When offshore oil drilling rigs are being installed, serviced or dismantled, the workers typically stay in cabins located on adjacent floating platforms. These semi-submersible platforms are towed into place (or travel under their own power) and then their hulls are partially filled with water, allowing them to remain somewhat stable in the pitching seas. Now, a ship is being built to serve the same purpose, but that will be a much more mobile alternative. It will keep from rolling with the waves by generating its own waves, inside its hull.
Called the Offshore Accommodation Vessel, the ship will have water-filled tanks built into the bottom and sides of its hull. In cross-section, the tanks will have a U shape, hence their being known as “U-tanks.”
Air valves located at the top of each U-tank will control the direction and rate at which the water inside sloshes back and forth. The idea is that by setting the motion of the water in the tanks to counteract the motion of the waves in the surrounding ocean, the rolling of the ship will be minimized. As can be seen in the following video of a model in a test tank, the concept does indeed appear to work.
The ship will also be equipped with six pod-like azimuth thrusters, each of which can rotate independently to allow the vessel to hold its position relative to the rig. This will be particularly important as workers are walking back and forth between the two, via the ship’s telescoping 55.5-meter (189-ft) gangway. Those thrusters will also provide the forward propulsion while the ship is in transit.
The 155-meter (509-ft) Offshore Accommodation Vessel was designed by Norway’s Salt Ship Design and MARINTEK, which is a division of the SINTEF research group. It is being built by Hyundai Heavy Industries under contract for Edda Accommodation. Once completed in June 2015, it will be able to accommodate 800 people. Along with cabins and office space, it will also feature a gym, sauna, two swimming pools, conference rooms and an auditorium.
A second similar vessel may also be on the way.