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

Treat the CDO and desalination lines as live marine construction with two completed pipe-pull operations, not as a fully accepted water system. The notice status supports the end of those discrete tow, pull-in and diving windows. It does not establish completed head structures, as-built alignment, pressure testing, environmental acceptance, desalinated-water production or handover of the temporary plant.

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

Evidence reviewed

20 July 2026

Controlled taxonomy

Related sectors

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Knowledge context

Related knowledge topics

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Project anatomy

Scope, counterparties, and execution logic

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 read
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.
Next milestone
Cancellation or replacement of the active three-line notice, followed by evidence for head-structure installation, as-built surveys, hydraulic and integrity testing, environmental compliance, connection to the temporary desalination plant, first production water and owner acceptance.

Material change register

What changed, when, and what it proves

Only source-bound events that alter phase, scope, counterparties, delivery, completion, commissioning, or acceptance are listed. A change can advance the record without proving the next gate.

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.

Read controlling 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.

Read controlling 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.

Read controlling 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.

Read controlling source

Delivery watch

Evidence needed next

Cancellation or replacement of the active three-line notice, followed by evidence for head-structure installation, as-built surveys, hydraulic and integrity testing, environmental compliance, connection to the temporary desalination plant, first production water and owner acceptance.

Decision watchpoints

  • 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

Project availability status

This public project page is a tracked market record rather than a tender notice, contractual commitment, or commercial instruction. Use the status, confidence label, evidence-review date, and source note together when judging how much weight to place on the signal.

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Record control

Evidence reviewed date is not a project-event date.

IMWO changes phase, participants, scope, completion, or acceptance wording only when a dated source establishes the change. The review date records when the evidence set was checked; it does not claim that physical work changed on that date.

Evidence reviewed 20 July 2026 Sources in this record 9

Source record

The active-notices register keeps Notice 11 in force and places Notices 19 and 24 in the inactive list. Notice 11 covers the three-line boring and lining sequence and names the principal marine plant. Notice 19 scheduled the CDO pipe pull for about 16 April; Notice 24 scheduled two desalination pipe pulls for about 3 June and states that the notice would be cancelled on completion. The harbour programme includes CDO and desalination dredging, pipe installation and head structures. Sizewell C's water page says the temporary desalination plant will supply most construction water against peak demand of four million litres per day. Planning documents provide assessment-stage head locations and removal intent; they are not as-built records.

Phase: construction. Evidence reviewed: 20 July 2026.

Project sources 9