Feasibility and benefit of different lashing arrangements for sea transport of containers on weather deck

Abstract

Nowadays, seagoing container ships carry cargo containers stowed on weather deck up to eight or even nine tiers high. In heavy sea conditions or in case of improper stowage, deck containers and their securing equipment are exposed to extreme forces caused by sea induced ship motions, gravity, wind, and green water. An inadequate dimensioning of container securing system may result in container damages and losses. An effort has been made by shipping industry and classification societies to prevent failure of container securing systems and, thus, to minimize container damages and losses at sea. On the other hand, due to economic reasons, shipping companies aspire to optimize the utilization of their fleet. Consequently, suppliers of container lashing equipment develop alternative methods for container securing on weather deck, such as external lashing, promising higher container capacity of existing ships. To ensure safe and reliable container transport on board ships, a particular verification of those novel container securing systems is required for approval of container stowage and lashing plans by classification societies. The basic difference between the often used internal lashing to the alternative external lashing system is that the first one secures the compressed side of the stack, while the second one secures the lifted side of stack. Thus, with internal lashing the compressive load acting on container posts is increased by the vertical component of the lashing force, limiting the allowable stack weight. This limitation does not occur when external lashing is used, allowing higher stack weights and better cargo distribution. In the other hand, the lashing force can be a limiting point due to the overloading caused on the lashing rod by uplifting, especially when high vertical clearance is allowed on the locks. Parametric studies were conducted with FEM models of container stacks using external lashing system. Parameters as lashing rod pre-tension, twist lock stiffness, lashing rod and lashing bridge stiffness and vertical clearance of twist locks were analyzed. The effect of these parameters on the operational loads acting on containers and lashing equipments was studied, and the parameters of influence and their sensitivities were determined. Using the outputs of the parametric studies, practical design criteria for stowage systems using external lashing were derived. The vertical clearance of twist locks was the most relevant design criteria for single stacks, having a strong influence especially on the lashing forces due to the uplifting. The determination of the lashing forces as a function of relevant system parameters, e.g. height of lashing bridge and twist lock type, was necessary. Comparative analyses of internal and external lashing regarding allowable stack weights and cargo distribution were performed. The comparison was made for each stack configuration (stack height and lashing bridge configuration), showing the benefits and drawbacks of each lashing system for the shipping companies.