선박 추진축계 정렬에 영향을 미치는 선체 변형에 관한 연구

Abstract

Modern ship hulls of large oil carriers and container carriers have become more flexible with the scantling optimization and an increase in ship length. On the other hand, as the demand for power has increased with the ship size, shaft diameters have become larger and stiffer. Consequently, the alignment of the propulsion shafting system has become more sensitive to hull girder deflections, resulting in difficulties in analyzing the alignment and conducting the alignment procedure. Accordingly, the frequency of shafting alignment related bearing damages has increased significantly in recent years. The alignment related damages are mostly attributed to inadequate analyses, changes in the design of the vessel, inadequate practices of the shipyard in conducting the alignment, and a lack of well defined analytical criteria.

Hull girder deflections are the most significant disturbance that affects the bearing offset after vessel construction. Inability to account for hull deflections may result in poor alignment design with serious consequences on the bearing life. The problem, however, is the difficulty in predicting and evaluating hull deflections.

Hull deflections can be estimated by an analytical approach and by measurements. The analytical approach is time-consuming and expensive. It requires detailed modeling (e.g., finite element) of the vessel, in particular the stern part, with a comprehensive model of engine room structure, engine and shafting. The analytical approach is seldom undertaken solely for the purpose of investigating hull deflections' effect on the alignment. It is more common to take advantage of the full scale vessel modeling conducted for the dynamic loading analysis (or similar) to extract the data on hull deflections that may be applied in alignment analysis.

The hull deflection analysis by using analytical and measurement approach was conducted for a container vessel and similar results from both methods were obtained. The measurement approach has been the proffered method to analyze hull deflections due to the substantial time and cost involved in the analytical approach.

Hull deflection analysis and the verification of analysis by measurements have been carried out by shipyards as a joint investigation with class societies. However, only one or two vessels have been studied for such research purposes. Accordingly, the analysis results from these research are not adequate for future application.

The purpose of this paper is to make a database of hull deflections on vessels of various type and size through direct measurements. This paper will introduce the hull deflection analysis method using the measurement approach and show the analysis results on the actual vessels. Where hull deflection data obtained by this research will be used for the shafting alignment analysis of similar or identical vessels, time and expense will be reduced, and the bearing damage will be prevented.

This paper consists of 7 chapters.

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

In chapter 2, the theoretical analysis method of propulsion shafting alignment is explained. The three-moment theory method, the matrix finite element method and the transfer matrix method are normally used for the shafting alignment analysis. Details of the matrix finite element method used in this research are explained in this chapter.

The results of the alignment calculations contain bearing reactions, shear forces and bending moments along the shafting, slope boring details (if applicable) and detailed description of alignment procedure. The alignment calculation is to be performed for theoretically aligned cold and hot conditions of the shafting system with specified alignment tolerances.

In chapter 3, details for the jack-up method and strain gauge method to get the bearing reactions are described.

Bearing reactions are measured directly and indirectly. The most commonly applied methods to measure the alignment condition are jack-up and strain gauge method. The strain gauge procedure is an indirect method to measure the deflections and strains in the shaft and those measurements are correlated to the bearing reactions. The jack-up measurement is a direct reaction measurement where a hydraulic jack is used to lift the shaft and measure the load at the particular bearing. Due to its simplicity, it is the most widely applied method in the shipbuilding industry.

However, jack-up results are sometimes different from the actual bearing load due to insufficient experience. The bearing reaction measurement method by jack-up is explained in three parts.

The strain gauge method can provide relatively accurate information on the loading condition of the bearings which are not accessible for jack-up measurements. Once the strain gauges are mounted, measurement can be easily repeated within a very short time. The disadvantages of the strain gauge method are as follows

It requires a relatively long time for equipment installation, the accuracy of the data depends on system modeling and it requires relatively sophisticated and expensive equipment for measurements.

In chapter 4, the method to get the actual bearing offset using the measured data on each different operating condition of the vessel is introduced. A genetic algorithm is applied to the reverse analysis program as a search engine. The genetic algorithm and the application method of reverse analysis program are explained in this chapter. Also, the strain gage data analysis program and the calculation procedures of actual bearing offset are introduced. The hull deflection results obtained by the finite element method are compared with those of the measurement method and the reliability of this study is verified.

In chapter 5, the bearing offsets obtained by the measurement method on the propulsion shafting system of the actual vessels are compared and investigated. Vessels selected include a 320K DWT VLCC, an 159K DWT oil carrier, an 105K DWT product carrier, a 47K DWT oil/chemical carrier and an 175K DWT bulk carrier, which are representative of the typical ships in worldwide shipyards. Measurements for all oil carriers have been carried out in Korean shipyards and measurements on the bulk carrier have been carried out in a Taiwan shipyard.

In chapter 6, hull deflections are calculated by using the actual bearing offsets obtained in chapter 5. And also, since the bearing offset is adjusted finally after launching, differences in hull deflection between before and after bearing final adjustments are calculated as correction values. The relative hull deflections between drydock condition and each operating condition are calculated and the upper and lower limits of hull deflections are described for future application.

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

(1) The methods to calculate the actual bearing offset on the installed bearing and to get the hull deflection values using the bending moments of the shaft and bearing reactions are introduced.

(2) The hull deflection results obtained by the finite element method are compared with those of the measurement method and reliability and utility on the hull deflection data obtained by the measurement method are verified.

(3) The bearing reaction at the aftmost main engine bearing is designed as an unload condition after bearing final adjustment due to the consideration of thermal expansion of the main engine L.O. sump tank and hull deflections. According to the results of this study, the current design method is suitable for over 150,000 DWT oil carriers. However, the bearing reaction of aftmost main engine bearing of oil and bulk carriers less than 100,000 DWT has to have a suitable load after final adjustment of the bearing to get the proper bearing load in the operating condition.

(4) The bearing offset is not changed from drydock to after launching in case of the 320,000 DWT oil carrier and 175,000 bulk carrier. Presently, the shafting alignment work is normally completed after launching because hull deflection from drydock to after launching can not be obtained. However, the shafting alignment work of the aforementioned vessels can be completed in the drydock.

(5) Even when the conditions of a bulk carrier changes from drydock condition to variable operating conditions, hull deflections are not large. Also, hull deflections of large bulk carriers are relatively smaller than those of small oil carriers. Accordingly, even if hull deflections are not considered in the shafting alignment calculations, it appears that the bearing damage caused by hull deflections will not occur.

(6) In case of oil carriers, hull deflections of bigger ships are larger than those of smaller ships. And since hull deflections of the oil carriers cannot be ignored, shafting alignment calculations should take into consideration hull deflection to prevent bearing damage.

Where the hull deflection data provided from this research will be used for the shafting alignment calculations for identical or similar vessels, shafting failures due to hull deflections could be reduced. Also, since the hull deflection data provided is based on the drydock condition, the shafting alignment work can be completed in the drydock and this means that the ship construction schedules can be reduced. However, measurements and analysis for five vessels have been conducted in this paper. Accordingly, where the hull deflection measurements and analysis for vessels of more types and sizes will be carried out by the method introduced in this paper and these data will be accumulated and formulated, the shafting damage from hull deflections will be prevented in advance.