November 12, 2011: Fall Back, In the Rain

By Dallas Murphy
Photos by James Campbell

It’s raining hard as the science team deploys another long mooring. That doesn’t make for comfortable conditions, but the wind’s light, and that’s what really matters. When mariners say “weather,” they mean wind. Wind causes waves. And in this on-rush of current, waves get up quickly when a southwesterly blows against it. Wind determines the quality of life aboard. So we’ll always trade a downpour for a blow.

Fall back
Fall back

Lisa has chosen precise geographical positions on the seabed, denoted in latitude/longitude, where she wants to place her moorings. With today’s technology, this specialized ship and the talent aboard, it’s no trick to position the transom directly above that point and stop. Trouble is, other variables crowd the equation. There is, for instance, the matter of “fall back.”

Dr. Lisa

mooring deployment Click on figure for larger version

In the time-tested manner of deployment, the top float goes in the water first, and as the wire is paid out, the instruments, current meters in this case, additional floatation and the acoustic releases, are attached step by step. Meanwhile, the mooring streams out behind the ship growing longer and longer, often so far you can’t see the top float with the naked eye. The anchor, a stack of railroad-car wheels weighing in at 4,000 pounds for the longer moorings, is the last item to go overboard.

At this writing, Melville floats 3,500 meters (11,483 feet) above the bottom. The mooring structure streams aft as we ease up current at 1.5 Knots toward the drop point. However, the drop point and the landing point are not the same. Lisa, Robert, and Adam have calculated that the anchor will take 30 minutes to reach the bottom. En route, the flotation will drag the anchor “backwards” as it sinks. Therefore, the ship will need to overshoot and the landing point before dropping. But it’s still more complicated. The ship sets up bow in the wind. But that puts the current on her port beam with the mooring streaming slightly away to starboard. So the ship sort of crabs sideways to the drop point. Lisa has just returned from the bridge where she and the ship drivers discussed these variables, including the length of the mooring wire, the rate of sinking, about 130 meters per minute, and as always the velocity and direction of the current.

Somebody in the rain

On the radio: “Deck, bridge. One hundred meters to go.”

“Roger that, one-zero-zero.”

The deck crew has swung the anchor out over the water, ready to go. Mark stands by on the trip line. It seems to take ages before: “Deck, bridge: we’re on drop site.” A yank on the line, down it goes.

We see the big orange float water skiing toward the ship leaving a churning, white wake. Ten minutes later it dives beneath the surface. Now the question is, where will it actually land? Lisa, Robert, and Adam gather in the main lab to listen to acoustic signals from the releases as they report their position on the way to the bottom. Meanwhile, the bridge has shut down the engines, letting Melville drift so as not to interfere with the signals. I try to visualize the structure as it speeds through the gin-clear depths, passing into twilight, then opaque darkness, and the explosion of sand and mud as it comes to a sudden stop.... Meanwhile, we’re telling sea stories about other ships and other cruises.

“It’s on the bottom,” Robert announces.

They’ve landed it 230 meters from the target. Brilliant. We go to lunch.

Reading this website, you might be inclined to ask why? All this effort and expense (ship time alone, no science, runs about $40,000 per day), all this technology and talent devoted to learning the total transport of water in an Indian Ocean current many people have never heard of—why bother? It’s a reasonable question, and the answer isn’t singular. We’ll discuss it in depth as the trip proceeds.

A job well done