Offshore Installation of Ocean Outfalls Diffuser Assemblies
Authors: Arnold Michael S. , Tait Stuart A. 
Published: December 2007

The Sydney Water Board commissioned the design and construction of threesubsea ocean outfalls to handle the sewage disposal needs of the area for thenext 100 years. The outfalls consist of three underground tunnels, extending upto 3 miles [5 km] offshore, that connect with 93 riser shafts capped with dilluser heads and supported by caissons. This paper discusses the methods andequipment used to install those ocean outfalls.

Introduction

Sydney is a coastal city on the Pacific Ocean. North and south of the cityare more than 30 recreational beaches. Three treatment plants in the area treatthe waste water from 2.1 million people and nearby industries. The treatedwaste water is discharged into the ocean just beyond the shoreline and slightlybelow sea level. This method of discharge has caused an unacceptable level ofpollution on the beaches. pollution on the beaches. The Sydney Water Boardauthorized the construction of submarine ocean outfalls to overcome thisproblem (Fig. 1). The outfalls will discharge the effluent up to 3 miles [5 km]offshore at seabed level. The effluent plumes are expected to remain below seaplumes are expected to remain below sea level for most of the year. The saltwater and sunlight will naturally purify the waste water, further reducing thepossibility of pollution on the beaches. Other possible pollution on thebeaches. Other possible means of disposal were considered but rejected on thebases of cost and environmental impact.

The Water Board installed one outfall at each treatment plant. Each outfallconsists of a tunnel connecting the existing treatment plant with risers anddiffuser heads installed plant with risers and diffuser heads installedoffshore (Table 1). The tunnels, excavated with full-faced boring machines andlined with precast concrete segments, have finished diameters of 7.5 to 11.5 ft[2.3 to 3.5 m]. Minimum rock cover for the tunnels is 148 ft [45 m]. The rockformations are sandstone and claystone with seabed covers varying from finesilt to rock outcrops.

The riser assembly consists of the caisson, the riser, and the diffuser head(Fig-2). The steel caisson supports and protects the diffuser head. The conicalsection of the caisson is used to deflect ships' anchors. The installedposition of the caissons must be determined to within +/-2.3 ft [0.70 m] ofstatewide grid system and to within +/ - 1 f[0. 30 m] of a prescribed heightdatum. The risers connect the tunnel with the diffuser heads. The riser isfabricated of glassreinforced plastic to resist the chemicals used in thewaste-treatment process. An extemal line is installed from the diffuser head tothe ported riser shoe to cement the riser in place. The diffuser head connectsto the riser and directs the effluent away from the installation site. The headis made of glassreinforced plastic encased in concrete with all fittings madeof a high-nickel alloy. The assembly has a minimum design life of 100 years andis expected to meet population requirements past the year 2030.

The diffusers are evenly spaced along the diffuser zone. The line ofdiffusers is located north of and parallel to the main outfall tunnel line. After installation of the diffusers and completion of the tunnels, eachdiffuser is connected to the tunnel with an ocean tap. The tap is a smallertunnel between the main tunnel and the riser shoe. After the riser shoe is cutaway, an L-shaped section is installed, from which pipe is run through the taptunnel and connected to the main outfall tunnel. Mass concrete infill surroundsthe L-shaped section and the pipe in the ocean tap.

Riser Assembly Installation Concept

Establishment of a Positioning System. A seabed/site survey initially isconducted within the area of the diffuser zone with a single line ofbathymetry, a sonar and subbottom profiler through the proposed diffuser sites, and the anchor locations. Three cross lines are run to verify the data.

Before the drilling unit is moved to location, an eight-transponder networkis deployed on the seabed. A microwave positioning system is used to locate thedrop positioning system is used to locate the drop sites accurately. Eachtransponder is equipped with depth, salinity, and temperature sensors andinclinometers. This intelligent acoustic network positions the drilling vesseland the seabed template. Each transponder is calibrated relative to the othertransponders to determine the geometry of the network and to fix thecoordinates of the transponders nominally. The transponders are then calibratedabsolutely to determine the position of the transponders with respect to theIntegrated Survey Grid of New South Wales, which establishes a commonpositioning basis for the installation work that positioning basis for theinstallation work that follows. The absolute calibration is carried out withelectro-optical distance-measuring units located at survey stationsonshore.