How It Works

Most conventional floating platforms acquire their floatation forces by directly displacing the water with their hulls. A pneumatic floating platform utilizes indirect displacement, in which the platform rests on trapped air that displaces the water. The primary buoyancy force is provided by air pressure acting on the underside of the deck.

The PSP is a distinct type of pneumatic platform, one in which the platform is composed of a number of cylindrical shaped components packed together in a rectangular pattern to form a module. Each cylinder is sealed at the top, open to the ocean at its base, and contains air at a pressure slightly above atmospheric pressure. Modules can be of a size that are relatively easy to manipulate, as shown in the simplified drawing below.

Another aspect of the PSP design is that, when needed, air is allowed to flow from a cylinder to its neighbors through a manifold or connecting orifices. The airflow provides a mechanism to help reduce the peaks in the pressure distribution beneath the structure and provide platform stability as well as a mechanism for dissipating wave energy. Directing the moving air through turbo-generators to produce electrical energy is one means of converting wave energy into electricity with a PSP. More recently, a new collector has been designed specifically for use with a PSP that is expected to significantly increase the percentage of wave energy converted to air. See the "Wave Energy" page.

An assembly of cylinders results in enclosed interstitial regions between cylinders, which may be filled with air, foam or other material. These regions are isolated from the air pockets within the cylinders to provide additional buoyancy and righting moment. As long as design loads are not exceeded, this important feature enables the PSP to endure catastrophic air pressure loss In comparison to conventional floating platforms, the designers of a pneumatic platform can modify the distribution of the flotation force as needed to minimize the hogging moment or in response to large concentrated loads on the deck. Further, it is possible, for a particular sea state, to tune the oscillation of the water columns inside the cylinders to minimize the overall hydrodynamic loading to which the platform is subjected.

This brings up the fact that the PSP, as with other floating platforms, is sensitive to its environment in form and function, and must be designed taking that into consideration. There is not a "one size fits all" PSP. However, that said, they can be segregated in two broad categories, open ocean platforms, and protected water platforms. These differ markedly in component size and complexity as illustrated in the following examples.

 

 

 

San Diego Prototype Component

20 x 20 x 40 feet

 

DARPA Contract

In January 1993, The Naval Surface Warfare Center, Carderock Division, took an interest in the PSP's potential to serve as a floating military base (See DARPA Contract on the "How it works" page). Float's proposal was for a three phase effort to construct a 100 x 300 foot prototype that would be deployed off the coast of San Diego, California, to demonstrate the PSP technology. When it became clear that it would not be possible to obtain sufficiently firm estimates for the costs of construction and deployment without completing the phase I & II research and design effort, the Defense Advanced Research Project Agency (DARPA) elected to proceed with the first two phases, deferring the third until their completion. DARPA awarded Float a contract as part of their Maritime Platform Technologies program. This contract, for $1.5 million, was funded in August 1995.

Phase I was conducted between August 1995 and December 1996. The first task was the development of the hydrodynamic and structural computer models needed for the design of the prototype platform. Next, a 1:22.85 physical scale model of the prototype was designed, built, and tested at the Offshore Model Basin wave tank in Escondido, California.

As it was not practical to fully scale air pressure, the first test model, which was comprised of 75 cylinders, was designed to have only five active cylinders. Consequently, the Phase I model tests were limited to confirming the computer models' prediction for the compliance of the air contained in the five cylinders (the "air pocket factor") and the platform's motion in a seaway.

Early in 1997, following a revision in DARPA's objectives Float's effort was transferred to the Office of Naval Research (ONR) for inclusion in their newly formed Mobile Offshore Base (MOB) program. Work was suspended during this change of administration, thus phase II did not commence until August 1997. The shift necessitated several changes in the direction of the research effort. Most important was the replacement of the 100 x 300 foot prototype demonstration platform with a 500 x 5000 foot platform as the principal study objective. Further, Float was requested to examine the application of the PSP technology to a variety of smaller ancillary support platforms.

Extensive model tests were conducted at the Offshore Model Basin in Escondido, California during June and July of 1998. A scale of 1/48.73 was selected and models representing platforms of 600 x 400 and 200 x 1200 feet in prototype scale were constructed. Tests were conducted with the models constrained (fixed to a truss spanning the basin) and free floating. Several air exchange (manifold) configurations were studied. 30 wave sets, with periods from 6 to 20 seconds, wave lengths from 180 to 2050 feet and wave heights from 3.5 to 68 feet, all in prototype scale, were used. Installed sensors measured cylinder air pressure, water pressure at the base of the model, wave height within the cylinders, motion of the models in 6-degrees of freedom and the forces exerted by the model on the supporting truss.

Although the air pressure was not scaled, these tests were designed to study the performance of a PSP when all cylinders were active, i.e., able to exchange air. The test data showed the wave attenuation to be very rapid; the height of the waves being significantly reduced after passing but a few cylinders. Further, the data clearly established the relationship between the air distribution, wave attenuation and platform motion.

The rapid attenuation indicates that the largest hydrodynamic loads will be confined to areas near the perimeter of the platform, which is expected to ease its structural requirements. It also suggests that the focus on air handling and energy conversion will be in cylinders near the perimeter of the platform and that the central areas of large platforms can be constructed more economically.

 

 

08-01-06

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