Dynamical Sliding Mode Control for Robust Dynamic Positioning Systems of FPSO Vessels
Round 1
Reviewer 1 Report
Article is interesting, the subject is current and has useful value. In order to enhance the article quality, I suggest the following remarks be taken into account:
- Please mark the vectors in bold.
- I suggest that for a better understanding of the paper content and for an easier implementation of the proposed algorithm it would be necessary to rewrite the section 3 by including a flowchart of the algorithm and its algorithmic presentation with all the steps that need to be taken.
- Figures 6-8 should be modified because it is illegible. Please improve resolution.
- It is good idea to add a few sentences about future analysis.
- The authors are suggested to have Discussion section to investigate the weakness, strength, and potential enhancement of proposed scheme.
- The References should be extended to include the publications on intelligent solutions in shipping that refer to the discussed subject, for instance:
- Veksler, T. A. Johansen, F. Borrelli and B. Realfsen, "Dynamic Positioning With Model Predictive Control," in IEEE Transactions on Control Systems Technology, vol. 24, no. 4, pp. 1340-1353, 2016
- Borkowski P. „Inference engine in an intelligent ship course-keeping system” Computational Intelligence and Neuroscience vol. 2017, art. no. 2561383, 2017
- Peng, D. Wang and J. Wang, "Cooperative Dynamic Positioning of Multiple Marine Offshore Vessels: A Modular Design," in IEEE/ASME Transactions on Mechatronics, vol. 21, no. 3, pp. 1210-1221, 2016
Author Response
Responses to Reviewers’ Comments (Manuscript Number: JMSE-1620321)
The authors of this paper want to thank the reviewers for their thorough and constructive review. We have revised the paper according to their suggestions and comments. We hope our revision has improved the paper to a level of their satisfaction. The answers to their specific comments/suggestions/queries are provided below. The reviewer’s comments are shown in italic fonts, while our responses are in normal fonts. The revisions in the paper are also explained in this document and highlighted in the revised manuscript.
Specific changes in the manuscript
The authors have updated figures that illustrate the flowchart and block diagram of the sliding mode controller, and the boundary layer for chattering reduction. Therefore, Figs. 4 – 6 were included according to the contents of the SM controller. The image quality of Figs. 9 – 12 was enhanced according to reviewers’ comments. Fig. 13 was added to explain that the trajectories in phase planes converged to the sliding surface. The general changes are highlighted as green in the revised manuscript.
Responses to Reviewer #1 Comments
Comments to the Author
Article is interesting, the subject is current and has useful value. In order to enhance the article quality, I suggest the following remarks be taken into account:
Response: The authors are grateful to the reviewer for encouraging comments. We try to do our best to apply your comments and improve the quality of the manuscript. The changes are highlighted as cyan in the revised manuscript according to your comments
Comment 1: Please mark the vectors in bold.
Response: The vectors were changed in bold according to your comment 1.
Comment 2: I suggest that for a better understanding of the paper content and for an easier implementation of the proposed algorithm it would be necessary to rewrite the section 3 by including a flowchart of the algorithm and its algorithmic presentation with all the steps that need to be taken.
Response: According to your comment 2, we included a flowchart and a block diagram of the SM control algorithm as shown in Figs. 4 and 6 and proper explanation in Section 3 in the revised manuscript for a better understanding of this study (Please, see Section 3).
Comment 3: Figures 6-8 should be modified because it is illegible. Please improve resolution.
Response: The resolution of Figs. 9-13 (previous Figs. 6-10) were improved according to your comment 3.
Comment 4: It is good idea to add a few sentences about future analysis.
Response: Future work was described in the last section of the conclusion according to your comment 4 (Please, see lines 434 – 442).
Comment 5: The authors are suggested to have Discussion section to investigate the weakness, strength, and potential enhancement of proposed scheme.
Responses: According to your comment 5, the discussion section describing the strength, weaknesses, and potential enhancement of the proposed method was included in the conclusion.
The proposed DP system with SM controller has the function of adjusting the control performance through the selection of control parameters, which intuitively consider the response characteristics of vessels to ensure the robustness of the system against external disturbance.
However, the weakness of this study is that there is no validation procedure of simulation results. As a next step, the simulation results should be validated by model tests with a SM-controller-based DP system.
In the future, the proposed DP system needs to be updated to have a function of prediction of incident Sea loads such as swell and gust in future events. And then, the DP system combined with a function of future prediction may ensure much more robustness against harsh ocean environments.
Comment 6: The References should be extended to include the publications on intelligent solutions in shipping that refer to the discussed subject, for instance:
Veksler, T. A. Johansen, F. Borrelli and B. Realfsen, "Dynamic Positioning With Model Predictive Control," in IEEE Transactions on Control Systems Technology, vol. 24, no. 4, pp. 1340-1353, 2016
Borkowski P. „Inference engine in an intelligent ship course-keeping system” Computational Intelligence and Neuroscience vol. 2017, art. no. 2561383, 2017
Peng, D. Wang and J. Wang, "Cooperative Dynamic Positioning of Multiple Marine Offshore Vessels: A Modular Design," in IEEE/ASME Transactions on Mechatronics, vol. 21, no. 3, pp. 1210-1221, 2016
Responses: According to your comment 6, three references were added to the list of references. Additionally, the recent references describing intelligent solutions in shipping were included in revised manuscript with appropriate explanation (Please, see lines 56 – 63, 72 – 86, and 478 – 497).
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This paper discusses the application of slide mode control method to DP. However, there are some problems, which are listed as follows.
1. The paper states that “it is difficult to find examples of SM controllers applied to DP systems” (line 70). However, the application of slide mode control method to DP is not new at all. A lot of similar researches have been reported even by commonly used search web tools, such as bing.com, goole.com, not needless to say the academic literature databases.
2. Most methods, such as extended Kalman filter (EKF), slide mode control, vessel motion dynamics, and etc., in this paper, come from the other papers. Then, what are main contributions of this paper?
3. Generally speaking, slide mode control will cause unpleasant chattering in control input. However, this paper discusses very little about this problem.
4. Some pictures are not clear, such as Fig6-11, which make it quite difficult to understand the paper.
5. Most of references are much too old. Recent references should be included.
Author Response
Responses to Reviewers’ Comments (Manuscript Number: JMSE-1620321)
The authors of this paper want to thank the reviewers for their thorough and constructive review. We have revised the paper according to their suggestions and comments. We hope our revision has improved the paper to a level of their satisfaction. The answers to their specific comments/suggestions/queries are provided below. The reviewer’s comments are shown in italic fonts, while our responses are in normal fonts. The revisions in the paper are also explained in this document and highlighted in the revised manuscript.
Specific changes in the manuscript
The authors have updated figures that illustrate the flowchart and block diagram of the sliding mode controller, and the boundary layer for chattering reduction. Therefore, Figs. 4 – 6 were included according to the contents of the SM controller. The image quality of Figs. 9 – 12 was enhanced according to reviewers’ comments. Fig. 13 was added to explain that the trajectories in phase planes converged to the sliding surface. The general changes are highlighted as green in the revised manuscript.
Response to Reviewer #2 Comments
Comments to the Author
This paper discusses the application of slide mode control method to DP. However, there are some problems, which are listed as follows.
Response: The authors are grateful to the reviewer for critical comments. We have revised the paper according to your comments to improve the quality of the manuscript. The changes are highlighted as yellow in the revised manuscript.
Comment 1. The paper states that “it is difficult to find examples of SM controllers applied to DP systems” (line 70). However, the application of slide mode control method to DP is not new at all. A lot of similar researches have been reported even by commonly used search web tools, such as bing.com, goole.com, not needless to say the academic literature databases.
Responses: We admitted your comment 1 and deleted the sentence “it is difficult to find examples of SM controllers applied to DP systems” in this manuscript. We also found a number of similar researches from the academic literature databases and insert them as references into our manuscript. The references and appropriate explanations were added in this revised manuscript (Please see, lines 72 – 85, 484 – 497).
Comment 2. Most methods, such as extended Kalman filter (EKF), slide mode control, vessel motion dynamics, and etc., in this paper, come from the other papers. Then, what are main contributions of this paper?
Responses: The methods used in this study are extended Kalman filter (EKF) to separate the low-frequency motion of the vessel model and estimate the external forces on the ocean environment, sliding mode control for robust dynamic positioning against external disturbance, and FPSO vessel dynamics with sensor models. Those schemes are basic elements to build the SM control-based DP system of the FPSO vessel model.
The main contribution of this study is to find the possibility to apply the sliding mode control scheme to a DP system of a FPSO vessel performing turret access operations in the sea. This algorithm developed for DP can be applied to FPSO vessels considering the uncertainty of the FPSO vessel dynamics, unknown time-varying environmental disturbances, and transient performance. In addition, a series of simulations were carried out to find a possible controllable range of DP systems in comparison with SM and PD controllers. The results of simulations figured out that the SM controller is much more stable and robust than the PD controller.
Comment 3. Generally speaking, slide mode control will cause unpleasant chattering in control input. However, this paper discusses very little about this problem.
Responses: Chattering is the main issue in the sliding mode controller and it is essential to reduce this phenomenon with appropriate measures. To reduce the chattering phenomenon, the boundary layer was set as changing from discontinuous to continuous forms of control inputs. According to your comment 3, the discussion of the chattering issue and solution was added in this revised manuscript (Please, see lines 222 – 229 and Fig. 5).
Comment 4. Some pictures are not clear, such as Fig6-11, which make it quite difficult to understand the paper.
Responses: Figs. 9 – 13 (previous, Figs. 6-10) were updated with high resolution of images in the revised manuscript.
Comment 5. Most of references are much too old. Recent references should be included.
Responses: According to your comments 1 and 5, the recent references were added in the revised manuscript (Please see, lines 478 - 497).
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I accept amendments made by authors. From my side the work is accepted in this new version.
Author Response
Responses to Reviewer #1 Comments
Comments to the Author
I accept amendments made by authors. From my side the work is accepted in this new version.
Response: The authors of this paper want to thank reviewer #1 for their thorough and constructive review. The authors are grateful to the reviewer for encouraging comments and acceptance of this paper. We will try to do our best to conduct the future work that we described in the manuscript.
Author Response File: Author Response.pdf
Reviewer 2 Report
The authors have given some responses for the previous edition. However, there are still some problems in the present paper.
- The authors state that the main contribution is the algorithm developed for DP can be applied to FPSO vessels. What are main differences between DP and FPSO? When the algorithm for DP is applied to FPSO, what are the main difficulties? This problem is very important. Clear explanations are expected.
- The equation (3) is the 3 DOF equations of the motion of the vessel (Line 132). The equation (10) is also “the dynamic equation of the motion of the vessel” (Line 191). What’s their main usage? Why do they look so different?
- This study uses EKF to estimate uncertain external forces. In the illustrative example, how does EKF work? What are the EKF parameters? How about the performance of EKF estimations?
- What’s the meaning of parameter “J”? In the controller equation (18), how can we get the value of “F”? Is “F” chosen by fine hand tuning?
- In the illustrative example, the FPSO matrix values in (10) and (3) should be given.
- The figures and picture are still not quite clear except Figures 14-17.
Author Response
Response to Reviewer #2 Comments
Comments to the Author
The authors have given some responses for the previous edition. However, there are still some problems in the present paper.
Response: The authors would like to to thank reviewer #2 for their thorough and constructive review. The authors have tried to do their best to apply for the reviewer for critical comments in the revised manuscript. We have revised the paper according to your comments to improve the quality of the manuscript. The changes are highlighted as yellow in the revised manuscript. In addition, this author's response to the review report contains some equations and figures that cannot upload to this note. Therefore, please see the attachment for responses to reviewer #2 comments.
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
The authors have given responses to all the questions, and added some new materials to the paper. One more suggestion is as follows.
Some results about EKF in the responses should be integrated into the paper to illustrate the usage of EKF.
Author Response
Response to Reviewer #2 Comments
Comments to the Author
The authors have given responses to all the questions, and added some new materials to the paper. One more suggestion is as follows.
Some results about EKF in the responses should be integrated into the paper to illustrate the usage of EKF.
Response: The authors of this paper would like to thank reviewer #2 for the thorough and constructive review. The authors are grateful to the reviewer for encouraging comments. Some equations, explanations, and results of EKF that estimate uncertain disturbances are integrated in this revised manuscript according to your comments. The changes are highlighted as yellow in the revised manuscript (Please see lines 188 – 209 and 291 – 302).
Author Response File: Author Response.pdf
