| Administrative History | Semi-submersibles, like Iolair, are stable floating structures used during exploratory drilling, diving operations and laying underwater pipelines. During the construction of new facilities offshore, they provide temporary galley/dormitory services, helicopter landing pads and storage space. This requires a great deal of stability, which is achieved by ballasting the pontoons until they are submerged some depth below the surface, and using anchors and/or dynamic positioning using thrusters and axial propellers. The movements once semi-submerged are small enough to be compensated for by mechanisms in the drilling or diving equipment targeting specific stationary positions on the seabed.
Iolair was designed as a safe command and rescue vessel and was capable of getting close to and cooling burning oil platforms using her water cannon. She was equipped with a telescopic boom which could be extended to reach those trapped. A heavy-duty jib crane on the main deck was able to plumb a moonpool. She was also fitted with a workshop, heli-deck and saturation- and free-diving equipment.
Semi-submersibles can either be towed or move slowly under their own power. The pontoons are de-ballasted and rise to the surface of the water. However, as Iolair was required to travel as fast a possible to the scene of any major incidents, and so her design differed slightly. Instead of simple horizontal bracing between pontoons, Iolair's pontoons had diagonal bracing rising from the foot of each column to the underside of the double skinned superstructure base. Attention was paid to the shape of the pontoon surfaces and the propellers' steering nozzles, designed to enhance the thrust, and thus her speed. |
| Description | Photograph 1 - Iolair shortly before her launching from Scott Lithgow's Kingston Yard in Port Glasgow, Renfrewshire.
Photograph 2 - Iolair negotiating the narrow channel in the River Clyde near Greenock prior to undergoing shipbuilder's trials. She is shown at, or very close to, her transit draught with the pontoons not quite submerged.
Photograph 3 - Iolair in dry dock at Brest, Brittany, France. The after ends of the pontoons are shown, each having a thruster propeller in transverse tunnel and the main propellers in their Kort-type steering nozzles.
Photograph 4 - Iolair in Loch Long, showing her fire fighting monitors, using sea water drawn using pumps in the pontoons. |