Municipality of Metropolitan Seattle
Puget Sound, WA, U.S.

Renton Effluent Transfer System

At the time, the effluent outfall lines Fluor designed for the Municipality of Metropolitan Seattle were the deepest large-diameter lines in the world. The twin 64-inch diameter steel lines with their diffusers extended 10,000 feet from shore to a 630-foot water depth in Puget Sound. The average design discharge rate was 44 million gallons of treated effluent per day.

Fluor was responsible for the engineering, design, permit support, bid evaluations and construction support. To address the steep and irregular seabed and sensitive environmental conditions, Fluor developed a group of non-linear stress history curvature and ovalization programs specifically for these outfall lines.

Client's Challenge

​The Municipality of Metropolitan Seattle sanctioned this capital investment to accommodate the city's expanding needs. A steep and irregular seabed, sensitive environmental conditions and the large pipeline diameters required high-confidence pipeline span and stress predictions. Daily construction challenges included coordination with local area commercial and residential fishing vessels along with awareness of ocean swell/storm conditions.

Fluor's Solution

​During the detailed design phase of the project, Fluor performed computer modeling that established the feasibility of surface towing of the long rings of pipe under a variety of conditions. As a result, remote fabrication yards were used and 500-foot sections were towed to the site for string installation. This assembly method significantly lowered the project cost and provided greater safety and quality control.

Throughout the course of the pipeline project, Fluor coordinated with the client and various government agencies to assist permit approval, provide data to interested organizations and summarize bid documents for potential contractors. This multi-faceted communication was key to ensure all parties were aligned to meet the project's demanding schedule.


​The diffusers were designed to ensure 50 years of operation life against corrosion and fouling. The diffuser support system prevents differential settlement to the pipeline and allows clearances for siltation over the design life.​