As we graduate into major cranes, new complications arise. The majority of vessels only perform lifting operations in calm weather. If the waves are up and the wind blows, we don’t lift that day. Most ships limit their lifting operations to nearly static conditions. But the offshore oil and gas industry frequently deploys large equipment to the seabed, on the open ocean, in waves. Waves introduce a new level of complexity: dynamic forces.
First, the waves bounce the ship, which bounces the heavy load attached to the crane line. But this is no normal crane line. Lowering something to the seabed means we have a heavy load attached to a 1000 m long line. At that length, the minor elasticity of the crane line adds up and behaves like a giant spring. The hook and attached equipment start bouncing up and down a few meters. This creates dynamic forces 50% – 100% above the static weight of the load, in some cases. Plus, that 1000 m of crane line also weighs quite a bit, which reduces our crane capacity. All those extra factors add up. A crane with a capacity of 1000 MT at the surface may only hold a hook load of 200 MT at the seabed.
But the worst dynamic force comes from the ship itself. That equipment we lower through the water column doesn’t like to move very fast, thanks to something called added mass. Any rapid movements (like movement from waves) create huge reaction forces. Conceptually, we are not a free ship lowering a load. Our ship anchored itself to a fixed point in the middle of the ocean, through the crane. Our load is the anchor. Back on the surface, the ship wants to rise and fall with the ocean waves, but the load on our crane hook resists that movement. Now we have the entire ship trying to pull against this crane line. HUGE dynamic forces.
The cranes also get much smarter. To reduce these dynamic forces, we add in active control. The basic goal behind active control is to isolate the hook load from ship motions. If the hook load moves slower, the dynamic forces get much smaller. A computer senses the motions of the ship and calculates the resulting vertical motion at the tip of the crane boom. The computer then compensates by automatically adjusting the line length. If the boom tip rises by 1.2 m., massive hydraulic systems very quickly pay out an additional 1.2 m. of crane line. This isn’t easy. It requires fast hydraulics and a complicated control system, where the computer almost predicts the future (at least for a very short time in advance).
Once we create that active control, fun things happen. The advanced cranes use this in different ways:
- Active Heave Compensation (AHC): The tip of the crane boom moves, and the system compensates to hold the load at one position. This is useful when transitioning through the ocean surface or approaching the seabed.
- Constant Tension (CT): The movement of the crane boom generates large dynamic forces. The computer adjusts the load position to minimize these dynamic forces and keep a constant tension on the crane line. This is useful when lowering through the water column.
Even the best active control doesn’t completely eliminate dynamic forces. There is no single solution to this problem. The engineering decisions behind a dynamic crane hint at a complexity on par with the rest of the ship. That complexity also yields added capability; stormy seas no longer hold us back. They diminish to just a normal day at work.
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Nate Riggins2024-05-14 09:00:002026-06-01 10:09:22Surviving the Arctic: Polar Class Icebreakers
