With the development of the global economy, more and more cargoes show the trend of large-scale and heavy-duty, from the initial electrical equipment to the current jack-up platforms, warships, engineering vessels, yachts, and semi-submersible platforms, TLP platforms and other deep-water floating platforms, etc. The weight of cargoes also rises from 100 tons to 1,000 tons or even 10,000 tons, therefore, the market of large marine transportation comes into being.
For large engineering vessels and other floating body cargoes, the form of dry towing by semi-submersible vessels is mainly used for ocean transportation. Dry towing refers to the use of semi-submersible vessels for loading and transporting floating body cargo, its forward resistance is small and the transport speed is faster, especially dry towing through self-propelled semi-submersible vessels, the transport speed is faster and less affected by weather, which can obviously save time cost.
For the ocean transportation of cargoes, its lashing and fixing are especially important, and the feasibility of the lashing and fixing scheme is directly related to the safety and economy of the ocean transportation route of cargoes.
1 Conventional lashing and fixing form of cargoes
Cargo transportation at sea will move together with the ship and is influenced by waves, currents, wind and other weather. The lashing of cargoes can resist the mutual movement between cargoes and ships, thus ensuring the integrity of cargoes and the safety of navigation. Depending on the type and size of the cargo, the form of lashing will be different. The following are two typical types of lashing and fixing for transportation of bulky cargoes.
·Fixed platform: Usually welded with fascia plates and diagonal ties, which connect the barge deck with the upper group block.
·Large floating cargo: usually fixed standard parts are used for lashing and fixing. After the semi-submersible is loaded with cargo in position, the fixing standard is installed on the outside of the cargo according to the designed position. The fixed standard parts are welded above the deck of the semi-submersible vessel to reduce the welding between the fixed structure and the cargo and maintain the integrity of the cargo structure.
2 Lashing and fixing solution for extra-wide engineering ship
In the process of ocean transportation, the arrangement of the engineering vessel on the deck of the semi-submersible directly affects the design of the lashing and fixing solution. For the engineering ship that meets the following two conditions, it is classified into the category of extra-wide engineering ship.
·The width of the engineering vessel is greater than or equal to the width of the semi-submersible ship deck, and the length of the engineering vessel is less than the length of the semi-submersible ship deck.
·The width of engineering ship is less than the width of semi-submersible ship, and the length of engineering ship is greater than or equal to the width of semi-submersible ship. When the engineering ship is dry towed, its length direction is arranged along the semi-submersible ship width direction.
For the above two cases of extra-wide engineering ship, the limitation of semi-submersible ship deck width makes it impossible to lash and fix the cargo on the left and right sides of the semi-submersible ship, and then it is necessary to consider designing the special type of lashing and fixing structure in the direction of the semi-submersible ship deck width to make it meet the strength and transportation requirements.
2.1 Basic principles of lashing and fixing design
For the design of lashing structure for ocean transportation of extra-wide engineering vessel, the following basic principles should be followed with reference to the relevant requirements of Noble Denton Code.
·The load-bearing capacity of the lashing fixed structure must be sufficient to resist the ultimate design load of the cargo obtained from the design calculation.
·The semi-submersible will have a mid-arch or mid-drop under the wave load, and there is a certain relative motion between the cargo and the deck of the semi-submersible. Therefore, the lashing fixed structure needs to meet the requirements of relative displacement when it is designed. Usually, the lashing fixing structure releases this part of relative displacement.
·The lashing fixing structure is allowed to bear positive pressure in one direction only.
·cargo is not designed with lashing fixed structure in vertical direction and is supported in vertical direction only by sleepers, which are fixed above the deck of the semi-submersible vessel using fixed structure.
·The lashing fixed structure of an engineering vessel is usually designed at the bottom, and its design load only considers the horizontal motion load of the engineering vessel, while the moment load is not considered
·The lashing fixed structure is usually made into the form of standard parts, and the number and type of which are combined according to the calculation result of the ultimate design load of the engineering ship and the design bearing capacity of a single standard part. The design bearing capacity of a single standard member cannot be too large and needs to meet the requirements of engineering ship and semi-submersible ship on the local strength of the structure, and also needs to meet the requirements on the welding strength.
2.2 Detailed design of transverse lashing fixed structure
Integrating the above basic principles, the transverse fixing standard parts of engineering ship can be designed into the structural form of upper and lower combination, and at the same time can save certain space.
In order to ensure that the structural strength of cargo and semi-submersible ship meets the design requirements, the design capacity of single piece of transverse lashing fixed standard member cannot be too large, and the maximum design is 200 tons/pc. The selection of structural dimensions of transverse fixed standard members is carried out in accordance with the specification hand calculation, which mainly contains the following aspects: calculation of the compressive capacity of the cross section of the circular tube, shear, tensile, combined stress and flexural capacity verification of the square shear plate, and weld strength verification at the welding of the square shear plate and the engineering ship (or semi-submersible ship).
According to the type, weight and outline size of engineering ship, the transverse lashing fixed standard members can be combined according to the corresponding angle and quantity, so as to carry out the design of transverse lashing fixed structure. The combination of fixing standard members is carried out in accordance with the following basic requirements.
·The combination of lashing fixed standard parts of the same group, the axis of its circular tube must be guaranteed in the same horizontal line. If the profile of the goods in the direction of lashing fixing changes, the upper part of the shear plate of the fixed standard parts is released in accordance with the actual situation.
·In the same group of fixed standard parts, the number of fixed standard parts of the lower part welded with the deck of the semi-submersible vessel is one more than the number of fixed standard parts of the upper part welded with the bottom of the engineering vessel
·The space at the bottom of the engineering ship is limited, if a group of fixed standard parts is designed separately on each side, its quantity cannot meet the design requirements: it can be designed in the form of multiple groups combined on each side, and the staggered design between each group can ensure the feasibility of welding work and welding strength.
The structural form can resist the load in both directions of port and starboard at the same time and play the purpose of limiting the lateral movement of the engineering ship.
2.3 Detailed design of longitudinal lashing structure
The longitudinal fixing structure of engineering ship can be designed directly by using the following fixing standard parts.
For the same consideration, the design capacity of the single piece of longitudinal lashing fixed standard parts should not be too large, and the maximum design is 150 tons per piece. The selection of structural dimensions of longitudinal fixed standard members is carried out in accordance with the specification hand calculation + finite element calibration.
The size of adapter structure, according to the outline of the bottom shape size of the engineering ship for the actual release, the contact position between the adapter and the fixed standard is designed as flat plate structure, and the rubber liner is added at the end of the flat plate structure, through the mutual extrusion between the rubber liner and the fixed standard to achieve the purpose of limiting the movement of the engineering ship. Due to the longitudinal lashing fixed structure, it can only resist the horizontal motion load in a single direction; at the same time, the motion of the ship is not definite when the semi-submersible is sailing at sea, which determines the uncertainty of the motion direction of the engineering ship it carries. Therefore, similar structures need to be designed at both the bow and stern of the engineering vessel (in the direction of the captain of the semi-submersible) so as to meet the transportation requirements.
3 Conclusion
This lashing and fixing solution, with an extended design based on the conventional structural lashing and fixing solution, satisfies the design requirements of relevant codes, and also meets the requirements of structural integrity of cargo and structural strength of semi-submersible, which provides some reference for ocean-going transportation of special cargo.
