Construction of Submarine Outfalls

Service Specifications for Submarine Outfalls

Submarine outfalls enable the safe conveyance of water flows into the marine environment via pipes and tunnels constructed partially from land. Trenchless technologies—microtunneling, pipe jacking, and Direct Pipe®—reduce environmental impact and ensure stability in coastal areas. This service is defined according to the following technical parameters:

• Main applications: Treated wastewater outfalls, pipelines to treatment plants, deep-sea outfalls, return pipelines, and underground crossings beneath beaches, boardwalks, or port areas.

• Construction methods used:
– Remote-controlled microtunnel, ideal for launching from land and reaching the underwater area with precision.
– Pipe jacking, for large structural diameters or great depths.
– Direct Pipe, when permanent steel pipe must be installed in a single push.
The choice depends on the diameter, distance from shore, geotechnical conditions, and required depth.

• Geotechnical conditions and marine environment: Saturated sands, silts, coastal deposits, and soils with a high water table. Internal pressures, soil permeability, slope stability, and coastal dynamics are taken into account.

• Diameters and materials used: Non-accessible pipes made of concrete, steel, or HDPE, depending on flow rate and pressure. For outfalls with higher capacity, accessible tunnels lined with segmental linings may be used.

• Typical lengths: From tens of meters below the shoreline to several hundred meters out into the open sea, depending on the required water quality, environmental regulations, and the configuration of the diffusers.

• Land–sea transition: A critical point in the project, where the microtunnel or pipeline connects to the offshore section. This transition can be carried out using subsea bells, submerged joints, prefabricated sections, or operations involving professional divers or ROVs.

• Civil engineering requirements on land: Manhole and receiving pits, platforms near the shoreline, access points for machinery, a sludge treatment plant, pumping systems, and areas for welding and handling large-diameter pipes.

• Integration with marine structures: Anchoring, ballasting, final alignment of the diffuser, elevation verification using an ROV, and protection of the outfall against waves, currents, and scouring.

• Environmental control: Minimizing impact on the coastline, complying with water quality regulations, protecting sensitive ecosystems, and scheduling work based on sea conditions.

• Quality control and verification: Monitoring of chamber pressure, thrust forces, guidance, excavated volume, and pipe stress (in Direct Pipe®); CCTV inspection of the onshore section; leak tests; and underwater verification following the installation of the diffuser.