How to Perform Marine Lashing of Cargo for Ocean Transport
With the development of the global economy, more and more goods are becoming larger and heavier. From the initial electrical equipment to today's jack-up platforms, warships, engineering ships, yachts, semi-submersible platforms, TLP platforms, and other deep-water floating platforms, the weight of goods has increased from hundreds of tons to thousands of tons or even tens of thousands of tons. Thus, the market for maritime heavy cargo transportation has emerged. During the process of ocean freight transportation, the lashing and fastening of cargo is particularly important. The feasibility of a marine lashing and fastening plan is directly related to the safety and economy of the ocean transportation route.
Conventional Forms of Marine Lashing and Fastening for Cargo
During maritime transportation, cargo moves together with the ship and is significantly affected by weather conditions such as waves, currents, and wind. Lashing the cargo can resist the mutual movement between the cargo and the ship, thus ensuring the integrity of the cargo and the safety of navigation. Based on the different types and shapes of cargo, marine lashing and fastening techniques also vary. Typical forms of marine lashing and fastening for two types of heavy cargo transportation include:
Fixed Platforms (jackets and topsides)
Usually welded and secured using stiffeners and diagonal braces. The diagonal braces connect the barge deck with the topsides (or jackets).
Large Floating Cargo (engineering ships)
Typically secured using standard fixing components. After the semi-submersible ship is loaded and the cargo is positioned, standard fixing components are installed on the outer side of the cargo according to the design position. The fixing components are welded on top of the semi-submersible ship's deck, reducing the welding between the fixed structure and the cargo, thereby maintaining the integrity of the cargo structure.
Marine Lashing and Fastening Solutions for Ultra-wide Engineering Ships
During ocean transportation, the arrangement of engineering ships on the deck of semi-submersible ships directly affects the design of the marine lashing and fastening plan. Engineering ships that meet the following two conditions fall into the category of ultra-wide engineering ships:
Engineering ship width is greater than or equal to the width of the semi-submersible ship deck, and the engineering ship length is less than the length of the semi-submersible ship deck.
Engineering ship width is less than the width of the semi-submersible ship, and the engineering ship length is greater than or equal to the width of the semi-submersible ship. During dry tow, the engineering ship is arranged lengthwise along the width of the semi-submersible ship.
For the above two types of ultra-wide engineering ships, due to the width limitation of the semi-submersible ship deck, lashing the cargo is not possible on the port and starboard sides. Special types of lashing and fastening structures designed in the width direction of the semi-submersible ship deck are required to meet strength and transportation requirements.
Basic Principles of Marine Lashing and Fastening Design
The load-bearing capacity of the marine lashing and fastening structure must be sufficient to resist the extreme design load of the cargo obtained through design calculations.
The semi-submersible ship experiences hogging or sagging under wave load, and there is a certain relative movement between the cargo and the semi-submersible ship deck. Therefore, the marine lashing and fastening structure design needs to meet the requirements for relative displacement. Typically, the marine lashing and fastening structure compensates for this relative displacement.
The marine lashing and fastening structure is only allowed to bear uni-directional positive pressure.
The vertical direction does not have a marine lashing and fastening structure. In the vertical direction, only skids are used for support. The skids are fixed on the top of the semi-submersible ship deck using a fixed structure.
For engineering ships, the marine lashing and fastening structure is usually designed at the bottom. The design load only considers the horizontal movement load of the engineering ship, not the bending moment load.
Marine lashing and fastening structures are usually made as standard components. The number and type are combined based on the engineering ship's extreme design load calculation results and the design load-bearing capacity of single standard components. The design load-bearing capacity of a single standard component should not be too high and must meet the local strength requirements of the engineering ship and semi-submersible ship structures, as well as the welding strength requirements.
