THE SHIPPING ENVIRONMENT

 

The first step in load planning is to understand the forces to which the cargo will be subjected in transit. Each mode of transport presents a different shipping environment that must be accommodated in the load plan. A container carried on a chassis (highway), for example, will be subject to different forces than that carried on a rail flat car, and thus may require a different system of securing the load. Consequently, for intermodal cargo movements, all transportation environments to be encountered should be considered and cargo secured for the most severe transportation mode to be encountered. An intermodal shipment may combine all three transportation modes, as follows:

 

• Shipper loads a container which is then transported by highway to a rail head;

 

• Container is transferred to rail car and transported by rail (COFC) to a seaport;

 

• Container is transferred to freight vessel (containership or RORO) and transported across the ocean to the port of

destination;

 

• Container is transferred to surface transportation mode(s) and received by the consignee and unloaded.

 

FORCES AFFECTING CARGOES IN SURFACE AND MARITIME

TRANSPORTATION

 

GENERAL

 

While each method of transportation presents its stresses and hazards to cargo in transport, some cross modal boundaries. During the design of a load plan, the types and degrees of stress most likely to be encountered should be considered. Some of the publications that discuss transportation forces and provide blocking and bracing guidelines are provided in the Bibliography of this guide. Additionally, a representative intermodal standard is provided as Table I.1 below:

 

***Source: American Bureau of Shipping, Rules for the Certification of Cargo Containers

 

Note: The ABS standard is representative of several similar standards. However, some differences in acceleration values exist among the standards. e.g., IMO/ ILO/UN/ECE’s “Guidelines for the Packing of Cargo Transport Units” use a 4G longitudinal acceleration value for rail. Acceleration values are expressed as multiples of the standard acceleration (1G) due to gravity of 32 feet per second. (9.8 meters per second2). As an example of the effect of acceleration on a cargo load, consider a cargo weighing 4000 pounds experiencing a 2G acceleration. The resulting force would be 4000 pounds x 2G = 8,000 pounds.

 

RAIL

 

While there are some differences in the shipping environments of Trailer on Flat Car (TOFC) and Container on Flat Car (COFC) rail shipments, they are considered to be the same for the purposes of this guide. Rail transportation subjects the cargo primarily to longitudinal shocks. Trailers or containers may be carried in backwards or reverse direction. Therefore, impact can come from either direction - to the nose or the doors of the container – and load planning must prepare for impact from both directions. Rail loads characteristically experience stresses from the following:

 

• Coupler slack can lead to individual cars accelerating or decelerating at rates different from the whole train. This

causes longitudinal forces on the load.

 

• Coupling impact or shock causes longitudinal forces on the load.

 

• Suspension system and track dynamic vibration, which can produce frequencies as high as 5 cycles/sec with ‘G’

forces up to 1.25.

 

• Sway or side to side motion from curves or uneven track causes lateral forces on the load.

 

HIGHWAY

 

The characteristic forces on lading during highway transport are due to the following:

 

Primary hazards. Major hazards include:

 

• Vertical shocks from surface irregularities from rough roads, bridge crossings, etc.

 

• Vibrations, particularly vertical, from road conditions, speed, and vehicle/cargo characteristics.

 

Secondary hazards. Less significant hazards include:

 

• Longitudinal shocks from impacts against loading docks, coupling impacts, braking, and accelerations.

 

• Lateral shocks and sway from running over curbs or other abrupt surface irregularity encountered by one side

of the trailer

 

CARGO HANDLING AT THE PORT INTERFACE

 

The port interface is the waterfront facility where the maritime and surface transportation modes converge. Characteristic forces are dependent upon the method of freight handling required for the vessel type:

 

Roll on/Roll Off vessels (ROROs) generally impose less severe container movements than that for container ships during loading and off loading. Instead of being handled by cranes, trailers and rail cars roll directly onto specially fitted ships and consequently experience the forces of the surface modes of transportation while being loaded or unloaded.

 

Container ships require specially designed handling equipment at waterfront facilities, such as yard haulers and container cranes, resulting in cargoes experiencing the following forces:

 

• Vertical shocks from lifting and landing containers at the facility and the container ship.

 

• Vibrations, particularly vertical, from road conditions, speed, and vehicle/ cargo characteristics.

 

Longitudinal shocks from braking, and accelerations by container handling equipment.

 

21

BACKGROUND

Vessel Motions at Sea

VESSEL MOTIONS AT SEA

 

Shipments are typically subject to a number of independent forces from ship movement. A ship at sea may move in all of the following six directions at once due to wave action (see Figure I.1, Forces Affecting Maritime Shipments, below):

 

• Roll (motion about the vessel’s longitudinal axis).

 

• Pitch (motion about the vessel’s transverse axis).

 

• Heave (vertical bodily motion of the vessel).

 

• Yaw (motion about the vessel’s vertical axis).

 

• Surge (longitudinal, fore and aft, bodily motion).

 

• Sway (lateral, side to side, bodily motion).

 

Furthermore, there are two common combination movements:

 

• Slamming (a combination of heaving, surging, and swaying).

 

• Pounding (a combination of heaving and pitching).

 

In addition, loads can be affected by:

 

• Wave impact (shocks to the vessel from heavy seas).

 

• Water entry (faulty container).

 

• Condensation (from lading or container).

 

Through movements such as those described above, cargo may be subjected to vertical, athwartship, and fore/aft shifting within the freight container. Because a freight container may be stowed on a ship with its longitudinal axis in the athwartship direction as well as in the more common longitudinal configuration, the possible effects on a container and its lading from ship movement are many. Further, their repetitiveness tends to break down cushioning and bracing. Tight loading and adequate bracing is imperative to prevent damage.