securing systems

13CONTAINER TECHNICS N.V.

Haven 219 Emdenweg 27 2030 Antwerpen Belgium24/24 tel.: +32 (0)3 546 41 00 fax: +32 (0)3 541 18 01 [email protected] www.containertechnics.com

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IONCONTAINER TECHNICS N.V.

TO PRACTICE

2. Stowage of containers in line load mode

A. Definition

A container is stowed in line load when it is resting on the 2 longitudinal bottom girders instead of on the 4 bottom corner castings.

B. Purpose

When a container is stowed in line load , the stack load is distributed uniformly along the bottom of the container: this system avoids the high point loads in way of the bottom corner castings of a container stowed in the classic way. The line load system leads to a lighter construction of the supporting structure (for example: hatch covers).

C. Installation principle

In the supporting structure (for example : hatch covers), a recess is made in way of the 4 bottom corner castings, sufficiently deep in order to prevent the corner castings from touching the structure or securing fittings: by doing this, the weight of the containers is not supported by the corner castings but by the longitudinal bottom girders of the container. Securing of the container against horizontal shifting and eventual vertical movement (floating, tilting) is done by stackers or twistlocks, installed in the recesses in way of the 4 bottom corner cast-ings. In case only floating and shifting is expected (1 tier of containers), at least 2 bottom points per container have to be locked by locking pin through the stacker or by locking the twistlock, or an equivalent securing system has to be used with tension/pressure bridge fittings connecting adjacent stacks or with lashings. In case also tilting is expected (more than 1 tier of containers), all four corners of the container have to be locked: because in most cases not all four container corners are easily accessible, use of stackers is not appropriate and twist-locks have to be used.

D. Restrictions

Because a container is not designed for line load, the classi cation societies impose weight restrictions or limitation of some forces acting on the containers. As an example, the conditions mentioned in the rules of Lloyds Register, Germanischer Lloyd and Bureau Veritas are given herewith :

1. One tier of containers : no restrictions.2. More tiers :

a) LRS : compression load on vertical corner post maximum 12 t. for 20 and 15 t. for 40-container.b) GL : total weight of tiers above the bottom tier maximum 24 t. for 20 and 40-containers.c) BV : total weight of tiers above the bottom tier maximum 24 t. for 20 and 40-containers.

E. Design

1. One tier, securing with stackers

Originally, the system was developed for occasional transportation of one tier of containers on smaller vessels with light hatch covers. Removable stackers with low thickness baseplate were installed in fixed guides in the recesses in the hatch cover. Eventual locking for vertical movements was done by inserting some locking pins through the stackers. Depth of recess in the hatch cover was limited to maximum expected protrusion of the corner casting below the longitudinal bottom girder plus the thickness of the bottom plate of the stacker plus the clearance required between the corner casting and the bottom plate of the stacker for preventing point load. Typically, such depth can be 17 mm (maximum protrusion) + 20 mm (bottom plate of stacker) + 5 mm (clearance recommended by class) = 42 mm total. This depth is not very critical : locking pin can still be installed even when containers with less protrusion are loaded : due to the el-liptical shape of the side openings of the corner castings, the locking pins have ample clearance to cope with a big range of protrusion.

2. Securing with twistlocks

When securing with twistlocks is specified (which is necessary when more tiers are intended), the depth of the recesses and the twist-lock have to be designed taking into account the range of protrusion of the container corner castings and the type of twistlock used. Further, the deck or hatch cover has to be reinforced in way of the 4 corner castings, not for downwards forces but for uplift forces cre-ated by tilting of the stacks.

14 CONTAINER TECHNICS N.V.

FROM THEORY

3. Protrusion of corner castings

Corner castings may protrude 4 to 17.5 mm below the longitudinal bottom girders of the container. Some containers are flush at the bot-tom (protrusion = 0 mm). The bigger part of the containers has a protrusion between 10 and 12 mm.

4. Twistlock

A standard bottom twistlock is designed so that the corner casting is resting on top of the body and the cone is locking with limited clearance (+/- 2 mm) into the bottom corner casting. For line load function however, clearance is needed between the twistlock body and the corner casting; this clearance is highly dependent on the protrusion of the corner casting. This arrangement requires a length-ened shaft of the twistlock. The amount of lengthening is defined by the depth of the recess and the expected range of protrusion. Lengthening is typically 10 to 20 mm. A consequence of this lengthening is that the clearance of the locked cone in a corner casting with big protrusion can be considerable (15-20 mm), leading to tilting movement of the stack with rolling vessel.

F. Precautions

1. Limiting criteria

For weight restrictions, depth of recess, etc.: see above.

2. Tolerances

The use of twistlocks is a requirement for a line load system when more than one tier is loaded. This system however is very sensitive to combination of building tolerances of hatch covers and hatch cover recesses and fixed twistlock shoes in recess, extreme low or high protrusion of corner castings. So, it may happen that due to an unfavorable combination of these factors, locking of twistlocks proves to be impossible. In that case, additional securing is required: this can be lashing with chains, rods or wire at the not-secured container end, or linking the not-secured stack to one ore more adjacent well-secured stacks by tension/pressure bridge fittings, or a combination of both lashing and bridge fittings.

Note: For clarity, in some of the above sketches the U-frame, twistlock and ISO hole of the corner casting have been turned 90 from normal alignment.



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1. Independent stacks with twistlocks and lashing rods

A. Description

The containers are stowed in a stack and are connected to the foundation and to each other by 4 twistlocks in the bottom corner castings of each container. There is no transversal connection with adjacent stacks. All forces acting on the stack are transmitted through the corner posts, corner castings and twistlocks to the lower container and so to the foundation. The front end and the door end of the container are connected by the longitudinal bottom girders and top girders, walls, roof, and bottom structure: however these elements are not considered strong enough to transmit considerable load from one end to the other: for standard container calculations, front and door end are considered as independent, and securing of each end has to be evaluated separately.

When admissible racking forces or uplift forces are exceeded, it may be necessary to install lashing rods with turnbuckles at both container ends. Minimum free space at the container ends for tting of the lashing rods with turnbuckles is 550 mm, but 750 mm is considered more suitable for doing a proper lashing.

B. Advantage

Loading and discharging can be done stack by stack or layer by layer.

Mixture of containers with different heights is no problem. Standard securing fittings. OSHA possible.

C. Disadvantage

Stack weights limited by strength of lower container(s).

II. SECURING


FROM THEORY

2. Block of stacks with side supports and stackers

A. Description

The containers are stowed in a block and are connected to the foundation and to each other by single and double stackers in the bottom corner castings of each container. All stacks are transversally interconnected by double stackers between the tiers. The complete block is supported sideways by pressure supports or tension/pressure supports. These supports are in line with the transversal double stackers, and tting in dedicated recesses (in case of tension/pressure supports ) or landing at reinforced areas (in case of pressure supports ) in the longitudinal bulkheads of the hold. All forces acting on the stacks are transmitted through the end frames, corner castings, single and double stackers and side supports to the longitudinal bulkheads and to the foundation. For standard container calculations, front and door end are considered as independent, and securing of each end has to be evaluated separately.

Calculation of reaction forces will show if side supports are required at each intermediate level and eventually also on top of the block. Weight of side supports should be limited to what can be safely handled, so in general the distance between the longitudinal bulkhead and

the container block should not be more than some 500 mm: 200 mm is a common value.

B. Advantage

Because forces are transmitted to sides as well as to bottom, high stack weights can be achieved.

C. Disadvantage

Each level in a block needs to have the same height, so mixture of containers with different heights needs careful planning. Loading and discharging can only be done layer by layer, and not stack by stack. Non-OSHA. Transversal distances may depend from ship to ship, so standardization of fittings is not 100%, and may require longer delivery time in

case of replacements.



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3. Independent stacks in cell guides

A. Description

The containers are stowed on top of each other in a cell with a vertical guide at each corner. No connecting ttings are required between the containers and between the lowest container and the foundation. The cell guides are strongly connected to the ships structure. All forces acting on the stacks are transmitted through the end frames, corner castings, to the cell guides and to the foun-dation.

Total clearance in the cell guides is speci ed in the class require-ments: 38 mm fore/aft; 25 mm athwartships.

B. Advantage

Because forces are transmitted to cells as well as to bottom, high stack weights can be achieved.

No fittings required. Loading and discharging can be done stack by stack or layer

by layer. Mixture of containers with different heights is no problem. OSHA.

C. Disadvantage

Only one size (length/width) of containers can be stowed.


FROM THEORY

4. Independent stacks 20/30 in 40 cell guides

A. Description

The 20 or 30 containers are stowed on top of each other in a 40 cell, with one end supported by the vertical guides, and the other end free standing. The lowest container is secured at the bottom at the cell end by fixed cones, preventing longitudinal movement and at the free end by bottom guides, or bottom stackers or an equivalent arrangement, preventing transversal movement. The containers are connected between each other by single cones at the free end, and in case of single cones with flange, also at cell end for leveling. All forces acting on the stacks are transmitted through the end frames, corner castings, to the cell guides at one end and also transmitted through the longitudinal construction of the containers and the single stackers to the other end and so to the lowest container and to the foundation.

B. Advantage

Loading and discharging can be done stack by stack or layer by layer. Mixture of containers with different heights is no problem. Limited number of fittings required, all fully standard. OSHA.

C. Disadvantage

Because forces are transmitted through the weak longitudinal construction of the containers, limited stack weights can be reached. Only containers with one width can be stowed.

* No selfhanging stacker required at cell guide, in case angeless stackers are used et free end.

securing systems

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