Taking the construction of 20,000TEU lashing bridge as the main object of analysis, the process of lashing bridge is analyzed with the accuracy control as the core, the accuracy control requirements in the process of lashing bridge construction are proposed, and the problems arising in practice are summarized and some suggestions for optimization and improvement are put forward.
With the container ship becoming larger and larger, the maximum container weight on the deck of the ship is increasing, the lashing bridge structure requirements are getting higher and higher, the so-called lashing bridge is a plate frame structure composed of columns and thin plates, the structure itself is relatively weak if the size continues to increase, it is bound to affect the strength and stiffness of the lashing bridge. Of course, the manufacturing process of lashing bridge also has a significant impact on its stiffness and strength, must be in the manufacturing process, and strictly grasp the process requirements to ensure the strength and stiffness of the lashing bridge.
1. What is lashing bridge
In the lashing bridge construction process, the initial ground sample line and tire frame design layout play a very critical role in the subsequent assembly of the components, on the one hand, the rationality of the ground sample line and tire frame design layout will be directly related to the lashing bridge construction efficiency. On the other hand, it is necessary to strictly control the accuracy to ensure the high precision assembly of the segmental and total segmental construction process.
2. Lashing bridge process characteristics
At present, in the construction of the lashing bridge for mega container ships, there are problems such as poor level, poor clearance when the platform plates are closed together, poor spacing of individual columns, poor angle of inclined insertion plate, and too much spare space under the columns. The platform gap problem is because the size shrinkage is not standard, the precision control of the end face is not carried out in the site construction, the shrinkage occurs in the welding after stripping and the level is out of balance, which causes the size deviation. The large spacing of the column may be due to the sub-section shrinkage is not standard, the shrinkage is out of control, the welding is not carried out according to the welding sequence requirements, and after the sub-section is stripped, the welding volume is large, which causes the lack of shrinkage control factors. The problem of inclined insert plate is mainly not assembled according to the requirements, the margin is too large is the process precision control is not in place.
The design and layout of the ground sample line can effectively ensure the positioning efficiency of the main structural components and improve the assembly accuracy of the lashing bridge, so the pre-design and construction process of the lashing bridge layout of the ground sample line is a very critical link, the dimensional accuracy of these reference lines must be guaranteed, the accuracy requirements shall not deviate from the standardized size of 1mm. tire frame generally has a special purpose, itself does not have the universality of the construction of other segments, in order to meet the rigidity Under the requirement of meeting the stiffness, the tire frame should be refined and simple, easy to build and modify, and the cost should be low, which are the main considerations of the tire frame design. Of course, because the main structure is more slender girders, the reasonable arrangement of the tire frame support points has a significant impact on the control of member deformation and overall accuracy. Generally, the main structure of the segmental tire frame of the tied bridge is truss type, welded by the section steel, platform ground and column ground intersection position will generally use I-beam as support, and T-beam for vertical connection, because of the structural characteristics of the tied bridge, the height of the support points will be inconsistent, generally consider the relative height of the support points. Relative height is generally 500mm from the horizontal surface as the benchmark to determine, the benchmark height is set on the side of the steel pipe, and other points to this benchmark height as the benchmark to calculate the relative height.
3 lashing bridge construction process
3.1 Lashing bridge segmental assembly and precision control
After the segmental assembly and welding of the lashing bridge, after the accuracy check, the segmental production can be carried out on the tire frame. The assembly process is mainly divided into the following parts: stern square steel tube and I-beam column, stern walkway side plate and shear wall, walkway plate, bow walkway side plate and walkway side plate, and bow square steel tube and I-beam column. According to the arrangement of the ground sample line and tire frame, the assembly is carried out in sections, and the accuracy is required to be controlled, mainly with the following key points.
The alignment accuracy of the rectangular steel pipe and the ground sample line must meet the requirements. The upper and lower edges of the rectangular steel pipe must be aligned with the top reference line of the ground sample line and the top reference line of the hatch cover, while the side edges need to be aligned with the reference line of the guide column, and the alignment accuracy requires ±2mm.
Guiding column spacing, delineating the ground sample line guiding column datum line process, must ensure that the delineation accuracy to meet the requirements, shall not deviate from the calibration size ± 1mm, and the alignment accuracy between the rectangular steel shall not deviate from ± 2mm, but because of the guiding column vertical span problem, the assembly period may appear local spacing deviation, so the requirement of guiding column spacing deviation shall not exceed the calibration value ± 2mm.
Platform plate accuracy, guide column arrangement is completed, and accuracy report acceptance, install the walkway side plate and platform plate, against the benchmark of the fourth layer platform, scribe the platform plate installation accuracy line on the rectangular steel, and install the platform plate based on the accuracy control line, the verticality of the platform plate and guide column should be controlled within ±3mm, flatness is also required ±3mm, other walking platforms also take the fourth layer platform as the benchmark to determine the platform The installation position of the plate and the spacing, the deviation is required to be controlled within ±2mm.
3.2 Segmental pier resting
After the assembly of ship side segmental lashing bridge, it is necessary to lift the ship-side segmental out of the tire frame and put it on the special pier, the pier arrangement must be reasonable, otherwise, it will lead to deformation. It is generally required to determine the load according to the weight of the lashing bridge, and determine the number of piers. The spacing between the front and rear of the piers and the left and right of the piers need to be determined according to the structure of the lashing bridge, to ensure the stability of the lashing bridge structure and prevent deformation under the premise of completely bearing the self-weight of the lashing bridge. The shelving pier must be easy to remove, and the removal speed should be fast. In general, 150t pillar shelf pier, 18 can meet the 20000TEU ultra-large container ship lashing bridge use, layout position generally in the first platform, the fourth platform and the walkway side plate intersection position, because these positions structure strength is relatively large, the placement of shelf pier can ensure structural stability and not deformation.
3.3 Segmental combination assembly
During the installation of the segments, because they are flat on the tire frame and lying down, the segments need to be turned over 90° before the total assembly process, so that the segments are in an upright position, so the segments must be installed with welded turning bodies and temporary lifting lugs. The specific line will be lifted and turned over, the symmetrical centerline is vertically aligned with the reference line on the tire frame of the total group, pay attention to ensure that the centerline of the tied bridge corresponds to the centerline of the ground hull, after the basic in place, adjust the precise position. The bottom of the segmental column and tire frame channel steel for welding, 10mm welding foot, using steel wire rope tension fixed segmental piece body, crane loose hook, so lifting segmental according to the above operation steps to complete the segmental combination. The overall accuracy requirement is within ±3mm of the level, the overall dimensional tolerance is within ±8mm, the tolerance of the centerline distance from the guiding column is ±5mm, the flatness between the walking platform and the guiding column should be 7~9mm±0.45mm, the flatness between the tying platform and the guiding column should be 4~6mm±0.3mm, the flatness of the shear wall surface is 6~9mm±0.45mm, the platform The deflection between the item chain guide column is ±5～10mm±0.5mm.
4 Tie-up bridge-related process extension
Because of the large span and weak structure of the lashing bridge structure of mega container ship, the process of lifting, transporting and placing may affect the structure, and the process of controlling the process purely by accuracy cannot guarantee the final total group accuracy requirement to a certain extent, so some special processes are needed as support.
Firstly, because there are more deformation factors for the lashing bridge of 20,000 TEU ultra-large container ship, strengthening the support is essential. According to the previous description, the bottom of the lashing bridge should be made to strengthen the structure with reinforcement. In fact, when making the lashing bridge in the early stage, it will be strengthened to some extent, such as adding 1 crane code, but in fact, a crane code is obviously insufficient, in practice, after the steps of turning over, placing and lifting of the 20,000TUE container ship lashing bridge, there will be serious deformation, deformation up to 70mm, so the crane code must be strengthened, can increase the crane code to 4, improve the rigidity of the lashing bridge, reduce the deformation factor.
Secondly, during the lifting of the tied bridge, because the structure is weak, it is obvious that the square tube will appear in the arch phenomenon, this middle arch can not be recovered at a later stage, which will cause horizontal deformation, therefore, attention must be paid to the lifting process, by increasing the lifting points and reasonable arrangement of points, to ensure smooth lifting and not to damage the square tube.
Finally, the shelf pier placement optimization, because the segmental assembly process is more complex, need to move the tied bridge components several times, the shelf pier position will be displaced because of moving the tied bridge, need to reposition the shelf pier, this process will appear many deformation factors, and the tied bridge force point is limited, if the shelf pier position deviation, must cause deformation problems. Therefore, in order to further optimize the above arrangement of piers, by increasing the number of piers, increasing the contact point between piers and tied bridges, reducing the probability of deformation factors, and reasonably arranging the piers, mainly by adjusting the height of piers to correspond to the inconsistent height of tied bridges, and further monitoring the accuracy of the position of piers, and immediately correcting the displacement.
5 Conclusion
The process of lashing bridge of super-large container ship is relatively complicated, the core point of which lies in the assembly accuracy, the accuracy control is not good, the lashing bridge is bound to be problematic, affecting the whole ship tethering performance. For the analysis of lashing bridge accuracy control process, it is necessary to control the accuracy of the ground sample line and tire frame, while thinking about the optimization of the process accuracy process improvement, can further provide some reference to the mega container ship lashing bridge assembly accuracy, of course, there may be some shortcomings in the analysis of this paper, in the subsequent practice will be a further summary of experience, this continuous improvement.
