Dossier Océan et énergie - Énergie Thermique des Mers
Sommaire IOA News Letters
INNOVATIVE SOLUTIONS FOR MPOP INFRASTURCTURE
Chairman, T. Y. Lin International and Member, IOA
Much research and innovative effort have been devoted so far to the perfection of the power generation system by OTEC. This is importantly so,the system being the sine qua non. However, when a planned land-based OTEC plant is located in areas where site conditions are not ideal in terms of technical and/or economic feasibilities, it is equally important that we also bring our innovative talents to bear on the design of the infrastructure to save cost and other-wise make sure it does not become the stumbling block to the realization of the plan itself. ROC's MPOP pilot plan on the east coast of Taiwan is a cast in point.
This article draws attention to the problems confronting the design and the installation of the cold water pipe (CWP) system for the pilot plant that extends up to four kilometers into the ocean to reach the desired water depth of one kilometer. As is well known, the island is prone to typhoons and earthquakes.
Added to these is the rugged terrain occurring at the deeper section of the CWP. The following are some of the unusual problems they pose as a result:
Problems Associated with Typhoons:
Storm wind and waves effects on the upper portion of the CWP, i.e. the portion above the water depth of approximately half a wave length. Scouring of sea bottom undermining pipe stability.
Problems Associated with Earthquakes:
Distortion of CWP alignment, and pipe damage due to differential ground displacement, differential anchoring stiffmess, and underwater mudsides. Liquefaction of soil transferring pipe load from the anchor-support to the next pipe section above, and so on until failure occurs.
Problems Associated with Rugged Bottom Terrain:
Precise installation work to enable pipe to run as straight as possible on supports placed at irregular intervals. Energy loss from pipe bends over irregular terrain.
Problems Associated with Constructional Risks:
Typhoons will damage incomplete offshore construction, if not adequately tied down and secured. The preferred installation method is therefore one that provides for the complete installation of the CWP within one construction season of a few months.
The Optimized Solution
There is obviously not a situation that can be resloved by normal, standard methods. However, since every method has its pros and cons, the adopted solution is likely to be one that combines the various desirable features into an optimized whole. An ideal solution is one that works with nature, not against it.
Let us consider the various viable solutions and see how they fit into our scheme of things. First, (1) buried, (2) bottom supported, and (3) floating submerged. Method (1) is safe against wave actions, but not practical for deep water. Method (2) is suitable for deep water, but vulnerable to earthquakes, current and wave actions. Method (3) is ideal for rugged terrain, and erathquake and current resistance, in deeper water, not for the shallow end that is subjected to storm wind and waves.
The best combination in our case is to use Method (1) as far as possible for the shallower end of the CWP, and Method (3) for the depper portion, and Method (2) transmitting in-between.
While the floating, submerged solution goes a long way in resolving problems due to terrain, earthquakes, etc., it also has special problems of its own. They are related to (1) construction materials, (2) configuration, (3) expediency of fabrication and installation, (4) durability, and (5) economy. There are others that are less critical, such as the applicability of the solution to a much larger pipe for the prototype plant a later time. But these do not impact the fesibility of a solution for the MPOP's CWP at this time.
The floating submerged CWP that had evolved from the above considerations is a concrete pipe that is made buoyant by constructing the pipe wall with buoyant materials sandwiched between two layers of ferrocement. The pipe floats at a pre-determined higth from the sea floor, and is held in position by a series of vertical or inclined cables anchored to gravity blocks on the sea floor.
To ensure the pipe will remain buoyant through its ifespain, it is necessary to ensure the ferrocenemt wall will permanently resist the intrusion of water under hydrostatic head of the order of 1,000 meters. To deploy the CWP within one construction season, it is envisioned that the pipe is first produced in modules of up to 75-meter long. The modules are assembled and joined by prestressing on shore into sections of up to1,000-meter long. The sections are further assembled in the floating mode to the greatest length possible in sheltered waters.They are then towed out for final assembly and installation at sea.