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Southwest Ocean Outfall (SWOO) End Gate Replacement


4.5 miles off Coast of San Francisco, CA


Power Engineering was contracted to remove an existing end gate cover plate on the San Francisco Southwest Ocean Outfall (SWOO) and install a modified monel end gate.  The purpose of the modified end gate is to provide future diver access to the SWOO for inspection and repair.  The work included clearing sea bed debris, removing a temporary steel plate installed by Power Engineering in 2008, lowering and positioning the new gate on the outfall pipe, and coring and installing new hold-down anchors.

The end gate is positioned on the end of SWOO, approximately 4.5 miles off the San Francisco coastline, in 85 feet of water.  The gate is eight feet in diameter and weighs 6,110 lbs. Installation of the end gate was coordinated with the SFPUC’s Oceanside Waste Water Treatment plant to allow for normal plant operation.  The plant allowed diving for 12 hours per day, opening the SWOO at night to pump down the plant.

The end gate was installed in October to take advantage of calm ocean conditions. A 3D side scan sonar was used to locate the end of the outfall and a multiple diver rotation with surface decompression was used to make the most of the limited window of time.  One unique challenge was determining a method for core drilling new hold down holes in the existing concrete outfall pipe, while working with limited bottom time.  Power Engineering designed and installed a custom mechanism for holding and aligning a deep water coring machine, allowing our divers to quickly drill and install replacement hold down anchor bolts.

The end gate is now securely in place and the treatment plant back to full working capacity.