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What Is The Problem?
The container ports within the United States and throughout the world have reached grid-lock. They  are all operating at maximum capacity. All available dock space is being utilized to berth container ships for loading/unloading. Consider the port of Oakland, California show below as an example.

    

The rate at which current cranes can load/unload container ships limits or governs the shipping schedules. More and more container ships are being built. These container ships continue to get larger and larger and faster and faster in order to satisfy the ever increasing commercial import/export traffic.

The container ports in general have a sufficient number of cranes to span the berthed container ships and usually place multiple cranes over each ship to maximize the loading/unloading rate. In the far majority of cases these cranes are Post-Panamax cranes, single trolley single boom cranes, and up to 5 or 6 cranes may be positioned over the ship. The left picture below shows four Post Panamax cranes being positioned over a ship and the right picture below shows eight Post Panamax cranes servicing two ships at the Oakland California container port.

   

So Why Are These Solutions Inadequate?
These three proposed solutions have two major problems, the first is obvious, the second  less obvious.

 The first major problem is that all these solutions require complete replacement of  current cranes. This would be extremely expensive not only in money, but in time and scheduling of replacement while maintaining port productivity.

The second and less obvious problem is the perceived increased productivity provided by these proposed solutions. Is the increased productivity really there? Assume that the unloading/loading rate of a single current Super Post Panamax crane is 40 containers per hour (cph) and that the clearance width is 80 feet. Productivity Factor (PF) = 1. A multiple crane configuration of 6 cranes would be 240 cph with a total width of 240 feet with a Productivity Factor (PF) = 6. Consider the following table that compares the three proposed solutions to the current Super Post Panamax crane.

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CONCLUSIONS

(1) Double trolley single container cranes A double trolley single container, PF =1                Comment: This is clearly not a good solution. The productivity factor is the same as current cranes. An increase in width would make the multiple cranes configuration even less productive.
(2) Twin container cranes                         (The crane superstructure to handle two containers at a time would require a crane of increased size and width compared to the current Post Panamax crane.) A single Twin container crane,  PF =2                  Comment: The productivity factor is the twice that of the current cranes, so it is a possible solution except for the high cost of a complete crane replacement. An increase in crane width of 30% would make the multiple cranes configuration  less productive. (PF = 4.5 compared to 6)
(3) Double trolley twin container cranes (The crane superstructure to handle two containers at a time in conjunction with an intermediate staging area would require an even greater increased in size and width compared to the current Post Panamax crane.) A double trolley twin container, PF =2                  Comment: The productivity factor is the twice that of the current cranes, so it is a possible solution except for the even high cost of a complete crane replacement. An increase in crane width of 50% would make the multiple cranes configuration  less productive. (PF = 4 compared to 6)


The Proper Solution is A Single Boom Dual Trolley System
The proper solution is to retain the current Panamax cranes, a single boom single trolley crane system, and double their individual loading/unloading rate by adding a second trolley. This converts the Panamax single boom single trolley crane into a single boom dual trolley system. This modification doubles the loading and unloading rate of each individual crane. Since the width of each current operational crane is unchanged,  the same number of cranes in a multiple crane configuration may be placed over the ship. This results in truly doubling the loading/unloading rate of the entire multiple crane configuration.

Possibly the most important additional feature of this solution is that  current operational cranes are modified, not replaced. They are modified while in use at a cost estimated to be 30% of the cost of a new crane replacement. This solution is further cost effective in that the port remains in operation while the current  cranes are being updated.

How Is This Solution Accomplished
This modification uses a horizontal trolley with a vertical spreader traveling on a lower set of tracks to move a container from the ship to the shore while simultaneously  a second horizontal trolley with an empty vertical spreader traveling on an upper separate set of tracks moves from the shore to the ship in an unloading sequence. The empty spreader is then loaded and the sequence is reversed to continue the unloading process. These two trolleys travel on the current Super Post Panamax boom and intersect each other at a location along the boom that does not over-stress the current boom structure.

How do two independent trolleys intersect while traveling on the same boom?

The upper trolley travels on a set of tracks on the top of the boom and has a center vertical spreader that can rotate 90 degrees so that the spreader may be either parallel or perpendicular to the ships longitudinal centerline. 

The lower trolley travels on a set of lower tracks with a vertical spreader supported by a left and right set of cables. The lower spreader does not rotate and has an unobstructed space called the by-pass zone between the left and right set of vertical spreader cables.

When the upper spreader is rotated perpendicular to the ship centerline and is above the lower spreader, the upper spreader can intersect the lower spreader through the by-pass zone. The two trolleys move back and forth in this synchronized manner to either load or unload the ship.  

The crane model shown in the figure below a 1 min 30 sec video of a computerized HO scale model of a Super Post Panamax Crane utilizing the MindStorm technology from Lego. The video shows two containers being unloaded from a container ship.

It shows a container being transported from the ship by the upper trolley as its loaded spreader is lifted vertically and rotated so as to pass through and above the lower trolley empty spreader that has just deposited a container on the dock and is moving towards the ship to retrieve a container.

The lower trolley spreader having picked up a container lifts vertically as it begins to  move towards the dock. The upper trolley having deposited it container on the dock  lifts its empty spreader vertically  and rotates preparing to pass through and above the loaded lower spreader as it moves towards the ship to retrieve another container. The cycle repeats itself as it continues to unload or load the ship.