Passive Heave Compensator Design and Numerical Simulation for Strand Jack during Lift Operation in Deep Water

Round 1

Reviewer 1 Report

The authors have studied a passive heave compensator for strand jack lifting system, while considering the nonlinear characteristic of the compensator stiffness.

The proposed manuscript will contribute to the development of an efficient heave compensator, which must be useful in various practical operations in the ocean.

For the reasons mentioned above, I recommend the proposed manuscript be published in Journal of Marine Science and Engineering.

However, before the manuscript be accepted, I have some comments and questions, to which I recommend the authors pay due consideration and revise the manuscript accordingly.

 

1. First of all, explicit descriptions on what are new in the authors’ work compared to the works conducted by other researchers on the same subject should be given in the manuscript. It is also desirable to show concrete results that manifest the distinct superiority of the authors’ work to those conducted by others.

 

2. In page 17, line 150,

It is described that ‘m_a (added mass) can be given as 0.15m_l (mass of the lifted structure)’.

It seems that it conforms to DNV standard, but I think the value 0.15m_l is quite smaller than actual added mass in general. For example, the added mass of a sphere is 0.50 times the mass of the displaced water or the added mass of a ship in heave motions could be as large as 1~2 times the displaced mass of water. Anyway, an added mass of a submerged objected has nothing to do with its own mass but it depends on its geometry.

Since the inertia force due to the motion of a lifted structure is proportional to its virtual mass (its own mass + added mass), accurate estimation of an added mass is quite important.

 

3. In Section 3, a numerical simulation is conducted while assuming a simple sinusoidal motion, but, lifting operations in real seas are conducted in irregular waves, which, in turn, should induce irregular motions of a lifting structure. Will the proposed passive compensator work in irregular waves as well?

 

Author Response

     Thank you for your questions about our manuscript during your busy schedule. After careful consideration, the answers are as follows.

     For the first question,the research object of this article is the passive compensator of the twisted jack lifting system. This article establishes a coupled nonlinear mathematical model and conducts numerical simulation. While proving the function of the compensator, the response of the compensator under different parameters is accurately determined.Prediction helps to find the feasible gas volume of the compensator.This article inherits the research of other researchers in this field, improves the mathematical model of the compensator, and analyzes the response of the compensator under different parameters, which is very helpful for the design of PHC.The above content is introduced in the introduction section of the text.

     For the secound question,in DNV standard, the additional mass of vertical motion of three-dimensional object in the infinite fluid is defined as A₃₃=ρC_AV_R, where C_A is the additional mass coefficient, V_R is the reference volume, and V_R is usually the displacement volume. The additional mass of the structure is affected by the free surface. In table A2 of Appendix A, we select C_A when c / a and b / a are both 1. Because most of the offshore structures are made of stainless steel, the density of the representative 304 stainless steel is 7.93g/cm³, It can be deduced that the relationship between m_a and m_l is m_a = 0.15m_l.

     And for the last question,firstly, according to the theoretical and experimental analysis, the random wave can be regarded as the result of the superposition of infinitely many random simple sine (cosine) waves, and the mean value of the wave surface height of the composite wave is zero, which satisfies the condition that the mean value of the wave random process is constant; Secondly, the variance of wave height is directly proportional to the wave potential energy per unit area of vertical water column, so the variance of wave height can be regarded as a measure of wave energy. When the wave is in the stable stage, the change rate of energy is very small, and the variance of wave height changes slowly, so the random wave can be regarded as a quasi-stationary process; Finally, from the analysis of autocorrelation function (reflecting the correlation degree of the two random function values of the interval time t in the random process), it can be proved that the random wave has the experience of each state. Through the above three points analysis, it is proved that random waves have stationarity and experience of each state. It can use the wave records of a single or a few measuring points to select the appropriate time period from any time for statistical analysis, and calculate the relevant statistical eigenvalues to represent the total body eigenvalues of waves. Therefore, the numerical simulation of simple sine (cosine) motion can reflect the effect of a certain part of the random wave on the system. Therefore, the proposed PHC has the conditions to work in irregular waves.

      Thank you again for your points.

Reviewer 2 Report

The paper analyzes the operation of a passive heave compensator used in combination with a strand jack for lifting heavy loads from the depths of the sea (above 300 m). A dynamic and mathematical model was designed, which takes into account the nonlinear stiffness characteristic of the compensator. To evaluate the efficiency of the compensator, numerical simulations were performed for specific design dimensions of the compensator (its functional parts).

In the beginning, an analysis of the use of strand jacks for installations of equipment at sea resp. for rescue operations. It explains the principle of operation of the jack, its advantages. Furthermore, the need to compensate for the lifting of loads from great depths and on stormy seas is explained. The types of compensators are listed. The authors chose a passive compensation system for further analysis due to its high reliability and low energy consumption.

A schematic arrangement of the lifting system is shown. Simplifying assumptions are given when creating a dynamic model. The stiffness of the compensator is expressed as a non-linear function of the pressure, the volume of air in the compensator and the area of the piston. The volume of gas in the compensator is optimized in regard to its rigidity and the extreme positions of the piston. The damping model of the compensator, the strand model and the hydrodynamic damping coefficient are also given. In the numerical simulation for specific values of the compensator construction of  and waves of the sea, the efficiency of the compensator and the influence of key parameters on the efficiency of the compensator are evaluated with a discussion of the results.

Positives:

Very nice, clear, logically arranged sequence arranged ideas included in individual chapters.

The partial results are clearly grouped, the text is suitably supplemented by figures, tables and graphs. The solution procedures are chosen correctly.

Comments:

In Fig.3 (row 113) the Additional gas bottles should be correct

In Fig. 5a (row 172) I might add a small hole at the bottom, as indicated in the attached picture.

The pressure P_a in the gas chamber is always the constant atmospheric pressure (row 174). This makes it clear that it is not possible to press it.

According to relation (2) (row 183) and logically according to the text, the direction of the arrow of the applied force F should be opposite. And the force F should be given with the index p → F_p (row 181).

In relation (2) (row 183), also in relation (6) (row 199), (7) (row 200), (15) (row 279,280) the pressure P_a cannot be multiplied by the area A_d, which is expressed in row 183. It should be multiplied by the value of the whole area of the piston A = π.D_p² / 4

At atmospheric pressure P_a = 100,000 Pa (N / m²) with this cross-sectional change, the difference in force per piston is approximately 6.1 kN for the selected dimensions of the piston (500 mm) and the piston rod (280 mm) and this is no longer a negligible value.

 

Comments for author File: Comments.pdf

Author Response

Thank you for your questions about our manuscript during your busy schedule. After careful consideration, the answers are as follows.

We corrected FIG.3 and FIG.5.  It is worth mentioning that in FIG.5, in order to make the direction of the force positive when building the geometric model, we used to set the direction of the force to be the same as the positive direction of the x-axis. We realized that this setting is not convenient for readers.  , Has been modified in the manuscript.

For your third suggestion,the air chamber is always connected to the atmospheric environment, which is achievable because the passive compensator will not be immersed in water.

And for the fifth suggestion,we has noticed our error and has been changed in the manuscript.

For your last suggestion,the mass of the structure in this paper is 400 tons, the force acting on the piston rod by atmospheric pressure is calculated to be 6.15kN, and the acceleration of gravity is 9.8m/s², which is equivalent to 627.6 kg, or 0.6276 tons, which only accounts for the mass of the structure 0.1569%, so this is a negligible small amount.

Once again, sincerely thank you for your amendments to us

Author Response File: Author Response.docx