steel structures are prone to PLC failure and it occurs in conditions where the ice speed is less than 0.04 m/s.
Point4:Explain more about the load panels used on the lighthouse in Figure 6.
The Norstromsgrund lighthouse in the Baltic Sea, shown in Figure 5, was equipped with ice load panels, accelerometers and video cameras to measure the ice action. Many PLC events were measured in the 1980s and 2000s. The lighthouse had a diameter of about 7.5 m at the water line and a total height of 42.3 m. Nine sets of load panels, each with an area of 1.2 m by 1.6 m, covered about 162 degrees of the structure surface at the waterline. There is a more detailed description of load panels in the cited paper
Point5:Provide more references from recent years in relation to this topic
Already provided
Point6:Provide more references from recent years in relation to this topic
Modifications have been made
Reviewer 2 Report
- The abstract should state briefly the purpose of the research, the principal results and major conclusions. An abstract is often presented separately from the article, so it must be able to stand alone.
- The quality of figures 1 to 5 is not acceptable. I would ask you to use the original source of these figures.
- All symbols and parameters should be defined, please check.
- Page 7: the following paragraph is unclear, so please reorganize that:
“The field data shows that response of Molikpaq to dynamic ice load is different to lock-in vibration of lighthouse and jacket structures. The difference is that Molikpaq platform is a caisson on the sand core which creates large damping. The system is close to an over damped or critically damped system which returns to equilibrium position after unloading without oscillating. The inertia force of the platform caused small or even no negative displacement against the ice movement direction after unloading.”
- The major defect of this study is the debate or Argument is not clear stated in the introduction session. Hence, the contribution is weak in this manuscript. I would suggest the author to enhance your theoretical discussion and arrives your debate or argument.
- Much more explanations and interpretations must be added for the Results, which are not enough.
- It is suggested to add articles entitled “Basack et al. Flow Characteristics through Granular Soil Influenced by Saline Water Intrusion: A Laboratory Investigation” and “G. Russo et al. Hybrid Energy Piles as a Smart and Sustainable Foundation” to the literature review.
- More suitable title should be selected for the table 1 instead of “Typical PLC conditions measurement on offshore structures.”.
- Methods section determines the results. Kindly focus on three basic elements of the methods section.
a. How the study was designed?
b. How the study was carried out?
c. How the data were analyzed?
- Please make sure your conclusions' section underscore the scientific value added of your paper, and/or the applicability of your findings/results, as indicated previously. Please revise your conclusion part into more details. Basically, you should enhance your contributions, limitations, underscore the scientific value added of your paper, and/or the applicability of your findings/results and future study in this session.
Author Response
Response to Reviewer 2 Comments
Point 1: The abstract should state briefly the purpose of the research, the principal results and major conclusions. An abstract is often presented separately from the article, so it must be able to stand alone.
Response 1: his paper addresses a detailed analysis of the ice-structure interaction process of the phase locked ice crushing (PLC) against offshore structures. Directly measured ice load, structure response data and in situ observation from the field measurements on Molikpaq, lighthouse and jacket platform were used in the study. The paper summarized a new ductile damage–collapse (DDC) failure mechanism for the PLC process. The DDC mechanism shows that the ice failure is a discrete ductile crushing process rather than a ductile – brittle transition process. The analysis identifies that the ice has a failure length in PLC and this failure length plays an important role for understanding the inter-action. It reveals that PLC can occur on most vertical sided offshore structures when the velocity of the ice sheet falls within the range of failure length divided by natural period of structure. The paper proposes that this relationship between ice failure length and natural period of structure can be used as one of the PLC occurrence conditions. The DDC failure mechanism provide basis for another technical route to solve the ice PLC problem.
Point 2: The quality of figures 1 to 5 is not acceptable. I would ask you to use the original source of these figures.
Response 2: The image has been replaced in the revised article
Point 3 : All symbols and parameters should be defined, please check
Response 3: All symbols and parameters have been defined in the revised article
Point4 : Page 7: the following paragraph is unclear, so please reorganize tha.
Two phenomena observed in the field measurements can be highlighted as fea-tures of the PLC process. One is the stop and run movement of ice during the PLC pro-cess, and the other is the simultaneous saw tooth ice forces measured by load panels. The stop and run phenomenon means that the ice movement before the structure looks like it suddenly stops and runs. The reason behind this is that when PLC occurs, the ice moves at a low velocity which is close to the structure’s velocity. When the structure moves in the same direction as the ice, it seems there is no relative motion between ice and structure and no obvious ice failure can be observed. The ice appears to stop before the structure. When the structure moves opposite to the ice, the ice collapses and ex-trudes. It appears that the ice runs before the structure.
The ice load data measured by different load panels plotted in Figure 11 and Fig-ure 12 show the simultaneous phenomenon of saw tooth ice loads during PLC. In Figure 11 from 6700s to 6705s, the ice load has a saw tooth shape, which indicates PLC happened. Due to direction of the ice acting on the structure, only load panels No.7, 8, and 9 were under ice action in this event. The load data recorded by these three load panels all have the same phase change during the locking in vibration. It should be noted that the width of load panel is 1.2 m in the lighthouse measurement. This implies that ice sheet breaks simultaneously throughout contact surface.
Point5: The field data shows that response of Molikpaq to dynamic ice load is different to lock-in vibration of lighthouse and jacket structures. The difference is that Molikpaq platform is a caisson on the sand core which creates large damping. The system is close to an over damped or critically damped system which returns to equilibrium position after unloading without oscillating. The inertia force of the platform caused small or even no negative displacement against the ice movement direction after unloading.”
For wide structures such as the Molikpaq caisson platform,Jefferies and Jordaan et al. have reported dynamic responses induced by phase-locked loading. The ice-structure interaction process experienced by Molikpaq platform in 1986 was studied by Jefferies, M. and Spencer, P. and Jefferies. Based on the analysis of ice load and acceleration data shown in figure 3 and 4, they suggested that although the ice load was dynamic, the system was not in an oscillation mode. This implies that the dynamic unloading in a highly damped situation would be a more accurate de-scription of ‘dynamic ice loading. The field data show that the response of Molikpaq to dynamic ice load differs from the lock-in vibration of lighthouse and jacket structures. The difference is that Molikpaq platform is a caisson on a sand core which creates large damping. The sys-tem is close to an over damped or critically damped system which returns to equilib-rium position after unloading without oscillating. The inertia force of the platform causes small or even no negative displacement against the ice movement direction af-ter unloading.
Point6: The major defect of this study is the debate or Argument is not clear stated in the introduction session. Hence, the contribution is weak in this manuscript. I would suggest the author to enhance your theoretical discussion and arrives your debate or argument.
As shown in Table 4, the probability of PLC occurrence for fixed offshore wind turbines in Bohai sea is less than 0.05%, based on the ice speed factor alone. However, other factors such as ice thickness and ice type also affect the occurrence of PLC. Tak-ing into account the random distribution of these factors would further reduce the probability. The range of ice speed (0.24cm/s-0.54cm/s) that causes PLC on fixed off-shore wind turbine in this paper is much narrower than the 5cm/s or 10cm/s assumed by related studies, and its probability is also much lower than the 1.25% calculated by Wang Shuaifei et al. This result demonstrates that the DDC failure mechanism can distinguish PLC occurrence on different structures.
This paper defines the ice failure that occurs during the PLC process as DDC fail-ure mode by studying the field measurement data of frequency lock-in vibration of the jacket and light house structures and the phase locked loading on the caisson structure. The DDC failure mechanism provide a different explanation for the PLC process. Key feature of the DDC mechanism is the failure is a discrete process and each time the ice break with a limited length. The failure length is an important parameter for under-standing and analyzing the PLC process., the ice failure length has not been intensively investigated in the past because the failure process has not been clearly recognized. The field measurement indicates that the failure length is less than 5 cm and does not depend on the ice thickness. Supposing the failure length is an inherent feature of PLC process, the PLC will occur when ice speed close to the ratio between structure natural period and ice failure length. That will give a much narrow ice condition window for PLC occurrence for wind turbine structures compare with results obtained from other mechanisms.
Point7:Much more explanations and interpretations must be added for the Results, which are not enough.
More explanations and interpretations have been added in the revised article.
Point8: It is suggested to add articles entitled “Basack et al. Flow Characteristics through Granular Soil Influenced by Saline Water Intrusion: A Laboratory Investigation” and “G. Russo et al. Hybrid Energy Piles as a Smart and Sustainable Foundation” to the literature review.
Articals have been added in the revised article.
Point9:More suitable title should be selected for the table 1 instead of “Typical PLC conditions measurement on offshore structures.”.
The title has been changed to “Typical PLC conditions measurement on Molikpaq , JZ9-3 MDP and Norstromsgrund Lighthouse”
Point 10 Methods section determines the results. Kindly focus on three basic elements of the methods section.
- How the study was designed?
- How the study was carried out?
- How the data were analyzed
These methods have been focused on in the revised article
Point11: Please make sure your conclusions' section underscore the scientific value added of your paper, and/or the applicability of your findings/results, as indicated previously. Please revise your conclusion part into more details. Basically, you should enhance your contributions, limitations, underscore the scientific value added of your paper, and/or the applicability of your findings/results and future study in this session
As shown in Table 4, the probability of PLC occurrence for fixed offshore wind turbines in Bohai sea is less than 0.05%, based on the ice speed factor alone. However, other factors such as ice thickness and ice type also affect the occurrence of PLC. Tak-ing into account the random distribution of these factors would further reduce the probability. The range of ice speed (0.24cm/s-0.54cm/s) that causes PLC on fixed off-shore wind turbine in this paper is much narrower than the 5cm/s or 10cm/s assumed by related studies, and its probability is also much lower than the 1.25% calculated by Wang Shuaifei et al. This result demonstrates that the DDC failure mechanism can distinguish PLC occurrence on different structures.
The field measurement indicates that the failure length is less than 5 cm and does not depend on the ice thickness. Supposing the failure length is an inherent feature of PLC process, the PLC will occur when ice speed close to the ratio between structure natural period and ice failure length. That will give a much narrow ice condition window for PLC occurrence for wind turbine structures compare with results obtained from other mechanisms.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
The paper summarizes ductile damage collapse failure mechanism of the phase locked ice crushing against both wide offshore structures, however is necessary to clarify some process used to perform this analysis. Also, some factual clarifications will be helpful as highlighted in the comments below.
- What are the units of Figure 7? Accelerations (g)? Ice force ( )?
-Please include the units on all the figures (7,8,…).
-Page 11, Line 263. Describe fatigue, in your analysis are you estimating the fatigue assessment? (see suggested references)
-Is a crush or crash analysis? Explain it.
-to understand the measurements, please describe the instrumentation, the hardware (load cells, LVDT, ……) and software. Also the measurements characteristics (sample rate, filters…)
-Why is used a different acquisition time in the responses?
-In the response there is a resonance?, if so, how affect the results?
-What is the impact direction?, the other directions don´t have an influence?
- Based on the pile length and the inertial forces, the phenomenon is solved with an implicit or explicit process? Please explain it.
-Is possible to apply the layer characteristics as material granular (clay and sea-bed)?
-How is the interaction fluid-structure?
-How can the corrosion degrade the mechanical response? (see suggested references)
Please review the next papers and include it in your references.
- Fatigue damage assessment of mooring lines under the effect of wave climate change and marine corrosion, Ocean Engineering Volume 206, 15 June 2020, 107303 https://doi.org/10.1016/j.oceaneng.2020.107303
-Harbor and coastal structures: A review of mechanical fatigue under random wave loading, Heliyon 7 (2021) e08241 https://doi.org/10.1016/j.heliyon.2021.e08241
Author Response
Response to Reviewer 3 Comments
Point 1: What are the units of Figure 7? Accelerations (g)? Ice force ( )
Response 1:The unit of Accelerations is m/s^2,unit of Ice Force is MN in Figure7.
Point 2: -Please include the units on all the figures (7,8,…).
Response 2: The units has been included in the revised article
Point 3 : Page 11, Line 263. Describe fatigue, in your analysis are you estimating the fatigue assessment? (see suggested references)
Response 3: In our analysis didn’t estimate the fatigue damage.We just raise a disscussion for potential application. We will present the fatigue analysis methodology in another paper. .
Point4 : Is a crush or crash analysis? Explain it..
The paper focus on the discussion of crushing ice failure against offshore structures. The ice failure caused dynamic ice load on the structure and in the PLC mode, the movement of structure coupled with ice failure process.
Point5: to understand the measurements, please describe the instrumentation, the hardware (load cells, LVDT, ……) and software. Also the measurements characteristics (sample rate, filters…)
The measurement system were addressed in the reference. And to focus on the common feature of the ice failure process, only related information were extracted from the reference.
Point6: Why is used a different acquisition time in the responses
The response and ice load data were measured at the same time.
Point7: In the response there is a resonance?, if so, how affect the results
The response is a coupled and lock-in process between ice failure and structure movement, looks like an steady resonance process, but it is not.
Point8: -What is the impact direction?, the other directions don´t have an influence?
Driven by the wind and current, the ice movement direction changes at different time. But only the side of structure faces the ice movement direction will interact with ice sheet and exposed to the ice load. The other directions don’t have influence.
Point9: Based on the pile length and the inertial forces, the phenomenon is solved with an iplicit or explicit process? Please explain it..
A simplified explicit method was presented to solve the problem by establishing an relationship between the ice failure length and magnitude of the structure vibration.
Point 10 -Is possible to apply the layer characteristics as material granular (clay and sea-bed)?
Yes it is possible. There is DEM method been used to study the ice-structure interaction. Our study mainly use the field measurement result. The results can be used to calibrate the DEM method.
Point11: How is the interaction fluid-structure
The sea water has not been considered in the ice - structure interaction. Generally we consider the ice failure is not affected by water.
Point12: How can the corrosion degrade the mechanical response? (see suggested references)
The corrosion degrade will be further considered when the fatigue damage accumulation been considered. But for the global vibration of the structure, the degradation is not considered.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The paper was corrected.
Reviewer 2 Report
The article has been revised very well, so I would suggest to accept in its present form.
Reviewer 3 Report
The requested changes have been done.
