Dan W

Software Developer

Postcards #5: Our Friend the Twistlock

Feb 4, 2015

Coming alongside at Yangshan Port, near Shanghai, things move quickly. Mooring lines are cast ashore. The gangway lowers and the pilot departs. Towering cranes roll into position and lower their booms over the ship’s bays. Trucks already wait on the quay to collect our containers.

It’s been barely 20 minutes since the tugs helped push the Maersk Seletar into our berth and we are ready to discharge. Within 15 hours our cargo will be unloaded and added to the 12 million TEU passing through the port every year. Yangshan’s cranes pick up two 40-foot long containers at a time, four TEU per load.

I watch from the bridge as a crane lowers its spreader. One task remains before it can collect the first container — something I hadn’t considered. A stevedore steps off the spreader and walks onto the stack of containers, about 60 feet above the deck of the ship. He clips his harness onto one corner of a container. Leaning over the edge he uses a metal operating rod to pull a steel cord below him, manually unlocking the mechanism holding the container to the ship.

Containerisation is a system, and the container itself is only one component. Twistlocks are another vital piece. They’re the means by which containers are attached to things: ships, train cars, trucks, cranes and other containers. Without them, stacks of containers would topple into the sea as the ship rolls in the swells. Think of building a tower from wooden blocks versus Lego bricks. Both stack well, but a slight tilt would see the wooden blocks come apart.

On deck Third Officer Bao shows me how the twistlocks work, passing me a hefty, dusty lump of metal. Squinting, I can see why some aboard have described them as small gray elephants. They’ve certainly got the weight of one. I’ve heard that if you get on someone’s nerves while aboard you might find a lock or two slipped into your suitcase — pushing it over the higher-than-normal airline baggage allowance seafarers are permitted.

The twistlocks aboard the Seletar seem like simple devices. Hooking into the oval slots in the corner posts of containers, they’re locked and unlocked by pulling a steel cord. Pulling out and down spins them 90 degrees into a locked position; pulling out and up unlocks them. Cleverly, the twistlock is not part of the container. As a moving part it’s liable to break or jam on occasion; the removable lock can therefore be quickly replaced without sending the entire container for servicing.

While the twistlock has improved over the years, welcoming automatic and semi-automatic versions that reduce the stevedores’ labor, it remains largely the same. The harsh sea environment and heavy loads from stacks of containers are barriers to more high-tech replacement attempts.

Another barrier to replacing the twistlock is the problem of who should take responsibility should it be replaced. The ship carries its own baskets of twistlocks. They’re the first thing unloaded when arriving in port and the last cargo back onboard. On the quayside, dockworkers remove locks from the bottom of each discharged container and attach them to those being loaded. If a new type of lock were to speed up this process, the terminal would see the gains but the costs of replacement would fall on the shipping line.

Departing Yangshan we pass into the remains of a dissipating typhoon. The ship begins to roll and pitch. Only by a few degrees, nowhere near our point of vanishing stability but enough to make me unsteady on my feet. Outside the dining room the containers creak and groan as we tilt, held firmly together by twistlocks. Their design may have originated in the 1960s, but it’s still a cornerstone of containerisation.

Elsewhere: Learn more about Keith Tantlinger, inventor of the twistlock. (1, 2)

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