(2) Wave Conditions. Predominant sea conditions vary in direction and intensity 

 seasonally with prevailing winds. Deepwater waves from northeast during northeaster storms 

 and from the southeast through southwest during hurricanes present the most adverse sea 

 conditions. Waves higher than 40 feet have been recorded. 



(3) Currents. Currents in the area are influenced by the Gulf Stream, a prevaiUng 

 northeast flow off the coast throughout the year. Surface currents at the site have been 

 reported in excess of 4 knots. 



(4) Winds. The wind at Cape Hatteras averages 8 to 11 knots year round. Prevailing 

 winds are southerly during April through August and northerly September through March. 

 Maximum recorded windspeed is 110 miles per hour. 



(5) Storm Surge and Tides. 



Extreme high tide (approximate), 7.6 feet above MLW 

 Extreme low tide (approximate), 2.0 feet below MLW 

 Mean normal tidal range, 3.4 feet 

 Mean spring tidal range, 4.1 feet 



(6) Littoral Transport. Not applicable (structure far from shore). 



(7) Water Depth at Structure. 54 feet below MLW. 



(8) Foundation Conditions. Structure is located on a sand ridge. Borings at site 

 reveal that soils are basically granular consisting of fine to medium sand approximately to 

 elevation —122 feet underlain by sUty sand with some clay seams and pockets to elevation 

 —220 feet. Shell fragments are prevalent throughout the sands. 



e. Basic Structural Features (Fig. 47). 



(1) Dimensions of Basic Structure (U.S. Coast Guard, 1964). 



(a) Steel Jacket Structure. 80 feet high by 70 feet square at base with 39 -inch 

 diameter by 0.75-inch wall-pipe legs at each corner battered 1:12 to outside. Legs serve as 

 sleeves for pipe piles. Framing members 18 inches in diameter. 



(b) Steel Pipe Piles. 33-inch diameter about 225 feet long (1.12-inch wall from 

 elevation -1-15 to —100 feet and 0.75-inch wall from elevation -100 feet to -210 feet). 



(c) Steel Deck Section. 56 feet high by 56 feet square at top with 33-inch 

 diameter by 1.12-inch wall pipe at corners. Framing members 14 and 18 inches in diameter. 



(d) Machinery and Quarters Deck. 70 by 70 feet. 



(e) Deck Elevations (referred to MLW). Boat landing, -f 11.8 feet; maintenance, 

 +61.0 (10 feet above maximum wave elevation); quarters, -1-79.0; helicopter, +90.3; and 

 lantern platform, +117.4 feet. 



(2) Unusual Structural Features. Template-type structure developed from successful 

 use in Gulf of Mexico and adapted to open Atlantic Ocean. Components of structure were 

 prefabricated onshore to simplify erection at sea. Tower consists of a template structure 

 fixed to the bottom by pipe piles driven through the legs, surmounted by a deck section 

 welded to these piles, which supports the storage tanks and deckhouse. 



86 



