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4.2 SUBSURFACE INTAKES 71
FIGURE 4.3 Two Ranney wells for Salina Cruz SWRO plant, Mexico.
and extending from the surface entry point underground past the mean-tide line. This type of wells has found application mainly in seawater desalination installations.
One of the most widely used HDD well intakes today is the Neodren well intake system. A general schematic of Neodren HDD collectors is shown on Fig. 4.4. A detailed description of typical configuration of the HDD wells is presented elsewhere (Peters and Pinto, 2008).
HDD-well intakes collect source water via a number of perforated high-density polyeth- ylene (HDPE) pipes with 120-mm pore openings at a relatively slow rate. The collected saline source water is naturally filtered through the ocean bottom sediments before it reaches the desalination plant. Typical collector pipe size is 450 mm (18-in.). However, collector pipes of diameters as large as 710 mm (28-in.) could be installed. Typically, the individual HDD col- lector pipes deliver the source water into a common wet well from where it is pumped to the desalination plant for further processing. The individual HDD well collectors of this type usually yield between 50 and 150 Lps (1.1e3.4 MGD).
The collector pipes are installed usually at a depth between 5 and 10 m (16e33 ft) below the surface of the ocean bottom in separate boreholes by drilling under the ocean seabed 200e600 m (660e2000 ft) into the coastal aquifer to a location that can yield seawater of
FIGURE 4.4 HDD intake.