Air Conditioning Pipelines

Cornell University’s Lake Source Cooling Project

Alignment and bathymetry of the 63″ intake pipeline for Cornell’s Lake Source Cooling Project.

In 1998 Makai was hired by Gryphon International Engineering Services and Cornell University to design a 63″ diameter HDPE intake and outfall pipeline in Lake Cayuga near Ithaca, New York to provide 20,000 tons of centralized cooling for the University.  This was a large innovative development for Cornell that made good economic sense, and by eliminating mechanical chillers that used ozone- threatening Chlorofluorocarbons, also made good environmental sense.

Makai’s responsibility was for all aspects of the marine design of 63″ intake pipeline from the Heat Exchanger Facility located 300 feet onshore to the intake structure two miles out in the lake and for a 48″ outfall pipeline from the Heat Exchanger Facility to a point 700 feet offshore. The intake design capacity was 32,000 gpm of 5ยบ C water drawn from a 250 foot depth.

Custom designed recoverable intake structure for the Cornell intake.

A plan view of the designed pipeline alignment is shown above. The pipeline runs through a tunnel from the heat exchanger facility to the lake shore where it is initially dredged into the bottom to maintain clearance from small boats and winter ice floes. When the pipe reaches water depths of 15+ feet it sits directly on the sediment covered lake bottom making a broad turn to the North as it heads for deeper water.  The lake bottom drops off very gradually at first requiring a mile of pipe to reach a depth of 35 feet.  Then from this depth the slope increases and the pipe is laid over the most gradual portion of the steeper slope down to the intake depth of 250 feet.

This was the first pipeline Makai had ever done in a fresh water lake, and it held several design challenges. While the currents and waves predicted in the lake were small compared to those experienced in the ocean, they were large enough to require a unique anchoring system to hold the pipe safely  due to the weak sediments which cover the lake bottom.  Steel shear plates were designed to be coupled to the concrete pipe anchors, and these plates were driven into the bottom after pipeline deployment.  Makai subcontracted Haley and Aldrich, Inc. of New York to assist in the geo-technical investigations and onshore aspects of this work.

The pipeline design life of 75 years and the concerns for existing aquatic species in the lake required careful design consideration. Makai decided early in the design that it was both prudent and technically feasible to design a recoverable intake structure for this pipeline. This would allow both detailed inspection and maintenance to be performed on this critical portion of the pipeline over its entire service life without the need for decompression diving. The natural flexibility of polyethylene pipe made such a development possible.  The last 600′ of pipe were lightly weighted, and the intake structure was designed with special buoyancy modules that could be air-filled to allow it to float on the surface.  The intake structure on the bottom is shown above. The intake can be recovered from the surface using a derrick barge or barge mounted winch.  In addition, a stainless steel screen structure was designed that can be separately recovered; its purpose was to keep fish and other aquatic species out while allowing large water flows to pass through at minimal head loss. Separate recovery of this screen will allow it to be cleaned of Zebra and Quagga mussel growths when necessary.

The deployment of the Cornell intake was challenging because of the large diameter of the pipe and the limited access to the lake for large tugboats.  The contractor used Makai’s controlled submergence process to deploy the pipeline in multiple sections.  Makai carried out extensive analysis to analyze several possible deployment scenarios and also conducted scaled model tests of the HDPE flange connections to ensure that they could safely withstand the expected bending and tensions during deployment. The pipeline was installed in the Fall of 1999 with final completion during 2000.

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