Sewer Outfalls

A marine outfall is a pipeline or tunnel that discharges municipal or industrial wastewater, stormwater or other effluent to the sea. Sewage outfalls are common inclusions in the infrastructure of coastal cities and towns. Near-ocean industries may also make use of such pipes. With the help of sea outfalls, wastewater can be transported away from the coastline and discharged at locations where diffusion, dispersion and decomposition are enhanced.

With the exception of relatively few, nearly all metropolitan areas in the world are located next to coastal waters. In addition, more than half the world’s population currently lives within 60km of a coastline. Therefore, tunnelled or directionally drilled outfall constructions are often an effective and sustainable way to improve the quality of life in coastal areas.

 

Outfall projects present complex environmental, engineering, and construction challenges. Historically outfalls using trenchless methods have been used where traditional installation methods were not viable because of depth or ground conditions. The technical advances in the last 20 years, in both microtunnelling and HDD equipment and techniques, have now made their use viable for sewer outfalls; technological furtherance which the team at Terra Solutions embrace and utilise.

The advantages of trenchless installation of sewer outfalls over open cut trenching include:

• Environmental impact is greatly reduced  - minimal surface disruption, lower emissions, no deterioration of seawater quality

• Impact on community & infrastructure is significantly reduced – roads, beaches and shipping can continue as normal, no disturbance of tourism, localised seabed impact

• Location of the launch site for the outfall is much more flexible; behind the foreshore, on top of a cliff etc.

• Higher lifetime of the pipeline due to less risk of settlement, higher seismic safety, cyclonic weather conditions and protection of pipeline (against shipping damage)

• Trenchless methods are less affected by bad weather, the only weather concerns are for the relatively short period during the final breakthrough and when installing the diffuser

Horizontal Directional Drilling Long Sea Outfall Installations start from the landward side with the rig located on the coastline. The pilot bore is drilled to the sea location using advanced guidance equipment such as gyroscopic steering equipment. Usually a barge with a crane is moored at the exit point, where divers recover the cutting and guidance assembly for removal.  A reamer (if required) is then attached, and several reaming steps may take place depending on the pipe diameter. The reamer is pulled towards the rig. Once the borehole has reached the dimensions necessary for the product pipe, the reamer is connected to a swivel attached to the towing head of the product pipe and pulled into the pre-formed hole. The product pipe is floated out to sea beforehand.

Microtunnelled Sewer Outfalls are often carried out in diameters between 1.2 and 2.5m. The TBM (tunnel boring machine) is launched from a land based shaft and can be recovered at its sea destination. To recover the TBM, it is necessary to install a bulkhead equipped with a valve in the end of the line. A reception pit is created by a dredged excavation in the bed of the ocean into which the TBM is driven. Closing the bulkhead allows the TBM to be jacked off the pipe and independent of the pipeline. Divers can connect the machine so it can be recovered onto a barge. The remainder of the work is completed using traditional ocean-going craft and equipment. For deep installations, however, a reception shaft may be created in the ocean to allow the TBM to be recovered; it also acts as a riser shaft for the outfall.

Marine outfalls can be installed using trenchless methods in nearly every geographical and topographic condition, and Terra Solutions offer both tunnelling and directional drilling services in this regard.