Project record

Sizewell C temporary desalination intake, outfall and CDO

Official Sizewell C marine works map showing the combined drainage outfall, temporary desalination intake and outfall, marine freight facilities and permanent cooling-water routes

Sizewell C Harbour Authority; official project marine works map

Sizewell C Harbour Authority is overseeing three sub-seabed utility lines off the Suffolk coast: a combined drainage outfall for treated construction-site wastewater, a seawater intake for the temporary desalination plant and a return line for surplus seawater. The harbour master's February notice started sequential boring and lining. Separate notices then scheduled the CDO pipe pull for April and the desalination intake and outfall pipe pulls for June. Those two operation-specific notices are now inactive, while the wider three-line construction notice remains active.

Project signal

Construction in United Kingdom

Current status Three sub-seabed utility lines remain under an active harbour construction notice; the CDO and desalination pipe-pull notices have closed Source confidence Confirmed

Location

United Kingdom / Europe

Segment

Marine utilities

Status

Three sub-seabed utility lines remain under an active harbour construction notice; the CDO and desalination pipe-pull notices have closed / Confirmed

Last checked

20 July 2026

Coverage

Related sectors

Marine utilities is the primary work classification. Related sectors connect this record to adjacent marine delivery and market coverage.

Subject index

Related subjects

These subject pages bring together related projects, technical articles, reports, and evidence methods.

Project anatomy

Contract scope and delivery roles

Client / authority
Sizewell C Limited, with marine operations regulated by the Sizewell C Harbour Authority
Delivery parties
Sizewell C Civil Works Alliance - wider civils delivery alliance including marine and tunnelling works, NP497, C-Odyssey, Paul D, MTS Valour and supporting workboats and guard vessels - named in harbour notices for the pipe-pull operations, Package-level tunnelling, pipeline and head-structure contractors - the harbour notices do not identify them
Contract route
Sizewell C formed a Civil Works Alliance with Balfour Beatty, Laing O'Rourke and Bouygues Travaux Publics. Its published remit includes marine and tunnelling works, but the alliance announcement does not assign the temporary desalination or combined-drainage lines to a named member or disclose the package contract. Harbour notices supply the construction dates, methods and named vessel spread.
Execution window
Notice to Mariners 11 set commencement on or about 19 February 2026 for sequential boring and lining of all three sub-seabed lines. The CDO pipe pull was scheduled from about 16 April and the desalination intake and outfall pipe pulls from about 3 June. The operation-specific notices have since moved to the inactive register. The wider construction notice remains active and the harbour programme continues to show dredging, pipe installation and head structures through 2026.
Works package
Construct a treated-wastewater combined drainage outfall plus temporary desalination seawater intake and surplus-water outfall below the seabed. Planning documents place the desalination outfall head about 385 metres seaward of the hard coastal defence and the intake head about 100 metres farther offshore. The temporary heads are intended to be removed after use. Harbour notices describe approximately 500-metre floating pipes, jack-up barge NP497, four-point mooring, towage, pull-in and diving operations.
Marine interfaces
The package links sequential tunnel boring and lining, pipe storage and towage from the River Stour, jack-up and multicat positioning, four-point mooring, diving, pull-in, offshore head placement, shore-side connections and harbour traffic control. The desalination lines also interface with temporary construction-water demand, while the CDO serves treated wastewater from the main construction site. These temporary systems remain separate from Sizewell C's permanent cooling-water intake and outfall works.
Commercial evidence boundary
Marine construction is under way rather than open for a new owner-level tender. Remaining work may include head structures, offshore connections, diving and survey support, testing, environmental monitoring, temporary-system integration and later removal. Package values, subcontract awards, installed quantities, as-built geometry and the acceptance regime are not public, and the alliance announcement does not allocate this package to a named member.

Project timeline

Material developments

Dated events that changed scope, counterparties, delivery, commissioning or operating status.

Open this project in the Change Log

The general construction notice remains active after two pipe-pull notices closed

The harbour register lists Notice 11 as active and the CDO and desalination pipe-pull notices as inactive. This supports completion of the two discrete pull-in windows, while head structures, testing, plant integration and final acceptance remain open.

Source

Desalination intake and outfall pipe pulls entered the marine work sequence

Notice 24 scheduled approximately 500-metre floating pipes for towage, jack-up-assisted pull-in and diving operations. The notice later moved to the inactive register; it does not certify the complete desalination marine system.

Source

The combined drainage outfall reached pipe pull

Notice 19 scheduled a 48-72 hour harbour closure around the CDO pull-in using NP497 and associated vessels. Its later inactive status supports the end of that operation, not final outfall acceptance.

Source

Boring and lining started across three sub-seabed utility routes

Notice 11 set commencement for the CDO, temporary desalination intake and desalination outfall and named the initial vessel spread. The notice remains active on the harbour register.

Source

Evidence gap

What remains unverified

Outstanding evidence

  • Cancellation, replacement or extension of the active CDO and desalination construction notice
  • Completion and as-built evidence for all three bored and lined routes
  • Desalination intake, desalination outfall and CDO head-structure installation
  • Pressure, integrity, flushing and hydraulic testing
  • Environmental monitoring and discharge compliance
  • Connection to the temporary desalination plant and first production water
  • Later isolation and removal of temporary offshore heads

Related records 2

Reporting status

Construction, confirmed confidence

Last checked 20 July 2026 across 9 cited sources. The dated timeline above carries the project events.

Last checked 20 July 2026 Cited sources 9

Sources and related records

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Source collections

Project sources 9