on July 13th, 2020

How do engineers move something as heavy as decommissioned offshore oil rigs? How about moving two at the same time? Oil and gas rigs can weigh up to 50,000 tons, far outweighing what tugboats are capable of moving. Transporting monumental cargo out at sea is complex work that requires complex vessels to do the heavy-lifting. The engineering solution lies in semi-submersible ships. A marvel of engineering, semi-submersible ships go about solving the conundrum of heavy cargo transportation amid battling the elements and the unpredictability of the ocean. Engineers can appreciate the incredible engineering that goes into these vital players in the marine engineering world.

A company formed in 2014 named GPO specializes in Semi-Submersible Heavy Lift Vessels. The vessels have been described as part-cargo ship and part-submarine. One of the four submersibles they have engineered is named the Amethyst. These kind of ships help with the transportation of:

  • Jack-up drilling rigs
  • Semi-submersible drilling rigs
  • Dredging equipment
  • Cranes
  • Barges
  • Other floating cargo
  • Offshore & onshore modules

How a semi-submersible works 

The Amethyst, and its twenty-six-man crew, when dispatched to load heavy cargo, have to first consult the meteorologists to determine if the weather will be conducive to heavy cargo lift. Once the crew is happy, they embark on their journey, battling winds out at sea on their way to pick up their target.

Once the ship reaches its destination, the semi-submersible performs the operation that gives it its name. The vessel is partially sunk into the ocean.

The Amethyst is one of the biggest of its kind in the world. The entire ship’s hull, at multiple levels, is lined with ballast tanks. The hull is split into 76 ballast tanks, distributed in three layers within the ship. These tanks take sea water in and weigh the ship down, submerging it in the water. 160,000 cubic yards (49 Olympic swimming pools) of water is pumped into the hull, in a very precisely engineered way to prevent the capsizing of the ship. At this point, the ship is at serious risk of sinking. Two towers at the stern of the ship, with empty ballast tanks, help keep the vessel afloat. Each tower holds ballast tanks that weigh 3,370 tons. Equally as important is the bow section that keeps the ship horizontal and not tipping.

Heavy lifting, heavy power 

Loading the cargo onto the deck is the next part in the operation. The key to submersibles is the flatness of the deck that takes on the incredible weight of its cargo. The Amethyst’s deck is six hundred feet long and one hundred eighty feet wide. The thickness of the steel, whilst some might think needs to be majorly thick to support immense weight, is actually the opposite. They are in fact super-strong, ultra-flat decks. The first deck is, in fact, only one inch thick. Underneath the first deck, however, lies a second deck with steel in the shape of I-beams. The force of the cargo the ship is lifting pushes down on the thinner top deck, with the second latticed I-beam deck absorbing the rest of the weight.

The cargo load needs to be balanced perfectly to ensure that the deck does not cripple underneath the immense weight of whatever is being transported. For something as big as an oil rig, three tugboats need to pull and position the rig over the sunken submersible ship. To lift the semi-submersible ship back up out of the water, with the cargo carefully balanced, the ship has to eject the water from its flooded ballast tanks back into the ocean. Pumps with high-speed impeller blades use centrifugal force to eject the water from the ballast tanks.The pumps can fully empty in 7 hours. Before that, however, engineers have to ensure the final position of the cargo they are transporting, and have to be millimeter accurate, otherwise the semi-submersible will be damaged. Once the semi-submersible is up out of the water, the ship can journey onwards.

The Amethyst has an incredibly powerful propulsion system. Diesel is the fuel source of choice. Each engine weighs 132 tonnes. Each of the four Diesel engines push out an impressive 9,600 horsepower. The ship can do one full round-world trip with full tanks.

The engines untypically reside under the bridge and in the bow – the only place where they could go in a vessel that partially sinks. The engines’ power feeds into electric generators that push electricity through cables to electric motors at the back of the ship and, in turn, powers the propellers. The 18-feet tall propellers are specifically designed to efficiently cut through water. The high-tech propellers can also reverse course immediately to bring the mammoth ship to a halt in only 500 yards of stopping distance.

Moving extremely heavy cargo out at sea was once an engineering challenge. The arrival of the semi-submersible to maneuver below heavy floating cargo and move it, was an engineering solution. The GPO Amethyst as seen above is capable of moving two decommissioned oil rigs each weighing north of 13,000 tons. What was once impossible, has now become possible, thanks to engineering. Encouragingly, instead of leaving decommissioned oil rigs out at sea, an engineering solution can ensure they are returned onshore and be disposed of in the right ways.

Works Cited

“Fleet.” GPO Heavylift, www.gpo-heavylift.com/fleet/.

“Watch Superstructures Engineering Marvels Season 1 Episode 2 on Disney  Hotstar VIP.” Disney  Hotstar, www.hotstar.com/in/tv/superstructures-engineering-marvels/s-2203/ultimate-cargo-ship/1000237821.,

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