Bending moment analysis of super large ore carrier based on graphical interface development

. In order to improve the safety of shipping process, this paper designs a graphical user interface for calculating the static water bending moment of super large ore carrier. Due to the complexity of the actual loading conditions, the long hull girder and the mobility and sinking of cargo, the irregular distribution of gravity and buoyancy along the direction of the captain of the VLCC is prominent. In the actual loading process of ships, compartment loading is adopted in many cases, and the position where the maximum shear force and bending moment appear is generally not in the corresponding position. Traditional ship strength analysis methods can be divided into two kinds. One is to establish the ship's finite element model to calculate the ship's strength. The other is to calculate the ship's strength according to the simple beam bending theory. In this paper, a 360m VALEMAX ore carrier is taken as an example to calculate the local bending moment of the ship at a specific position by using QT software. Compared with the ship loading manual, the error of the program is small, and the program has guiding significance in the actual shipping process.


Introduction
The calculation of the shear force and bending moment of the ship is an important premise for the safety of the ship navigation and the foundation of the automatic loading of the ship. The calculation of static water shear and bending moment of large open deck ships such as bulk cargo and container is a mandatory requirement of SOLAS Convention. SOLAS Convention requires that all bulk carriers with a master of not less than 150m and large open deck ships as defined in classification society specifications shall be equipped with corresponding computer software to calculate the static water shear and bending moment under various loading conditions. [1] Even if the super large ship is optimized in hull strength, the probability of local shear and bending moment is higher than that of small ships due to the long captain. The probability of grounding and permanent damage of super large ships in the actual shipping process is also greatly improved due to the deformation of the hull.
At present, the ship strength calculation methods can be divided into two kinds: one is to establish the finite element model of the ship to calculate the ship's strength. The other is to calculate the ship strength according to the bending theory of simple beam. Gao Maojin uses the software of classification society of Norway and Max surf software of Australia to calculate the bending moment and shear force of the ship. The conclusion shows the approximate position of the maximum shear force and bending moment along the direction of the captain. [2] Based on the three-dimensional STL model of hull and cabin, Liu Chunlei and Yinyong proposed a calculation method of floating state, static water shear force and bending moment of the ship under any loading condition, which made up for the deficiency of the conventional method [1] .In the program module forcecalculator, zhangguangfa and liuyujun introduced the calculation method of checking the local stress and distribution of the total longitudinal bending moment of the hull girder in the launching analysis of large ships. [3] In summary, the method of calculating ship strength based on simple beam is mainly based on the static hydraulic, tank capacity table and bond capacity curve provided by the ship design department. In this paper, the trapezoidal distribution method is used to design the graphical user interface to calculate the local bending moment of the ship.
As shown in Figure 5, some results of the program calculation are shown. The third column is the corrected bending moment value of longitudinal bulkhead calculated by integral. Through the relationship between theoretical bending moment and actual bending moment in the sixth column, it can be found that the overall ratio is about 1, and the ratio near the stern fluctuates greatly. Considering the structure of stern type, the ship is loaded in five cargo tanks, and the ratio near the stern is not important. The results obtained by the iterative calculation of the piecewise trapezoidal method are not different from the actual values. During the shipping process, the shear force and bending moment of the corresponding position can be estimated through this program, and the points exceeding the allowable value in the specified value can be calculated accurately and the loading can be reasonably arranged.

Fig. 5. Program calculation results
As shown in Figure 6, the user inputs the shear force at the corresponding position and clicks the "calculate bending moment" button, and the calculation results will be displayed in the bending moment table of the ship. On the basis of the bending moment results, click the "visualization" button, and the bending moment data in the table will be displayed on the ship bending moment curve. There is also a maximum bending moment curve in the figure, which is used to identify whether the bending moment value at the corresponding position exceeds the allowable range, and has the characteristics of simple visualization.

Experimental Result
As shown in Table 1, the calculation results of the program are compared with some experimental data in the code. It can be seen from the ratio relationship in the table that the overall ratio relationship of this trapezoidal calculation method is 1. The ratio of the ship close to the stern fluctuates greatly, which may be related to the fact that most ships adopt the stern structure. As shown in Figure 7, there is no significant difference between the side shell modified bending moment curve and longitudinal bulkhead modified bending moment curve at different captain positions and the actual ship bending moment curve, which has guiding significance in the actual shipping process. Considering the influence of wave, hull wear and other errors in the actual sea conditions, the local allowable load will fluctuate to a certain extent compared with the ex factory ship, so the actual influence of the error in the figure will not be great. The diagram is a comparison diagram of bending moment under load condition, and the local shear force can also be obtained by integral calculation of the program. The program has guiding significance in calculating local shear force and local bending moment.

Conclusion
For the super large ship with non-uniform compartment loading, the maximum still water bending moment is not necessarily located near the middle of the ship, which is directly related to the loading state of the ship. Similarly, the maximum shear force of a ship may not be at the quarter of the ship's length from the fore and stern. In order to ensure the safe and economic navigation of the ship in the actual sea conditions, the ship loading should meet the allowable range and favorable range specified in the code.
The shear force and bending moment calculated by QT software segmented trapezoidal method have little difference with the actual value. In the actual shipping process, the graphical user interface is simple and visual, which has certain guidance. This paper does not consider the calculation error caused by Hull wear and bulkhead correction. In the future, we can reduce the error by model, simulation and physical model.