How is a microtunnel subsea outfall designed and executed from land?

Q: How is a microtunnel subsea outfall designed and executed from land?

The design of a submarine outfall with a microtunnel starts with the definition of the exit point under the seabed and the geometry of the route from a well on the coast. The project considers bathymetry, geotechnics, water table, hydraulic slope and implementation space. The execution is carried out with a remote-controlled tunnel boring machine (EPB or hydroshield), controlling chamber pressure, alignment and elevation to advance under the seabed without dredging or offshore works. When the subsea window is reached, a controlled break is made and the subsea branch is connected. This method improves safety, reduces environmental impact and guarantees a precise slope for catchments or spills.

The design of a subsea outfall executed from land with microtunneling begins by defining the function of the outfall (discharge of treated water, desalination brine, aquaculture returns, etc.) and the best discharge area into the sea. From there, a remote-controlled tunnel is traced from an onshore shaft to an outfall point below the seabed, avoiding dredging and open trenches on the seabed. An overview of these assets is explained in what is a submarine outfall.

Preliminary analysis and layout design

Before defining the alignment of the microtunnel, they are studied:

Bathymetry and oceanography (depths, waves, currents, turbidity).
Environmental sensitivity of the environment (protected areas, bathing areas, marine habitats).
Operating conditions: flow rates, discharge regime, dilution requirements.

The article how to plan a subsea outfall from land summarizes these steps and explains how to combine microtunneling or Direct Pipe® in the onshore section with marine techniques in the offshore section.

In parallel, the submerged applications reviews the types of projects in which these solutions are applied: outfalls, dam outfalls, catchments and crossings of marine protected areas.

2. Offshore attack well and associated civil works

The attack shaft is designed to house the TBM, pusher bed and pipe chain, and is usually close to the treatment plant or desalination plant. It must guarantee:

Stability of the enclosure against water table and wave action.
Space for sludge equipment, cranes and surface logistics.
Heights that allow reaching the seabed with the foreseen coverages.

In projects of sea water catchments, This civil work is integrated with pumping chambers and service galleries, as detailed in the following table. seawater catchments with microtunneling, HDD and Direct Pipe.

3. Choice of method and shield type

Depending on the geotechnics under the coast and seabed, it is selected between:

Microtunnel with hydro-shield, when saturated sands and silts predominate and pressure control is crucial.
Microtunneling with EPB, in cohesive or mixed soils with lower permeability.

For desalination, it is common to combine microtunneling with other techniques in the catchment systems, as explained in seawater harvesting systems for desalination.

4. Excavation of the microtunnel under the seabed.

They are continuously monitored during excavation:

Chamber pressure, to balance the front and avoid water ingress or loss of fines.
Thrust and torque, to maintain the feed rate without overloading the pipeline.
Alignment and elevation, with laser or gyroscopic navigation systems.

In some cases, sections are executed in curves or with section changes similar to those seen in reference works, such as the Moncofa submarine outfall or the desalination plant SDP, The project was completed with a combination of onshore and subsea tunnels for brine immisaries and outfalls.

5. Exit window and connection to the marine section.

When the exit point is reached, a controlled breakage The front of the front under the seabed, usually inside a caisson, cofferdam or other protective structure. The subsea branch (diffusers, discharge heads or collection inlets) is then connected.

The complete process of laying and connecting offshore pipelines can be reviewed in the guide how the construction of submarine outfalls is carried out, which explains the marine phases of fabrication, transport, anchoring and protection of the outfall.

6. Benefits of microtunneling in submarine outfalls

Compared to conventional methods, microtunneling from the ground provides:

Lower environmental impact, by avoiding large ditches and dredging in sensitive seabed areas or marine protected areas.
Increased safety, reducing the exposure of personnel and equipment to adverse offshore conditions.
Accuracy in dimensioning and scribing, critical for the hydraulics of the outfall and the correct dispersion of the discharge.
Reduced coastal impact, The project will maintain beaches and seafronts in service during the work.

To see real examples of outfalls and catchments executed with this philosophy, you can consult the projects highlighted in the section of works of Eurohinca and the cases of efficient catchments and returns for aquaculture, The report details solutions in fish farms and demanding coastal environments.