is moved to a protected deep water site and moored. Construction con- 

 tinues, usually by slip-forming the tank and tower walls on a round-the- 

 clock basis. Sand and water ballast maintains the working level at about 

 a 30- to 40-foot freeboard for the major part of the construction phase.L 13 J 



The Brent B CONDEEP platform, built in Norway by the above method, is 

 representative of the concrete offshore platforms. Brent B has an installed 

 displacement of about 400,000 tons. Its base, composed of 19 cells, is 330 

 feet across; three 525-foot— tall concrete towers support the steel super- 

 structure. The structure was towed 250 miles across the North Sea and 

 emplaced in 460 feet of water in August 1975. £ 5 J 



Because of the present offshore construction activity, concrete 

 societies around the world have committed much effort to defining the state- 

 of-the-art and developing recommended standards of practice for concrete 

 ocean structures. L ~ J This work is continuing. 



Also of particular interest for OTEC applications are the outstanding 

 advances made in the past decade in construction of large concrete structures 

 on land such as ultrahigh-rise buildings, nuclear reactor containment ves- 

 sels, and liquid natural gas (LNG) storage tanks. 



Material 



Well— designed, high quality concrete is an excellent marine construc- 

 tion material for massive, floating structures. Experience has shown a 

 relatively long life for many marine structures with a minimum of mainte- 

 nance and an ease of repair. 



The following topics are various material considerations that have 

 importance to OTEC-type concrete structures. 



14 



