Abstract
This paper focuses on the issues of low work efficiency, high safety risks, and long construction periods during the installation of steel casting brackets for tilted twin-shaft systems at the launching stage. Taking a segmented twin-shaft rudder system of a Ro-Pax ship as the research object, this paper analyzes its precision control standards and requirements. A precision quality control method for the pre-assembly of steel casting brackets for the shaft rudder system during the block construction stage is proposed and implemented during the construction process. The implementation results show that this precision quality control method fully meets the later requirements for ship main engine usage, significantly improves ship production and construction efficiency, shortens the ship construction period, and improves the construction working environment.
Keywords: shaft rudder system; precision control; steel casting; concentricity; reverse deformation
1. Introduction
Steel casting brackets for ship shaft rudder systems are typically installed during the hull launching stage. This method poses significant challenges, including difficult construction, low work efficiency, extended construction periods, and high-altitude operations with considerable safety risks. Currently, many domestic shipyards lack experience during the initial construction of long-shaft engineering ships, leading to insufficient precision control awareness and multiple repair incidents [1].
Therefore, this paper takes a Ro-Pax ship as the research object. Firstly, combining the precision control characteristics of the twin-shaft rudder system segments, the steel casting brackets for the twin-shaft rudder system are moved forward to the pre-assembly stage during the segment construction. Secondly, starting from the splicing of steel castings for the shaft system, three-dimensional detection technology is employed to control the pre-assembly precision of key points to ensure efficient and precise installation of the ship’s shaft rudder system.
2. Formulation of Pre-assembly Precision Control Scheme for Steel Casting of Shaft Rudder System
The shaft system brackets of a certain Ro-Pax ship are all installed on the external hull. There are six shaft system steel casting at the stern to fix the main engine rotating shaft and two rudder system steel casting to fix the rudder rotating shaft. The shaft system steel casting are installed on six segments: two stern castings on segment ED11P/S, two “I”-shaped single-arm brackets on segment AG02C, and two “V”-shaped double-arm brackets on segment AG03C. The two rudder sleeves of the rudder system steel casting are installed on segment AG01C.
2.1 Pre-assembly Precision
To ensure the installation precision of the shaft rudder system during the launching stage and the normal operation of the main engine shaft system, it is necessary to control the concentricity of the steel casting axle holes within a range of 2 mm during the assembly and construction stage.
2.2 Construction Environment
Due to the extremely high precision requirements for the pre-assembly of steel casting for the shaft rudder system, the construction environment must also meet strict standards. To ensure that the ambient temperature during measurement remains basically the same, thereby ensuring measurement accuracy, the splicing and pre-assembly of steel casting must be conducted indoors.
2.3 Positioning Scheme
Before the pre-assembly of steel casting for the shaft rudder system, a positioning scheme must be formulated to clarify the reverse deformation addition and positioning scheme for each construction step of the eight steel castings during the pre-assembly and positioning process. The specific contents are as follows:
(1) Positioning requirements for two stern castings of segment ED11P/S:
- Front-back direction: Positioning is based on the distance between the end face of the steel casting head axle hole and the tail end face of the segment, with an error range of 0-3 mm.
- Left-right direction: Positioning is based on the center of the axle hole, with a concentricity of 0 mm and an allowable error range of ±1 mm.
- Height direction: Positioning is based on the center of the axle hole, with an added reverse deformation of 3-5 mm.
(2) Positioning requirements for two “I”-shaped single-arm brackets of segment AG02C:
- Front-back direction: Positioning is based on the front end face of the steel casting, ensuring that the single-arm bracket structure aligns with the ribs. A 4 mm reverse deformation is added to the “I”-shaped single-arm bracket towards the bow.
- Left-right direction: Positioning is based on the center of the axle hole, with a concentricity of 0 mm and an allowable error range of ±1 mm.
- Height direction: Positioning is based on the center of the axle hole, with an added reverse deformation of 3-5 mm.
(3) Positioning requirements for two “V”-shaped double-arm brackets of segment AG03C:
- Front-back direction: Positioning is based on the tail end face of the steel casting, ensuring that the double-arm bracket structure aligns with the ribs. A 5 mm reverse deformation is added to the “V”-shaped double-arm bracket towards the stern.
- Left-right direction: Positioning is based on the center of the axle hole, with the circle center of the steel casting deviated by 5 mm towards the shipboard side.
- Height direction: Positioning is based on the center of the axle hole, with an added reverse deformation of 5 mm.
(4) Positioning requirements for two rudder sleeves of segment AG01C:
- Front-back direction: Positioning is based on the position of the rudder hole center on the segment, with an error range of 0-1 mm.
- Left-right direction: Positioning is based on the center of the rudder hole, with a concentricity of 0 mm.
- Height direction: Positioning is based on the lower opening of the rudder sleeve, with an added reverse deformation of 3-5 mm.
3. Implementation of Precision Control Scheme for Pre-assembly of Steel Castings in Segments
3.1 Precision Control of Steel Casting Assembly and Splicing
The axle brackets of the “I”-shaped single-arm bracket and “V”-shaped double-arm bracket steel casting is spliced from multiple steel casting sections on a horizontal iron platform jig. To ensure the splicing precision quality, the following precision control requirements are imposed for the axle bracket splicing:
(1) Drawing the benchmarks: After drawing the center benchmark of the steel casting axle hole and the splicing positioning benchmark on the platform, the steel casting is lifted onto the jig for positioning. The plumb bob should ensure that the concentricity of the steel casting placement axle hole is 1 mm and coincides with the center and splicing seam position on the platform.
(2) Welding method: Symmetrical welding is adopted. After welding the bottom layer of the butt joint, double-person symmetrical welding is performed. During the welding process, real-time monitoring is conducted to check for any deformation of the bracket. If deformation occurs, the welding sequence should be adjusted.
(3) Post-welding measurement: After welding is completed, the steel casting is allowed to cool completely before measuring the axle hole and upper opening dimension data to ensure that the splicing precision of the steel casting is controlled within 2 mm.
3.2 Precision Control of Stern Casting Pipe Splicing
The stern casting pipe body is spliced from steel casting and steel pipes. The specific precision control method for the splicing is as follows:
(1) Assembly and positioning stage: Two steel casting tube bodies are connected with one steel pipe. During splicing and positioning, a 2-3 mm contraction allowance is added to each butt weld. During the tube body splicing process, a 2-3 mm welding reverse deformation is placed in the middle of the first and second butt weld positions to control the weld gap contraction and height direction deformation during welding.
(2) Post-splicing measurement: After the tube body is spliced and formed, the concentricity position detection point inside the tube body is moved to the outer surface of the steel casting tube body, and six detection marks are made. A three-dimensional total station is used to replace the traditional steel wire method for concentricity detection.
(3) Welding process monitoring: During the welding of steel casting, the outer points of the tube body are detected to analyze the changes in tube body straightness. Based on the straightness change data, on-site adjustment of the welding sequence is guided to ensure that the concentricity of the stern casting pipe body after welding is controlled within a 1.5 mm error range.
The control and detection method are illustrated in Figure 1.
Figure 1. Process Tracking Control Data Chart
3.3 Precision Control of Rudder Sleeve Pre-assembly
The two rudder bases on segment AG01C are located on two curved assemblies, respectively. The precision quality of the rudder base positioning directly affects the positioning precision of the rudder sleeve and the entire rudder system. To ensure the pre-assembly precision of the rudder sleeve, during the rudder sleeve positioning stage, the reverse deformation is added according to the rudder sleeve positioning scheme. By tracking the welding process data, the height direction data changes of the rudder base and rudder sleeve, as well as the concentricity changes in the front-back and left-right directions, are measured and observed. Based on the changes in the measured data, the welding sequence during the on-site construction process is guided to ensure that the concentricity precision of the rudder base is controlled within ±1 mm. The rudder sleeve precision control is illustrated in Figure 2.
Figure 2. Rudder Sleeve Precision Control (Unit: mm)
3.4 Pre-assembly Control of “I” and “V”-shaped Steel Castings in Segments
The axle brackets of the “I”-shaped and “V”-shaped steel casting is installed on segments AG02C and AG03C, respectively. Considering the high precision requirements of the shaft rudder system, it is necessary to ensure that the pre-assembly of steel casting is performed after the welding of all hull structures in the segments is completed.
(1) Selection of positioning benchmarks: After the welding of the hull structures of segments AG02C and AG03C is completed, a three-dimensional total station is used to conduct a complete measurement of the overall precision of the segments to clarify the front-back, left-right, and height direction benchmarks for the positioning of the “I”-shaped and “V”-shaped steel casting on the segments.
(2) Steel casting positioning: The steel casting is lifted to the corresponding positions on the segment structures. Crosshair marking points are set at the front and tail end faces of the steel casting axle holes, which serve as the positioning detection points for the steel casting axle brackets. Using three-dimensional detection technology and the pre-assembly positioning scheme, the steel casting is positioned. After the positioning data meets the requirements, the steel casting is assembled and welded.
(3) Steel casting welding: Clamps are used to fix the connection between the steel casting and the structures. Spot welding is used to fix the steel casting at strong structural positions, ensuring that the welding length between the hull strong structure and the end of the “V”-shaped steel casting is not less than 500 mm. Symmetrical welding is adopted to control the welding sequence and prevent axle offset. During welding, the outer plate and deck butt welds are completed first, followed by the welding of internal components. Simultaneously, the three-dimensional measurement method of the total station is used to monitor the center precision of the steel casting, with data detection performed at a frequency of once every hour. On-site adjustments to the welding sequence are made based on data changes.
(4) Special considerations for “V”-shaped steel casting: Due to the special structure of the “V”-shaped steel castings in segment AG03C, there is a deviation towards the shipboard side after welding. To ensure that the relative spacing and front-back direction error of the axle hole centers of the two “V”-shaped steel castings are controlled within the standard range after welding, the first steel casting on the port side is installed and welded first. After the welding of the first “V”-shaped steel casting is completed, the positioning and reverse deformation addition of the second “V”-shaped steel casting is performed based on the actual changes. During the welding process, the concentricity precision of the two steel casting should be considered to ensure that the error in the concentricity of the double axle holes is within ±1 mm.
3.5 Precision Control of Stern Casting Pre-assembly
(1) Selection of positioning benchmarks: After the welding of the hull structure of segment ED11P/S is completed, a three-dimensional total station is used to measure the overall precision of the segment to clarify the front-back, left-right, and height direction benchmarks for the positioning of the stern casting steel casting on the segment. According to the pre-assembly positioning scheme, the steel casting is positioned.
(2) Welding process control: After the positioning data meets the requirements, the stern casting steel casting is assembled and welded. The three-dimensional measurement method of the total station is used to monitor the center precision of the steel casting, with data detection performed at a frequency of once every hour. On-site adjustments to the welding sequence are made based on data changes to ensure that the concentricity of the stern casting steel casting is controlled within ±1 mm after welding.
4. Implementation of Launching Precision Control Scheme
During the launching stage, which is conducted in an open shipyard environment, changes in ambient temperature due to sunshine can lead to deviations in the positioning of segments related to the high-precision shaft rudder system, causing deviations between the centerline and the theoretical centerline. Therefore, the positioning and control of segments related to the shaft rudder system are performed during overcast days or at night without sunshine.
During the block assembly stage, the data of the twin-shaft rudder system are used for block positioning, and attention is paid to the arrangement of the axles倾斜 towards the stern to ensure that the spacing between the various shaft systems simultaneously satisfies the centering requirements [2]. The positioning benchmark for the axles is referenced based on the hull benchmark. Welding during segment launching can cause the segment to warp upwards or downwards due to welding stress, thereby affecting the hull baseline. In particular, the stern segments often experience excessive subsidence after launching. Therefore, reverse deformation is added to the segment launching, and the welding sequence is controlled to manage the subsidence of the stern segments in the height direction [3].
When positioning the stern segments of the shaft rudder system:
- Front-back direction: Positioning is based on the distance from the front end face of the steel casting to the reference frame, with a 4 mm reverse deformation added.
- Height direction: Positioning is based on the height of the main engine base, with an added reverse deformation of 8-10 mm.
- Left-right direction: Positioning is based on the center of the steel casting axle hole, with an added reverse deformation of 3-5 mm towards the shipboard side.
The positioning requirements for other segments are similarly detailed:
(1) Positioning requirements for segment ED11P/S stern casting:
- Front-back direction: A 4 mm reverse deformation is added based on the distance from the front end face of the steel casting to the reference frame.
- Height direction: An added reverse deformation of 8-10 mm is based on the height of the main engine base.
- Left-right direction: An added reverse deformation of 3-5 mm is based on the center of the steel casting axle hole towards the shipboard side.
(2) Positioning requirements for segment AG02C (double “I”-shaped steel castings):
- Front-back direction: A 4 mm reverse deformation is added based on the distance from the front end face of the steel casting to the front end face of the stern casting of segment ED11P/S.
- Left-right direction: Positioning is based on the axle and the concentricity of the steel casting, with an allowable error of ±1 mm.
- Height direction: An added reverse deformation of 8-10 mm is based on the height of the main engine base.
(3) Positioning requirements for segment AG03C/P/S (double “V”-shaped steel castings):
- Front-back direction: A 4 mm reverse deformation is added based on the distance from the tail end face of the steel casting to the front end face of the “I”-shaped steel casting of segment AG02C.
- Left-right direction: Positioning is based on the axle and the concentricity of the steel casting, with an allowable error range of ±1 mm.
- Height direction: An added reverse deformation of 8-10 mm is based on the height of the main engine base.
(4) Positioning requirements for segment AG01C (double rudder sleeves):
- Front-back direction: A 4 mm reverse deformation is added based on the distance from the rudder hole center to the tail end face of the steel casting of segment AG03C.
- Left-right direction: The concentricity of the two rudder hole centers is ensured, with positioning at ±1 mm.
- Height direction: An added reverse deformation of 13-15 mm is based on the height of the main engine base.
During the segment launching process, the positioning of segments related to the shaft rudder system is performed according to the positioning scheme. The three-dimensional measurement technology of the total station is used to position the segments. During the segment assembly and welding process, the changes in the concentricity data of the steel casting axle hole centers and rudder hole centers are monitored. Adjustments to the welding sequence are made based on data changes to ensure that the axle hole concentricity is controlled within an effective range. The data changes during the welding process are shown in Table 1.
Table 1. Data Changes During Welding Process (Unit: mm)
| Segment Name | Front-back Change Before and After Welding | Left-right Change Before and After Welding | Height Change Before and After Welding |
|---|---|---|---|
| AG02C | -11 | -2 | 12 |
| AG03C | 12 | -2 | 2 |
Table 1 provides an overview of the data changes during the welding process for the segments related to the shaft rudder system. The data changes reflect the precision control measures taken during the assembly and welding of these segments.
The table indicates the changes in the dimensions of each segment before and after welding, specifically in the front-to-back, left-to-right, and height directions. For instance, for segment AG02C, there is a decrease of 1 mm in the front-to-back dimension and an increase of 2 mm in the left-to-right dimension, while the height remains unchanged. Similarly, for segment AG03C, there is an increase of 2 mm in both the front-to-back and left-to-right dimensions, with no change in height. For segment AG01C, there are minor changes in all directions, but notably, there is an increase of 1 mm in the height.
These data changes are closely monitored and analyzed to adjust the welding sequence in real-time. By doing so, the precision of the axle hole concentricity and the overall installation of the shaft rudder system can be maintained within stringent tolerances. The effective control of these changes ensures that the final assembly meets the required precision standards, thus guaranteeing the smooth operation and reliability of the ship’s propulsion system.
The successful implementation of this precision control scheme not only improves the efficiency and quality of shipbuilding but also provides valuable technical insights and experience for future projects involving complex shaft rudder systems. The data collected during this process serves as a solid foundation for continuous improvement and innovation in shipbuilding precision control techniques.