Outfall Placed Through a Tangle of Marine Traffic

By Lucy Bodilly

Seattle - Sailing through the Sound dragging a 5,000 ft long sewage pipe is will take a license, planning and luck. Triton Marine, Bremerton, Wash., the design build contractor for the Brightwater Marine Outfall project needed all three early last month when it moved the outfall lines 17 nautical miles from the Snohomish River in Everett, Wash. to Point Wells, just north of Seattle.

The outfall represents the final leg of the journey for effluent that will leave King County's $1.6 billion Brightwater Treatment Plant, now under construction 13 miles north of Seattle. Constructed off-site, the 1-mile long pipe was constructed in Everett and tugged down to Point Wells in Seattle.

Construction of the project started in 2005 and will be completed in 2012. Highlights will include extensive environmental mitigation, North America's largest membrane bioreactor, and 13-miles of tunneling using TBMs and cut and cover. The advanced membrane bioreactor technology will clean the wastewater seven to 10 times cleaner than conventional treatment rocesses, treating nearly all the wastewater there to the state's highest reclaimed water standards. Reclaimed water from Brightwater will be used for irrigation and industry.

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Triton's contract includes construction of twin 5,420 ft HDPE (high density polyethelyne) pipes and diffusers to discharge treated effluent into Puget Sound about a mile off-shore at a depth of approximately 600 feet. Its proposal includes 417 feet of 84-in steel pipe and 5,018 feet of twin 63-inch HDPE pipes (placed using the controlled submergence method) with staggered 250 foot long diffusers.

Placing the HDPE pipeline is risky and was cancelled once for high winds. "We have to pass the ferry lanes, numerous shipping lanes and recreational traffic," says Steve Haskell, Triton assistant project manager. "We had the Coast Guard putting out a call to mariners and had have lots of escort boats to ward off trouble." Before the move took place, Triton worked with the fabricator to submerge the pipe and test for leaks. Then all the water was removed, to allow it to float again.

To move the pipe, tug boat operators attached it to two tug boats and towed it down the Snohomish River into Puget Sound. The tugs play an integral part in placement, by holding the pipe taut while it's filled with water and sinks to the bottom of the trench. Sixty-foot long pieces are pared at the ends to fit over the succeeding section. "The pipe is heated, then pushed together." The connections are stronger than the original pipe," says Haskell.

As the pipe got longer, it floated in the Snohomish River. At the outfall site Triton prepared the trench where the pipe will be placed. The company built a dock from the shore to the end of the trench. Then it placed a sheet metal wall to shore-up the trench. It chose to use twin pipes, instead of one large one, because "there would be less soil disturbance," says Gunars Sreibers, King County project manager.

Silt disturbance is a key factor controlling salmon habitat and the contractor wanted to minimize it." Using two pipes allowed for less disturbance in the low tide area, where most of the sensitive wildlife lives. Triton Marine Construction, under a design-build contract, will be the first major contractor to finish its share of the project. "The contract says we can take two years, but we wanted to try to get done in one, in case we had problems," says Haskell.

Among the difficulties are stringent environmental regulations both at the plant along the Snohomish River where the HDPE pipe is being fabricated and on the Puget Sound shore. Both are critical salmon habitat, which limits the contractors' access primarily from May to October, and puts tight controls on stormwater management and erosion. Native water plants on the Sound bottom were removed and carefully transplanted at a site further up the sound, to be replanted when work is complete. Sand and other material removed when the trench was being dug is also in storage, to be placed when work is completed.

Triton's involvement in the contract started in 2007 when it was one of five teams to respond to King County's RFP. Award of the design build contract was based on 11 factors, with price being weighted the heaviest at 20 percent.

"We wanted technology to be a strong factor in the scoring as well," says Jeff Lundt, King County project manager.

"Permitting for the environmentally sensitive project was already in place, with just some minor tweaking to be done," says Haskell.

"We wanted permitting to be approved as much as possible, to save time," says Lundt.

The team reviewed the contract and decided to go with a HDPE pipe instead of steel or concrete. The main factor was the price of the material and the cost of labor," says Haskell. The choice allowed the contractor to have two 5,000 lin foot pipes fabricated on shore. "Then we locked in the price with our supplier," Haskell says.