The rise of ultra large container ships has necessitated new lashing and twistlock solutions to optimise cargo capacity
Lashing and twistlocks are under the spotlight as never before, due to increase in the number of ultra large container ships (ULCS) able to carry stacks of 10 or more container tiers on deck.
Class societies have been active in their work within lashing and twistlocks for such vessels and have launched a number of initiatives in this area.
The results of research and development work carried out by structures and hydrodynamic experts at Bureau Veritas (BV) has led to a better understanding of the forces at play in large containerships – in terms of hull structures, cargo stowage, and propulsion systems.
The class society explained that this work enables greater flexibility in container loading and allows for heavier containers to be loaded higher in a stack than was previously possible.
Due to their long, slender hulls - which result from their operating speed requirements and the need for large deck openings to accommodate container bays - large containerships present specific structural challenges, according to BV technical director Jean-François Segretain. As container ships increase in size, so do these structural challenges, he said.
New BV structural rules for containerships, coupled with its new container securing and lashing software, VeriSTAR Lashing, have increased the understanding of the physics and engineering requirements of large containerships, Mr Segretain said.
“As container ships increase in size, so do the structural challenges”
He explained that acceleration loads applied in the new rules (arising from the surge, sway, heave, roll, pitch and yaw motions of the ship) have been more precisely determined, allowing them to be lowered. “Reduced accelerations mean less force acting on the lashing rods, allowing operators to optimise cargo capacity, not just in terms of being able to add more containers, but also in how the containers are distributed within each stack. This has the potential to save significant sums of money and time,” Mr Segretain said.
The revised wave loads, which now underpin container ship rules, have helped optimised ship structures. But an additional and highly significant consequence of their application is that they now also facilitate better container lashings, offering operators much more flexibility.
Within the new rules, there is a chapter about carrying out lashing calculations, based on the new acceleration calculations. VeriSTAR Lashing enables operators to apply these calculations as efficiently as possible.
“The strength of VeriSTAR Lashing is that it is linked to direct and powerful computation, using state-of-the-art hydrodynamics and real sea states – the same design wave computation that underpins the structural rules,” said Mr Segretain. “In the past, calculations for lashing were based on empirical formula; today, better calculations mean reduced accelerations and more flexibility,” he explained.
A key feature of the software is that it takes account of the additional forces on the lashing bars and rods, resulting from vertical and horizontal gaps in the twistlocks and between containers. “The influence of gaps is now clearly understood, though previously it had been disregarded,” Mr Segretain said.
Maximising stack potential
Elsewhere, Lloyd’s Register (LR) has been working closely with ULCS. It has classified numerous such vessels with TEU in the 20,000 range, leading LR lead specialist structural analysis, marine and offshore, Seb Brindley to comment: “With over 50% of their cargo on deck, the importance of on-deck stack security and safety is vital for commercial success.”
Seb Brindley (LR): LR's methodology means that the importance of the lashing bridge strength...and twistlock separation can be investigated
LR has developed a methodology which it says helps operators maximise their vessels’ potential “This unique methodology is specifically engineered and validated to capture the stack behaviour, allowing clients to quickly, easily and reliably ascertain the stack safety… the importance of the lashing bridge strength, container corner post rigidity and twistlock separation can be investigated,” said Mr Brindley.
“With over 50% of their cargo on deck, the importance of on-deck stack security and safety is vital for the commercial success [of ULCS”
Singling out the closed end of a typical 40 ft inner stack with a single external lashing, and homing in on twistlock separation, he said this is inherent in the twistlock design, due to a slight gap or play in its connection with the corner casting allowing it to be easily fitted and removed. The gap between two container corners connected with a twistlock can increase about 18 mm under tension. “For external lashing arrangements, this phenomenon not only causes a substantial increase in the lashing rods’ load, but also reduces the compressive load in the corner posts,” Mr Brindley said.
He noted for the examined stack there are four main states, due to the interaction between twistlock separation, lashing-rod loads and lashing bridge stiffness:
Mr Brindley summarised that “twistlock separation, lashing bridge deformation and corner post compression influence the stack behaviour.”
KR updates software
In October 2018, Korean Register (KR) released the latest version of SeaTrust-LS, its container-securing strength assessment software. KR said that SeaTrust-LS has been significantly enhanced by incorporating KR's new 2018 guidance for container securing and reflects practical insights and feedback from world-class lashing makers, SEC Bremen and German Lashing Robert Böck GmbH.
KR said that optimal container stowage and arrangement is “more critical than ever” as the trend for ever-larger containerships persists – with ULCS now carrying stacks of 10 or more container tiers on deck.
KR has revised its guidance for container stowage and lashing, after conducting ship motion analysis for different sizes of container ships from 1,000 TEU up to 23,000 TEU. Analysis included optimising accelerations, nonlinear analysis for calculating accurate external lashing forces, and computational fluid dynamics analysis under various possible scenarios relating to the application of reasonable wind forces.
The latest version of SeaTrust-LS contains all revisions of this latest guidance. The new SeaTrust-LS applies the semi-nonlinear calculation method to consider the twistlock separation effect of external lashing. This ensures the safe securing of external lashing, as well as speeding up lashing strength assessments. The new software provides a range of features, including the calculation of optimal acceleration and wind force, together with 13 route reduction factors enabling container ships to maximise their cargo capacities while ensuring the safety of the ships themselves.
SeaTrust-LS also includes a feature that automatically identifies maximum cargo capacity. This function helps users optimise design stack weight, lashing bridge design and container stowage arrangements, enabling the operator to select the best stowage arrangements under specified targets.