New Floating Platforms
Authors: Vilain R.H. 
Published: December 2007

I - GENERAL
In the oil days the first submersible platforms were built to be used in the shallow waters in the LOUISIANA bayous or in the MARACAIBO LAKE banks. They were towed to location, floating on their base pontoons, sunk to the bottom by ballasting of these pontoons for drilling.
In the type of platform, the base pontoons, and the drilling platforms are connected by piles, the diameter of which is usually not sufficient to ensure the stability of the platform when the base is half ballasted in the intermediate position.
Those platforms must be sunk in two stages, one end first, then the other end.
Some other types had piles large enough to ensure the static stability in the intermediate position. Incidentally a great dynamic stability in such intermediate position was discovered.
During the same period freighters were transformed into drilling ships and used in deeper water. The stability was obtained by the same way as for any other ships.
Then, theory and experience progressed, and it was discovered while the movements of semi-submersible platforms were reduced compared with other types of platforms. A second age came when the phenomena of compensation between surface and submerged volume effects was discovered, studied, evaluated and used as a main criteria for platform design.
Calculation methods were developed and checked by model testing to such an extent that the behavior of a platform can now be predetermined.
This period has reached its top results with the well-known platforms Sedco 700 or Penta 81. platforms Sedco 700 or Penta 81.But the oil-men are never satisfied; they want to work every day in whichever sea conditions and in ever deeper water.
This leads to the actual third age of floating platforms which are more stable, which involve built-in propulsion units to speed up the towing, and dynamic positioning to reduce down-time and a fourth age will probably come soon as the compensation effects will not be sufficient to give the required stability to the rig. Research and development studies are already been conducted to provide a greater stability by other means such as oscillating floats or vertical columns of great dimensions.
Some projects come now to the new concept of hydropneumatic connections between the floats and the platform itself. platform itself. Really, there is no mathematical differences between the rough surface of a road on which a car is driven at high speed, and the rough surface of the sea which moves under a floating platform moored on anchors or dynamically positioned. positioned. There is only a difference in scale, and the application of an hydropneumatic suspension to a platform needs a new technology for this problem which looks like the best known solution.
The fourth age floating platforms are now being studied in various engineering offices and I expect within a year to be able to show you a serious project.
Let us come back now to the existing types. In parallel with such good theoretical developments, it looks that architecture has not so much progressed. The structure is still made of vertical piles which makes with the floats and the deck quadrilateral figures.
This four-sided shapes induce bending moments in the joints. Most of the designers introduce diagonal bracings to make triangular figures but the small diameter pipes used as bracing increase the stresses in the joints.