The combination of containers stowed on deck and containers stowed in holds inherently results in a container securing conundrum. Containers stowed on deck sit on hatch covers, and the covers needs to be larger than the size of the hatch that fits the containers. That means that it is not possible to fit a fixed cell guide structure above a hatch cover, since the cover cannot be lifted without interfering with the cell guide.
In practice that means that deck containers cannot use cell guides and need to be lashed in place.
Lashing is very expensive (mostly due to labor costs and worker exposure to injuries) and in the relatively short history of containerization I have encountered about a half dozen attempts at avoiding lashing of containers on deck.
Sea-Land used stacking frames which were large frames that fitted between deck container tiers and that allowed a complete tier of containers to be locked together. They worked as long as all containers were identical (which were 35 foot Sea-Land containers), but were maintenance intensive and did not allow standard and hi-cube mixing on one hatch.
Some operators tried flip down stacking frames that were fitted on lashing towers, but they had similar size and mechanical restrictions.
In theory one can fit moveable cell guides on deck that can swing away when the hatch cover is lifted, but cell guides take an incredible beating and to make them moveable is asking for maintenance troubles.
For a while it looked like open top (hatchcoverless) container vessels might solve the problem, but while a number were built, they did not compete successfully.
There is a simple solution to the cell guide/hatch cover conundrum. If one only stows 40 footers below deck, and 45 or 48 footers above deck, deck cell guides do not interfere with the hatch covers. However, that results in rather serious cargo stowage flexibility restrictions.
Around 2008 I was dealing with a number of lashing failures and lashing tower personal injuries and felt there had to be a better solution.
I came up with a concept that I thought had promise and even registered it as an invention, but after some initial documentation and development work, I became distracted by other tasks and did not pursue it commercially. I also had a further realization that will become apparent at the end of this blog.
Quite recently one of our junior engineers working on a lashing failure case wondered out loud why container ships did not have cell guides on deck and I managed to find the 2008 file.
So here is a container hatch with deck cell guides and since the deck cell guides align with the hatch cell guides, the cover cannot be lifted off the hatch.
This cover is split into a fore and aft cover that is connected with an articulated hinge mechanism. (The hinge mechanism is the secret sauce and not shown in detail here)
When the spreader connects with the hatch cover lift twist lock fittings and starts to lift, the cover will partially fold and can now fit between the cell guides. The cover can be lifted ashore and the tracking wheels will allow the cover to store flat on the berth.
There is now an open hold and the crane can load containers in the hold along continuous cell guides (with additional lowering guidance closer to the container crane boom which actually speeds up loading rates).
Once the hold is full, the cover is fitted in the same way that it is lifted off and additional 40 footers can be stacked on the hatch without requiring deck lashing.
Those of you who have gotten this far in this blog will now probably think: “Wait a minute you just designed a 40 foot only vessel! That will not work in real life!”
So here comes the interesting part. Modern container ships are so big that they have four or more hatch covers across. If one only fits this system on the outboard hatches, it builds an unlashed, but very secure outer wall and the inboard hatches remain available for overlength containers and 20 footers.
If I design the outboard containers to be very rigidly secured in cell guides, I will need much less securing in the inboard stacks. Securing from the hatch cover top, and automatic twistlocks higher up, may very well be sufficient. As a matter of fact, while I am not completely convinced automatic twistlocks belong on outboard tiers, in these inboard tiers they may very well be sufficient to do the job.
So far so good, but once I went through this exercise, I had a further realization. With this approach I do not need to fold the hatch covers at all, these outboard covers can simply be slid out sideways slightly above the hatches. Now there is a solution that is cheap, rugged, and efficient!
Is there something wrong in my analysis of this problem, or has nobody actually figured this out? Is it possible that the slack in cell guides above deck simply allows too much lateral movement of containers when the ship starts to roll? Maybe this was the problem with hatchcoverless containers ships? I can’t think of any other reason not to use this approach.