East Anglia Three Closes Its Offshore Installation Phase — and Exposes the Supply Chain Behind 1.4 GW

Seaway7 has laid the last of 95 inner-array cables at East Anglia Three, more than 240 km of cable pulled into foundations across a 1.4 GW site roughly 69 km off the Suffolk coast. The milestone is the second and final structural headline for Seaway7's scope on the project, following completion of all 95 monopiles and transition pieces (TPs) in August. Cable installation, however, is not the same as cable commissioning, and the offshore construction phase is closing with a substantial amount of work still in the water.

What has actually been finished

The distinction matters for anyone reading the milestone as a proxy for project completion. Seaway7's own framing is careful: testing, termination and trenching continue, and the company speaks of progress towards completion of its full project scope, not completion of it.

The sequence is worth noting. The final cable lay landed after turbine installation had already begun, which is normal for large arrays where the electrical works follow the turbine erection front rather than precede it, but it compresses the electrical commissioning window ahead of the stated end-2026 operational date. Turbine installation finishing "this year" and full operation by year-end leaves very little float.

The cable scope is bigger than the headline number

240 km across 95 cables averages roughly 2.5 km per string. That reflects the geometry of a large array with wide turbine spacing and multiple strings routed back to the offshore substation platforms, not unusually long individual cables. The engineering load sits less in the laying than in the terminations — 95 cables require pull-in and termination at each end, and the termination and testing phase is typically where schedule risk accumulates on a project of this size.

Seaway7's award covered both foundations and inner-array cables, a bundled scope that ScottishPower Renewables structured to consolidate offshore vessel demand. Seaway7 classified it as a "very large" contract in Subsea7's disclosure bands, i.e. USD 500–750 million at award — a figure that should be read as covering foundations and cables, not the cable campaign alone (Seaway7 contract announcement; band definitions are Subsea7's).

The foundation campaign was executed by the jack-up Seaway Ventus, and East Anglia Three was the first foundation installation campaign that vessel completed.

Foundation steel: 200,000 tonnes, and almost none of it British

The most useful supply-chain fact in the project sits in the foundations rather than the cables. ScottishPower Renewables' chief executive Charlie Jordan has put the foundation steel content at more than 200,000 tonnes, spread across a seabed area of roughly 43,000 football pitches (Riviera Maritime Media, August 2026). Monopiles ran 1,200–1,800 t each, 67–84 m tall and up to 10.6 m in diameter — described by the project as the largest installed from a jack-up vessel in Europe. TPs added roughly 400 t each at 20 m height and 8 m diameter.

A rough sanity check on the tonnage: 95 monopiles at a midpoint of ~1,500 t gives about 142,500 t, and 95 TPs at ~400 t gives about 38,000 t, totalling around 180,000 t. Jordan's "more than 200,000 t" therefore plausibly includes secondary steel, boat-landing and J-tube structures, scour protection steelwork, or simply sits at the top of the monopile weight range. The figure is developer-reported and is best treated as an order of magnitude rather than a verified mass take-off.

The supply split is the part that rarely makes the press release:

Every monopile and every transition piece on a flagship British offshore wind project came from Spanish fabrication. That is not a criticism of the developer — the UK has no monopile rolling capacity of comparable scale, and the Haizea framework alone exceeded €200 million — but it is the structural reality behind the frequently repeated "almost £2 billion invested in the UK supply chain" figure. That £2 billion, which the developer attributes to items including Wood Thilsted foundation design, Siemens Gamesa blade manufacture in Hull, and vessel charters from Norfolk-based NR Marine Services and OEG, is real but heavily weighted toward services, design, blades and marine logistics rather than heavy steel fabrication. Readers should note the methodology for the £2 billion has not been published.

For steel market participants, East Anglia Three is therefore a useful data point on where European offshore wind heavy plate demand actually lands: Spanish and, historically, Dutch and German fabricators, with UK content concentrated downstream.

Ownership and the bigger picture

The £4 billion (≈€4.7 billion) project is owned and developed by ScottishPower Renewables, Iberdrola's UK arm, alongside Masdar. It is the second project under the Iberdrola–Masdar €15 billion partnership and the first with ScottishPower in the UK (Riviera). The 95 Siemens Gamesa machines at 14+ MW — Seaway7 cited 14.7 MW at award — make this one of the largest single fixed-bottom arrays in European waters, and the first UK project to deploy 115 m blades.

What to watch next

Three things will determine whether end-2026 holds:

• Termination and testing throughput. 95 cables × 2 ends is a large commissioning load, and it is the least visible part of the programme.
• Turbine installation completion. Begun in April and slated to finish this year; any slippage pushes first full-power operation into 2027.
• Trenching and survey sign-off. Cable burial verification and any remedial work are the classic late-stage schedule surprises on arrays of this scale.

Seaway7's cable milestone is a genuine achievement and a clean signal that the offshore construction phase is winding down. It is not, however, a completion signal — and the more consequential intelligence for the metals and fabrication supply chain is not the 240 km of cable but the 200,000 tonnes of Spanish-fabricated steel that preceded it.