초대형 원유운반선의 선체변형을 고려한 추진축계의 안정성 평가에 관한 연구

Abstract

As hull deflections have been one of the root causes for bearing damage of shafting system, a strengthened shafting alignment analysis considering hull deflections has been required among the interested parties. In recent days, the length of the vessel and the propulsion power tend to increase due to technological advance. The hulls have become more flexible whereas the shafts and propeller in shafting system have become larger and stiffer. Therefore, the shafting alignment analysis has become more sensitive to the hull deflections. It is a very important factor to secure the stability of shafting system against the variation of the bearing offset derived from the hull deflections during the life cycle of the vessel. However, due to difficulties to considering the hull deflections, the shafting alignment analysis turns out insufficient or incomplete, which might adversely effect the bearing lifetime.

This study aimed to theoretically calculate the hull deflections and analyze the measurement data for the very large crude oil carrier. For this, the whole structural analysis of the vessel was performed according to the draft changes of the vessel. Based on the hull deflections obtained from the analysis, the shafting alignment was analyzed. The whole structural analysis of the vessel was carried out under five(5) conditions according to the normal draft changes of the vessel, and it was confirmed whether the propulsion shafting system satisfied the permissible load, even under the influence of hull deflections. In addition, the results of the theoretical calculation method and the jack-up method were compared. The stability of the analysis was cross validated to investigate whether the shafting is stable under hull deflections.

This paper consists of 7 chapters.

In chapter 1, the historical background, objectives of this research and the structure of the paper are introduced.

In chapter 2, the problem of propulsion shafting arrangement and design criteria are reviewed.

In chapter 3, the theoretical analysis method for the shafting alignment is explained. Several methods are introduced for the theoretical analysis of bearing reaction but this chapter describes the matrix finite element method, which is widely used for complex structural analysis.

In chapter 4, the measurement method for propulsion shafting alignment condition is introduced as follows. (1) Displacement measurement method (2) Reaction measurement method

In chapter 5, the whole structural analysis of the vessel using the finite element method for the very large crude oil carrier is performed. Based on this, it can consider which the effect is existing on the propulsion shafting by the hull deflections.

In chapter 6, the shafting alignment analysis is performed depending on the displacement of the shafting obtained by the whole structural analysis carried out for five(5) conditions considering the draft changes of the vessel. Based on this, the measurements were carried out for five(5) bearings using jack up method and then measured results are compared with calculated results to review stability of shafting bearings.

In chapter 7, the achievements of this study are summarized as follows.

(1) Hull deflections are a very important factor affecting the each bearing offset that supports the propulsion shafting system, and it was confirmed that it effects the shafting alignment analysis.

(2) The finite element analysis confirmed that the transformation patterns due to the draft changes of the “hull” from the engine room bulkhead(FR#60) to forward bulkhead(FR#110) and the “engine room” part from the stern end where the shafting system is installed to the engine room bulkhead showed the opposite trend. That is, if the hull is in the hogging state, the engine room part is in the sagging state, and when the hull is in the sagging state, the engine room part is in the hogging state. Under the alignment draft condition, the hull is transformed into the hogging state, and the engine room is in the sagging state, and, under the loaded draft condition, the hull is in the sagging state, and the engine room is in the hogging state.

(3) Examining the relative displacement in terms of the bearing offset that supports the shaft by converting the amount of hull deflections over the entire length of the vessel on the basis of the imaginary reference line passing through the centers of two(2) stern tube bearings, it can be seen that the intermediate shaft bearing and the main engine bearings are placed on the vertical line above the baseline under alignment draft condition and ballast draft condition and on the vertical line below the baseline under scantling draft condition.

(4) The reaction force of the shafting supporting bearing showed a significant change from the stern tube bearing to three(3) main engine bearings on the stern side, and the change in the reaction force of the main engine bearing was negligibly small.

- The reaction force of the after stern tube bearing decreased with increasing draft, while the reaction force of the forward stern tube bearing increased. - In particular, it was seen that the bearing reaction force varied depending on whether the stern peak tank was loaded, even under the same draft condition. The reaction force of the after stern tube bearing showed a maximum value when the stern peak tank was loaded and minimum when it was empty. For the reaction force of the forward stern tube bearing, the minimum value was observed when the stern peak tank was loaded, and maximum when it was empty, again confirming the opposite trend. - It was confirmed that the reaction force of the intermediate shaft bearing is the same as that of the after stern tube bearing. - Reaction force change resulting from hull deflections showed the largest value at main engine bearing No. 8 and No. 7. For main engine bearing No. 8 and No. 6, the reaction force tended to increase with increasing draft and for main engine bearing No. 7, the reaction force decreased. However, it was confirmed that the reaction force change of the main engine bearing was hardly observed according to the stern peak tank loading state.

Under the hull deflections considering the change of the draft condition, it was confirmed that the reaction force variation of the shafting bearings satisfied the permissible load suggested by the engine and the bearing manufacturer. This means that, even if shafting alignment analysis is performed without considering hull deflections, the bearing reaction force can be within the permissible load. However, main engine bearing No. 8 is very sensitive to the variation of bearing reaction force according to the draft condition. Therefore, the reaction force of main engine bearing No. 8 should be set approximately zero(0) during shafting alignment analysis so that the values are within the permissible load proposed by the engine manufacturer, even under the influence of hull deflections.

For the very large crude oil carrier, it is necessary to perform shafting alignment analysis considering the hull deflections as it has long hull and draft changes that show pronounced characteristics depending on the loading state, unlike a small or medium sized carrier.

In addition, the hull deflections obtained in this study can ensure the stability of shafts and prevent shafting damage from hull deflections if one refers to the results during the shafting alignment analysis of similar or identical vessels.