Rena's New Phase: Container-Plucking

The removal of Heavy Fuel Oil, diesel, and lubricating oil from the Rena is complete enough that the next phase, container removal, has begun.

We're told that a grand total of 1,262 containers remain on the Rena, topside and in the holds. A crane on the Sea Tow 60 has pulled off eighteen to date, after workers freed the corner attachments with cutting torches. Here's a short NZ news item with a video clip.

Pulling containers from unstable stacks and lifting them with a crane that's sitting on another ship -- a ship that's moving independently from the Rena -- is difficult and dangerous, even in the best of weather. Getting close enough to use a cutting torch and attach cables means a rigger is close enough to get smashed. 

Despite the salvor's best efforts, it's likely that hundreds of containers will end up in the water anyway, particularly if a storm comes along. And the work is so difficult and dangerous it can't be rushed with the thought of beating the storms. The best information I've seen on the ups and downs of the salvage work can be found at this blog, Antipodean Mariner. Check it out!

Six to eight containers off would be a good day, so months of fair weather would be required to get all of them off. Not too likely.

Anticipating that likelihood, Svitzer's salvors have readied two hundred transponders, and attached them to containers most likely to become flotsam.

A more substantial crane-equipped vessel, the Smit Borneo, is on its way from Singapore and should arrive in a week or two. The Smit Borneo is a heavy-lifter, usually employed for pipelaying in the offshore oil patch. Here's an issue of a Smit publication, The Tug, that mentions the Borneo at work on a sunken drill rig. Here's the vessel, from VesselTracker.com:

A look at the Rena makes the challenge clear. Containers were stacked seven high aft of the deckhouse, and up to six high elsewhere. There are hundreds more containers secured in steel racks called "cell guides," under the decks.

For container geeks, here's a handbook for vessel masters about how containers are stowed on different types of ships, above and below decks. Here's a risk-advisory paper from marine insurers on the problems of container stowage above decks. (Item for future blog: marine insurers are concerned about a rising trend of containers lost from containerships while in transit; there are a variety of reasons for this; one is a scary phenomenon called parametric rolling.)

First, a brief summary about how generic containerships like the Rena stow their cargo. At port, crews begin by lowering and stowing containers in the holds (the cavernous areas below decks). When each hold is full, cranes lift heavily reinforced covers to seal the hatches. These hatch covers are the foundation for hundreds more containers on deck, usually six high. 

Containers in the holds: Each vertical column of containers is called a “cell.” As lowered by a crane, a container slides down a “cell guide" (a strong steel framework with vertical tracks) until it comes to rest either on the base of the cell guide, or another container. 

Containers are highly standardized, which allows the dimensions of the cell guides to be very precise. Precision is normally a good thing, because if there's slack between the cell guides and the containers, movement of the containers at sea will damage the cell guides.

Above decks, the first two or three tiers of containers are held down by steel rods called lashings. These are diagonal tension rods, securing corners of the containers to fittings on the ship structure. These attachments are quite strong ... assuming the containers and fittings are in good shape. If you look at the Rena photo above you can see some of the diagonal lashings -- they look like X's across the ends of the containers.

But containers in tiers rising higher than the lashed ones are more at risk. These upper containers attach to each other at the corners, not directly to the ship structure. The higher and heavier the stack, the more leverage to crush the containers below, or to snap their attachments. Crushed containers are visible at the bottom of the stacks on the stern.