Authors:
A. Ruiz, D. Jurich, G. Leary
North American Society for Trenchless Technology (NASTT); No Dig Show 2010 Chicago in May 2010; and North American Tunneling Conference 2010 (NAT), June 2010
Abstract
This paper discusses the drilling of large diameter inclined shafts into Lake Powell as part of a new deep water intake system for a coal fired power plant. A two pass drilling operation and down hole position locating technology combined with real time monitoring were used to maintain deviation to less than 1% and successfully hit a small target area located 250 feet underwater. The 2,250 MW coal fired Navajo Generating Station located in Page, AZ draws water from Lake Powell using submersible pumps installed in five inclined shafts. Drought threatens to lower the reservoir to below the existing intakes. New steel lined intake shafts located on the same site were required to ensure uninterrupted plant operation. The small size of the site and the need to keep the existing system in operation required detailed designs and placed significant limitations on construction equipment and activities. The Navajo Sandstone forms the near vertical shoreline of the reservoir, is 98% very fine grained quartz, highly abrasive, and contains highly fractured intervals that presented numerous challenges to drilling the new 500-foot deep 43-inch diameter intake shafts inclined 23 to 26 degrees from vertical. Spatial constraints, submersible pump design, and operational criteria required each shaft to have a unique inclination and orientation. Environmental requirements mandated drilling fluids introduced to the lake were kept to a minimum and grout was not allowed in to the lake under any circumstances. The shafts were video taped and the breakout locations were examined using an underwater ROV to verify rock conditions and location and to guide placement of pneumatic packers to ensure grout did not transmit to the lake during the steel liner installation. In anticipation of the invasion of Quagga mussels, the shaft design included an allowance for a chemical dosing system and a copper rich alloy at the exposed portions of the steel liner.
