Introduction:
The development of containerization was a giant step forward in carrying general cargo by sea. At the time, it was correctly predicted that unit costs would fall and cargo damage become a thing of the past. This has been true until recently.
To gain full advantage of containership operations, the container must be moved efficiently, speedily, and safely. This is accomplished by using special handling and lashing equipment as well as proper labeling on board vessels and in ports. The type of lashing systems varies from ship to ship. However, the most reliable system consisted of stacking cones, twistlocks, lashing bars, bridge fittings and turnbuckles (bottle screws).
This assignment is intended to provide information about the lashing equipment types, its ease of use and any cost savings that could be made by its utilization onboard a container vessel, so as to give a better understanding of the importance of these lashing systems.
Due to the rapid technical development in the container securing field, this assignment will also describe the effective use of software in order to reduce lashing costs.
The containers stowage:
To understand the aspects of the lashing systems, I should draw attention to the fact that Containers are generally stowed in the fore and aft direction. This stowage method is sensible with regard to the interplay of stresses in rough seas and the loading capacity of containers. Stresses in rough seas are greater athwartships than fore and aft and the loading capacity of container side walls is designed to be higher than that of the end walls. However, on many ships the containers are stowed in athwartships bays or are transported athwartships for other reasons. This must be taken into consideration when packing containers and securing cargo. Moreover, care must be taken to ensure access between containers for checking, lashing, and unloading (cargo notes, by Dhananjay Swadi).
The lashing equipment:
The components of the lashing system carried by the majority of modern vessels are generally type approved by one of the major classification societies. The wide range of lashing components each has different functions that form a part of the lashing system as a whole (TMC Marine Consultant). These components can be divided into two categories as follows:
1) Fixed fittings:
These are permanently attached to the ship. Below is a list of these fittings with their area of use and comments:
Flush sockets which are used for the locating of base twistlocks or stacking cones in the cargo hold. They are normally used over a small recess to ensure watertightness. Moreover, the ones used on deck are called Raised sockets.
Lashing plate or Pad-eye is used to tie down point for turnbuckle on deck or hatchcover. It is designed only for in-plane loading because out-of-plane load could bend the plate and may crack the connection weld.
D ring is an alternative tie down for a turnbuckle but has the advantage that it can be used for in-plane and out-of-plane loading unlike lashing plates. The ship’s crew should bear in mind that corrosion of the pin ends can weaken the D ring.
Dovetail foundations are used as base for sliding twistlocks. They must be checked for damage or wear before using.
Fixed stacking cones and Mid-bay guides are used to prevent horizontal and transverse movement of 20-foot containers in 40-foot cell guide, respectively. Fixed stacking cones can be found at the base of a cell guide and Mid-bay guides are fitted at tank top level and they do not interfere with general stowage of 40-foot containers.
2) Loose fittings:
Loose fittings are those that are not permanently attached to the ship. Loose fittings must be certified. However, they are not normally surveyed by the class surveyor during regular ship surveys but it is essential to follow the manufacturer’s instructions at all times. The list below shows the most commonly used components:
Lashing rods are used to provide support for container stacks on deck when used in conjunction with turnbuckles. Notably, very long lashing bars can be difficult to handle and difficult to locate in a container corner casting. They have the ability to resist tensile loads.
Extension pieces are used to extend a lashing rod when securing ‘high cube’ containers. They fit at the base of a lashing rod and connect to the turnbuckle.
Turnbuckles -also know as bottle screws- are used to connect a lashing rod to a lashing plate or D ring. Tightening puts tension into a lashing rod. They resist loads and used to keep the lashing tight. Cautionary, ensure the locking nut or tab is locked.
Penguin hooks are used as a supporting device in conjunction with a special lashing rod with an eye-end. Likely, fitted in place when the container is still ashore because of difficulty in fitting when aboard. Risk of injury is likely if it falls out when container is lifted aboard.
Stacking cones are placed between containers in a stack and slots into comer castings. They may be locked into bottom corner castings prior to lifting a container aboard. These cones are designed to resist horizontal forces.
Twistlocks are placed between containers in a stack and slots into corner castings. Currently, there are three different types of twistlocks namely,
· Manual twistlocks: they resist both horizontal and separation forces. Each fitting requires locking after fitting. Ship’s crew should be aware that there are left and right-hand types causing uncertainty whether a fitting is locked or open.
· Semi-automatic twistlocks: same as the manual type but these can be fitted on shore and automatically locks into the lower container when placed on top. It is easier to determine whether it is locked or not when compared to manual ones. When unlocking, this has to be manually.
· Fully-automatic twistlocks: a new and innovative designed. They are designed to be unlocked automatically. They usually opened by a vertical lift, with a twist/tilt.
It is highly important to ensure that the ship has only either left-hand or right-hand twistlocks to be used throughout. Mixing the two types can easily lead to serious incidents (A Master’s guide to container securing).
Sliding twistlocks are used to connect bottom containers to the ship. They fit into a dovetail foundation. They are used on hatchcovers and in holds when a raised socket can cause an obstruction.
Bridge fittings are used to link top containers of two adjacent stacks together and can be used on deck or in a hold. They resist tensile and compressive forces. The possibility of falling is quite likely. Therefore, ship’s crew and lashers should draw attention to this.
Mid-locks are placed between containers in a stack, and slots into corner castings. They are used on deck between 20-foot containers in 40-foot bays at mid-bay position. They resist lateral and separation forces. They are fitted to underside of container on shore and automatically lock into lower container when placed onboard.
There are some other lashing components but they are less commonly used such as, buttresses, double stacking cones and load equalizing devices (A Master’s guide to container securing).
The question now is: what is the mechanism for applying these lashing equipments?
The lashing systems:
Containers can be carried on and below deck, bearing in mind that every task has its own lashing requirements and features as will this paper show.
Below deck securing:
Virtually, container ships are provided with cell guides with vertical guide rails as securing means for hold cargoes. The cell guides are generally steel angle bars orientated vertically with entry guides at the top to assist with locating the container –the clearances, and hence construction tolerances, are very tight. 40-foot containers when in the right position in the hold need no further securing arrangement (A Master’s guide to container securing) except, perhaps, stacking cones between container tiers (cargo notes, by Dhananjay Swadi). However, when stowing 20-foot containers, this arrangement requires longitudinal and transverse support for them where they meet at the mid-length position. In this case, mid-bay guides must be used at the tank top, placing stacking cones between tiers of containers (A Master’s guide to container securing).
(Source from: A Master’s guide to container securing)
The greatest stress the containers are exposed to stems from stack pressure. Since the containers are not connected together vertically, lateral stress is transmitted by each individual container to the cell guides when positioned in such cell guides, individual containers are not usually able to shift. If the corner posts of one of the containers at the bottom of a stack collapse under excessive pressure, containers stowed above it generally suffer only slight damage. The risk of damage to containers in adjacent stacks is kept within tight limits (container handbook, 2006).
(Source from: containers handbook, 2006)
In the case of hatchcoverless container ships, all containers are stowed and secured in cell guides (A Master’s guide to container securing).
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On deck securing:
Containers are generally arranged in several rows and tiers so as to constitute blocks. The first tier, being the foundation for second and subsequent tiers, would be stowed on top (cargo work, by David J House). The arrangement of containers is to be such as to provide sufficient access to spaces on deck for operation and inspection of the lashing devices and for the normal operation of personnel (BV rules, 2005).
Today, large container ships are being built. In general terms, the lashing systems in use on this kind of vessels are very similar to those for old ones. They both have adopted the twistlocks and lashing bar/ turnbuckles system. But – because of these vessel’s large beam results in an unavoidable, relatively large GM (metacentric height), and 6-high stowage is common – the modern practice is to be fitted with a lashing bridge; a substantial steel structure running athwartships between each 40-foot container bay. This allows the second and the third tiers of containers to be secured to the bridge using lashing rods and turnbuckles, whilst the whole stow is secured throughout with twistlocks. This bridge allows the lashings to be more effective in reducing the tipping moments acting on a stack when a vessel is rolling heavily. However, the practice of fitting the bridges between 40-foot bays means that 20-foot containers can only take advantage of the bridge at one end. So, in effect, the 20-foot stacks have to revert to the limits of a conventional lashing system (UK P & I club, issue 7, 2004).
Securing on deck using stacked stowage method is the one used most frequently. Cargo handling flexibility is its key advantage. The containers are stacked one on top of the other, connected with twist locks and lashed vertically. No stack is connected with any other stack. The container lashings do not cross over the lashings from other stacks, except for the “wind lashings” on the outer sides of the ship. Obviously, a container stack of this kind can topple if it is not adequately secured (BV rules, 2005).
(Source from: A Master’s guide to container securing)
Cost savings and the implementation of software:
Every year the cost of damage to cargo sustained during sea transportation runs into hundreds of millions of dollars. Much of this loss is the result of inadequate lashing and securing systems that put not only the cargo, but the vessel and its crew at risk (Clive Woodbridge, 2002). Typical example of this happened onboard a large container ship where 42 containers were sent over the side caused by lashing failure, resulting in losses of over 20 million dollars (Robert D. Caligiuri, Exponents International Limited). Therefore, inadequate lashing systems and components is a serious matter, which can be a real danger to shipping and, of course, significant financial implications when overboard lost occur.
Lashing is a time consuming business, which can affect the length of a ship’s stay in port. It is thus an activity with an important influence on vessel operating costs (Strength and Strain article, Cargo Systems, 1998).
Some ship owners have mistakenly bought counterfeit lashing equipment in order to reduce the ship’s operating budget. These equipments may appear to function, but is of a poor quality. When such equipment is put under stress, even within normally acceptable limits, there may be failure. Consequently, over board containers falling is likely to occur and this obviously will lead to an unnecessary number of claims and therefore, costs. (Chris Hall, 2005).
Accordingly, ship owners and crew should bear in mind that applying proper lashing systems and equipments is not a choice. All equipments must be certified, tested and used as per the cargo securing manual and the manufacturer’s instructions.
Nowadays, the lashing requirements should be calculated by the use of a shipboard computer that can calculate both static and dynamic forces on container stacks. As a minimum, Lashings should be applied in accordance with the results from the computer calculation. One such system is the Cornlash software produced by shipboard informatics Ltd. This programme, widely used in industry, calculates lashing requirements based on the Lloyd’s Register requirements for container securing. Without a doubt, a better lashing method will result when requirements are calculated for each and every stow after reference to voyage stability data, actual container weights and stowage and anticipated roll angles. Lashing by this method may not eliminate every loss but it will go far to prevent them (The standard club’s publication marine matters, 2000).
The benefits of using such programmes can be summarised, therefore, as helping to ensure a safer carriage of deck-stowed containers, a saving on lashing requirements both in terms of usage and employment of lashing crew and the possibility of loading more cargo (UK P & I club, issue 7, 2004).
Conclusion:
There are different lashing systems implemented on container ships. These systems vary whether containers are secured on or below deck. To facilitate these systems, different components are used throughout the lashing process as mentioned before. Precisely, every component has its own unique function to ensure proper and safe lashing. On the other hand, these components can be our best friend or worst enemy if not maintained and applied in the correct manner. Finally, this assignment has briefly described how effective is using software to calculate the stresses acting on lashing systems.
Credit:ivythesis.typepad.com
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