Taking 37300DWT multipurpose bulk carrier as an example, the design optimization of deck loading timber lashing system of multipurpose bulk carrier is discussed. It focuses on the discussion and analysis of the problems encountered in the design and installation of the timber lashing system, and proposes reasonable design suggestions and solutions to improve the safety, reasonableness and economy of the timber lashing system of multipurpose ships through comparison and according to the actual production of shipyards.
International ocean-going bulk carriers In order to improve the function of bulk carrier loading, often bulk carriers are also used to transport timber. For bulk carriers not only require the loading of timber in the cargo hold but also on the open deck in order to improve the economic efficiency. IMO 2011 came into effect after all deck timber ships need to meet the “Practical Rules for the Safety of Ships Carrying Timber Deck Cargo” (hereinafter referred to as “the Rules” in the relevant provisions). The deck timber system is mainly composed of fixed columns, invertible columns, protective frames and lashing ropes arranged on both sides of the ship, see picture 1. Fixed posts are installed on the side of the main deck between the cargo holds, and reversible posts are installed between the fixed posts. When the deck surface is not loaded with timber, the reversible columns will fall down towards the transom in order not to affect the cargo lifting. When the deck surface is loaded with timber, the invertible columns on the side of the main deck are pulled to the vertical state by the cargo crane arranged in the middle of the ship between the cargo holds and connected to the fixed columns with steel ropes and unloading buckles. The timber on the deck can be fastened to the deck by arch back lashing, cross lashing chain and swinging rope. We focuses on the problems encountered in the design of deck timber system of 37300 multi-purpose bulk carrier under the premise of meeting the Rules, and proposes solutions to the problems through design optimization.
1 Optimization of column design
The deck columns are arranged on both sides of the main deck in the cargo area of the ship, and the columns are generally divided into fixed columns and invertible columns. The fixed column generally uses steel pipe, and the invertible column uses H-beam as picture 2.
37300 multi-purpose bulk cargo according to the “rules” in the preliminary calculation of the fixed column selected ?457 × 20 seamless steel pipe, easy to directly purchase, without too much processing, convenient construction. Invertable column selected is welded non-standard H section steel, the web is 350 × 12mm, the panel is 250 × 25mm steel plate, such welded H section steel shipyard need to purchase steel plate, cut and processed, assembled and welded, due to the thin web, under the thermal influence of welding assembly welding profiles often produce distortion, welding H section to be corrected by annealing, greatly increasing the workload and costs . The bending strength of H section in the X direction is mainly considered in the shipping timber voyage, and the assembled welded H section of this ship ensures the premise of section modulus, and the standard H section is selected by calculation instead of assembled section. The X-direction moment of inertia of assembled section H400×250/12×25 is calculated as follows
Formula 1 in B, H, b for the H section steel size code as picture 2. take the value in accordance with the original welded non-standard H section steel size.
X-directional section modulus calculation.
Formula 2 in H for H-beam size code as picture 2. take the value in accordance with the original welded non-standard H-beam size.
According to the formula 1, 2 calculated moment of inertia and cross-sectional modulus, the choice of standard steel is not less than the calculated results. Check the hot-rolled H-beam GB/T 11263-1998 standard, you can choose H506×201/11×19, according to the requirements of buyers, in the design stage can be appropriate to increase the cross-sectional modulus to offset the corrosion loss in the shipping process, etc.
The ship’s center of gravity tends to be high when timber is loaded on deck. In order to increase the stacking area of the deck surface between the columns and reduce the loading height of timber as much as possible, the columns are generally arranged at the widest point on both sides as far as possible, close to the gangway. During the loading of timber, the gangway will also support the invertible columns. This design often brings the invertible column base and the invertible column upper rotation device too close to the gangway, and the pin is difficult to construct in the installation stage. When the length of the pin at the top end of the invertible column exceeds the H-beam panel, the column will often touch the wall when the turning pin passes through the wall in the process of closing and placing, resulting in the column not being able to be placed or pulled up. The optimized design of the 37300 multipurpose bulk carrier has designed the base of the column and the pin plate of the upper turning device near the middle of the ship, and the other end of the pin near the side of the gangway does not exceed the H-beam panel as shown in picture 3.
37300 multi-purpose bulk carrier side column is 8.5m high, and the upper end of the inverted column can be found sometimes touching with the side wall during the pulling and placing process. When the cargo crane lifts the invertible column through the wire rope, the invertible column tends to sway from side to side, and when the invertible column swings to the side, the upper end of the invertible column is sometimes stuck by the gangway wall, and at this time the cargo crane is still lifting upward, and the cargo crane hook head is under great tension, which brings great hidden danger to the safety of site operation. The gap between the lower panel of the invertible column and the base was found to be 32mm, which gave the 8.5m long invertible column a large free swinging space. The gap was adjusted to 6mm by welding 13mm washers on both sides of the base support plate, and the span distance between the base plates of the column was reduced to control the large sway of the invertible column. At the same time, a suitable half-round steel was installed as a guide plate at the intersection of the pulling track and the gangway panel at the upper end of the invertible column. Through the improvement, the problem of collision between the invertible column and the gang wall during the pulling and releasing process is solved.
2 Optimization of connection system design
The invertible column is pulled and released by cargo crane on board. The columns are connected in series with steel wire rope, unloading buckle, loosening screw buckle and pulley. When the ship is loaded with timber, the ship’s cargo crane lifts the invertible column in series and pulls the invertible column on the deck to the vertical state, which is 90° with the limit plate set on the base, as in picture 4.
Adopt fixed length connecting wire rope and unloading buckle to connect the column in series. Such a design will also bring the invertible column can not be pulled to the vertical state after repeated use. For example, the 37300 multi-purpose bulk carrier is connected with fixed-length wire rope and buckle between invertible columns, and after repeated use, the wire rope is plastic deformed and stretched longer, which directly leads to slackness between invertible columns. The distance between the invertible columns is fixed, but the plastic deformation between the invertible columns occurs in the steel wire rope, and the limit plate is installed on the base, so the other invertible columns can not be pulled to the vertical state. Optimized by design, one end of the connecting wire rope is designed in the form of fixed pressure buckle and the other end is designed in the form of 3 sets of clip buckle, when the connecting wire rope is stretched after repeated use, can use the clip buckle to adjust the length of the wire rope.
One end of the lifting wire rope is connected to the upper end of the first column between the fixed columns with a discharging buckle and passes through the carriage at the upper end of the fixed column on the bow, and the other end is fastened to the hook head of the cargo crane, so that the columns can be pulled up in turn in the process of the cargo crane walking upward. The original intention of the design is to lift the wire rope in the process of lifting the hook head of cargo crane slowly stressed, in the state of stress the carriage is turned from down to up and the lifting wire rope runs in the carriage slot. As the carriage is downward under the action of gravity, often in the lifting process is not easy to turn upward, and sometimes the lifting process will also appear stuck situation. The 37300 multi-purpose bulk carrier is designed and optimized to arrange the same type of carriage on the main deck between the fixed column on the bow and the first invertible column. One end of the hoisting wire rope is connected with the eye plate unloader at the top end of the invertible column, and the other end passes through the carriage on the fixed column and the carriage on the deck in turn and then is fastened to the head of the cargo hook to change the direction of hoisting wire rope travel. Practice has proved that the lifting process is not only safe, but also easy to use, avoiding difficult operation and unsafe factors, and winning the buyers’ approval after design optimization.
3 Conclusion
The design of 37300DWT multi-purpose bulk carrier deck timber lashing system is optimized under the premise of satisfying the Rules, which makes the installation and use more simple, convenient and reasonable. It has been proved that the optimized design of 37300DWT multi-purpose bulk carrier with column system and lashing system saves time and cost for shipyard and avoids a series of problems in construction, installation and use, and also wins the affirmation of shipowner.
