Floating tied platform for offshore airport or port

Figures 1 and 2 show examples of conceptual design of floating airport and floating port.  Basically it comprises one or several moored bodies assembled from smaller units  The units can be pontoon type or semi-submersible, the latter has better performance in the open sea.  As the horizontal dimensions of the body are much larger than its thickness, it is sensitive to hydrodynamic actions from the surrounding water.  In the engineering discipline it should be designed and analyzed as Very Large Floating Structure or VLFS.  Connecting the smaller units in the open sea might lead to another technical difficulty.   A prototype of floating airport measuring 1000 m x 60 m with thickness of 3 m, referred as Mega Float has been constructed in Tokyo Bay, Japan.

My conceptual idea on the floating airport is shown in the Figure 3. The system comprises pre-cast floating hollow boxes but unlike the existing floating system they are not connected with each other, thus eliminating connecting problem in the open sea. Each box is tied to the concrete blocks placed on the seabed with diagonal-cable system.  The cables must be in tension under any condition and they make the boxes fixed in place. In contrast in the existing system the floating body will move vertically and laterally according to the water level and the forces acting on it.  The diagonal tying arrangement is chosen to resist lateral forces.  Figure 4 shows the floating box' side and top views and its cross-section.  The box curved shape or in engineering term known as shell-structure is chosen to increase its strength under the enormous hydrostatic pressure.  The thickness increases with depth following the magnitude of the water pressure.  The box has ballast tank at the bottom for the purpose that will be explained later and the remaining space can be used for other purpose like waiting rooms, store rooms etc. 

The proposed construction method is shown in Figures 5 to 8. We first place the concrete blocks on seabed with all the cables already connected to them.  The first floating box is towed to the site (Figure 5) and is lowered by filling the ballast tanks inside it with water (Figure 6).  Then we tied it with diagonal cable system (Figure 7).  After the water in the ballast tanks is pumped out, the box will move upward but the cables will hold it at certain designated level.  Thus the water buoyancy will produce tension in the cables.   The designated level of the tied box is selected to ensure that the cables will remain in tension under any condition. Once the first box is tied then the procedure is repeated for the adjacent boxes (Figure 8).  The fenders will protect the boxes when they hit each other during the construction. They also transmit any lateral force from one box to the next one. 

As the boxes are tied with pre-tensioned cable system and they are not connected the proposed floating tied system will more or like fixed in its place.  The vertical movement is limited to the one coming from the cables elongation.  Thus the proposed system behaves more like static structure and might be much less sensitive to hydrodynamic action from the surrounding water.

Fig. 1 US Navy's Conceptual floating airport
(picture is taken from US Navy website)

Fig. 2 Conceptual design of McDermott mobile floating port
(picture is taken from Coastal Operations Institute)

Fig. 3 Floating Tied System for Offshore airport

Fig. 4 The floating box

Fig. 5 The first floating box is towed to the site

Fig. 7 The first box is tied

Fig. 6 The first box is lowered by filling the ballast tank

Fig. 8 The cables are tensioned through emptying the ballast tank and the next floating box is towed to the site