Large Diameter Steel Pipe Design under Deep Water: Lake Travis Experience

Abstract

The City of Austin, TX is building a new water treatment plant, Water Treatment Plant No. 4 (WTP4), to provide added treatment and transmission capacity necessary to meet projected increases in demand on a schedule that accounts for increased water conservation. The WTP4 will have a 50 million gallons per day (MGD) initial capacity, be expandable to 300 MGD in the future and will provide the City with increased system redundancy and reliability. Raw water supply for WTP4 comes from a deep water intake facility located in Lake Travis. The intake facility consists of three large custom designed intake screens, which are located at three different elevations and separated horizontally by approximately 80 ft. Each of the screens is designed to withdraw 300 MGD flow. The three screens are interconnected by a 160 ft long, 9-ft diameter steel pipe manifold, which is installed along the sloping lake bottom. Lake water will be drawn through the sides of the screens, then flow through the sloping steel pipe manifold, and enter a 9-ft diameter steel riser pipe embedded 70 ft into the lake bottom, and eventually reach the water treatment plant via a series of tunnels and a pump station. The water depth at the intake area varies between 60 ft to 190 ft. This paper will focus on the unique challenges that were associated with the planning and design of the 9-ft diameter riser pipe at the “lake tap” and the complex 9-ft diameter steel pipe manifold. Some of the major challenges included: (a) constructability and logistics of the pipeline installation under great water depth, (b) design of riser pipe to facilitate the “lake tap” construction; (c) reliable anchorage of the steel pipe with minimum cost; (d) safety concerns and risk management during construction, and (e) corrosion protection of steel pipe in submerged environment.