Design and Construction Planning of Deepwater Ocean Outfall Riser Shafts and Diffuser Structures
Authors: Brooks R.P. , Perrone J.V. 
Published: October 2008

The design and construction planning of deepwater tunneled ocean outfalls involves both underground and offshore engineering. In particular, the demands placed on the outfall seabed riser structures requires consideration of many of the engineering disciplines associated with the offshore industry. This paper describes the investigations, design and construction planning that is required in the development of the seabed diffuser structures and riser shafts of deepwater tunneled outcalls. Explains how modem technology has been integrated with offshore construction and drilling methods typically found in the offshore oil and gas industry.

INTRODUCTION

For many years now coastal cities and communities around the world have been discharging their waste water and sewage into the surrounding coastal waters. Often this discharge or effluent has been untreated and in some locations discharged virtually at the shoreline through open pipelines or via shallow water Outfalls. The result of such practices, in many instances, has been a significant increase in the level of pollution of the marine environment to such an extent that the quality of life in those cities and communities is now becoming threatened.

Governments and regulatory authorities are coming under increased public pressure to rectify this environmental pollution, and in many instances actions have been taken and projects initiated to reduce the level of pollution of the coastal environment. A recognized method of reducing the pollution of coastal waters is to first treat the wastewater and sewage by means of processes which remove the toxins, greases and harmful bacteria, and then to discharge the treated effluent through deep water ocean outfalls.

A deepwater outfall is a conduit that takes treated effluent from an onshore treatment works to a predetermined seabed site some distance offshore. The effluent is discharged into the marine environment at this seabed site through a structure known as a diffuser. The area of discharge is known as the diffuser zone, The outfall conduit can be either a seabed pipeline or a submarine tunnel. Deep water outfalls usually require conduits in excess of four feet diameter, and discharge in water depths in excess of 60 feet.

This paper concentrates on the offshore engineering aspects of deep water tunneled outfalls, and in particular, the design and construction planning of the offshore riser assembly, Particular reference is made to the Sydney Ocean Outfall Project currently under construction off the coast of Sydney, Australia. There are three deep water tunneled outfalls involved in this Project and the outfalls are considered the most technically advanced of this kind ever undertaken.

Reference is also made to the planned Boston Outfall, which when constructed will be the largest tunneled outfall in the world.

OUTFALL CONFIGURATION

A tunneled outfall comprises the following components:An access shaft from the onshore treatment works to the start of the submarine tunnel.A submarine tunnel extending from the base of the access shaft to the offshore diffuser zone.A diffuser zone which consists of a collection of riser assemblies. Refer to Figure 1 for the general arrangement of a deep water tunneled outfall.