December 2001
Introduction
The practice of coupling two or more containers together vertically into a unit and then lifting the unit is known as vertical tandem lifting (VTL). The containers lifted by this method are known as VTL units. This practice has been in common use for several years. In the late 1980's and early 1990's the U.S. Occupation Safety and Health Administration (OSHA) permitted the lifting of two empty freight containers vertically coupled by twist locks. This authorisation was then widened to included the lifting of VTL units loaded with automobiles.
Following these authorisations VTL operations have become more common and the International Organisation for Standardisation (ISO) is being asked to produce standardised procedures, based upon a full engineering analysis, for handling VTL units. The ISO Technical Committee (TC104) is currently circulating a discussion paper that concludes containers, their fittings and twist locks specified in the standards have sufficient structural strength to allow VTL operations to be safely carried out within specified limits. Further, the Technical Committee determined that the design strengths of containers was such that partly loaded containers could safely be handled as VTL units. The final text has yet to be adopted by ISO member organisations; at the first round of voting, the text achieved acceptance with only one dissenting vote and it is anticipated that at the second round, due in the near future, voting will be unanimous.
General standards
In developing the ISO standards a structural safety factor of 5 has been used, based on the ultimate strength of the component twist locks, corner fittings, corner posts etc for a VTL unit . Additionally, extra safety factors were introduced using severe wind loadings and these considerations have led to the conclusion that a gross mass of up to 20,000 kgs could be safely handled as a VTL unit. However, if any of the containers in the unit is a thermal or refrigerated container, the maximum gross mass should not exceed 18,000 kgs. No tank containers, full or partly loaded with liquid, should be handled in a VTL unit; however, these may be handled in an empty condition.
Twist locks or latch locks used in VTL operations must be certified for lifting with a safe working load (SWL) of at least 10 mt. For the purposes of the ISO standards, to avoid confusion, twist locks and any other type of inter-box connector that may be used for lifting, are known as "lift locks". These "lift locks" will be considered to be loose gear, thus each lift lock must be of good design and construction, of adequate strength, maintained in good repair, used in a safe and proper manner, tested and thoroughly examined by a competent person. Further, lift locks must be periodically thoroughly examined by a competent person, such examinations to take place at least once every twelve months and to comply with ISO 3874 standards.
The effects of wind speed on VTL units has been determined and operations should not be permitted when speeds exceed 55 kph or 30 knots.
It should be noted that the heaviest container should be in the lowest position of a VTL unit and the lightest on the top.
The examination of containers is an existing International Maritime Organisation (IMO) requirement in accordance with the International Convention for Safe Containers (CSC) to maintain a high level of safety for human life and for the transport and handling of containers. It is quite obvious that owners of containers have no means of knowing whether or not their containers are likely to be lifted as a VTL unit. Therefore, all thorough examinations of containers must include the top and bottom corner fittings (which are often referred to as corner castings).
Ship Operations
Much of the onus of VTL operations is attached to the ship which is required to ensure that the lift locks used for these operations are certified by a competent person or national organisation. Further, the ship has a duty to maintain and examine lift locks regularly, at least once every 12 months, and the master will have the added responsibility of ensuring that a maintenance programme exists, is carried out efficiently and on time. It is the ship operator's responsibility to supply the lift locks and they have a duty to provide certificated evidence to terminal operators that each lift lock has been tested in compliance with International Labour Organisation (ILO) regulations
To complicate matters, twist locks and latch locks with an approved SWL of 25 mt for lashing and securing containers to the deck will not necessarily be certified for lifting nor will they automatically qualify as "lift locks" able to lift the required 10 mt SWL. The ship must either be equipped solely with lift locks or accept the responsibility for differentiating clearly between the lift locks and other twist locks that may be on board.
A critical area of safe VTL operations is planning: Stowage plans should clearly indicate the position and the gross mass of all VTL units in the stow.
Conclusions
The ISO Technical Committee analysis was performed on the assumption that no more than 3 tiers of ISO Series 1 freight containers would be handled in a single VTL unit and has indicated that VTL on a wide variety of container equipment is definitely feasible from a structural standpoint. Calculations justifying VTL handling have been based on chains of very conservative assumptions that would be unlikely to occur simultaneously in an operations environment.
The intended requirements for lift locks will be phased in over several years. At present, all lift locks must be tested under the direction of a competent person or organisation and certificated for an SWL of 10mt. After 31st December 2005 all lift locks will have to be marked so that they can be associated with an SWL certificate. After 31st December 2009 individual lift locks will be required to be identified by certificate and the person or organisation owning the lift locks will need to maintain records during life of the lift lock. These records will be inspected at the request of the competent authority.
